Download The Interaction between Dietary Protein and Bone Health

The Interaction between Dietary Protein and
Bone Health:
Dr. David Jesudason
Commonwealth Scientific and Industrial Research Organisation1
Department of Medicine, University of Adelaide2
Food and Nutritional Sciences
CSIRO
PO Box 10041, Adelaide, BC, South Australia 5000.
+ 618 83631155 (T)
+ 618 83632665 (F)
[email protected]
Professor Peter Clifton
Commonwealth Scientific and Industrial Research Organisation 1
Department of Medicine, University of Adelaide2
Introduction:
Bone consists of calcium and phosphorus as part of hydroxyapatite crystals which
00accounts for approximately 70% of its weight and approximately 20% protein such
as type 1 collagen.
Traditionally increased dietary protein has been thought to be harmful to bone health
predominantly due to its association with increased acid load and subsequent
hypercalciuria. However, over the last 40 years researchers have approached this
topic by studying animals as well as humans and performed cross sectional as well as
longitudinal studies. Some studies have focused directly on fracture risk whilst others
have used indirect measures such as bone density or increase in bone formation
markers such osteocalcin or decrease in bone resorption markers such as
deoxypyridinoline or pyridinoline. IGF1 which may promote bone formation has also
been measured.
This review considers what role dietary protein intake has in increasing or decreasing
bone strength and reducing fracture risk. The review will also address any separate
effect of vegetable protein from animal protein such as meat or milk protein. It is also
clear that the effect of dietary protein per se must be considered in the context of the
effect on acid base balance and interaction with dietary calcium and phosphorus.
Method Of Article Selection:
We did a search of PubMed looking at all articles generated by searching dietary
protein, soy protein and milk protein respectively and either bone, bone density or
fractures. All papers generally dealing with human subjects and the interaction
between dietary protein and bone density, bone markers or fractures whether crosssectional, longitudinal were deemed suitable for discussion in this article. There were
also a few studies not generated by this search but which were frequently cited by the
articles generated and these were also included when relevant.
Effects of High Protein Diet on Bone Mass and Bone Density:
There is evidence from studies that amino acids from dietary protein have an anabolic
effect on bone. As early as 1985, essential amino acids were shown by Clemmons et
al. to augment the IGF1 response to refeeding after fasting[1].
Cross-sectional Studies:
Several cross –sectional studies obtained from Pubmed comparing the effect of
dietary protein (usually derived from food diaries or validated questionnaires) have
looked for an association between dietary protein and bone mineral density (BMD) or
bone mineral content (BMC) at the traditionally measured sites. These are
summarised in Table 1. The studies are not directly comparable as the subjects vary
according to age, sex, mean daily protein intake, pre and post-menopausal status of
women, site for BMD measurement, statistical method to look for a relationship
between the two parameters and rigour of the study. Even mean daily protein intake
has to be interpreted with caution as it does not tell us mean intake per kilogram body
weight nor protein intake as percentage of total energy which is not always published.
In young and pre-menopausal women increased dietary protein seems to be positively
correlated with higher bone density at least at some sites particularly the radius[2-6]
In Teegarden’s study a complex relationship between calcium, protein and
phosphorus was obtained so that women with higher protein intakes were better off
with higher calcium supplementation.
In post-menopausal women, increased protein intake was associated with increased
bone density in most not all studies [3, 4, 7-9]. The sites where dietary protein was
correlated included radius, hip, spine and total body BMD. This included the study by
Chiu et al. of predominantly vegetarian women. Usually some but not all sites would
show an association in any given study. Moreover when statistical significance was
shown it was not always apparent the degree to which a higher dietary protein might
produce a clinical benefit.
In the three studies of men, the association between dietary protein and BMD is less
convincing [10-12]. In the one positive study, the mean protein intake was much
higher at 97.0 g protein daily or 1.17 g/kg body weight.
Longitudinal Studies:
Table 2 summarises longitudinal studies looking at dietary protein intake and future
BMD or bone loss at commonly measured sites. These studies usually adjusted for
osteoporotic confounders such as age, weight, height, weight change, total energy
intake, smoking, alcohol intake, caffeine, physical activity, calcium intake, and, for
women, current estrogen use. Most studies are either positive or neutral for bone
when increased protein consumption is noted. There are several interesting
observations.
Firstly the importance of dietary protein intake during adolescence when peak bone
mass is being accrued is suggested by the Saskatchewan Paediatric Bone Mineral
Accrual Study[13]. An effect of increased dietary protein was not shown in the study
by Beasley JM et al with a slightly older but still pre-menopausal population who
may have already largely attained peak bone mass [2].
In older cohorts of men and particularly pre-menopausal women such as the Rancho
Bernardo study, the Framingham Osteoporosis Study and the hospital study by
Geinoz et al. a favourable effect was seen for increased dietary and particularly
animal protein [14-17]. The only neutral study in an older cohort was the study by
Rapuri et al. [9]. This aspect of the study may have been underpowered. In fact the
major negative relationship of increased protein intake and decreased BMD was seen
for vegetable protein in the Rancho Bernardo Study. This study also suggested that
subjects with lower calcium intake might benefit more from higher protein intake.
This is at odds with other studies suggesting calcium adequacy is synergistic with
higher protein intakes such as the Saskatchewan Paediatric Bone Mineral Accrual
Study and as discussed in the calcium subsection below.
Protein Supplementation:
Dietary protein has also been shown to be effective later in life. This was shown when
an oral protein supplement consisting of 90% milk proteins or isocaloric placebo
containing maltodextrins was given to 82 women with a mean age of 80.7 +/- 7.4
years with recent osteoporotic hip fracture [18]. Treatment was for 6 months and
calcium and vitamin D were also supplied. The protein supplemented subjects had
significantly greater increases in serum levels of IGF 1 (85.6% +/- 14.8% vs. 34.1%
+/- 7.2% at 6 months; p = 0.003) and an attenuation of the decrease in proximal femur
bone mineral density (-2.29% +/- 0.75% vs. control -4.71% +/- 0.77% at 12 months;
p= 0.029).
Overall based on these studies there seems to be a small beneficial effect of dietary
protein on BMC and BMD particularly in women in adolescence and early adulthood
when bone mass is being attained as well as after the menopause when bone loss is
accelerated. This was recently tested by Darling et al. who performed a systematic
review and meta-analysis of dietary protein and bone health studying sixty-one crosssectional surveys, cohort and ecological studies [19]. In the meta-analysis most
studies showed a positive influence or no influence of dietary protein on BMD and
BMC with only a minority showing a negative relationship. The 18 cross-sectional
surveys with correlation co-efficients were pooled, by population subtype and by
outcome. The pooled values for population subgroup (eg men, pre and postmenopausal women) and for almost all bone sites were positive and were significant
at most sites including radius BMC and BMD, hip BMD, lumbar spine BMC and
BMD and total body BMD.
Only 6 protein supplementation trials could be used for the meta-analysis and only
then for lumbar spine BMD. For all protein (total protein and milk basic protein), a
significant effect of protein supplementation on lumbar spine BMD was seen
(weighted mean difference + 0.02; 95% CI: 0.00, 0.04; p = 0.04). No effect was seen
on lumbar spine BMD of MBP or soy protein alone. Overall the effect of protein on
bone was modest with the authors estimating that protein intake could account for 12% of BMD.
With regards to the elderly specifically, the beneficial effects of higher protein diets
are not just an attempt to correct protein deficient diets. Gaffney- Stomberg et al. have
shown that the anabolic effect of the body to dietary protein is reduced in the elderly,
so that the amount of protein needed for anabolism is greater and would exceed the
0.8g/kg/day which is the recommended daily intake. In fact they suggest that the
Recommended Dietary Allowance of dietary protein to be increased to 1.0-1.2g/kg
day in the elderly to maximise its anabolic effect on muscle and bone[20]. This would
be consistent with the findings of previously quoted studies such as by Geinoz et al.
where a beneficial effect was shown for total protein intakes exceeding >1g/kg/
day[14]. This is analogous to the study by Wengreen discussed in detail below where
the lowest odds ratio for fracture were in the quartile of 50-69 year olds with protein
intakes as a percentage of energy intake of 17.4-30.8 % as opposed to the quartile
with percentage protein intake of 5.6-13.9%[21].
Relationship between Dietary Protein and Fractures
Epidemiological studies do not provide a clear answer about the fracture risk with
higher protein diets as the results are contradictory and not easy to reconcile. These
studies are displayed in Table 3.
For example the Nurses Health Study suggested increased dietary protein
consumption may increase fracture risk [22]. In this large cohort study for 12 years,
dietary protein was associated with an increased risk of forearm fracture (relative risk
(RR) = 1.22, 95% confidence interval (Cl) 1.04-1.43, p for trend = 0.01) for women
who consumed more than 95 g per day compared with those who consumed less than
68 g per day. This association was linked to animal but not vegetable protein
consumption. The increased rate of forearm fracture was slightly reduced in those
consuming more calcium but there was no reduction in fracture risk when women
with a high ratio of dietary calcium to total protein (≥ 11) were compared to women
with a low ratio (<5.5) (RR = 0.91, 95 percent CI 0.78-1.06).
In a group of 19,752 women and 20,035 men from Norway followed for an average of
11.4 years, a semi quantitative dietary questionnaire was completed at baseline[23].
There were 213 hip fractures identified during follow up with no overall association
between calcium intake or non-dairy animal protein intake and hip fracture in this
cohort. In this instance there was a calcium interaction in that there was an elevated
risk of fracture in women with a high intake of protein from non-dairy animal sources
in the presence of low calcium intake (relative risk = 1.96 (95% confidence interval
1.09-3.56) for the highest quartile of non-dairy protein intake and the lowest quartile
of calcium intake vs. the three lower quartiles of protein intake and the three higher
quarters of calcium intake). These two studies involved pre-menopausal women but
the numbers were very small in the high protein/low calcium group
In older women some of the studies are more convincing for the benefit of a high
protein diet.
Dietary nutrient intake by food frequency questionnaire was assessed in a cohort of
Iowa women aged 55-69 y at baseline and followed for 104338 person-years [24].
There were 44 cases of incident hip fracture during follow up. In this case fracture
risk was negatively associated with total protein intake and animal protein. In a
multivariate model with inclusion of age, body size, parity, smoking, alcohol intake,
estrogens use, and physical activity, the relative risks of hip fracture decreased across
increasing quartiles of intake of animal protein as follows: 1.00 (reference), 0.59
(95% CI: 0.26, 1.34), 0.63 (0.28, 1.42), and 0.31 (0.10, 0.93); P for trend = 0.037.
There was a non-significant trend for higher vegetable protein intake to increase
fracture risk.
In the Adventist Health Study 2, the incidence of wrist fractures were determined in
1865 peri and post-menopausal women who had completed two lifestyle
questionnaires 25 years apart [25]. Amongst the vegetarians, those who consumed the
least vegetable protein intake were at highest risk for fracture. As levels of plantbased high-protein foods increased the wrist fracture decreased with a 68% reduction
in risk (hazard ratio (HR) = 0.32, 95% confidence interval (CI) 0.13-0.79) in the
highest intake group. In meat eaters, amongst those with lowest vegetable protein
consumption, increasing meat intake decreased the risk of wrist fracture with the
highest consumption decreasing risk by 80% (HR = 0.20, 95% CI 0.06-0.66).
In a subset of the Shanghai Women’s Health Study, 24 403 post-menopausal women
aged 40-70 were followed for a mean of 4.5 years[26] . Their dietary intake including
of soy protein was determined on several occasions by food frequency questionnaire
and a link with fracture risk was sought. After adjusting for confounders the relative
risks (95% confidence intervals) of fracture were 1.00, 0.72 (0.62-0.83), 0.69 (0.590.80), 0.64 (0.55-0.76), and 0.63 (0.53-0.76) across quintiles of increasing soy protein
intake (P<.001 for trend). This trend was particularly marked for women soon after
the menopause.
In men such as in the study by Mussolino et al. the effect of dietary protein on hip
fracture seems less convincing [27].
In the study above by Schurch, there were seven vs thirteen new vertebral deformities
respectively amongst patients who received protein supplements vs controls, (p > 0.2)
[18].
Dietary protein is also anabolic later in life. A study from Utah also supported the role
for a higher protein diet for bone health [21]. Wengreen et al. recruited subjects aged
50-89 with previous hip fractures and matched them with controls of the same age and
sex in a case-control study. The 2501 subjects’ diets were assessed using a picturesort food frequency questionnaire. Protein intake was stratified into quartiles and
logistic regression was applied controlling for gender, smoking, oestrogen, calcium
and vitamin D use as well as body mass index and level of physical activity. There
was no association between protein intake and hip fractures for the 70-89 year old age
group. However, for participants aged 50-69, the odds ratio of a hip fracture decreased
from an odds ratios of 1.0 [reference] for the lowest protein intake quartile to 0.35
[0.21-0.59]; (p < 0.001) for the highest protein consuming quartile. In this study,
stratifying by high and low calcium did not alter results.
Despite the promising results from some of these studies , in the meta-analysis
discussed above, there was no overall effect of protein intake on hip fracture risk [19].
Disappointingly however only 4 studies were able to be used (the Nurses’ study, the
Norwegian Study, the Iowa study and NHANES 1 study) and then only hip fracture
examined. Hence it is hard to draw definitive conclusions from the meta-analysis
about fracture risk and it is possible that a small beneficial effect of dietary protein
may have been missed.
It is possible that whilst higher protein diets may be beneficial in improving bone
mass, the effect is modest and dependant on interactions with dietary acid and calcium
as discussed below and easier to demonstrate at times of increased bone formation
and resorption and in women. With regard to fracture risk, as bone mass is only one
factor in the aetiology of fractures, it is harder to demonstrate a beneficial effect of
high protein diets when there is interplay with other factors including falls risk,
muscle strength, and other nutrients such as Vitamin D. Nevertheless a higher protein
diet seems to show a more convincing effect on reducing fractures in older rather than
younger subjects, perhaps when its anabolic effect may be more important.
Relationship between Dietary Protein, Acid and Bone:
Traditionally high dietary protein was thought harmful to bone health. Wachman and
Bernstein hypothesised in 1968 that the increased incidence of osteoporosis with
ageing was due to “the life-long utilisation of the buffering capacity of the basic salts
of bone for the constant assault against pH homeostasis” due to the western meat diets
containing “acid ash”[28]. It has long been hypothesised that diets high in acid, cause
a low grade metabolic acidosis and mobilise base from the skeleton and thereby
contributing to bone loss with ageing.
As early as 1994, potassium bicarbonate was administered to post-menopausal
women for 18 days whose diet was otherwise controlled for calcium and protein [29].
There was an improvement in calcium and phosphorus balance (due to reduction in
urinary excretion of these elements) and reductions in urinary hydroxyproline and
increases in osteocalcin suggesting a beneficial effect on bone turnover. When
potassium citrate supplementation was given to post-menopausal women for three
months, a significant reduction in urinary deoxypyridinolines and hydroxyproline-tocreatinine ratios was achieved [30].
In terms of dietary protein, it is the effect of the sulphate acid load derived from the
amino acids methionine and cysteine that have been postulated to have a
demineralising effect.
The relationship between meat intake and acid load and also urine pH has been
examined in several studies. In a cross-sectional study, 161 post-menopausal women
had DEXA measurement of bone density at the lumbar spine and total hip[31]. The
dietary intakes of sulphur containing amino acids, protein and minerals were also
assessed. Although dietary protein did not predict BMD at lumbar spine, a positive
relationship was demonstrated after adjusting for the negative effect of sulphate,
suggesting that non sulphur containing amino acids are beneficial to bone. The hip
BMD was positively associated with dietary protein; the relationship did not change
after adjustment for sulphate.
Several groups have tried to reduce dietary protein and acid load and study the effect.
Ince et al. studied the effect of lowering protein content in the diet in 39 healthy,
premenopausal women consuming ad libitum diets[32]. Mean daily protein was
decreased from 1.1 g/kg to the US recommended dietary allowance of 0.8g/kg daily
on an isocaloric diet. There was no change to concentration of other minerals such as
819 mg (20.5 mmol) Ca, 1152 mg (37 mmol) P, 129 mmol Na. This required the
substitution of carbohydrates and fats for protein as well as using phosphate
supplements. Bone markers were measured before and one week after the switch. As
the dietary protein was reduced, mean urine pH increased from 6.3 to 6.8 (P < 0.001),
and net renal acid excretion (NRAE = urine ammonium plus titratable acids minus
bicarbonate) decreased 68% (21.4 mEq/d; P < 0.001. Mean urinary calcium decreased
32% [42 mg (1 mmol)/d; P < 0.001], and bone resorption urine N-telopeptides
decreased 17% (74 micromol bovine collagen equivalents/d; P < 0.001).
Although in this study, reduced dietary protein had a beneficial effect on bone
markers and reduced calciuria due to decreasing acid load, the authors acknowledged
that dietary modifications, such as increasing vegetable and fruit intake, could result
in sustained reductions in NRAE without reducing protein intake. This is a valid
insight as other studies have shown a beneficial effect of dietary protein when these
factors are considered.
In the Framingham Osteoporosis Study discussed above, the potentially catabolic
effect of dietary acid on bone was considered [15, 17]. As expected the dietary intake
of basic foods were positively associated with BMD at baseline and with reduced
bone loss in men. However higher rather than lower protein intake was associated
with less bone loss reflecting the interaction of protein with other components in a
mixed diet.
Alexy et al. conducted a prospective study of 229 children and adolescents aged 618[33]. Every year for four years, dietary intake was measured from 3 day weighed
dietary records followed by bone measurement on one occasion by quantitative
computed tomography at the proximal radius. Using an algorithm utilising dietary
phosphorus, protein, potassium and magnesium, the potential renal acid load (PRAL)
was determined as a marker of dietary acid load. The authors adjusted for confounders
such as sex, age, energy intake, body mass index, growth velocity and stage of
puberty. Children with low intake of alkalinizing minerals (i.e. with a higher dietary
PRAL)had significantly less bone mineral content (p < 0.01) and had significantly
less cortical area (p <0.05). In addition the long term protein intake was significantly
associated with markers of bone modelling and remodelling such as periosteal
circumference (p < 0.01), bone mineral content (p < 0.01), cortical area (p < 0.001)
and polar strength strain index (p < 0.0001).
Sebastian speculates that this trade off between the potential anabolic effect of a high
protein diet and the catabolic effects of a high acid diet interact to determine the
overall net effect of any given diet[34]. Thus according to him the ideal diet from a
bone viewpoint in order to achieve peak bone mass in early adulthood would be more
akin to that of prehistoric hunter gathers. This would combine a high protein diet with
a high alkaline load such as fruit and green leafy vegetables, stalks and roots rather
than acid producing cereal grains. Not only are fruits and vegetables a good source of
dietary potassium which has an alkalinizing effect but cereals are also a major source
of phosphorus – an excess of which can increase parathyroid hormone secretion and
bone resorption. The importance of fruit and vegetables was also recently emphasised
by Heaney and Layman[35].
Protein and Calcium Adequacy:
There is evidence that dietary protein has a more favourable effect in the presence of
calcium adequacy [13]. Besides the issue of acid-base status, one concern about high
protein diets are that they cause calciuria. Theoretically, this could lead to negative
calcium balance in the absence of supplementation and cause secondary
hyperparathyroidism and calcium loss from bone.
In a well quoted study, Dawson-Hughes et al. reviewed the protein intake of 342
healthy subjects aged over 64 who had completed a 3 year study of calcium citrate
malate and Vitamin D supplementation or placebo[36]. Overall the mean (+/-SD)
protein intake of all subjects was 79.1 +/- 25.6 g/d whilst the mean total calcium
intakes of the supplemented and placebo groups were 1346 +/- 358 and 871 +/- 413
mg/d, respectively. In this study only in the group supplemented with calcium was a
higher protein intake significantly associated with a favourable 3-y change in totalbody and femoral neck BMD. It should be noted however that New raised the
possibility that some of the beneficial effect of calcium may be related to the
alkalizing effect of the citrate rather than the calcium alone[37].
There is also some reassuring evidence that the hypercalciuria associated with a high
protein diet is accompanied by and due to increased gut calcium absorption rather
than due to release from bone in a buffering response to the acid load from the
protein. In an elegant experiment, Kerstetter JE et al. studied 13 women who received
10 days of an experimental diet after a two week run in period. These women received
either a moderate (1.0 g/kg) or high (2.1 g/kg) protein diet in random order[38].
Calcium kinetics was studied using dual stable calcium isotopes. Urinary calcium
increased during the high-protein diet in comparison with the moderate (5.23 +/- 0.37
vs. 3.57 +/- 0.35 mmol/d, P < 0.0001, mean +/-sem) but this was due to increased
intestinal calcium absorption (26.2 +/- 1.9% vs. 18.5 +/- 1.6%, P < 0.0001, mean +/sem). Hence the high protein diet did not affect net bone balance and in fact caused a
significant reduction in the fraction of urinary calcium of bone origin and a non
significant trend toward a reduction in the rate of bone turnover.
Type of Protein:
Much research has also gone toward investigating what might be the best source of
protein for bone health in both animals and humans.
Soy Protein:
Soy protein is notable for its low fat content and for containing phytochemicals such
as isoflavones, saponins and phytic acid. Phytates are naturally occurring compounds
particularly found in whole grains. Despite concern that they may reduce calcium
absorption, there is some evidence that they are beneficial for bone health [39].
Soy protein studies are of three types; firstly studies aiming to see if a higher soy
protein intake per se is beneficial usually by cross-sectional analysis, secondly
longitudinal studies comparing isoflavone rich versus isoflavone poor soy protein and
lastly comparing soy with another protein such as milk protein [40-53].
In the cross-sectional studies by Ho and Horiuchi respectively in post-menopausal
women, increased dietary soy protein was associated with increased BMD/BMC at
hip, total body and spine respectively. However when high versus low isoflavone
concentration was compared in longitudinal studies varying from 3-12 months, a
beneficial effect of isoflavones was mainly seen on markers of bone formation and
resorption. In a few studies a beneficial study on spine BMD was shown for the
higher isoflavones [40, 50, 51]. In the two studies involving men, no effect was seen
for BMD. In comparison with milk derived proteins no beneficial effect was seen on
bone density.
In the subset of the previously mentioned Shanghai Women’s Health Study, women
in the lowest quintile of soy intake actually had a relative risk of 0.63 for hip fracture
compared to the highest quintile which is at odds with all the other studies suggesting
no effect or a beneficial effect of soy protein. This was the only major fracture study
looking at soy protein so it is hard to know how to interpret this result. Overall based
on these very mixed results there is little to definitely suggest that soy protein itself
has any intrinsic properties making it superior to other proteins. Once again dietary
interactions may be important.
Roughead suggests soy protein may be more favourable at doses above 40g/day and
in the presence of calcium supplementation (eg 650-1400mg in previous studies) such
as in some of the studies above[40, 41, 51, 52].
Milk Protein and Bone:
The effect of milk whey protein supplementation (particularly its basic protein)
fraction on bone health has also been studied. The Copenhagen Cohort Study was a
cross-sectional study performed on 63 seventeen year old girls and 46 boys[54]. A
seven day diet analysis was performed and BMC and serum markers of bone turnover
as well as IGF1 were measured. Both total and milk (0.3g/kg) protein intake, but not
meat protein intake (0.4g/kg) was positively associated with size-adjusted BMC (P≤
0.05). After correcting for calcium intake, physical activity and energy, the positive
association between milk protein intake and size adjusted BMC remained significant
(P≤ 0.01) and did not seem to be mediated via current IGF-1.
A series of studies have been performed in Japan some of whose authors are from
Snow Brand Milk Products Co Limited[55-59]. They have variably studied healthy
women, post-menopausal women, and healthy men and looked at the effect of doses
of 40-300mg of milk basic protein supplementation for e.g. periods of 6 months. They
have found suppression of bone resorptive markers such as urinary cross-linked Ntelopeptides of type-I collagen/creatinine and deoxypyridinoline/ creatinine and
variably increased markers of bone formation like ostocalcin. They have also
demonstrated modest effects on bone density compared to control at sites including
calcaneus, lumbar spine and radius. These results are somewhat surprising. For
example even modest doses of 40mg increased the mean (+/- SD) rate of left
calcaneus BMD gain of women in the MBP group by 3.42 +/- 2.05% compared to the
BMD of women in the placebo group of 2.01 +/- 1.75%, p = 0.042.
However not all studies have shown a convincing a benefit of MBP. In one study 84
healthy young women were divided into 3 groups receiving placebo, whole milk or
milk containing 40mg MBP for eight months[60]. BMD was measured at lumbar
spine and forearm at baseline and 8 months as well as indices of bone turnover. The
total BMD actually significantly increased compared with the baseline values in all
groups. However, no significant difference on the mean rate of gain of total BMD was
observed among the MBP group (2.19%), the whole milk group (2.63%) and the
control group (1.61%). There was no significant increase in markers of bone
formation like bone – specific alkaline phosphatase in either milk group. Serum
(NTx) in MBP group at 8 months and in whole milk group at 6 months were
significantly decreased from baseline. Although there was no significant difference
between the whole milk group and MBP group, after combining the milk groups, NTx
had significantly decreased from baseline.
Animal vs Vegetable Protein:
Attempts to demonstrate the superiority or inferiority of animal versus vegetable
protein to bone health has been confounded by the fact that communities consuming
more vegetable protein tend to also consume more fruit, vegetables and potassium
which tends be more alkaline with beneficial effects as described above.
The effect of varying amounts of dietary calcium is also an issue. Hence
epidemiological studies can tend to favour vegetable protein For example a global
analysis was performed by recording hip fracture rates per 100 000 person years in
women aged over 50, from 33 different countries. [61]. Those 11 countries in the
lowest tertile of hip fracture rates had the lowest animal protein consumption with
vegetable protein intake exceeding animal protein. Of the 11 countries in the highest
risk tertile for hip fractures, animal protein consumption exceeded vegetable protein
intake in 10 countries. Overall hip fracture rates varied directly with total (r = +.67, p
< .001) and animal (r = +.82, p < .001) protein intake and inversely with vegetable
protein intake (r = .37, p < .04). The vegetable/animal protein intake accounted for
70% of the total variation in hip fracture rates. The authors concluded that the critical
determinant of hip fracture risk in relation to the acid-base effects of diet is the net
load of acid in the diet both from dietary protein as well as from other sources such as
fruit and vegetables. There are several weaknesses with cross-sectional between
country studies such as these. Firstly the links were associational and confounding
factors such as physical activity, and cultural variations were not controlled for.
Additionally the study required using dietary data for the whole country and applying
it to one subset of the population i.e. post-menopausal women.
In the EPIC Potsdam Study, the relationship between bone health and diet,
particularly animal and vegetable protein and their interaction with calcium was
studied[62]. The 8178 females had bone measurements performed on their right os
calcis by broadband ultrasound attenuation (BUA) and the relationship between BUA
and dietary protein was assessed by a linear regression model after adjusting for
confounding factors. A high vegetable protein intake correlated with an increased
BUA (beta = 0.11; p = 0.007) whilst a high intake of animal protein was associated
with decreased BUA values (beta = -0.03; p = 0.010). The authors admitted however
that they could not exclude the possibility that some other factor such as the
alkalinizing effect of the diet (e.g. from vegetables) rather than the amount of protein
per se was responsible. Moreover, the apparent detrimental effect of animal protein
on bone was attenuated in those subjects with a high calcium intake. There was a
significant interaction between calcium and animal protein but not calcium and
vegetable protein.
Sellmeyer DE et al. followed 1035 women aged over 65 and followed them
prospectively for a mean (+/-SD) of 7.0+/-1.5 years[63]. Protein intake whether of
vegetable or animal origin was noted from a food frequency questionnaire whilst
BMD was measured by dual-energy X-ray absorptiometry. They found that women
with a higher ratio of animal to vegetable protein had greater bone loss at the femoral
neck (p=0.02) and greater risk of validated hip fracture (relative risk = 3.7, P = 0.04)
after adjustment for confounding factors including calcium and total calcium intake.
In a subsequent clarification, they explain that they were not suggesting that animal
protein per se is harmful to bones. However the animal to vegetable protein ratio of a
community is a surrogate for a higher consumption of animal foods with less
accompanying foods such as vegetables which are base rich. [64].
Conversely many of the longitudinal studies in table 2 showed a beneficial effect of
animal protein and with the Rancho Bernardo study vegetable protein actually had a
deleterious effect. [16].
In the Adventist Health Study 2 discussed above, both increasing vegetable and meat
protein appeared to attenuate wrist fracture risk [25].
One concern with meat protein is that it may have a negative effect on calcium
balance as discussed above compared to vegetable protein or compared to a low meat
diet. Two studies by Roughead et al. are reassuring. Roughead et al. compared the
effects of a high or low meat diet (20 vs. 12% of energy as protein) on calcium
balance in 15 post-menopausal women [65]. Women received 600mg calcium
supplementation and either diet for 4 weeks. Calcium retention was measured by
extrinsically labelling the 2-d menu with (47) Ca, followed by whole body
scintillation counting for 28 Days. Calcium retention was not different during the high
and low meat dietary periods (d 28, mean +/- pooled SD: 17.1 and 15.6%, +/-0.6%,
respectively; P = 0.09). The diets did not affect urinary calcium loss nor indicators of
bone metabolism. An initially higher renal acid excretion in subjects consuming the
high meat compared with the low meat diet decreased significantly with time.
Another carefully designed feeding study followed 13 post-menopausal women [52] .
These women consumed one of two similar diets except that 25g of high isoflavone
soy protein was compared with a similar amount of meat protein for 7 weeks in a
randomized cross-over trial. No beneficial or harmful effect was noted on calcium
homeostasis as determined after a 47Ca retention study followed by whole body
counting for 28 days. No difference was noted in bone specific alkaline phosphatase,
osteocalcin, serum IGF1, serum IGF-IBP3 nor tartrate resistant acid phosphatase.
Calcium was similarly retained during the control and soy diets (d 28, percent dose,
mean +/- pooled sd: 14.1 and 14.0 +/- 1.6, respectively). Although during the soy diet,
there was a 15-20% lower renal acid excretion, urinary calcium loss was unaffected
by diet.
Conclusion:
On balance, dietary protein by itself appears to have an anabolic effect on bone and
has variably shown favourable effects on bone markers including IGF1, bone density
and much less consistently on fracture risk where other non-dietary factors such as
falls risk are important.
Dietary protein may be more important at key times in the life cycle such as
adolescence when bone mass is being acquired, after the menopause when increased
bone loss occurs and in late life when dietary protein intake might be otherwise low.
The benefit of increased dietary protein may be harder to show in men than women
because their baseline osteoporosis and fracture risk is lower.
Early fears about protein induced hypercalciuria have been somewhat allayed by
studies showing increased gut absorption of calcium. However several studies have
suggested that dietary protein works best in the presence of calcium sufficiency. Also
essential is the presence of adequate fruit and vegetables which will reduce the acidity
of the diet as a whole.
Protein from vegetable, milk and meat sources has each been (inconsistently)
demonstrated to aid bone health. The relative superiority of one source over another
particularly in epidemiological studies may relate more to other components of the
diet such as calcium and fruit and vegetables rather than to characteristics of the
protein itself. There is not enough evidence to suggest that one protein source (e.g.
soy, milk protein) should be favoured over another source.
Because the overall benefit of dietary protein on bone is modest, recommendations
about its use should be targeted to higher risk groups such as the elderly and only in
the presence of adequate calcium and fruit and vegetables.
Moreover further trials aimed at longitudinally studying the effect of high protein
diets using hard end-points like fracture reduction are indicated. Ideally these should
be performed in high risk groups like the elderly in the presence of calcium and acid
base sufficiency.
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Table 1: Cross-sectional Studies linking protein with BMD/BMC
Study Name Age
Number
Diet
Endpoint
Results
Composition
Teegarden D
18-31
215 women
et al. [6]
Usual diet
BMD FN,
Protein,
(mean protein
spine, hip
intakes positively
76.0g/day)
radius, TB
correlated with
and BMC
radius,
spine,
spine BMD and
radius, TB
spine BMC
Protein calcium
interaction.
Beasley JM
Women 14-40
560 women
et al. [2]
Baseline AP and
LS, HIP
No relationship
VP intake from
and TB
between TP, AP
questionnaire as
BMD.
and adjusted BMD.
% of energy
Low VP associated
intake.
lower BMD hip
Mean TP was
(0.03) and TB (p=
15.5% total
0.04).
energy.
Michaelsson
K. Et al. [5]
(Sweden)
28-74
175
Usual diet
TB, L2-
Protein associated
women
(mean protein
L4,
TB and FN BMD
59g/day)
proximal
not spine with
femur
linear regression
BMD
Lacey JM et
89 35-40
178
Usual diet
Mid-radial
Protein intake was
al. [4]
years, and 89
Japanese
(mean protein
bone
a positive correlate
55-60 years
pre and post
70.9 and
mineral
of MBMC for
menopause
72.6g/day in pre
content
pre and post
women
and post-
(MBMC)
menopause
menopausal)
and
density
(MBMD)
Cooper C et
Premenopausal 72 pre and
Usual diet
BMD LS,
Protein intake
al. [3]
mean age 39
218 post
( pre-
prox
correlated with
Postmenopaus
menopausal
menopausal
femur,
BMD distal radius,
al mean age 68
women
mean protein
distal and
proximal femur in
75g/day,
mid radius
premenopausal.
Chiu et al.
Mean 60.8
[7]
Ilich et al. [8] Mean 68.7
postmenopausal
No relationship in
72 g/day)
post-menopausal
258 pm
Usual diet
LS and FN Dietary protein
Buddhist
(vegetarian)
BMD
women
(mean protein
BMD in regression
61.0g/day)
analysis
associated LS
136 PM
Usual diet
TB BMD,
Protein associated
women
(mean protein
TB BMC,
with TBBMD,
70.7g/day)
Hip ,
TBBMC, Ward’s,
forearm,
hand in regression
hand
analysis
BMD
Rapuri P.B.
68-77
[9]
Whiting et
Quartile 1 mean
BMD
Quartile 4 had
Mean daily
protein 53.7
spine,
higher BMD spine,
protein
g/day cf Quartile radius,
mid-radius total
71.5/day
4 71.2 g/day
femur,
body cf quartile 1
total body
&2
Usual diet
TB, hip
Protein predicted
(mean protein
and spine
TB, spine not hip
97.0g/day)
BMD
BMD
Usual diet
Femoral
No association with
(mean protein
neck BMD dietary protein
39-42
489
57 men
al. [12]
Jaime et al.
Mean age 62.3
277 men
[10]
86.5g/day)
Kyriazopoul
18-30
300 men
Usual diet
BMC and
No association with
os et. al. [11]
(mean protein
BMD
protein from meat
(Greece)
196±63 g/week)
radius
and fish
FN – Femoral Neck, LS - Lumbar Spine, TB – Total body, AP- Animal Protein,
VP= Vegetable Protein
Table 2: Longitudinal Studies linking protein with BMD/BMC
Study Name
Age
Number
Diet
Endpoint
Results
Duration
Saskatchewan
8-15 at start
133 young
Usual Diet
TB BMC,
Adult protein
Paediatric
Mean age 23
adults (74
Annual 24 hour
TB BMC
intake predicted
Study [13]
at end
females and
record
net gain
TB-BMC net gain
59 males)
including
from peak
β=
protein
height
0.11; P = 0.015).
velocity
TB BMD
annually
Rancho
55-92
Bernardo [16]
388 men
Usual diet from
BMD HIP, BMD increased at
and 572
questionnaire
FN, Spine,
hip (p = 0.005), at
TB,
FN (p = 0.02), at
women
4 year
spine (p = 0.08),
follow up
and at TB(p = 0.04)
for every 15-g/day
increase in AP but
decreased at hip (p
= 0.03), FN and (p
= 0.04), and spine
(p = 0.02) for every
15-g/day increase
in VP.
Framingham
69-97
855 subjects Dietary TP,
Hip,
Bone loss in
Osteoporosis
followed for as % energy
forearm
Quartile 1 (lower
Study [15]
4 years
BMD
TP) greater than
intake from
questionnaire
Quartile 4 at N
divided into
(P<0.001) and LS
quartiles.
(P< 0.05). Similar
Mean % protein
results for AP.
intake 15.9%.
Beasley JM et
Women 14-
560 women
Baseline AP and
LS, HIP
No relationship
al. [2]
40
3 year
VP intake from
and TB
between % increase
Mean age
follow up
questionnaire as
BMD.
in dietary protein
24.3 at start.
% of energy
and BMD
intake.
longitudinally
Mean TP was
15.5% total
energy.
Geinoz et al.
Men mean
48 women,
Diet record after
BMD FN,
Higher protein
[14]
age 80
26 men
28 days hospital
LS
group had higher
Women mean
Divided into TP
BMD at FN
age 82
>1g/kg/ day or <
(P<0.05). Higher
1g/kg/ day
protein men had
higher BMD at LS
(p<0.05).
Rapuri PB. et
68-77
96 women
7 day food
BMD
No association
al. [9]
Mean daily
3 years.
diaries at start
spine,
between BMD loss
and end.
radius,
and protein intake.
protein
71.5/day
femur,
total body
FN – Femoral Neck, LS- Lumbar Spine, TB – Total body, AP- Animal Protein,
VP= Vegetable Protein
Table 3: Dietary Protein and Fractures
Study
Age
Number
Name
Diet
Duration Endpoints
Results
12 years
234 hip, 1628
AP > 95g/d
distal forearm
cf 68 g/d
fractures
RR 1.22
Composition
Nurses
35-
89 500
Health
59
women
Usual diet
Study
[22]
Forearm #
No
association
with hip #
Norweg mean
19,752
Usual diet
47.1
women
from baseline
20,035
questionnaire
ian
[23]
11.4
213 hip
RR hip #
fractures
1.96 for
highest
men
quartile
non-dairy
protein,
lowest
quartile
calcium
Iowa
55-
32050
Usual diet
104338
44 hip
Multivariate
[24]
69
Women
from
person
fractures
analysis.
questionnaire
years
RR highest
quartiles AP
0.31 cf
lowest
quartile
Adventi mean
1865
Usual
peri/post
baseline diet
Health
menopa
from
highest VP
Study 2
usal
questionnaire
cf lowest.
[25]
women
then 25 years
80% RR if
later
meat > 4x
st
52.2
25 years
216 wrist
Vegetarians;
fractures
68% RR in
week
Shangh
40-
24 403
Soy protein
4.5
1770 incident
RR 0.63 in
ai
70
post-
from
years
fractures
lowest cf
Women
menopa
questionnaire
including
highest SP
’s
usal
wrist (17.6%),
quintile
Health
women
vertebrae
Study
(14.9%), and
[26]
hip (3.3%).
Utah
50-
1167 hip Usual
Cross-
1167 hip
OR hip
Hip
89
fracture
baseline diet
sectional fractures
fracture
Fractur
cases
from
0.35 highest
e Study
(831f,
questionnaire
protein
[21]
336m)
quartile vs
matched
lowest
to 1334
protein
controls
quartile for
subjects 5069 only
Nhanes
45-
2879
Dietary
Up to 22
71 hip
No
1
74
men
protein from
years
fractures
association
follow
questionnaire
hip #
up
study
[27]
Geneva
80.7
82 men
Usual + 550
6
13 vertebral
Non-
Protein
or
calcium daily,
months
deformities in
significant
Supple
women
200000 units
treatment vs 7
mentati
with
D3 once
in control
on
recent
+ 20g protein
Study
hip
daily or
[18]
fracture
isocaloric
control
AP – Animal Protein, VP- Vegetable protein, SP- Soy Protein
Table 4: Soy Protein intake and bone health
Study
Subjects
Duration
Intervention
Results
Ho et al.
454 Chinese
cross-
Usual diet
No difference
(cross-
post-
sectional
Soy protein
in BMD/BMC
sectional) [46]
menopausal
quartiles
spine. For hip
women. Mean
Q1: 1.38±0.81
and TB
55.1 y.o.
g/day
BMD/BMC
Q4: 19.41±11.58
significant dose
g/day
response for
increased soy
in women.
4years PM
Horiuchi T. et
85 post-
cross-
Usual diet
SP associated
al. [47]
menopausal
sectional
Mean SP 12.4 ±
with L2-4 and
(cross-
women mean
5.4 g/day
UDPYR z
sectional)
age 66.9 y.o.
score on
regression
analyses
Alekel et al.
69 peri-
[40]
24 week
Isoflavone rich
Isoflavone rich
menopausal
soy (SPI+) vs
soy only
women.
Isoflavone poor
attenuated bone
Mean age
soy (SPI-)
loss in LS in
50.2 y.o.
Vs whey placebo
perimenopausal
40g SP daily
No effect on
Dalais et al.
106 PM
3 months
[43]
women
(including 118mg
urine DPYR
(longitudinal)
Mean age 60
isoflavones) or
/PYR
placebo
Gallagher JC
65 PM
et al. [45]
(longitudinal)
9 months
SP with 96mg
No effect on
women
isoflavones vs
BMD after 15
Mean age 55
52mg vs < 4mg
months
isoflavones
Vupadhyayula
203 PM
et al. [53]
women
24 months
(longitudinal)
25g SP/ 90mg
No change in
isoflavones vs
hip/LS bone
25g of SP/0 mg
loss
without
isoflavones or 25g
of milk protein/
casein and whey
Kreijkamp-
202 PM
Kaspers et al.
[49]
12 months
25.6 g of soy
No effect on
women aged
protein/ 99 mg of
BMD
60-75
isoflavones
(longitudinal)
vs placebo
Arjmandi BH
71 PM
et al. [41]
women
3 months
40g SP or MBP
Urine DPD
decreased more
(2003)
and IGF1
(longitudinal)
increased more
for SP vs MP
Arjmandi BH
87 PM
et al. [42]
women
(2005)
(longitudinal)
12 months
25g SP / 60mg
Soy improves
isoflavone vs
IGFBP3 cf
placebo
placebo, no
other change in
BMD or DPD
Evans EM et
61PM
al. [44]
women
9 months
(longitudinal)
25.6g SPI/91.2mg
No difference
isoflavones vs
in BMD.
25.6 g MPI
SPI improved
bALP and
decreased Cterminal crosslinked
telopeptides
Potter SM et
66 PM
al. [51]
6 months
40g SP with 1.39
Increased
women
mg isoflavones vs
BMD and
Mean age
40g SP with
BMC in higher
60.8
2.25mg??
isoflavone
isoflavones vs
group in LS
40g casein/ nonfat only.
milk protein
Newton KM et
145 subjects
12 months
SP with 83 mg
No difference
al. [50]
(22 women,
isoflavones(+ISO) for BMD in
123 men)
vs SP with 3mg
men nor hip in
Aged 50-80
isoflavone (−ISO)
women.
+ISO women
had modest
benefit in
spine.
Khalil DA et
Men
al. [48]
59.2 +/- 17.6
3 months
40g SP vs MP
SP
significantly
y
increased IGF1
cf MP. No
effect on other
bone markers
SPI- Soy Protein Isolate, MBP – Milk Based Protein, MPI- Milk Protein Isolate, PMPost-Menopausal