Download 1 Dietary Considerations of Protein Quality

Dietary Considerations of Protein Quality
Lynne Kammer, MSE, MA
The University of Texas at Austin
Protein is a key macronutrient that has direct effects on overall health. Although it is most
commonly associated with muscle, it also provides structure for other cells and is involved in
biochemical processes. Protein forms include skin, hair, hormones, enzymes, cell receptors,
transporters, immune system cells and plasma. Proteins consist of amino acids (AAs) that are
either synthesized in the body (non-essential amino acids) or obtained in the diet (essential
amino acids, or EAAs) (Table 1). Cysteine and tyrosine are synthesized from methionine and
phenylalanine respectively, and will be deficient if these primary EAAs are not consumed.
Children cannot synthesize arginine, which must be included in their diet. Availability and rate of
appearance of EAAs vary by protein source and must be considered when developing a dietary
plan. Most amino acids are digested in the small intestine and processed by the liver, but
isoleucine, leucine and valine, the branched-chain amino acids (BCAAs) bypass the liver and
are processed by muscle.
Table 1 Amino Acids
Histidine
Isoleucine
Leucine
Lysine
Essential amino acids
Methionine ➛ Cysteine
Phenylalanine ➛ Tyrosine
Threonine
Tryptophan
Valine
Non-essential amino acids
Alanine
Glutamine
Arginine
Glycine
Asparagine
Proline
Aspartic acid
Serine
Cysteine
Tyrosine
Glutamic acid
Excess EAAs are not stored, but are converted into other amino acids, used for energy, or
stored as fat. In the latter two situations, the nitrogen portion of the molecule is excreted from
the body, primarily as urea. Biochemical processes that depend upon EAAs will not occur if
EAAs are not available; therefore protein must be consumed multiple times during the day. Daily
dietary protein requirements depend upon age, activity type and level, weight and energy
balance (Table 2). Protein intake should match the metabolic demand, but there may be
variation between individuals and even within individuals. The current protein RDA for adults is
0.8 g/kg body weight (Institute of Medicine, 2005) and no additional allowance was made for the
needs of exercise. However, this position is in contrast with the views of most sports nutrition
researchers. As shown in Table 2, the general consensus is that athletes require up to double
the RDA and sometimes more still if energy and/or carbohydrate intake is restricted.
1 Table 2 Daily protein requirements1
1
Population
Institute of
Medicine RDA for
Protein
(g/kg body
weight/day)
Infants to 1 yr
1.5
Children age 1-3 yr
1.1
Children age 4-13 yr
0.95
Children age 14-18 yr
0.85
Adults age 19-59 yr
0.8
Adult endurance athletes
0.8
1.2-1.4
Adult strength athletes
0.8
1.2-1.7
Vegetarian adult athletes
0.8
1.3-1.8
Elderly ≥60
0.8
1.0-1.3
Dieting adults
(goal is fat loss while maintaining lean)
0.8
1.2-1.6
Post-diet maintenance
(goal is maintenance of fat loss)
0.8
1.0-1.6
First half pregnancy
0.8
Second half pregnancy
(based on pre-pregnancy weight)
1.1
Lactation
1.3
Range of Protein to Optimize
Body Composition and Health
(g/kg body weight/day)
De Souza, 2010; FAO/WHO/UNU, 2007; Institute of Medicine, 2005; Layman, 2009; Lejeune, 2005; MSSE, 2009; Mojtahedi, 2011
The amount of protein consumed each day should meet daily requirements to maintain or
improve health while minimizing excretion. It is available from animal sources such as meat,
dairy and eggs and also vegetable sources including grains, rice, nuts and beans; however,
quality, or the availability of EAAs, varies by source. The Food and Agriculture Organization of
the United Nations (FAO) and World Health Organization (WHO) define protein quality by
assigning a Protein Digestibility – Corrected Amino Acid Score (PDCAAS) to protein sources.
PDCAASs are accepted internationally and are derived by assessing digestibility and comparing
the protein source to an ideal mix of EAAs, termed a reference protein (FAO/WHO/UNU, 2007).
The first quality component within the PDCAAS is digestibility, or the percentage of protein that
is extracted from a protein source. It is determined by measuring nitrogen loss in feces during
both protein-only and protein-free diets. The second component of each PDCAAS is an amino
acid score (AAS), which is calculated by dividing the limiting EAA in a protein source by the
amount of this EAA contained in the reference “ideal” protein and multiplying by 100. For
example, if the protein source contains 98% of leucine contained in the reference protein and
86% of lysine, the AAS is 86 because the amount of lysine in the protein would limit protein
synthesis. Note that the AAS is a function of the relation of the protein source to the reference
protein, not the total percentage of EAAs in the protein source. Table 3 shows the PDCAAS
2 (Percent digestibility x AAS) for various protein sources. Since digestibility is always less than
100%, FAO/WHO truncates the PDCAAS, which clusters many protein sources.
Table 3 PDCAAS values of protein sources1
Protein
Source
Beans
Egg
Beef
Cow Milk
Brown Rice
Soy protein
Wheat
1
Digestibility
(%)
78
97
98
95
88
95
86
AAS
102
121
94
127
66
96
40
PDCAAS
(Truncated)
78
97
92
95
58
91
34
PDCAAS
(Not Truncated)
80
118
92
121
58
91
34
FAO/WHO/UNU, 1985; FAO/WHO/UNU, 2007; Schaafsma, 2000; Sarwar, 1997
Although both animal and vegetable sources may be identified that provide all EAAs with
respect to the reference protein (AAS = 100 for each EAA) (Table 4), digestibility affects the
total volume that must be consumed. For example, to supply the same net amount of available
protein in an all-bean versus all-milk diet, the bean eater would have to consume 1.2 times
(digestibility of milk divided by digestibility of beans) the amount of milk protein. This difference
must be considered to ensure that net available protein meets dietary goals; however, typical
diets contain a mixture of different protein sources, affecting both total digestibility and total
AAS, which may balance the relative deficiencies of some proteins.
Using the PDCAAS scoring model is sufficient for healthy adults who perform moderate
exercise but probably not for individuals who participate in regular high-volume endurance or
resistance exercise (Millward et al., 2008). One of the BCCAs, leucine, is not only important for
protein structure, but also triggers pathways within the muscle that are necessary to build
muscle. Additional leucine beyond the amount in the reference protein may be necessary to
support muscle development, and except soy, plant proteins generally contain less leucine as
compared to animal proteins, especially when the lower digestibility of most plant proteins is
considered. Finally, leucine oxidation during hard exercise can rise, particularly when
carbohydrate stores are low.
Muscle recovery is optimized when protein is consumed 30-60 minutes after exercise, but not all
highly-digested proteins are tolerated after a long and/or intense workout. Supplement
manufacturers have recognized this and include protein that is quickly absorbed such as soy
and/or whey into drinks, bars and powders. Since muscle building continues beyond the shortterm recovery window, proteins that are digested more slowly will further increase muscle. For
example, milk can be separated into fast-acting whey and slow-acting casein. Similar to soy
and whey, casein has a high PDCAA, but it does not provide as many BCAAs as soy or whey
and digests more slowly. While studies comparing the effects of soy and whey are inconsistent,
the additional leucine in whey appears to stimulate greater muscle anabolism after exercise, at
rest and in the elderly (Wilkinson et al., 2007; Millward et al., 2008; Tang et al., 2009).
Alternatively, soy may also affect total antioxidant status and reducing oxidative stress after
exercise (Paul, 2009). A combination of both fast-acting and slow-acting protein sources in
supplements or recovery foods can supply EAAs for both immediate and extended muscle
building while providing other health benefits (Wilkinson et al., 2007; Paul, 2009; Tang et al.,
2009).
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