Nutrition & Health Info Sheets for Health Professionals - Protein Requirements

Nutrition & Health Info Sheets contain up-to-date information about nutrition, health, and food. They are provided in two different formats for consumer and professional users. These resources are produced by Dr. Rachel Scherr and her research staff. Produced by Kristen James, BS, Anna M. Jones, PhD, and Rachel E. Scherr, PhD.

What is protein?

Protein is a nutrient found in animal and plant foods that provides the body with amino acid building blocks used for many biological roles. Amino acids are composed of 3 main parts: an amine group, a carboxyl group, and a distinguishing R-group. The R-group influences the amino acid’s chemical properties (e.g. hydrophobic, hydrophilic, ionic charge), the protein’s structure, and the protein’s biological function. When protein is consumed in the diet, it is broken down into amino acids and absorbed into the body, where it is used to create new proteins that fulfill the body’s needs.

Why does the body need protein?

Dietary protein provides the body with amino acids and nitrogen. The body uses protein for a variety of needs that can be summarized by three broad categories: structural, regulatory , and energetic.

Structural

The most abundant protein in the body is collagen, a structural protein found in the extracellular matrix and in connective tissue. There are many types of collagen proteins, which are distributed in different tissues (e.g. skin, bone, tendon, cartilage, heart) [1]. Another example of a structural protein is actin, an abundant protein that also comes in many types. In vertebrates, actin is a critical component of muscle fibers [2]. Overall, structural proteins can be thought of as critical to maintaining the dynamic elasticity and shape of cells and tissues.

Regulatory

Proteins also serve regulatory roles as they orchestrate communication within and between cells. A major class of hormones are made of protein and play critical roles in the endocrine system. Examples of protein hormones include prolactin, growth hormone, and insulin. Protein hormones also communicate with other systems such as the immune system, which is important in the immune response [3]. 

Energetic

Apart from serving structural and regulatory roles, proteins also provide the body with a source of energy. In this process, amino acids are deaminated (the amine group is removed) and the carbon skeleton becomes an intermediate in glucose metabolism to generate energy. The body does not have an amino acid storage system; therefore, amino acids that are not used in structural, energetic, or regulatory roles are deaminated and their carbon skeleton is converted to fat.

Dietary protein also provides the body with nitrogen. Nitrogen is common to all amino acids and is removed through either deamination or transamination. Transamination, a reversible reaction, involves the transfer of the amino nitrogen to intermediates of glucose metabolism that enter circulation as nitrogen carriers. Deamination removes amino nitrogen creating ammonia, which is converted to urea in the liver [4]. Urea is transported to the kidneys where it is excreted.

What are essential and non-essential amino acids?

Many amino acids can be synthesized in the body from other biological molecules and do not need to be consumed in the diet; however other amino acids cannot be made by the body and have to be eaten in the diet to sustain health. Amino acids that must be eaten in the diet are called essential amino acids whereas those that the body can make are called non-essential amino acids [5].

There are 9 essential amino acids, which include: histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan, and valine. The most common non-essential amino acids include alanine, arginine, asparagine, aspartic acid, cysteine, glutamic acid, glutamine, glycine, proline, serine, and tyrosine. During growth, arginine is also considered an essential amino acid. There are certain conditions that may require consumption of nonessential amino acids, which are called conditionally essential. For example, in burned individuals, the metabolic demand of proline surpasses the body's capacity to synthesize the amino acid [6].

Many food protein sources differ in their amino acid profile. A complete protein is a food that contains all 9 essential amino acids in quantities to sustain the body’s need. Examples of complete proteins include animal sources (meat, poultry, fish, eggs, and dairy) as well as soybeans. In contrast, an incomplete protein is a food that does not contain all of the essential amino acids or a food that contains all 9 essential amino acids but not in the quantities needed to sustain the body’s need. In an incomplete protein, the essential amino acid found in the lowest relative quantity is called the limiting amino acid [5].

What are protein requirements for different ages and life stages?

The 2015-2020 Dietary Guidelines for Americans state that the Acceptable Macronutrient Distribution Range (AMDR, macronutrient range suggested to reduce risk of chronic disease) is 5-20 percent of daily calories for children ages 1 to 3 years, 10-30 percent of daily calories from years 4-18, and 10-35 percent of daily calories from years adults over the age of 18 regardless of sex [7]. The Dietary Guidelines also state that the Recommended Dietary Allowance (RDA, the amount of a nutrient needed to meet basic nutritional requirements) is 0.8 grams of protein per kilogram bodyweight. The protein RDA for an average weight individual in each sex and age group can be found in Table 1.

Table 1:  Recommended Dietary Allowance for an average weight individual in each sex and age group.

Table based on Appendix 7-1 of the 2015-2020 Dietary Guidelines for Americans [7]

Biological states that will increase the protein requirement include growth, pregnancy, lactation, injury, sickness, and activity level. In pregnancy and lactation, the RDA is increased to 71 g/day [8].

The daily value for protein is 50 g. This is a standard value used on Dietary Food Labels in reference to a standard daily 2,000 calorie diet.

What are current intakes of protein in the US?

According to the 2007-2010 NHANES survey, What We Eat in America, “intakes of protein foods are close to amounts recommended for all age-sex groups.” The results of this survey suggest that most common protein sources are 1) meats, poultry and eggs; 2) nuts, seeds, and soy products; 3) and last, seafood [7].

The 2015-2020 Dietary Guidelines suggest shifting protein intake to increase seafood and nut and seed intake by substituting these foods for meats, poultry and eggs [7]. This shift will also increase the intake of healthy fats, while decreasing sodium and saturated fat intake.

Are there any groups at risk for low protein intake?

People with low intake of protein, inefficient digestion or absorption of protein, or experiencing rapid growth are at risk of protein deficiency. Because animal source foods are the most common sources of complete proteins, people who exclude animal source foods, such as vegetarians or vegans, might be at risk of low protein intake [9]. It is important that people following these eating patterns pair incomplete proteins that contain complementary amino acid compositions in order to obtain essential amino acids at levels to sustain biological processes. A classic example of pairing incomplete proteins is consuming beans and rice.

Pregnancy and lactation are physiological states associated with rapid growth and demand an increased protein requirement. Infants and children also undergo rapid growth and experience an increased average protein requirement in order to sustain tissue development. Athletes and people who perform rigorous activity have increased protein needs to repair and maintain muscle tissue.

Sarcopenia, age-related loss of muscle mass, is commonly observed. Beginning as early as 25 years, muscle mass begins to gradually decline with an increased decline between 40 and 80 years [10,11]. Amino acids are responsible for signaling muscle synthesis and other anabolic functions. However, with age cells become less reactive to amino acid signaling and require higher concentrations to generate the same response [12,13]. Therefore, the recommended daily allowance of 0.8 g protein/kg/day in elderly populations is likely too low to maintain lean body mass. Research suggests that an intake of 1.5 g protein/kg/day would provide 15-20 percent of total energy requirements, achieve maximal stimulation of muscle protein synthesis, and is still within the Dietary Reference Intakes (DRI) recommendation of consuming 10-35 percent of total energy from protein [12,14].

What are the potential consequences of consuming too little protein?

General consequences of consuming too little protein include impaired cognitive and physical development, stunting, edema, intestinal problems, suppressed immune responses, fatty liver, and reduced muscle synthesis [15,16]. These phenomena are due to the critical role essential amino acids and nitrogen play in each of these processes. In addition, many of these consequences, such as intestinal problems and suppressed immune response, are due to the rapid cell turnover, which makes these systems more sensitive to protein deficiency [16].

Kwashiorkor is a disease of severe protein malnutrition in infants and children. It is characterized by edema, muscle atrophy, abdominal distension, round face, peeling skin, fatty liver, low height-to-age ratio, and low levels of antioxidants such as glutathione [16]. Kwashiorkor is different than marasmus or wasting, which is described by overall energy malnutrition—not limited to only protein. Kwashiorkor is rare in the United States and more common in low-income countries and places experiencing famine.

Overall, the severity of the consequence of consuming too little protein is related to age and physiological status of the individual. Particularly, the consequences of consuming too little protein for infants and children and the elderly are the greatest. Infants and children who consume too little protein risk cognitive and developmental impairment while the elderly risk decline in lean body mass, increasing the risk of falls and loss of independence [11,17].

What are potential consequences of overconsuming protein?

The AMDR is to consume 10-35 percent of total energy from protein. On a standard 2,000 kcal diet, this consists of an intake of 200 to 700 kcal from protein.  Consuming more than 35 percent of total energy from protein is generally thought of as safe, however there is some evidence that overconsuming protein may have no benefit or some risk [18]. In a review of 21 experimental human studies and 11 reviews related to consequences of consuming a high protein diet, it was reported that overconsuming protein was associated with disordered bone and calcium homeostasis, disordered renal function, increased cancer risk, increased coronary artery disease risk, and altered microbiome composition [19]. Purported mechanisms linking high protein intake and disordered bone homeostasis may be related to increased acid excretion by the kidneys due to the amino acid load, which drives bones to release calcium as a blood buffer.

Overconsuming protein places an increased demand on the kidneys as they deaminate and filter nitrogenous compounds for elimination. This may be of concern to individuals with kidney impairment [19].

Elucidating the consequences of protein overconsumption is difficult for many reasons. One reason is that not all protein foods are nutritionally equivalent. Individual amino acids have different effects on cells and protein foods have different amino acid contents. Secondly, isolating the effect of protein in a food apart from the food’s other nutrients is difficult. For instance, animal source proteins are more likely than plant-based foods to contain saturated fat, a nutrient associated with cardiovascular disease. Within animal-sourced proteins, foods such as beef, poultry, and seafood all have diverse nutrient profiles.

Another challenge in studying the effects of protein overconsumption is accounting for whether the protein was consumed in one sitting or throughout the day. For instance, the amount of protein absorbed in one sitting may be less than the amount of protein absorbed if consumed throughout the day.9 Protein that is not absorbed by the body acts as a substrate for the gut microbiome and may promote the growth of bacteria with the abilities to utilize such fuel [20]. The health effects of these bacteria are an active area of research.

Finally, overconsuming calories, regardless of their macronutrient source, will lead to weight gain.

Are there additional considerations for vegetarians and vegans?

Most protein-containing plant-based foods do not contain all of the essential amino acids or contain all of the essential amino acids but not at concentrations required by the body and are considered incomplete. Because of this, vegetarians and vegans need to pair protein foods with complementary essential amino acid profiles [21]. Examples of complementary proteins include:

• Rice (low in lysine) and beans (low in methionine)
• Peanut butter (low in methionine) and whole grain bread (low in lysine)

How do protein needs differ for athletes?

Strength, power, and endurance athletes have an increased protein need to maintain nitrogen balance. Nitrogen balance is a method for determining protein homeostasis. A positive nitrogen balance, when intake is greater than loss, is indicative of growth, tissue repair, and an increase in the body’s protein pool. A negative nitrogen balance is indicative of injury, sickness, and loss of lean body mass.

To sustain nitrogen balance, protein intake for strength and power athletes are recommended between 1.4-1.8 g/kg per day, whereas in endurance athletes are between 1.2-1.4 g/kg per day [5,22,23].

Protein powder supplementation for athletes

Protein powders are commercially available and composed of different types of animal- or plant-based proteins. Commonly available protein powders include whey, casein, and pea protein. In a study of amino acid content of 35 protein supplement samples, plant-based protein such as soy, pea, and corn did not meet the requirements for essential amino acid intake whereas animal-based proteins such as whey, casein, and egg did [21]. In a pilot study assessing the effect of whey versus pea protein on body composition, muscle thickness, force production, and strength in trained athletes, there was no significant effect of protein source when consumed before and after each workout [24].

The International Society of Sports Nutrition (ISSN) recommends that healthy, exercising individuals consume protein before and after resistance exercise to stimulate muscle protein synthesis (MPS); in an even distribution across the day; and in whole foods and in supplementation to ensure intake of protein intake quality and quantity. Th ISSN also recommends achieving adequate intake of leucine to most effectively stimulate MPS [25,26].

What foods are good sources of protein?

Examples of plant and animal-sourced protein foods are presented in Table 2.

Table 2. Protein foods and their caloric and protein content.

Table adapted from Academy of Nutrition and Dietetics Protein in Vegetarian and Vegan Diets and USDA FoodData Central [27,28].

References:

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Inquiries regarding this publication may be directed to cns@ucdavis.edu. The information provided in this publication is intended for general consumer understanding, and is not intended to be used for medical diagnosis or treatment, or to substitute for professional medical advice.

California's CalFresh Healthy Living, with funding from the United States Department of Agriculture’s Supplemental Nutrition Assistance Program – USDA SNAP, produced this material. These institutions are equal opportunity providers and employers. For important nutrition information, visit www.CalFreshHealthyLiving.org.