The Great Protein Fiasco? Understanding Plant Proteins in Human Nutrition

This article by Dr Caroline Hodges is shared in two parts, the second part will be posted on Thursday the 4th of August 2016. Caroline teaches nutrition at the University of Brighton and is a member of the BSUFN steering group.

The following description of what we should eat might surprise many people: “Households should select predominantly plant-based diets rich in a variety of vegetables and fruits, pulses or legumes, and minimally processed starchy staple foods.”

You would be forgiven for thinking this comes from some vegan, vegetarian or alternative ‘nuts and sandals’ health group, but the source is The World Health Organization and the Food and Agriculture Organization of the United Nations.

In my previous blog I presented the so called ‘myths’ relating to dietary protein which I encounter in biochemistry and nutrition textbooks. In this one I would like to discuss two specific events which contributed to the perpetuation of these myths and consider the long term negative impact on our health and the environment. First I will describe the early experiments on rats to determine optimum protein requirements and second, I will outline the discovery and repercussions of a disease called kwashiorkor.

Before I unravel the effect of these two events I would like to explain what initiated my concerns about this subject. The following multiple choice question is used in my nutrition exam. (Correct answer is C, but A is the answer given by approx. 50% of students).

What is the primary source of protein for most of the world’s population?

  1. Meat
  2. Dairy
  3. Grains and vegetables
  4. Fruits

The number of students who get this wrong always surprises me, despite lectures which should lead them to the correct answer and me going so far as to announce that this, or a similar question, will be in the exam! Every year I go further; this year I told the students that approx. 50% of students might get the answer wrong, in the vain hope of alerting them to this error.

It is not absenteeism from lectures that causes the mistake, so I find it both disturbing and revealing to ponder the reason. It seems so many students, even those at medical school, are so convinced by our need for animal protein that whatever else they read does not register. I still find comments in the exam suggesting that we cannot survive without animal protein and in its absence we become ill. It seems the perceptions of my students (and the public) have been shaped by decades of poor information on this subject.  It does not help that textbooks still describe proteins as ‘complete’ (animal sources) and ‘incomplete’ (plant sources). The insidious and seemingly pervasive implication being that we cannot survive without the ‘complete’ ones. After all, these textbooks can’t be wrong, can they? Most dictionaries define incomplete as ‘lacking a part’, but as applied to plant protein this is not so, every one of the essential amino acids is present, just in varying proportions in different plants. If we only ate one food item all day, every day, that proportion would be hugely important, but in most countries this is unlikely to be the case. So what has caused this misunderstanding?

Rat experiments

The first of the two subjects I would like to describe is related to animal experiments conducted over a century ago to determine the optimum protein requirement for humans, the legacy of which still prevails. The myth, described by (Young et al, 1994) is as follows:

‘Animal procedures can provide good indices of the human nutritional value of food proteins’.

In 1914 Osborne and Mendel studied the protein requirements of laboratory rats and demonstrated nutritional requirements for the individual amino acids of which proteins are made. At that time it was not known that rats have much greater protein requirements than humans (Rose 1948) because, by comparison, they have a much more rapid tissue growth. This difference in protein requirements is further demonstrated by the comparison of breast milk from both species; the protein content of rat breast milk is 10 times greater than the milk intended for human babies (Bell 1959; Reeds 2000).

The other damaging outcome of this animal-based work is the concept of complete and incomplete proteins, also referred to as first class or superior (from animal sources) and second class or inferior (from plants sources) proteins. These descriptions are based on the premise that animal products provide the most ideal pattern of essential amino acids for humans, which is now known to be incorrect. These animal experiments and subsequent definitions of protein quality are much less relevant in human nutrition, and our metabolic requirements are quite different.  This is substantiated by the difficulty in demonstrating in normal healthy adults any difference in nitrogen balance (an indication of appropriate protein intake) between diets based on plant protein and those based on animal sources (Rand et al, 2003).

So over misinterpretation of animal experiments (Ioannidis, 2012) and inappropriate extrapolation to humans has encouraged both inflated estimates of protein requirements, especially in children, and erroneous distinctions between the quality of plant and animal protein. The estimated protein needs of children are now half as much as they were in the 1940’s and it is becoming apparent that the much greater risk (in the West at least) is over-consumption of protein.

It seems that comparisons of ‘complete’ and ‘incomplete proteins’ are much more academic than practical and require rethinking, as described so well by Bender, (2014, 255):

‘While protein quality is important when considering individual foods, it is not relevant when considering total diets because different proteins are limited by amino acids, and hence have a relative excess of others. The result of mixing different proteins in a diet is an unexpected increase in the nutritional value of the mixture…..The average Western diet has a protein score of 0.73,  whilst the poorest diets in developing countries, with a restricted range of foods, and very little milk, meat, or fish, have a  protein score of 0.6’. (The difference is minimal.)

Miller and Payne (1969) concluded that ‘almost all dietary staples contain sufficient protein to meet human needs and that even diets based on very low protein staples are unlikely to be specifically protein-deficient. Webb (2012, 279) points out that since 1969 this view has become the nutritional consensus. It seems unfortunately that this message has not permeated to the lay public.

The second event known to have a huge influence on policy and recommendations led to what is called the ‘great protein fiasco’. Caroline’s article on the ‘great protein fiasco’ will be posted on Thursday the 4th of August 2016.


Arnold, D. (1994) ‘The Discovery of Malnutrition and Diet in Colonial India’, The Indian Economic and Social History Review, 31:1, 1–26

Bell, G. (1959) Textbook of Physiology and Biochemistry, 4th ed., Williams and Wilkins, Baltimore, p. 12

Bender, D. (2014) Introduction to Nutrition and Metabolism, 5th ed. Chapter 9 (CRC press) p255

Briend, A. (2014) Kwashiorkor: still an enigma – the search must go on. In: CMAM Forum Technical Brief

Brock, J.F. and Autret, M. (1952) Kwashiorkor in Africa, World Health Organization Monograph, Series No. 8. Geneva: WHO

Gopalan, C. (2007) From ‘Farms to Pharmacies’: Beginnings of a sad decline. Econ Pol Wkly, 42: 3535-3536

Hartley, S. (2016) Scientists hungry to deliver food system paradigm shift, BBC article

Ioannidis, J. (2012) Extrapolating from animals to humans, Sci Transl Med, 4: 151

McLaren, D.S. (1974) ‘The Great Protein Fiasco’, Lancet, 304(7872), 93-96

McLaren, D.S. (2000) ‘The Great Protein Fiasco Revisited’, Nutrition, 16: 464-5

Miller, D and Payne, P. (1969) Assessment of protein requirements by nitrogen balance. Proceedings of the Nutrition Society, 28: 2, 225-234

Newman, J.L. (1995) ‘From Definition, to Geography, to Action, to Reaction: The Case of

Protein-Energy Malnutrition’, Annals of the Association of American Geographers, 85: 2,233–45

Osborne, T. and Mendel, L. (1914) Amino-acids in nutrition and growth. J Bio Chem, 17: 325-49

Rand, W.M, Pellett, PL. Young, V.R. (2003) Meta-analysis of nitrogen balance studies for estimating protein requirements in healthy adults, American Journal of Clinical Nutrition, 77: 109-127

Reeds, P.J. (2000) Protein nutrition of the neonate, Proc Nutr Soc, 59:1, 87-97

Rose, W. (1948) Comparative growth of diet containing ten and nineteen amino acids, with further observation upon the role of glutamic and aspartic acid, J Bio Chem, 176: 753-62

Sathyamala, C. (2016) Nutritionalizing Food: A Framework for Capital Accumulation. Development and Change 47:4, 818-839.

Webb, G. (2012) Nutrition: Maintaining and improving health, 4th ed. Chapter 11, Taylor & Francis Group, LLC

Young, V and Pellett P. (1994) Plant proteins in relation to human protein and amino acid nutrition. Am J Clin Nutr, 59 (suppl):1203S–1212S


One thought on “The Great Protein Fiasco? Understanding Plant Proteins in Human Nutrition

  1. Pingback: Kwashiorkor and the Great Protein Fiasco? Understanding Plant Proteins in Human Nutrition, Part 2 | Brighton and Sussex Universities Food Network

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