Protein in the Age of the New Plant Based Canada Food Guide

By: Glenda Courtney-Martin, PhD, RD

Clinical Dietician and Associate Scientist, Hospital for Sick Children
Assistant Professor, University of Toronto

Protein is an essential nutrient, which means that it must be obtained from the diet. Proteins are part of the structure of all living organisms and are involved in all biological processes that take place within cells. The word protein is derived from the Greek word protos, meaning “first”. The importance of protein is largely due to its structural and functional role in the body. Proteins serve as part of the structure of skeletal muscle, bone, connective tissue (skin, collagen and cartilage) organs (such as kidney, liver, heart, lungs) blood cells, hemoglobin, hair and nails. It is also important for the repair and maintenance of all such tissues damaged by illness or injury or as a normal part of the body tissue turnover. Because of this, protein has always been given special importance in the diet.

Functional Role of Protein

This nutrient is the main component of the thousands of enzymes in the human body, including the enzymes in the intestinal track which are responsible for breaking down the food that we eat. Proteins are also important components of hormones, like insulin growth hormone, as well as many other important chemicals in the body, including antioxidants which provide protection against oxidative stress. Proteins help maintain acid-base and fluid balance, and contribute to a feeling of satiety (being full during a meal). They also serve as a source of energy, providing 4 calories per gram. The body of a 70 kg male is approximately 16% protein (~11 kg). About half of that 11 kg of protein is found in skeletal muscle, while the other half is distributed among organs, blood, enzymes and other protein substances.

Protein Composition and Function

Amino acids are the building blocks of proteins. Proteins are composed of twenty amino acids, and every protein in the body is made up of a unique combination of these twenty amino acids. Nine of the 20 amino acids are essential, which means that they must be obtained from foods, while the other 11 amino acids can be made by the body and are considered nonessential. However, all 20 amino acids are required by the body to build and maintain proteins. Therefore, the body’s ability to fulfill its dietary requirement for protein is largely dependent on the presence of the constituent amino acids, particularly those which are essential.

Adult Protein Requirement

The protein requirement recommendations for the United States of America and Canada are set by the National Academy of Medicine (which was previously called the Institute of Medicine), and the recommendations are known as the Dietary Reference Intakes (DRI)[1]. The requirements are based on nitrogen balance studies which were formerly viewed as the gold standard for estimating protein requirements[2]. Nitrogen balance is known to underestimate requirements[3], and the current recommendations of 0.8 g/kg/day for all adults ≥ 18 years is believed to be too low. Using the indicator amino acid oxidation (IAAO) method, our group estimated the requirements in young[4] and older adults (65 years and over)[5]^,[6] to be 1.0 and 1.2 g/kg/day respectively. Based on the current DRI recommendation, a 70 kg adult should consume 56 grams of protein/day. If the IAAO estimate is used, the same 70 kg adult would require 70 to 84 grams/day. Both the DRI recommendations of 0.8 g/kg/day and the IAAO derived estimates of protein needs are based on high-quality protein from animal sources.

However, plant-based diets have been considered to be of lower protein quality since plant proteins are limiting in one or more essential amino acids and the amino acids from plant foods are not as easily available to the body as those from animal proteins. Therefore, adults on a plant-based diet could need to consume more protein to meet the body’s needs..

Protein quality

Dietary proteins are categorized as being either complete or incomplete with complete proteins being those which supply all of the essential amino acids in the amounts required by the body to maintain health and bodily functions. Incomplete proteins are those which are limiting in one or more of the essential amino acids. In general, animal proteins are complete proteins whereas most plant proteins are incomplete proteins. An incomplete protein is one which is lacking in one or more of the essential amino acids.

In human nutrition, the term “protein quality” relates to the value which has been ascribed to a dietary protein based on (1) its composition of essential amino acids (relative to their requirement) and (2) the amount of the amino acids which are available to the body after the protein has been digested and absorbed (bioavailability). Animal proteins generally have higher amounts of the essential amino acids which are more bioavailable than plant proteins[7]. Therefore, from a nutritional requirement perspective, animal protein have a superior quality to plant proteins.

Protein Quality and the Canada Food Guide Recommendation

In 2019, Health Canada published a revised version of the Canada Food Guide[8] which is predominantly plant-based. Two of the previous four food groups; meat and alternatives and milk and alternatives have been condensed into one group named “Protein Foods”. Within the “Protein Foods” group, nuts, seeds, tofu, legumes, meat, milk products, eggs, and fish are suggested to make up ¼ of the food intake, with other foods from plant sources; including vegetables and fruits, combined with grains, suggested to make up the other ¾ of our food intake. The recommendations to increase plant foods in the diet is not accompanied by guidance on how this should be done because it is largely assumed that Canadians are getting adequate protein[9] and that protein undernutrition is not a problem in Canada. However, this may not be the case for everyone, particularly older adults[10]^,[11]. United States data show that 38 and 41% of adult men and women are consuming less protein than the current recommendation^11,[12]. A shift therefore to a predominantly plant based diet could potentially lead to an increase in the number of persons getting too little protein. This is because more protein could be required when the protein source is from plants than when the protein source is from animals.

Quality of Plant Proteins

Although animal proteins can be superior in their amino acid composition and bioavailability, animal protein production is “more resource intensive” than plant protein production. In addition, foods that are high in animal protein are high in saturated fat which is linked to cardiovascular disease[13]. Plant foods, like pulses and nuts, are important alternatives to animal proteins, and their production has been shown to be less impactful to the environment, as well as benefiting human health[14]^,[15],[16]. However, pulse proteins (such as lentils,) are limiting in the essential amino acid methionine[17]. If combined with cereal grains such as wheat and rice (the mainstay of the human diet for thousands of years)[18] in the same meal, the quality of the combined protein can be improved to that of animal protein. This is because, whereas pulses are limiting in the amino acid methionine, cereal grains have high methionine content, and whereas cereal grains are limiting in the essential amino acids lysine and tryptophan¹⁸, pulses have high lysine and tryptophan.

Although this may seem relatively simple to accomplish in practice, there is one caveat. That is, amino acids in plant proteins are not always as available to the body as those from animal proteins. Plant foods may contain anti-nutritional compounds (e.g., tanins and phytates), which play a part in protecting plants against microorganisms and insects, but can also affect amino acid bioavailability[19]. Therefore, when plant foods are consumed in the diet, the proteins must be corrected for available amino acids for protein synthesis. Animal scientists have studied this for many years and many data exist from animal studies, particularly from the study of pigs[20]. Although the pig is an acceptable model for the study of protein quality in humans[21], comparison studies conducted in pigs (whose digestive systems are comparable to ours) and humans with the same foods prepared in the same way are urgently needed. Such studies are underway. In addition, there is a need to develop more affordable and quicker methods to study the availability of amino acids from foods consumed by humans. However all such methods should be validated against the human model.

One minimally invasive method, called the indicator amino acid oxidation method, was developed to study amino acid requirements and has been applied to study amino acid bioavailability in humans¹⁷. It is the only currently available method which can be used to provide guidance on how to combine plant foods in a complementary fashion to meet amino acid needs. So far the method has been applied to study lysine availability in rice[22], lysine and tryptophan availability in corn[23], and methionine availability in soy protein and casein[24]. However, more data is needed in humans. The results of these studies will be helpful when educating the public on how to combine plant sources of protein in a nutritionally sound approach.

Protein content of some commonly consumed foods

References

[1] DRI. The Institute of Medicine, Food and Nutrition Board, Dietary Reference Intakes: Energy, Carbohydrate, Fiber, Fat, Fatty Acids, Cholesterol, Protein and Amino Acids. Washington DC: The National Academy Press; 2005.

[2] Rand WM, Pellett PL, Young VR. Meta-analysis of nitrogen balance studies for estimating protein requirements in healthy adults. The American journal of clinical nutrition. 2003;77(1):109-27.

[3] Elango R, Ball RO, Pencharz PB. Recent advances in determining protein and amino acid requirements in humans. Br J Nutr. 2012;108 Suppl 2:S22-30.

[4] Humayun MA, Elango R, Ball RO, Pencharz PB. Reevaluation of the protein requirement in young men with the indicator amino acid oxidation technique. Am J Clin Nutr. 2007;86(4):995-1002.

[5] Rafii M, Chapman K, Elango R, Campbell WW, Ball RO, Pencharz PB, et al. Dietary Protein Requirement of Men >65 Years Old Determined by the Indicator Amino Acid Oxidation Technique Is Higher than the Current Estimated Average Requirement. J Nutr. 2016.

[6] Rafii M, Chapman K, Owens J, Elango R, Campbell WW, Ball RO, et al. Dietary protein requirement of female adults >65 years determined by the indicator amino acid oxidation technique is higher than current recommendations. J Nutr. 2015;145(1):18-24.

[7] WHO/FAO/UNU. Protein and amino acid requirements in human nutrition. Report of a joint WHO/FAO/UNU expert consultation. World Health Organization Technical Report Series. 2007.

[8] Health Canada. Canada’s Food Guide. Ottawa, Ontario K1A 0K9: Publications, Health Canada; 2019.

[9] Millward DJ. Optimal intakes of protein in the human diet. Proc Nutr Soc. 1999;58(2):403-13.

[10] Houston DK, Nicklas BJ, Ding J, Harris TB, Tylavsky FA, Newman AB, et al. Dietary protein intake is associated with lean mass change in older, community-dwelling adults: the Health, Aging, and Body Composition (Health ABC) Study. The American journal of clinical nutrition. 2008;87(1):150-5.

[11] Fulgoni VL, 3rd. Current protein intake in America: analysis of the National Health and Nutrition Examination Survey, 2003-2004. Am J Clin Nutr. 2008;87(5):1554S-7S.

[12] Pasiakos SM, Agarwal S, Lieberman HR, Fulgoni VL, 3rd. Sources and Amounts of Animal, Dairy, and Plant Protein Intake of US Adults in 2007-2010. Nutrients. 2015;7(8):7058-69.

[13] Horrigan L, Lawrence RS, Walker P. How sustainable agriculture can address the environmental and human health harms of industrial agriculture. Environ Health Perspect. 2002;110(5):445-56.

[14] Akibode S, Maredia M. Global and Regional Trends in Production, Trade and Consumption of Food Legume Crops. Michigan State University, Department of Agriculture FaRE; 2011 March 27, 2011. Contract No.: version 3/27/11.

[15] Hu FB. Plant-based foods and prevention of cardiovascular disease: an overview. Am J Clin Nutr. 2003;78(3 Suppl):544S-51S.

[16] Tuso PJ, Ismail MH, Ha BP, Bartolotto C. Nutritional update for physicians: plant-based diets. Perm J. 2013;17(2):61-6.

[17] Iqbal A, Khalil I, B., , Ateeq N, Khan M, Sayyar. Nutritional quality of important food legumes. Food Chemistry. 2006;97:331-5.

[18] Awika JM. Major Cereal Grain Production and Use around the World. In: Series AS, editor. Advanes in Cereal Science:Implications to Food Processing and Health Promotions. Washington, DC: American Chemical Society. 2011. p. 1-13.

[19] Gilani GS, Cockell KA, Sepehr E. Effects of antinutritional factors on protein digestibility and amino acid availability in foods. J AOAC Int. 2005;88(3):967-87.

[20] Swine CoNRo. Nutreint Requirements of Swine. 11th rev. ed. ed. Washington, DC: National Academics Press; 2012.

[21] FAO. Research Approaches and Methods for Evaluating the Protein Quality of Human Foods: Report of a FAO Working Group. Rome; 2014.

[22] Prolla IR, Rafii M, Courtney-Martin G, Elango R, da Silva LP, Ball RO, et al. Lysine from cooked white rice consumed by healthy young men is highly metabolically available when assessed using the indicator amino acid oxidation technique. J Nutr. 2013;143(3):302-6.

[23] Rafii M, Elango R, Ball RO, Pencharz PB, Courtney-Martin G. Metabolic Availability of the Limiting Amino Acids Lysine and Tryptophan in Cooked White African Cornmeal Assessed in Healthy Young Men Using the Indicator Amino Acid Oxidation Technique. J Nutr. 2018;148(6):917-24.

[24] Humayun MA, Elango R, Moehn S, Ball RO, Pencharz PB. Application of the indicator amino acid oxidation technique for the determination of metabolic availability of sulfur amino acids from casein versus soy protein isolate in adult men. J Nutr. 2007;137(8):1874-9.