Adequate protein is essential for good health, but you may be wondering what the best protein sources are. With so much conflicting research on animal and plant-based protein, choosing the best options can be challenging. Although nutrition research is a relatively young science and we still don’t fully understand which diet is optimal for human health, a large body of evidence suggests that a healthy diet includes a variety of whole, minimally-processed foods, including vegetables, fruit, beans and legumes, nuts and seeds, whole grains, dairy, seafood, lean meat, and poultry. A healthy diet also consists of the three essential macronutrients: carbohydrates, fat, and protein. (26) Keep reading to learn more about animal and plant-based protein and how to identify high-quality protein sources.
What is protein?
Protein is a macronutrient found in animal and plant-based foods. Protein facilitates several essential functions in the body, such as muscle and tissue repair, hormone regulation, facilitation of enzymatic processes, and transportation and storage of molecules such as oxygen. (2)(33) Both plant and animal protein sources consist of organic compounds known as amino acids that serve as building blocks for proteins. (33)
Did you know? Protein needs vary depending on your activity level, body size, and life stage. The recommended dietary allowance (RDA) for protein is 0.8 g/kg of body weight per day for the average, minimally active adult, while individuals with increased activity levels may require up to 1.2 to 2 g/kg per day. (3)
Complete vs. incomplete protein
Plant and animal-based protein sources have varying amounts of amino acids, making some protein sources complete and some incomplete. A food is considered a complete protein when it contains all nine essential amino acids, those that the body can’t synthesize on its own. In contrast, a food is considered an incomplete protein if it lacks any of these nine essential amino acids (isoleucine, histidine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan, and valine). (21)
Animal-sourced foods are complete protein sources, and plant-based proteins are incomplete proteins (with the exception of soy) because they lack some of the essential amino acids or don’t contain significant enough amounts of each essential amino acid. (21)
Examples of complete proteins include:
- Dairy (e.g., cheese, milk, yogurt)
- Fish and shellfish
- Soy (e.g., edamame, tempeh, tofu)
- Turkey (15)
Examples of incomplete proteins include:
Generally speaking, the highest protein foods are animal-based foods because they contain all essential amino acids. However, that’s not to say that plant-based foods are nutritionally inferior. Many plant-based protein foods contain protein and are also rich in other health-promoting dietary components, including fiber, antioxidants, vitamins, and minerals. (23)
Diets rich in whole, plant-based foods are linked to many positive health benefits. According to a 2020 meta-analysis, consumption of plant-based protein is significantly associated with a reduced risk of all-cause mortality and cardiovascular disease mortality compared to diets rich in animal protein. The analysis also noted that even a modest 3% increase in energy from plant-based protein foods per day reduced the risk of all-cause mortality by 5%. (20) These results suggest that a diet consisting of more plant-based protein versus animal-based protein may promote longevity.
When possible, aim to consume more whole food-containing meals featuring various high protein plant-based foods such as beans, legumes, nuts, and seeds. Consumer demand for plant-based meat alternatives has increased in recent years; however, many of these alternatives are highly processed and may be high in sodium and saturated fat. Keep in mind that many of these alternatives are not nutritionally equivalent to meat as they may lack certain nutrients such as vitamin B12, iron, and zinc. (4)
Shopping tip: Because the majority of soybeans grown in North America are genetically modified and grown with the use of potentially harmful pesticides and herbicides, it’s best to choose certified organic soy-based products such as tofu, tempeh, and soy milk. (1)(31)
Precautions for vegans and vegetarians
If you’re vegan or vegetarian, it’s possible to obtain enough protein from plant-based foods; however, you’ll need to pay close attention to your diet to ensure you’re consuming adequate protein. Most people tend to consume more than the RDA of protein per day, regardless of whether they adhere to a vegetarian or omnivorous diet. (17) Depending on your dietary habits and physical activity levels, you may need to consume slightly more protein beyond the RDA to make up for the lower digestibility (net absorption of amino acids) of plant-based proteins. However, research on the subject is inconclusive, and some experts proclaim that the differences in digestibility between plant and animal proteins are minuscule. (3)(17)
Furthermore, certain food preparation techniques, such as boiling, soaking, sprouting, and fermenting, can improve digestibility of some plant-based foods. (6)
Download a handout on anti-nutrients to learn more.
Combining incomplete proteins: is it necessary?
You might have heard that you should combine two or more incomplete, plant-based protein foods to form a complete protein; however, research demonstrates that combining plant-based foods in a single meal isn’t necessary as long as your diet provides enough variety throughout the day. (18) Focus on consuming a variety of plant-based foods, including fruits, vegetables, beans and legumes, nuts and seeds, and whole grains (e.g., barley, brown rice) and pseudograins (e.g., quinoa) each day to ensure you’re obtaining a full range of all of the essential amino acids.
Some earlier research has demonstrated that the consumption of red meat, particularly processed meat, is associated with an increased risk of total and all-cause mortality (11)(19)(27)(29); however, more recent research points to a different conclusion. According to the same 2020 meta-analysis that demonstrated the benefits of plant-based protein for longevity, consumption of animal protein was not significantly associated with an increased risk of cardiovascular disease or cancer mortality. (20)
It’s important to highlight that many of the studies investigating the health implications of meat consumption have some limitations. For example, some studies don’t consider other influencing lifestyle factors such as physical activity, weight management, and tobacco use. (24) Furthermore, many studies don’t consider the quality or livestock management practices of the meat consumed by the trial participants. Due to modern agricultural practices and a high demand for animal protein, much of the livestock raised in North America is fed grain rations and raised on industrial feedlots instead of on nutrient-rich pastures. As a result, feedlot meat is often nutritionally inferior to grass-fed, organically-raised meat. (22)(28)
According to one study, meat from grass-fed, organically-raised beef contained greater amounts of heart-healthy omega-3 fatty acids and a lower ratio of omega-6 to omega-3 fatty acids than feedlot meat. Why is this significant? A high ratio of omega-6 to omega-3 fatty acids, which is common in diets rich in processed foods, contributes to low-grade inflammation, oxidative stress, endothelial dysfunction (impaired functioning of the lining of blood vessels), and atherosclerosis (hardening of the arteries). (5)
When choosing meat, poultry, and fish, opt for high-quality options when possible. Whenever possible, tips for choosing high-quality animal protein include:
- Avoiding processed and deep-fried meats (e.g., deli meats, bacon, pepperoni, hot dogs, chicken nuggets)
- Choosing certified organic meat
- Looking for grass-fed or pasture-raised products
- Looking for unprocessed, nitrite-free, and low-sodium products
- Purchasing locally-raised meat and poultry from farms or farmers markets (7)
- Referring to the Environmental Working Group’s Consumer Guide to Seafood for safe and sustainable seafood options (9)
Tip: Optimize your diet by combining high-quality meat, poultry, and seafood with an abundance of fiber-rich foods such as non-starchy vegetables (e.g., broccoli, cauliflower, dark leafy greens), fruit, and whole grains. (19)
Environmental concerns of meat consumption
Did you know that animal agriculture accounts for approximately 16.5% of total greenhouse gas emissions? (30) These effects are due in part to mass deforestation (to provide enough space for livestock), nitrous oxide (N2O) emissions from fertilizer use and irrigation practices, and increased methane (CH4) production from ruminant animals. (32) The graph below shows the greenhouse gas emissions for many common foods.
To counter the harmful effects of animal agriculture on the environment, many experts have proposed some sustainable solutions. Regenerative agriculture, which involves several environmentally-conscious practices such as managed grazing, silvopasture (integration of trees, forage, and grazing livestock), and farmland restoration, is a more sustainable alternative to industrial feedlot operations. Research demonstrates that regenerative agriculture encourages enhanced biodiversity and may reduce and sequester greenhouse gas emissions. (22) Wondering where you can find food grown on a regenerative farm? Check out your local farmer’s market or community-supported agriculture (CSA), or visit the Regeneration International website for a map of farms near you.
As a consumer, you may feel as though your personal contributions may not make a difference; however, there are some small actions you can take to do your part. You don’t need to ditch the meat to reduce your carbon footprint. Simply reducing your intake of meat and animal products or opting for non-ruminant sources such as chicken or fish can have a significant impact. If you want to incorporate more plant-based foods into your weekly meal rotation, consider enjoying a plant-based meal each week in honor of “meatless Monday.” (25)
Assessing protein quality
A food’s protein quality is determined by its amino acid composition and digestibility. (10) Digestibility is highly variable and is influenced by numerous factors. For example, the presence of naturally-occurring anti-nutritional factors (e.g., tannins, phytates, lectins) or high levels of insoluble fiber can reduce protein digestibility. Furthermore, individual physiological factors can affect how efficiently you absorb protein, such as the composition of your unique gut microbiota and the amount of stomach acid you produce, which is required for proper protein digestion. (10)(12)(13)
Several methods can determine protein quality. Three commonly used methods include biological value, protein digestibility-corrected amino acid score, and digestible indispensable amino acid score.
Biological value (BV) is a measurement of how well the body utilizes a food’s protein. It determines protein quality by measuring the percentage of protein from food absorbed by the gut and retained in the body. The higher the BV, the greater the supply of essential amino acids. Generally, animal protein foods have a higher BV than plant-based foods since many plant-based protein sources provide insufficient amounts of one or more essential amino acids. BV does have some limitations as it doesn’t account for the interaction of other foods consumed at the same time. (15)
Protein digestibility-corrected amino acid score
The protein digestibility-corrected amino acid score (PDCAAS) also assesses protein quality. PDCAAS is calculated using a given food’s true fecal nitrogen digestibility, the level of nitrogen detected in feces relative to the amount of nitrogen ingested. Foods with low true fecal nitrogen digestibility and/or low levels of essential amino acids will have a lower PDCAAS, meaning the food is a lower-quality protein. Many plant-based foods generate lower PDCAAS considering that they have lower levels of one or more essential amino acids. (10)
PDCAAS also has some limitations; for example, PDCAAS doesn’t consider the bioavailability of each essential amino acid in a given food. The bioavailability of a food encompasses the digestibility, chemical integrity (proportion of an amino acid that is in a utilizable form), and a lack of interference in metabolism from other foods consumed simultaneously. (10) These limitations often lead to overestimations of PDCAAS values. (16) The PDCAAS method has been utilized to assess protein quality for over two decades; however, research suggests that protein quality should be measured differently. (10)
Digestible indispensable amino acid score
In response to growing criticisms of the limitations of PDCAAS, the Food and Agriculture Organization of the United Nations (FAO) now recommends the use of a different protein rating method known as the digestible indispensable amino acid score (DIAAS). Unlike PDCAAS, DIAAS uses the intestinal digestibility coefficients of each amino acid, which is equal to the difference between consumed amino acids and levels of amino acids detected at the end of the small intestine. (10) This method of determining protein quality has been shown to better reflect the amount of amino acids absorbed during digestion. (10) A food with a high DIASS (>100) is considered a high-quality protein source. (14)
Comparing plant and animal protein sources
The table below outlines the nutrition facts data and digestibility scores for some common protein sources.
The bottom line
A combination of animal and plant-based protein foods can be consumed as part of a healthy, varied diet. When possible, eliminate processed meat and replace some of your weekly animal-based meals with high-protein plant sources such as beans, peas, and lentils. If you choose to consume meat, opt for healthier, more sustainable options such as organic, pasture-raised meat and poultry. Protein needs vary depending on your activity level, body size, and life stage. Speak to your integrative health care provider before making significant dietary modifications.
- Bawa, A. S., & Anilakumar, K. R. (2013). Genetically modified foods: Safety, risks and public concerns-a review. Journal of Food Science and Technology, 50(6), 1035–1046.
- Berg, J. M., Tymoczko, J. L., & Stryer, L. (2002). Protein structure and function. W H Freeman.
- Ciuris, C., Lynch, H. M., Wharton, C., & Johnston, C. S. (2019). A comparison of dietary protein digestibility, based on DIAAS scoring, in vegetarian and non-vegetarian athletes. Nutrients, 11(12).
- Curtain, F., & Grafenauer, S. (2019). Plant-based meat substitutes in the flexitarian age: An audit of products on supermarket shelves. Nutrients, 11(11).
- DiNicolantonio, J. J., & O’Keefe, J. H. (2018). Importance of maintaining a low omega-6/omega-3 ratio for reducing inflammation. Open Heart, 5(2), e000946.
- Drulyte, D., & Orlien, V. (2019). The effect of processing on digestion of legume proteins. Foods (Basel, Switzerland), 8(6).
- Environmental Working Group. (2011a). Helpful tips for meat eaters – 2011 meat eaters guide. https://www.ewg.org/meateatersguide/helpful-tips-for-meat-eaters/
- Environmental Working Group. (2011b). Meat eater’s guide to climate change and health. https://static.ewg.org/reports/2011/meateaters/pdf/methodology_ewg_meat_eaters_guide_to_health_and_climate_2011.pdf
- Environmental Working Group. (2014). EWG’s consumer guide to seafood. https://www.ewg.org/consumer-guides/ewgs-consumer-guide-seafood
- Food and Agriculture Organization of the United Nations. (2011). Dietary protein quality evaluation in human nutrition: Report of an FAO expert consultation. http://www.fao.org/ag/humannutrition/35978-02317b979a686a57aa4593304ffc17f06.pdf
- Fraser, G. E. (1999). Associations between diet and cancer, ischemic heart disease, and all-cause mortality in non-Hispanic white California Seventh-day Adventists. The American Journal of Clinical Nutrition, 70(3 Suppl), 532S – 538S.
- Gilani, G. S., Cockell, K. A., & Sepehr, E. (2005). Effects of antinutritional factors on protein digestibility and amino acid availability in foods. Journal of AOAC International, 88(3), 967–987.
- Heda, R., Toro, F., & Tombazzi, C. R. (2021). Physiology, pepsin. In StatPearls. StatPearls Publishing.
- Herreman, L., Nommensen, P., Pennings, B., & Laus, M. C. (2020). Comprehensive overview of the quality of plant- and animal-sourced proteins based on the digestible indispensable amino acid score. Food Science & Nutrition, 8(10), 5379–5391.
- Hoffman, J. R., & Falvo, M. J. (2004). Protein – which is best? Journal of Sports Science & Medicine, 3(3), 118–130.
- Marinangeli, C. P. F., & House, J. D. (2017). Potential impact of the digestible indispensable amino acid score as a measure of protein quality on dietary regulations and health. Nutrition Reviews, 75(8), 658–667.
- Mariotti, F., & Gardner, C. D. (2019). Dietary protein and amino acids in vegetarian diets-a review. Nutrients, 11(11).
- Marsh, K. A., Munn, E. A., & Baines, S. K. (2013). Protein and vegetarian diets. The Medical Journal of Australia, 199(S4), S7–S10.
- Micha, R., Wallace, S. K., & Mozaffarian, D. (2010). Red and processed meat consumption and risk of incident coronary heart disease, stroke, and diabetes mellitus: A systematic review and meta-analysis. Circulation, 121(21), 2271–2283.
- Naghshi, S., Sadeghi, O., Willett, W. C., & Esmaillzadeh, A. (2020). Dietary intake of total, animal, and plant proteins and risk of all cause, cardiovascular, and cancer mortality: Systematic review and dose-response meta-analysis of prospective cohort studies. BMJ , 370, m2412.
- Nehete, J. Y., Bhambar, R. S., Narkhede, M. R., & Gawali, S. R. (2013). Natural proteins: Sources, isolation, characterization and applications. Pharmacognosy Reviews, 7(14), 107–116.
- Provenza, F. D., Kronberg, S. L., & Gregorini, P. (2019). Is grassfed meat and dairy better for human and environmental health? Frontiers in Nutrition, 6, 26.
- Rogerson, D. (2017). Vegan diets: Practical advice for athletes and exercisers. Journal of the International Society of Sports Nutrition, 14, 36.
- Rohrmann, S., & Linseisen, J. (2016). Processed meat: The real villain? The Proceedings of the Nutrition Society, 75(3), 233–241.
- Rust, N. A., Ridding, L., Ward, C., Clark, B., Kehoe, L., Dora, M., Whittingham, M. J., … & West, N. (2020). How to transition to reduced-meat diets that benefit people and the planet. The Science of the Total Environment, 718, 137208.
- Shao, A., Drewnowski, A., Willcox, D. C., Krämer, L., Lausted, C., Eggersdorfer, M., Mathers, J., … & Griffiths, J. C. (2017). Optimal nutrition and the ever-changing dietary landscape: A conference report. European Journal of Nutrition, 56(Suppl 1), 1–21.
- Sinha, R., Cross, A. J., Graubard, B. I., Leitzmann, M. F., & Schatzkin, A. (2009). Meat intake and mortality: A prospective study of over half a million people. Archives of Internal Medicine, 169(6), 562–571.
- Średnicka-Tober, D., Barański, M., Seal, C., Sanderson, R., Benbrook, C., Steinshamn, H., Gromadzka-Ostrowska, J., … & Leifert, C. (2016). Composition differences between organic and conventional meat: A systematic literature review and meta-analysis. The British Journal of Nutrition, 115(6), 994–1011.
- Takata, Y., Shu, X.-O., Gao, Y.-T., Li, H., Zhang, X., Gao, J., Cai, H., … & Zheng, W. (2013). Red meat and poultry intakes and risk of total and cause-specific mortality: Results from cohort studies of Chinese adults in Shanghai. PloS One, 8(2), e56963.
- Twine, R. (2021). Emissions from animal agriculture—16.5% is the new minimum figure. Sustainability: Science Practice and Policy, 13(11), 6276.
- U.S. Center for Food Safety. (2020). GMO crops, animal food, and beyond. U.S. Food and Drug Administration. https://www.fda.gov/food/agricultural-biotechnology/gmo-crops-animal-food-and-beyond
- U.S. Environmental Protection Agency. (n.d.). Sources of greenhouse gas emissions. https://www.epa.gov/ghgemissions/sources-greenhouse-gas-emissions
- U.S. National Library of Medicine. (2019). Amino acids. MedlinePlus. https://medlineplus.gov/ency/article/002222.htm