The protein myth


There are a number of food myths currently in circulation, some of which have been around for decades.  They generally revolve around particular micro- and macro-nutrients, their sources, and/or their imaginary abilities to cure us of all sorts of ailments ranging from the common cold to cancer.  Among these, we find the “protein myth” which, in a nutshell, states that protein from animal foods is superior because it contains all essential amino acids.

WHAT ARE PROTEINS?

Proteins are the most complex of the three macro-nutrients. They are composed of long chains of amino acids and, in some cases, include other components that are strung together in complicated formations consisting of carbon, hydrogen, oxygen, and nitrogen.  Every cell in the human body contains protein. It is a major part of the skin, muscles, organs, glands and all body fluids, except bile and urine.  Proteins in the body act as enzymes (catalysts), messengers (hormones), structural elements, immunoprotectors (immunoglobulins or antibodies), transporters, buffers, fluid balancers, or receptors on cell surfaces.  They also play a role in cell adhesion, storage of minerals in the body, and as conjugated proteins (glycoproteins) (1).

Three types of amino acids fold into acid chains to form proteins.  They are:

Essential or indispensable amino acids

  • Essential amino acids cannot be made by the body. As a result, they must come from the foods we eat.
  • They are: histidine (infants only), isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan, and valine.

Nonessential amino acids

  • “Nonessential” means that our bodies produce an amino acid, even if we don’t get it from the foods we eat.
  • They include: alanine, asparagine, aspartic acid, and glutamic acid.

Conditional amino acids

  • Conditional amino acids are usually not essential, except in times of illness and stress, and are made in the body.
  • They include: arginine, cysteine, glutamine, tyrosine, glycine, ornithine, proline, and serine.

The chains are held together by hydrogen bonds, and, sometimes, by ionic bonds, depending on how the chain is folded (i.e. positive and negative ions face each other), by van deer Waals dispersion forces, or by sulphur bridges.

HOW MUCH PROTEIN DO WE NEED?

Although the human body contains a large amount of protein, it does not need to consume large amounts to maintain itself.  According to the World Health Organization (WHO), the average adult needs to consume approximately 60 grams of protein per day (0.8 grams per kilogram of body weight or 10 to 15% of total calories (assuming daily caloric needs are met) (2). As far as amino acids are concerned, we do not need to consume all of the essential amino acids at every meal, but getting a balance of them over the course of a 24 to 48 hour period is important.

PLANT vs ANIMAL PROTEIN

Plants produce/contain all of the essential amino acids because they can not get them from their environment by consuming other living organisms (the exception being carnivorous plants such as the Venus Fly plant which “eats” insects).  The amino acid profile of each plant varies – for instance, beans are high in lysine, while grains are low in it, but both contain it.  There is no such thing as a plant that lacks one or more amino acids (3). It is surprising, and disappointing, to see that, in spite of all we have learned about nutrition in the past couple of decades, the notion that plants lack certain amino acids persists (often in places one would least expect to find such false assumptions).

It is not necessary to consume animal products to meet essential amino acid needs, as long as the diet includes plant foods from all the food groups and caloric needs are met. Keep in mind, there is enough protein in plants to grow elephants and Panda bears. Contrary to popular belief, animals don’t make the essential amino acids we require. They ingest them by consuming plants (or one another).

The terms “complete” and “incomplete” proteins, when referring to protein foods, are no longer considered accurate or useful, and educators are encouraged to abstain from using them in the classroom (4).  Such terms are misleading and can create confusion since “incomplete” proteins are often described as “lacking” one or more essential amino acids.  This, of course, is not true and can be easily verified by looking up the amino acid content of plant foods using the USDA National Agricultural Library Nutrient Database.

From Young and Pellet’s review on plant proteins in relation to human protein and amino acid nutrition – click on image for larger version (5).

Should you, for whatever reason, want to include so-called “complete” proteins in one single meal, you may consume any of the following:

  • Soy, or
  • Eggs, or
  • Dairy, or
  • Legumes + Grains (e.g. peanut butter sandwich, burrito), or
  • Legumes + Nuts  (e.g. lentils and cashews), or
  • Legumes + Seeds (e.g. hummus)

In a review of plant based diets and their adequacy in meeting amino acid needs, Millward concludes:  “it is clear that meat-free, largely plant-based diets available in developed countries can supply protein in the amount and quality adequate for all ages” (6).  Similarly, the American Dietetic Association (ADA), in its position paper on vegetarian diets, states the following:  “Plant protein can meet protein requirements when a variety of plant foods is consumed and energy needs are met. Research indicates that an assortment of plant foods eaten over the course of a day can provide all essential amino acids and ensure adequate nitrogen retention and use in healthy adults; thus, complementary proteins do not need to be consumed at the same meal(7).

In the same paper, the ADA further adds:  “Vegetarian diets are often associated with a number of health advantages, including lower blood cholesterol levels, lower risk of heart disease, lower blood pressure levels, and lower risk of hypertension and type 2 diabetes. Vegetarians tend to have a lower body mass index (BMI) and lower overall cancer rates. Vegetarian diets tend to be lower in saturated fat and cholesterol, and have higher levels of dietary fiber, magnesium and potassium, vitamins C and E, folate, carotenoids, flavonoids, and other phytochemicals. These nutritional differences may explain some of the health advantages of those following a varied, balanced vegetarian diet” (7).

Animal protein, on the other hand, has been associated with cardiovascular disease and cancer, even after confounders such as saturated fat have been taken into account (8,9).

A VEGETARIAN MENU

To illustrate the ease with which amino acid requirements are met on a meat-free diet, let’s look at a modest sample menu for a 170 lbs man in his 30’s.  The selection is modest on purpose, and not representative of the wide variety of plant based foods consumed by the average vegetarian or vegan.

Breakfast:

2 scrambled eggs (or, for vegans an equivalent amount of tofu scramble)
2 pieces of toast with margarine
1 glass of orange juice

Snack:

1 avocado

Lunch:

1 Frozen bean burrito, microwaved
1 Small salad (1 cup of shredded lettuce, 1 sliced tomato, with dressing)
6 oz. soy milk

Snack:

1/2 cup pistachio nuts

Dinner:

1 bowl vegetarian stew (peas, tomatoes, green beans, carrots, onions, parsnip, olive oil, seasonings)
1 cup mashed potatoes
1 tomato
2 slices of bread
1 small slice of cherry pie

Amino acid requirements for our subject:          Amino acid content of selected  menu:In this example, the vegetarian menu meets and surpasses the amino acid requirements of our hypothetical man.

Evidence that plant based diets can meet all of our essential amino acid needs abounds (1,3,4,5,6,7,9,10).  Yet, the myth persists.  Education, particularly in the medical community, is key to putting this silliness to rest.

REFERENCES

  1. Gropper SS, Smith JL, Groff JL. Advanced Human Nutrition, 5th Ed. 2009;179-182.
  2. World Health Organization. Nutrition Health Topics – Population nutrient intake goals for preventing diet-related chronic diseases. Available at: http://www.who.int/nutrition/topics/5_population_nutrient/en/index.html  Accessed February 21, 2012.
  3. Mangels R, Messina V, Messina M. The Dietitians’s Guide to Vegetarian Diets, 3rd Ed. 2011;65-83.
  4. Millward DJ.  The nutritional value of plant-based diets in relation to human amino acid and protein requirements. Proc Nutr Soc, 1999;58:249-260.
  5. Young VR, Pellett PL.  Plant proteins in relation to human protein and amino acid nutrition.  Am J Clin Nutr, 1994;59:1203S-12S.
  6. U.S. National Library of Medicine National Institutes of Health – Medline Plus Fact Sheets.  Protein in the diet. Available at:  http://www.nlm.nih.gov/medlineplus/ency/article/002467.htm  Accessed February 21, 2012.
  7. Position of the American Dietetics Association:  Vegetarian Diets. Journal of the ADA, 2009;109(7):1266-82.
  8. Preis SR, Stampfer MJ, Spiegelman D, Willett WC, Rimm EB.  Dietary protein and risk of ischemic heart disease in middle-aged men. Am J Clin Nutr, 2010;92:1265-72.
  9. Fontana L, Klein S, Holloszy JO. Long-term low-protein, low-calorie diet and endurance exercise modulate metabolic factors associated with cancer risk. Am J Clin Nutr, 2006;84:1456-62.
  10. Millward DJ, Fereday A, Gibson NR, Pacy PJ.  Human adult amino acid requirements: leucine balance evaluation of the efficiency of utilization and apparent requirements for wheat protein and lysine compared with those of milk protein in healthy adults. Am J Clin Nutr, 2000;72:112-21.
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