2.3 Protein

Nutritional Function of Proteins

Proteins are found in every cell and perform various functions in the human body. The digestion of proteins begins in the stomach with the enzyme pepsin and hydrochloric acid. The acidic environment denatures the proteins, and the enzyme divides the protein into smaller polypeptides, which are linear organic molecules consisting of many amino acid residues bonded together in a chain, forming part or all of a protein. The small intestine also releases digestive hormones, including secretin and cholecystokinin, which stimulate the enzymes to break down the proteins into individual amino acids. Accessory organs release additional enzymes and contribute to the breaking of complex proteins into smaller individual amino acids, which are transported across the intestinal mucosa.

In the final stages of metabolism, amino acids remain in the amino acid pool, a total number of essential amino acids and nonessential amino acids available for building proteins, which is regulated by the liver. Circulating amino acids recombine to form every protein required for maintenance and growth. In contrast, excess protein is converted to glucose or triglycerides depending on the body’s need for energy. The sequence of the amino acids and folding of the protein molecule determines the function (Figure 2.2).

Figure 2.2 The structure of a protein determines its function. (credit: modification of work from Biology 2e. attribution: Copyright Rice University, OpenStax, under CC BY 4.0 license)

Amino acids are classified as essential if they come from food and nonessential if the body can synthesize them. See Table 2.8 for a list of amino acids and their classification. Amino acids are continually needed to replace normal cell breakdown. Each protein has a specific sequencing of amino acids; when one is missing, protein synthesis stops until the muscle breaks down to free up the needed amino acids for the amino acid pool. The intricate folding of the protein molecule is based on the function required. For example, the proteins in nails, when compared to those in the eyes, are very different in structure because the functions are highly diverse.

Protein Intake for Wellness Promotion and Illness Prevention

Protein is essential for all tissues, and regular protein intake is essential to prevent proteolysis, a breakdown of proteins that causes loss of lean muscle mass. Because protein is essential for all tissues, it is a superstar of dietary intake. However, protein myths have elevated this macronutrient to an extreme level. High-protein diets, protein powder, and amino acid supplements are popular as individuals pursue well-muscled bodies. Most adults require only 1–1.2 g of protein per kilogram of body weight daily. For example, a person who weighs 165 lb (75 kg), should consume 75–90 g of protein daily. Protein is not stockpiled in the body. Therefore, a diet high in protein will not make for larger muscles or longer hair and nails, but an excessive amount of protein can contribute to weight gain. Unneeded protein is stripped of its amino group, the nitrogen is excreted in the urine, and the carbon remnant is converted to either glucose or triglyceride for use as energy.

Proteins from animal sources provide the highest quality proteins or “complete proteins” because they contain all the essential amino acids. Meat, fish, poultry, and eggs are complete proteins. However, individuals following a plant-based diet can achieve complete proteins by eating various legumes, grains, and nuts. Interestingly, soy is the only legume classed as “complete” because it contains all the essential amino acids (Metropulos, 2019). However, it is less digestible than animal protein, as humans cannot digest cellulose in the plant cell structure.

All animal proteins contain cholesterol, so a high-protein diet of beef, pork, poultry, and eggs will also be higher in fat and cholesterol. Lean meats cooked without added fat, poultry without skin, and egg whites instead of whole eggs will help control fat and cholesterol and provide adequate, high-quality protein.

Protein supplements are unnecessary if an individual is in good health, moderately active, and knowledgeable about nutrition. A well-balanced diet with lean proteins and necessary fat, carbohydrate, and micronutrients can provide adequate nutrients without purchased supplements. The bioavailability (the way food is absorbed) of the macronutrients and micronutrients is better for whole foods. For example, chocolate milk is a “close to perfect” postexercise supplement (Amiri et al., 2019). It contains the ideal ratio of carbohydrates to protein and includes the milk proteins casein and whey, which metabolize at different rates for replenishment in the short and long term (Garden-Robinson & Christenson, 2019).

Nitrogen balance is widely used as an indicator to assess protein loss or gain. This indicator can be especially useful following trauma or prolonged illness (Dickerson, 2016). If dietary nitrogen intake in the form of protein is greater than what is lost, a positive nitrogen balance is achieved, reflecting a gain of total body protein. Positive nitrogen balance is essential for growth and healing.

Conversely, a negative nitrogen balance reflects protein degradation, with protein loss higher than protein retention. A negative protein balance in acute care substantially impacts survival (Dupuis et al., 2022). Healthy adults achieve nitrogen balance when protein intake is the same as protein degradation. Table 2.9 lists the protein content of animal- and plant-based foods.

Special Considerations for Individuals Who Face Challenges Securing Adequate Protein Intake

Negative nitrogen balance occurs in several disease states, ranging from poor protein intake, which can be related to eating disorders or bariatric surgery, to hypermetabolic states requiring increased protein. Concurrently, carbohydrates and fats must meet energy needs to channel proteins specifically for tissue repair and restoration (Munoz & Posthauer, 2022). For example, more calories and protein are needed to repair and restore function during burn or wound healing, especially in children, in whom protein needs can be as high as 2.5–4.0 g/kg/day (Clark et al., 2017). Enteral nutrition within 24 hours of hospital admission is optimal, as oral meals cannot usually meet these metabolic requirements.

Metabolic-associated fatty liver disease has been traditionally treated with protein restriction, resulting in improved liver function. However, malnutrition has been noted in more than 50% of clients following a low-protein diet, contributing to low survival rates. Protein ingestion in liver disease must be individualized because although some clients with cirrhosis may tolerate normal protein intake, others may require targeted amino acid therapy to reduce the breakdown of lean muscle mass (Ampong et al., 2020).

Individuals with chronic renal disease also require protein in lower amounts. The kidneys eliminate protein waste products, so when an individual consumes less protein, the kidneys do not have to work as hard in filtering metabolites. The amount of prescribed protein in clients with chronic renal disease depends on their glomerular filtration rate (GFR). If the client progresses to dialysis, protein should be increased because of protein loss in the dialysate. Table 2.10 illustrates the decrease in recommended protein intake as the GFR decreases. When the GFR level drops below 15, the decision to begin dialysis depends on individual symptoms.