The main protein
foods in a vegan diet are the pulses (peas, beans and lentils),
nuts, seeds and grains, all of which are relatively energy dense.
As the average protein level in pulses is 27% of calories; in nuts
and seeds 13%; and in grains 12%, it is easy to see that plant foods
can supply the recommended amount of protein as long as the energy
requirements are met.
are not Rats
it that plant proteins are of a poorer quality than animal proteins,
because the essential amino acids are present in proportions which
may not be ideal for human requirements. In the early years of research
into protein quality this belief derived from experiments with laboratory
rats, when it became clear that amino acid supplementation of a
plant source of protein improved its biological value to the point
where it would support the growth of weaning rats. The parameters
of these experiments were set in such a way that differences in
the quality of plant and animal proteins were maximised; the second
major problem is that rats and humans have different nutritional
rat grows, relatively, at a much faster rate than the human infant
and therefore requires a more concentrated source of nutrients,
including protein. A comparison with human milk makes the difference
quite clear; protein comprises only 7% of the calorie content of
breast milk, while rat milk contains 20% protein. If weanling rats
were fed soley on human milk, they would not thrive. Using the same
logic as was applied in the early experiments, it could be argued
from this that breast milk is also inadequate for human infants!
Some early studies
further demonstrated the differences in nutritional requirements
between rats and humans. In 1955 (5) an experiment with three
male volunteers showed that the amino acid cystine is able to substitute
for 80-89% of the body's requirement for another essential amino
acid, methionine, whereas in rats the substitution value is only
terms 'first-class' and 'second-class' proteins are no longer used,
in some circles the belief persists that a vegan diet, containing
only plant proteins, may be inadequate. This is because cereals,
nuts and seeds contain less of the amino acid lysine, while being
high in methionine; and pulses are rich in lysine but contain less
methionine. This has given rise to concern that the amino acid present
in lower amounts in each food will limit the availability to the
body of the others, and the suggestion has been made (6),
and adopted quite widely - even among vegans - that complementary
protein foods, such as beans and grains, should be eaten at each
meal in order to enhance amino acid availability. Vegetarians are
also sometimes advised to ensure that they complement vegetable
proteins with dairy foods. Are these precautions necessary?
may reduce the amount of protein required to keep the body in positive
protein balance (7), but several human studies have indicated
that this is certainly not always the case. For example, over a
60-day period seven human subjects were fed diets in which protein
was derived solely either from beans, corn and refined wheat; beans,
rice and refined wheat; or a combination of the plant foods with
the addition of cow's milk (8). All subjects remained in
positive nitrogen balance (a measure of the adequacy of dietary
protein), and there were no significant differences in nitrogen
balance between the subjects eating only plant foods and those whose
diet was supplemented with milk.
looked at the nutritive value of a plant-based diet in which wheat
provided 76% of the protein (9). The aim was to determine
whether this regime could be improved by adding other sources of
plant protein - such as pinto beans, rice and peanut butter. The
diets were entirely vegan, containing only 46g of protein, and were
fed to 12 young men over a 60 day period, during which they continued
their normal daily activities. The researchers found that all subjects
remained in nitrogen balance, and that replacement of 20% of the
wheat protein with beans, rice or peanut butter did not result in
significant changes in the levels of essential amino acids in the
Even more startling
perhaps were the findings of a 59-day investigation with six male
subjects who consumed diets in which virtually the sole source of
protein was rice (10). At two protein levels (36g and 48g
per day) the diets comprised rice as the sol source of protein,
or regimes where 15 and 30% of the rice protein was replaced with
chicken. The partial replacement of rice with chicken protein did
not significantly affect the nitrogen balance of the volunteers
(in contrast to earlier experiments with rats which showed that
a rice diet did not sustain normal growth). In this human study,
even on the low-protein diet rice as the sole source provided between
2 and 4.5 times the WHO-recommended amounts of all essential amino
acids, except lysine - of which it supplied 1.5 times the suggested
level. On the higher protein diet, rice alone provided between two
and six times the essential amino acid levels suggested by the WHO,
and all subjects were in positive nitrogen balance.
was fed as virtually the sole source of protein to ten male volunteers
during a 100-day study it was found that at an intake of 6g of nitrogen
per day (approx. 36g protein) not all the subjects were in positive
nitrogen balance (11). Yet all the essential amino acids
were eaten in amounts which met or exceeded standard requirements,
with the exception of tryptophan - of which 91% was provided. These
results suggest that on a corn protein diet, non specific nitrogen
is the first limiting factor, not lack of esssential amino acids.
The 1988 position
paper of the American Dietetic Association emphasized that, because
amino acids obtained from food can combine with amino acids made
in the body it is not necessary to combine protein foods at each
meal. Adequate amounts of amino acids will be obtained if a varied
vegan diet - containing unrefined grains, legumes, seeds, nuts and
vegetables - is eaten on a daily basis.(12)
These and other
similar experiments show clearly that diets based solely on plant
sources of protein can be quite adequate and supply the recommended
amounts of all essential amino acids for adults, even when a single
plant food, such as rice, is virtually the sole source of protein.
The American Dietetic Association emphasizes that protein combining
at each meal is unnecessary, as long as a range of protein rich
foods is eaten during the day.
1.Food and Agriculture Organization/ World Health Organization/
United Nations University (1985). 'Energy and protein requirements', WHO
Technical Report Series 724. Geneva, WHO. 2.Department of Health and Social
Security (1979). Recommended Daily Amounts of Food Energy and Nutritients
for Groups of People in the United Kingdom. London, HMSO.
3.National Advisory Commitee on Nutrition Education (1983). Proposals for
Nutritional Guidelines for Health Education in Britain. London, Health
4.Vaghefi, S.B., Makdani, D.D. and Mickelsen, O. (1974). 'Lysine supplementation
of wheat proteins, a review', Am. J. Clin. Nutr. 27, 1231-1246.
5.Rose, W.C. and Wixom, R.L. (1955). 'The amino acid requirements of man.
XIII The sparing effect of cystine on methionine requirement', J. Biol.
Chem., 216, 763-773.
6.Lappe, F.M. (1976). Diet for a small planet. New York, Ballantine Books.
7.Kofranyi, E., Jekat, F. and Muller-Wecker, H. (1970). 'The minimum protein
requirements of humans, tested with mixtures of whole egg plus potatoes
and maize plus beans', Z. Physiol. Chem., 351, 1485-1493.
8.Clark, H.E., Malzer, J.L., Onderka, H.M., Howe, J.M. and Moon, W. (1973).
'Nitrogen balances of adult human subjects fed combinations of wheat, beans,
corn, milk, and rice', Am. J. Clin. Nutr., 26, 702-706.
9.Edwards, C.H., Booker, L.K., Rumph, C.H., Wright, W.G. and Ganapathy,
S.N. (1971). 'Utilisation of wheat by adult man; nitrogen metabolism, plasma
amino acids and lipids', Am. J. Clin. Nutr., 24, 181-193.
10.Lee, C., Howe, J.M., Carlson, K. and Clark, H.E. (1971). 'Nitrogen retention
of young men fed rice with or without supplementary chicken', Am. J. Clin.
Nutr., 24, 318-323.
11.Kies, C., Williams, E. and Fox, H.M. (1965). 'Determination of first
limiting nitrogenous factor in corn protein for nitrogen retention in human
adults', J. Nutr., 86, 350-356.
12.Havala, S. and Dwyer, J. (1988). 'Position of the American Dietetic
Association: vegetarian diets - technical support paper', J. Am. Diet.
Assn., 88, 352-355.
Extracts from "Vegan Nutrition, a survey of research"
by Gill Langley MA PhD