5.4: Why We Need Protein

Proteins serve a wide range of essential functions in the body, making them vital to overall health and physiological processes. One of their primary roles is structural, as they are key components of muscles, skin, hair, and connective tissues, making them extremely important during periods of growth and development. Proteins also function as enzymes, which catalyze biochemical reactions necessary for digestion, energy production, and cell signaling. Additionally, proteins play a critical role in the immune system as antibodies that help defend against pathogens. They are also involved in transporting molecules, such as hemoglobin carrying oxygen in the blood, and in maintaining fluid and pH balance. Some proteins act as hormones, like insulin, which helps regulate blood glucose levels. Because of these diverse functions, adequate protein intake is essential for growth, repair, and maintenance of body tissues.

In addition to consuming protein, our body is able to undergo protein turnover, the continuous process of breaking down and rebuilding proteins in the body. This dynamic cycle allows the body to replace damaged or old proteins, adapt to changing needs, and support growth and repair, without requiring enormous amounts of dietary protein. During protein turnover, the amino acids released from broken-down proteins can be reused to build new proteins or, if not needed, deaminated and used for energy or converted into other compounds. Closely related to protein turnover is nitrogen balance, which reflects the balance between nitrogen intake (mainly from dietary protein) and nitrogen loss (primarily through urine, feces, sweat, and other bodily processes). Because we are constantly breaking down and synthesizing protein, our bodies are always trying to maintain a balance of nitrogen. A person is in positive nitrogen balance when nitrogen intake exceeds loss, indicating that the body is building more protein than it is breaking down — a state common during growth, pregnancy, muscle building, or recovery from illness. Negative nitrogen balance occurs when nitrogen losses exceed intake, suggesting muscle or tissue breakdown, often seen during illness, injury, or malnutrition due to inadequate dietary protein intake or inadequate essential amino acid intake. Nitrogen balance means protein synthesis and breakdown are equal, which is typical for healthy adults who are maintaining their body weight and muscle mass.

Protein Recommendations

The recommendations for dietary amino acid and overall protein intake reflect the ultimate reasons for protein consumption — to supply adequate amounts of essential amino acids, and to supply additional nitrogen needed to make nonessential amino acids and other non-protein, nitrogen-containing compounds (such as DNA). The recommendations do not distinguish between consumption of high-quality or low-quality proteins but are designed to support the body’s needs for growth, maintenance, and repair.

Table 5.2 Protein needs may vary slightly in different populations.

Population Group	Recommended Protein Intake	Reason for Increased Need
General Adults	0.8 g/kg of body wt/day (RDA)	Supports basic maintenance and repair of body tissues
Endurance Athletes	1.2-1.4 g/kg of body wt/day	Supports muscle repair and energy needs due to prolonged activity
Strength Athletes	1.6-2.0 g/kg of body wt/day	Needed for muscle growth, repair, and recovery
Older Adults (65+)	1.0-1.2 g/kg of body wt/day	Helps preserve muscle mass and strength, supports immune function
Pregnant Women	+25 g/day above normal needs	Supports fetal growth and maternal tissue expansion
Lactating Women	+25 g/day above normal needs	Needed for milk production and nutrient transfer to the infant
Recovering from Illness/ Injury	1.2-2.0+ g/kg of body wt/ day	Supports healing, tissue regeneration, and immune response
Vegetarians/Vegans	May need ~10% more than RDA (~0.9g/kg/day or more)	To compensate for lower digestibility and amino acid profile of plant foods

The Recommended Dietary Allowance (RDA) for protein is 0.8 grams per kilogram of body weight per day for healthy adults. This amount is considered sufficient to meet the needs of nearly all individuals in a given population. However, needs can vary depending on factors such as age, activity level, and health status (Table 5.2). For example, some athletes, older adults, and individuals recovering from illness or injury may require higher intakes of protein. In addition to the RDA, the Acceptable Macronutrient Distribution Range (AMDR) provides a broader guideline, suggesting that 10–35% of total daily calories should come from protein. This range allows for flexibility based on individual energy needs and dietary preferences, while still supporting optimal health.

Protein Excess

Protein is often a macronutrient that individuals struggle to consume in adequate amounts. However, the continual increase in protein supplements in the form of powders, bars, ready-to-drink form, and others raises the question of whether a person can consume “too much” protein. Overall, the body can tolerate higher levels of protein intake, and this has not been causally associated with adverse health outcomes. However, consuming protein well above the recommended levels for an extended period may have potential drawbacks. High protein diets can put extra strain on the kidneys, particularly in individuals with pre-existing kidney conditions. Excess protein intake may also displace other essential nutrients in the diet, leading to nutritional imbalances. Additionally, very high intake from animal sources may be associated with increased intake of saturated fat and cholesterol, potentially raising the risk of heart disease. For this reason, it is important to choose a variety of lean and low-fat protein foods in one’s diet. In healthy individuals, moderately high protein intake is generally well tolerated and may even support weight management and muscle maintenance.

Protein Deficiency

Protein deficiency, on the other hand, can have serious health consequences, especially when it is prolonged. Inadequate protein intake can lead to muscle wasting, poor wound healing, weakened immunity, and delayed growth in children.

Protein-Energy Malnutrition (PEM) is a condition that arises from a deficiency in both protein and energy (calories), and it most commonly affects children in developing countries but can also occur in adults who are severely ill, elderly, or have eating disorders (Figure 5.6). PEM is not just a lack of protein alone; it involves insufficient overall nutrient intake, impacting growth, immunity, and organ function.

There are two primary forms of PEM:

1. Marasmus – Caused by a severe deficiency in calories and protein. This leads to extreme weight loss, muscle wasting, and fat depletion. The individual appears very thin with prominent bones, and there's little to no edema (swelling).
2. Kwashiorkor – Results from adequate or near-adequate calorie intake but insufficient protein. It is characterized by edema (swelling, especially in the abdomen), fatty liver, skin and hair changes, and a weakened immune system. Children with kwashiorkor may appear puffy despite being malnourished.

Both conditions can be life-threatening if not treated, and they often occur together in varying degrees. Even mild deficiencies can affect enzyme and hormone production, impair cognitive function, and lead to fatigue and increased risk of infections.

Figure 5.6 Protein Energy Malnutrition (PEM) results from insufficient caloric and protein intake. Though deaths from PEM have been on the decline since the 1990s, rates are still substantial and especially in children under the age of 5. Image by Our World In Data from IHME, Global Burden of Disease is licensed under the Creative Commons Attribution 3.0 Unported license. Access Interactive data from Our World in Data.

Health Note

Protein is essential for building and repairing tissues—including muscles, skin, and even hormones.

Fun Fact

Your body can’t store protein in the same way it does fat and carbs—so you need to eat it regularly!