28.2 : Proteins: Dietary Sources and Requirements

Consuming animal-based products offers high-quality proteins that contain optimal levels and combinations of essential amino acids, crucial for tissue repair and growth. Foods like eggs, milk, fish, and most meats are a source of complete proteins. Legumes and cereals are abundant in proteins; however, they typically lack a full range of essential amino acids. As a result, they are considered incomplete protein sources. Some plant sources like soybeans, quinoa, and amaranth do contain complete proteins. As a result, strict vegetarians must ensure a well-planned diet to avoid protein malnutrition. When consumed together, grain and legumes are a potent source of all essential amino acids. Proteins are crucial as structural materials such as keratin, collagen, elastin, muscle proteins, and functional proteins, including enzymes and some hormones. The utilization of amino acids for either protein synthesis or energy production relies on several factors:

The all-or-none rule states that for a specific protein to be synthesized, all necessary amino acids must be present in the right quantities. Essential amino acids not immediately used for protein building are either oxidized for energy or converted to carbohydrates or fats.

The diet must provide adequate energy sources like carbohydrates or fats for optimal protein synthesis. Without sufficient energy sources, dietary and tissue proteins are utilized for energy.

Anabolic hormones accelerate growth and protein synthesis. Other hormones, such as adrenal glucocorticoids, facilitate protein breakdown and the conversion of amino acids to glucose during stress.

In healthy adults, protein synthesis matches protein breakdown, creating a state of nitrogen balance. A positive nitrogen balance indicates tissue growth or repair, whereas a negative balance signifies protein breakdown exceeding incorporation into tissues due to stress, poor diet, or starvation.

Dietary proteins not only provide essential amino acids but also serve as raw materials for nonessential amino acids and various nitrogen-bearing substances. Protein requirements vary based on age, size, metabolic rate, and the need for new protein synthesis. As a general guideline, about 0.8 g of protein per kilogram of body weight is recommended daily.