Sources of Protein

Sources of Protein

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.  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)  

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)

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. (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)

Many plant-based protein foods contain protein and are also rich in other health-promoting dietary components, including fiber, antioxidants, vitamins, and minerals. (23)

Sources of proten

Plant Protein

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)

Animal Protein

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, impaired functioning of the lining of blood vessels, and 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 
  • 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)

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

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)

Greenhouse gas

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 produced, 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

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)

Digestible indispensable amino acid score

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

In Conclusion

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.

Plant protein

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