16.6 The Basics of Protein

Protein is Essential

Protein is an essential nutrient because it serves as the body’s sole source of the nine essential amino acids, which are required by the body for many vitally important functions. The nine essential amino acids must be consumed because the body can’t produce them (Institute of Medicine, Food and Nutrition Board, 2005).

Protein is the building block for all types of body tissue, including bone, skeletal muscle, connective tissues, skin, organs, red blood cells, hair, and fingernails. It’s important for building, maintaining, and repairing all types of tissue, including muscle.

Not only does protein provide structure for the body’s many tissues, but it also plays a role in almost all biological processes throughout the body, including maintaining optimal organ function and influencing immune system performance. It’s the basic component of enzymes and hormones. Enzymes facilitate nearly all chemical reactions in the body, and hormones coordinate many important activities. For example, protein is essential in the production of hemoglobin, which transports oxygen through the blood. Protein is also required for the body to produce neurotransmitters, which transmit messages between neurons or from neurons to muscles.

Protein deficiency impairs almost all aspects of body function and can contribute to or result in the following (Santos et al., 2017; Wu, 2016; Institute of Medicine, 2006; Bonjour, 2011):

• Compromised immune function, which increases the risk of infection and decreases the ability to fight infections such as the common cold
• Increased susceptibility to metabolic and infectious diseases
• Thin and fragile hair and appearance of grey hair color
• Dull, pale, dry or flaking skin
• Loss of muscle mass and physical weakness
• Lower bone density, increased bone fragility, and increased risk of fracture
• Dental abnormalities
• Anemia
• Edema or swelling due to fluid retention
• Cardiovascular dysfunction
• Stunted growth and impaired cognitive development in children
• Emotional disorders ranging from moodiness and irritability to severe depression and anxiety
• Insomnia
• Headache
• Fainting
• Loss of libido, reduced fertility, and embryonic loss
• Advanced aging

For these reasons, consuming protein is essential.

Adequate, Optimal, and Safe Amounts of Protein

The daily RDA for protein is 0.8 grams per kilogram of body weight, or 0.36 grams per pound. This is the minimum amount of protein deemed adequate to prevent deficiency, which is not necessarily the same amount that may be required for optimal health and functioning (Institute of Medicine, Food and Nutrition Board, 2005; Wolf et al., 2017; Cuenca-Sáchez, Navas-Carillow, & Orenes-Piñero, 2015). Protein requirements vary on an individual basis depending on factors such as age, sex, body weight and lean body mass, physical activity level, and health status (Wolfe et al., 2017). Research has shown that amounts of protein greater than the RDA are necessary for some individuals, including people recovering from injury or certain illnesses, physically active adults and athletes, individuals who are pregnant or lactating, aging and older adults, and individuals seeking to achieve or maintain significant weight loss (Rodriquez & Miller, 2015; Bauer & Diekmann, 2015; Jager et al., 2017; Lonnie et al., 2018; Wolfe et al., 2017; Wu, 2016). Protein intake above the RDA benefits bone health and improves markers of cardiovascular risk (Wolfe et al., 2017; Wu, 2016), and levels approximately twice the RDA have been associated with the promotion of satiety, muscle synthesis, healthy body composition, preservation of lean body mass during weight loss and aging, functional ability with aging, and metabolic function (Rodriquez & Miller, 2015; Westerterp, 2004). It’s worth noting, if one’s goal is to increase muscle synthesis or lean body mass, this cannot be done solely by consuming protein; it also requires physical activity or exercise (Tome, 2012).

Because the body can’t store amino acids, excess protein beyond what the body can use is converted to glucose for energy or fat for storage or excreted in the urine. Processing excess protein can burden the liver, kidneys, and intestines. Some researchers have debated the safety of high protein diets, with concern that long-term high protein consumption may be linked to adverse effects such as kidney stones, renal insufficiency, liver dysfunction, gastrointestinal discomfort, nausea, diarrhea, dehydration, fatigue, headaches, weak bones, osteoporosis, seizures, increased risk of heart disease and cancer, and even death (Delimaris, 2013; Wu, 2016; Cuenca-Sáchez et al., 2015).

Unfortunately, what’s considered “high protein” has not always been very clearly and consistently defined, and historically, many people’s high protein diets have tended to be very low in carbohydrates and high in meat and unhealthy fats, which could be the real cause of adverse effects.

A number of studies and comprehensive literature reviews have concluded that, among healthy adults, protein intake above the current RDA appears to pose more benefit than potential risk (Rodriguez & Miller, 2015; Cuenca-Sáchez et al., 2015; Wu, 2016; Wolfe et al., 2017; Jager et al., 2017). Healthy adults can tolerate long-term consumption of up to 2 grams of protein per kilogram of body weight per day (Wu, 2016). This is more than two times the RDA. Some well-adapted healthy adult athletes have tolerated up to 3.5 grams per kilogram per day (Wu, 2016; Jager et al., 2017). However, protein intake above 35% of total daily calories is not considered safe and therefore not recommended (Institute of Medicine, 2006; Wu, 2016; Wolfe et al., 2017; Jager et al., 2017). Further, people with chronic kidney disease or other kidney dysfunction, susceptibility to kidney stones, liver dysfunction, or gout may need to limit their protein intake (Cuenca-Sánchez et al., 2015; Bauer & Diekmann, 2015; Wu, 2016). Anyone with a health condition should consult with a trusted health professional to determine what’s appropriate for their individual circumstances.

In summary, a reasonably high level of protein intake is appropriate and safe for most adults.

Sources of Protein: Are Some Better Than Others?

Protein is present in many foods. Assessing the quality of a source of protein takes into account whether it offers complete or incomplete protein, the digestibility of its amino acids and their availability for protein synthesis within the body, and other factors such as the presence of other beneficial nutrients and association with disease risk.

Some sources of protein are considered complete proteins, meaning they provide adequate amounts of all nine of the essential amino acids. Others are referred to as incomplete proteins because they lack one or more of the essential amino acids; fortunately, the amino acids from incomplete proteins can complement one another within the body when eaten together or within the same day.

All animal-based foods, including the following, are sources of complete protein:

• Fish and seafood
• Poultry, such as chicken, turkey, and duck
• Meat, such as beef, pork, and lamb
• Eggs
• Dairy products, such as cottage cheese and yogurt

The following plants offer complete protein:

• Soybeans, edamame, and soy products such as tofu, tempeh, and textured vegetable protein
• Hemp and chia seeds
• Quinoa, amaranth, and buckwheat
• Spirulina

Many other plants also provide protein, albeit incomplete protein. They include:

• Legumes, including beans, such as black beans and chickpeas, lentils, peas, and peanuts or peanut butter
• Nuts, such as almonds and pistachios
• Seeds, such as hemp, pumpkin, flax, and sunflower
• Nut and seed butters such as almond butter, sun butter, and tahini
• Whole grains
• Some vegetables such as potatoes, broccoli, mushrooms, and kale
• Plant-based meat substitutes including mock duck or seitan, which is made from wheat gluten

Regardless of whether a particular plant food is a source of complete or incomplete protein, the amino acids from plants are less digestible and less available for protein synthesis compared to the amino acids from animal-based foods (Tome, 2012). Further, animal-based foods provide some important nutrients not sufficiently present in plant foods—such as heme iron, which is found only in meat, poultry, fish, and seafood, and vitamin B12; this means that individuals who avoid or minimize consumption of animal-based foods need to be a little more thoughtful about their intake of certain nutrients like vitamin B12. However, they can get adequate protein, or potentially even achieve a relatively high protein diet, if that’s desired, by consuming generous amounts of a sufficient variety of plant-based sources of protein every day (Tome, 2012). Plant foods high in protein offer not only protein, but also fiber and other valuable nutrients not found in animal-based foods, and diets made up of primarily plants are associated with health benefits and decreased risk of disease (Katz & Meller, 2014; Yokoyama, Barnard, Levin & Watanabe, 2014; Wang et al., 2015; Huang et al., 2012; Mertens, Markey, Geleijnse, Lovegrove, & Givens, 2018; Mozaffarian, 2016; Sacks et al., 2017; Obersby, Chappell, Dunnett, & Tsiami, 2013). Additionally, although red and processed meats are high in protein, they are also high in saturated fat and cholesterol and have been shown to increase the risk of cardiovascular disease, diabetes, cancer, and early death (Harvard T. H. Chan School of Public Health, 2015c; Rohrmann et al., 2013; World Cancer Research Fund, American Institute for Cancer Research, 2007; St. Jeor et al., 2001; Bingham, 1999; Bouvard et al., 2015; Chan, 2011; Norat & Riboli, 2001). So, as you can see, both animal-based and plant sources of protein offer pros and cons.

Another source of protein that has become more popular among many people is supplemental protein powder. Some protein powders are made from animal proteins, such as whey protein. Others are made from plant proteins, such as soy, pea, or hemp proteins, and these are generally labeled as vegan. Supplemental protein can be convenient, and it is considered safe as long as its use doesn’t result in protein intake greater than 35% of total daily calories (Jager et al., 2017). However, nutrients in foods don’t occur in isolation, and the risk of adverse effects from excess protein intake from whole foods appears to be very low; in general, it’s ideal for us to get most or all of our protein from whole food sources (Jager et al., 2017; IOM, 2006). Further, some high-protein whole foods may offer a similar level of protein and convenience and be less expensive compared to a typical protein supplement; greek yogurt is an example.

In summary, it’s recommended that we:

• prioritize consumption of plant-based sources of protein,
• if desired, include eggs, dairy, fish, seafood, poultry, and/or lean meats and
• minimize our intake of red and processed meats.

Frequency of Protein Consumption

Because so many important bodily functions require protein and the body can’t store amino acids, it’s recommended that we consume adequate protein on a daily basis. Further, distributing protein intake throughout the day, including some at each meal or snack every three to four hours, will maximize its absorption and utilization and can help with feelings of fullness between meals (Wu, 2016; Jager et al., 2017; Symons, Sheffield-Moore, Wolfe, & Paddon-Jone, 2009; Mamerow et al., 2014; Loenneke, Loprinzi, Murphy, & Phillips, 2016; Farsijani et al., 2017).