Download Carbohydrates: is the advice to eat less justified for diabetes and

Carbohydrates: is the advice to eat less justified for diabetes and
cardiovascular health?
Jim Manna,b and Kirsten McAuleya
Purpose of review
Recent randomized controlled trials examining diets of
varying carbohydrate composition recommended for
people with diabetes and cardiovascular disease and those
at risk are summarized.
Recent findings
Severe carbohydrate restriction results in appreciable initial
weight loss and improvement in risk factors. After a year,
however, the beneficial effects are equal to or less than
those achieved on conventional alternatives. Some people
develop elevations of LDL cholesterol. Modest
carbohydrate restriction with relatively high intakes of cisunsaturated fatty acids and protein is acceptable to many
people and is more likely to produce sustained benefit in
terms of weight loss and cardiovascular risk indicators.
Summary
Diets involving moderate carbohydrate restriction are
suitable alternatives to high-carbohydrate, high-fibre diets
for weight loss and reduction of cardiovascular disease and
diabetes risk, as well as to treat individuals with the
conditions. As such diets are generally high in protein and
unsaturated fatty acids, they are not recommended for
those with established or incipient nephropathy.
High-carbohydrate, high-fibre diets remain appropriate for
use in all those situations, provided carbohydrate is derived
principally from minimally processed wholegrain breads and
cereals and intact vegetables and fruit. Lower carbohydrate
options may be preferable for markedly insulin-resistant
individuals.
Keywords
cardiovascular disease, diabetes, low-carbohydrate diet,
nutrition
Curr Opin Lipidol 18:9–12. ß 2007 Lippincott Williams & Wilkins.
a
Edgar National Centre for Diabetes Research, University of Otago, Dunedin,
New Zealand and bDepartment of Human Nutrition, University of Otago, Dunedin,
New Zealand
Correspondence to Jim Mann, Edgar National Centre for Diabetes Research,
University of Otago, PO Box 56, Dunedin, New Zealand
Tel: +64 3 479 7719; fax: +64 3 474 7641;
e-mail: [email protected]
Current Opinion in Lipidology 2007, 18:9–12
ß 2007 Lippincott Williams & Wilkins
0957-9672
Introduction
The suggestion that low-carbohydrate diets might be
appropriate for people with diabetes is not new. In the
eighteenth century, the diabetic dietary prescription in
Britain involved severe carbohydrate restriction [1]. As
recently as the 1970s, in most Western countries, it was
generally recommended that around 40% total energy be
derived from carbohydrate, with sugars being eliminated
as far as possible [2]. The demonstration of a substantially
increased risk of cardiovascular morbidity and mortality
and the potential atherogenicity of diets high in saturated
fat, led to the questioning of this approach [1]. A series of
studies in the 1970s and 1980s demonstrated, in both type
1 and 2 diabetes, that high-carbohydrate, high-fibre diets
low in saturated fat could improve glycaemic control and
a range of cardiovascular risk factors when compared with
the standard low-carbohydrate dietary regime [1]. By the
1990s, nutritional recommendations for people with
diabetes suggested a much more liberal approach with
regard to carbohydrates, with organizations such as the
European Association for the Study of Diabetes, through
its Nutrition Advisory Group, suggesting that carbohydrate might provide as much as 60% of total dietary
energy [3]. The epidemiological evidence suggesting
that populations consuming high-carbohydrate diets have
low rates of cardiovascular disease, and the generally
favourable effect of low-fat, high-carbohydrate diets on
cardiovascular risk factors, especially total and low
density lipoprotein cholesterol, have led to a widespread
acceptance that such diets are cardioprotective for the
population at large, as well as people with diabetes [4].
Unfortunately, the importance of the nature of the carbohydrate-containing foods has not been sufficiently
emphasized when recommending relatively high carbohydrate intakes. Epidemiological and experimental evidence suggesting the cardioprotection and improved
glycaemic control, associated with high-carbohydrate
diets, is based upon studies in which vegetables, fruits
and wholegrain cereals rich in nonstarch polysaccharides
(dietary fibre) predominate [5]. When rice, potato and
highly refined cereals constitute a high proportion of the
carbohydrate-containing foods, a high-carbohydrate diet
may result in elevation of triglycerides, reduction of HDL
cholesterol and deterioration of glycaemic control
(in people with diabetes) [6]. These observations and
the widespread promotion of a range of commercially
driven low-carbohydrate diets have prompted a reappraisal
9
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10 Nutrition and metabolism
of the benefits in terms of cardiovascular and diabetic
health and potential adverse effects of low-carbohydrate
diets. There is no universally accepted definition of a lowcarbohydrate diet, but it seems that it is used for diets
including as little as 20 g carbohydrate daily, as well as for
those in which around 40% total energy is derived
from carbohydrate.
Severe carbohydrate restriction
The Atkins diet has been the most popular of the diets
involving severe carbohydrate restriction, typically
starting with no more than 20 g carbohydrate daily,
gradually increasing to 50 g or 60 g per day. This approach
has principally been recommended as a weight-loss diet
but it has been claimed to have beneficial effects on
cardiovascular risk factors. Two recent randomized trials
have been particularly helpful in establishing the place of
such an approach. Dansinger et al. [7] randomized
160 overweight participants (BMI: 27–42) with at least
one cardiovascular risk factor to four groups receiving one
of four sets of dietary advice: the Atkins diet, the Zone
diet (40% carbohydrate, 30% fat, 30% protein), Weightwatchers diet (strictly calorie controlled), and the Ornish
diet (10% fat, vegetarian). The study was continued for
12 months, but dropout rates were very high: only about
half of those recommended the Ornish or Atkins diet
completed the 12-month study, with modestly higher
(65%) retention rates on the other two diets. Among
those who completed the study, weight loss at 1 year
was around 2 kg for Atkins participants and 3 kg for those
on the other diet groups. The ratio of LDL to HDL was
reduced by about 10% in each of the diet groups. Changes
in lipoprotein-mediated risk (as measured by total/HDL
cholesterol), C-reactive protein and insulin levels were all
significantly associated with weight loss, with no significant differences between diets. Amount of weight loss
was associated with self-reported dietary adherence level,
regardless of diet type. The study by McAuley and
colleagues [8,9] included 96 overweight insulin-resistant women who were randomized to receive dietary
advice based on the Atkins approach, the Zone approach
or a high-carbohydrate, high-fibre diet. They too were
followed for a year, but the aims and method of dietary
instruction differed, in that during the first 8-week
period, the intensive intervention carried out under strict
supervision aimed to achieve weight loss, and the second
phase (also 8 weeks) aimed for weight maintenance.
During the final 8 months, participants were encouraged
to maintain weight loss, but were offered no further
dietary advice. Retention rates after 1 year were relatively
low in the Atkins group (77%), among whom an interesting trend was apparent with regard to a number of
outcome measures. During the first 2 months, the
reductions in total body weight and fat mass were greater
in this group than in the other two groups. Thereafter,
however, these measures of adiposity tended to trend
upwards towards initial levels. Several cardiovascular
risk factors, including triglycerides and insulin levels,
followed a similar trend.
These observations are broadly compatible with a metaanalysis [10] published in February 2006 that included
the Dansinger study and four earlier studies. Although
weight loss after 6 months was greater ( 3.3 kg; 95%
confidence interval: 5.3 to 1.4) on the low-carbohydrate than on the high-carbohydrate diets, after
12 months there was no difference. In the meta-analysis,
triglycerides and HDL cholesterol tended to be more
favourable on the low-carbohydrate diets whereas total
and LDL cholesterol were significantly lower on the lowfat, relatively high-carbohydrate diets. Retention rates
were generally poor. Thus in terms of longer-term weight
loss, there appears to be little merit in the severely
carbohydrate-restricted Atkins approach. The adverse
effects noted on total and LDL cholesterol argue rather
strongly against the use of this method, despite the
potentially beneficial changes in some other cardiovascular risk factors. It has been argued that for those who
find it a palatable method for weight loss, it is acceptable
in the short term. This matter remains to be finally
resolved but, in our opinion, the generally poor retention
rates and the effects on raising total and LDL cholesterol
in some people, even during the weight-loss phase, argue
strongly in favour of excluding this approach from the
nutritional armamentarium. If it is to be used, monitoring
of total and LDL cholesterol is necessary. There would
appear to be no justification for its long-term use.
Modest carbohydrate restriction
More modest degrees of carbohydrate restriction have
been studied in the context of the Zone, Southbeach and
Commonwealth (of Australia) Scientific and Industrial
Research Organisation (CSIRO) diets, which tend to be
relatively high in protein and fat. As indicated earlier, the
Dansinger et al. study [7] reported broadly similar
findings on all diets. On the other hand, McAuley et al.
[8,9] found the most favourable overall set of results
among those allocated to the high-protein, lower-carbohydrate (Zone) diet. Initial losses of weight and fat mass
( 6 kg and 4 kg) were generally maintained over the
entire 1-year period as were the improvements in metabolic measures. Retention rates were highest in this
group (93%). Although those randomized to the highcarbohydrate, high-fibre diet were also able to retain the
benefits achieved in body mass and risk factors, all
improvements were less marked. This is perhaps because
compliance was relatively poor compared with the degree
of compliance with the lower-carbohydrate, highprotein approach.
High-protein diets (28% total energy) relatively low in
carbohydrate (42% total energy) have also been compared
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Carbohydrates advice Mann and McAuley 11
with relatively high-carbohydrate diets (55% total
energy) in people with type 2 diabetes [11]. Glycaemic
control did not differ on the two diets, but LDL cholesterol was lower on the high-protein diet and, in women,
reduction in fat mass was greater. In this study, however,
total dietary fibre intake did not differ appreciably on
the two diets. Thus it is conceivable that, as with the
McAuley study, there was insufficient consumption of
the appropriate high-fibre, carbohydrate-containing foods
(by those randomized to the higher carbohydrate diet)
to produce the benefits repeated elsewhere of such
diets [5].
The group from Adelaide, South Australia, have carried
out the longest study to date in which two relatively lowfat diets were compared in obese hyperinsulinaemic
subjects: one provided 15% total energy from protein,
55% from carbohydrate, and the other 30% protein, 40%
carbohydrate. Intensive dietary advice relating to weight
loss was given during the first 12 weeks. This was followed by a 4-week period of instruction regarding weight
maintenance. Participants were then followed for a
further 52 weeks but no professional support was offered.
Long-term compliance without active ongoing advice
was poor but participants in both groups achieved modest
weight loss, a reduction in fasting insulin and insulin
resistance, as well as some improvement in inflammatory
markers [12]. Using a similar experimental design, this
group has also investigated subjects with type 2 diabetes,
and reported a more favourable carbohydrate risk profile
on the higher protein diet [13].
There appears to be little difference when protein or
monounsaturated fatty acids predominate in a carbohydrate-restricted diet. Luscombe-Marsh and colleagues
[14] compared 35% carbohydrate diets that were
relatively high in protein (34% total energy) or monounsaturated fatty acids (24% total energy) in obese hyperinsulinaemic women. Weight loss and improvements in
insulin resistance and cardiovascular risk factors were
similar in the two diets.
Thus with regard to diabetic and cardiovascular health, a
relatively low-carbohydrate diet (35–40% total energy)
appears to be a reasonable option for weight loss and
improvement in cardiovascular risk factors and glycaemic
control. The apparently better results than those
observed on high-carbohydrate, high-fibre diets seem
to relate to improved compliance.
Thus for those who find it difficult to consume the
appropriate carbohydrate-containing foods, a higher
intake of protein or unsaturated fatty acids in conjunction with a reduced intake of carbohydrate may
facilitate compliance and, as a consequence, more
favourable outcomes.
Safety of low-carbohydrate diets
Issues of both efficacy and safety have been the subject of
a recent systematic review [15]. Few randomized studies
have continued for as long as a year and only one for
longer. Thus it is difficult to offer definitive comments on
safety. Severe carbohydrate restriction as occurs at least
initially in the Atkins approach is associated with ketosis.
The extent to which this has deleterious effects in those
with abnormalities of carbohydrate metabolism or indeed
in those who do not is unclear. There is no doubt,
however, that at least some individuals will respond to
the high saturated-fat intake, which almost invariably
accompanies the substantial reduction of carbohydrate,
by increasing LDL levels, sometimes markedly. While
we do not recommend such diets, if they are used it would
seem prudent to measure lipids and lipoproteins. With
modest carbohydrate restriction, usually associated with
an increase in protein and unsaturated fatty acids, there
would seem to be a small likelihood of untoward effects,
at least in those who do not have diabetes or renal disease.
Indeed, even in those who have diabetes but normal renal
function, a moderate increase in protein appears to be
well tolerated [16]. In the presence of renal disease,
however, perhaps especially in those with diabetes, an
appreciable increase in protein may not be appropriate
[17]. Further research in this regard is necessary since no
definitive studies have been conducted.
Conclusion
The question posed does not lead to a single unequivocal
affirmative or negative response. Carbohydrate restriction
to the extent that induces ketosis may result in short-term
weight loss. Such advice is probably not justified, however, given that long-term benefits in terms of reduced
adiposity and cardiovascular risk factors are no greater on
those diets than on those more closely aligned to eating
patterns to which humans have long been adapted.
Furthermore, some people consuming such diets will
develop raised LDL levels and there are potential
adverse effects of high intakes of saturated fatty acids
(e.g. reduced insulin sensitivity-enhanced thrombogenesis), which have not been well studied and may be
especially relevant after the weight-loss phase. On the
other hand, modest carbohydrate restriction in association
with somewhat higher than usual intakes of protein and
cis-unsaturated fatty acids can facilitate long-term
reduction of excess adiposity, enhance insulin sensitivity
and favourably influence several cardiovascular risk factors. It also seems to be acceptable to those accustomed to
a typical Western dietary pattern. Thus it would appear to
be a reasonable alternative to the high-carbohydrate,
high-fibre diet, especially as many people appear to
prefer the food choices associated with a lower carbohydrate intake to the fibre-rich wholegrain cereal and
breads, vegetables and fruit that are an essential component of high-carbohydrate, high-fibre diets. Rather
Copyright © Lippincott Williams & Wilkins. Unauthorized reproduction of this article is prohibited.
12 Nutrition and metabolism
than recommend carbohydrate restriction, however,
health professionals should inform patients and those
at risk of diabetes and cardiovascular disease, that alternatives are available.
References and recommended reading
Papers of particular interest, published within the annual period of review, have
been highlighted as:
of special interest
of outstanding interest
Additional references related to this topic can also be found in the Current
World Literature section in this issue (p. 73).
1
Mann JI. Lawrence lecture: Lines to legumes: changing concepts of diabetic
diets. Diabet Med 1984; 1:191–198.
2
Truswell AS, Thomas BJ, Brown AM. Survey on dietary policy and management in British diabetic clinics. Br Med J 1975; ii:7–11.
3
Mann JI, De Leeuw I, Hermansen K, et al. Evidence-based nutritional approaches to the treatment and prevention of diabetes mellitus. Nutr Metab
Cardiovasc Dis 2004; 14:373–394.
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World Health Organisation. Diet, nutrition and the prevention of chronic
diseases. Report of a Joint WHO/FAO Expert Consultation (Geneva).
WHO Technical Report Series 2003; 916.
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Mann J. Dietary fibre and diabetes revisited. Eur J Clin Nutr 2001; 55:919–
921.
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Venn BJ, Mann JI. Cereal grains, legumes and diabetes. Eur J Clin Nutr 2004;
58:1443–1461.
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43–53.
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and demonstrated similar effects of each, improvements being related principally
to extent of reported compliance.
7
8
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11 Parker B, Noakes M, Luscombe N, Clifton P. Effect of a high-protein, highmonounsaturated fat weight loss diet on glycemic control and lipid levels in
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trial. Diabetologia 2004; 47:1677–1686.
14 Luscombe-Marsh ND, Noakes M, Wittert GA, et al. Carbohydrate-restricted diets
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loss and improving blood lipids. Am J Clin Nutr 2005; 81:762–772.
15 Bravata DM, Sanders L, Huang J, et al. Efficacy and safety of low-carbohydrate diets: a systematic review. J Am Med Assoc 2003; 289:1837–1850.
16 Nuttall FQ, Gannon MC. Metabolic response of people with type 2 diabetes to
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17 Pedrini MT, Levey AS, Lau J, et al. The effect of dietary protein restriction on
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