Protein is a macronutrient the body needs in large amounts for energy, structure, and system maintenance. It is involved in nearly every bodily process, from cellular function to overall physical structure.
The Building Blocks and Their Assembly
Proteins are complex molecules constructed from smaller units known as amino acids. There are 20 different types of amino acids commonly found in most proteins. These amino acids link together in specific sequences, forming long chains called polypeptides.
The connections between amino acids are strong chemical links known as peptide bonds. Once these polypeptide chains are formed, they fold into unique three-dimensional structures. The precise order and arrangement of amino acids determine the protein’s final shape and activity.
Diverse Functions in the Body
Proteins perform many functions within the human body, contributing to its structure, regulation, and defense.
They provide structural support for cells and tissues. Examples include collagen, which forms the connective framework of muscles, bones, and cartilage, and keratin, the main structural component of hair, nails, and skin. These fibrous proteins give rigidity and stiffness to various body parts.
Many proteins act as enzymes, biological catalysts that speed up biochemical reactions without being consumed. These enzymes facilitate thousands of reactions, such as those involved in digestion, where they break down nutrients into smaller units the body can use. Specific enzymes, like salivary amylase, hydrolyze carbohydrates, while others, like lipase, break down fats.
Proteins also play a role in transport and storage. Hemoglobin, for instance, is a protein in red blood cells that carries oxygen from the lungs to various body tissues. Other transport proteins move substances like vitamins, minerals, blood sugar, and cholesterol into, out of, or within cells. Ferritin is another example, storing iron and releasing it in a controlled manner.
The immune system relies on proteins, as antibodies formed in white blood cells identify and attack harmful bacteria and viruses. These defensive proteins recognize specific foreign molecules, known as antigens, to neutralize threats. Hormones, many of which are proteins, act as chemical messengers, coordinating activities between different cells, tissues, and organs. Insulin, for example, is a protein hormone that regulates blood glucose levels.
Proteins are also involved in movement, with actin and myosin being two examples found in muscles. These contractile proteins work together to enable muscle contraction and relaxation, facilitating physical movement. Proteins also help maintain fluid and acid-base balance within the body by acting as buffers, releasing or taking up hydrogen ions to keep blood pH within a normal range.
Dietary Sources and Quality
Protein is obtained from a variety of dietary sources, categorized into animal-based and plant-based options. Animal sources include fish, poultry, eggs, beef, pork, and dairy products. These are considered “complete proteins” because they contain all nine essential amino acids the human body cannot produce on its own.
Plant-based protein sources include legumes, nuts, seeds, whole grains, and various vegetables. Most plant proteins are “incomplete,” meaning they are low in or lack one or more of the nine essential amino acids. However, certain plant foods like soy (tofu, tempeh, edamame), quinoa, hemp seeds, chia seeds, buckwheat, and spirulina are considered complete proteins.
For individuals following vegetarian or vegan diets, combining different plant proteins can ensure all essential amino acids are obtained. This concept, known as protein complementarity, involves pairing foods deficient in certain amino acids with others that contain them in higher amounts. For example, combining grains (low in lysine) with legumes (rich in lysine), such as rice and beans, or peanut butter on whole-grain bread. Eating a varied plant-based diet throughout the day provides all necessary amino acids without needing strict meal-by-meal combinations.
How Much Protein Do You Need
The amount of protein an individual needs daily varies based on age, physical activity level, and specific physiological states. For most healthy adults, the Recommended Dietary Allowance (RDA) is 0.8 grams of protein per kilogram of body weight per day. This translates to about 54 grams of protein for a 150-pound (68 kg) adult.
Certain groups require higher protein intake. Growing children and infants need more protein per kilogram of body weight to support rapid tissue development. Pregnant or lactating women also have increased protein requirements, with the RDA rising to 71 grams per day during these periods.
Physical activity influences protein needs. Individuals with moderate physical activity may need 1.0 to 1.3 grams per kilogram of body weight daily. Athletes involved in intense training or strength sports may require 1.2 to 1.7 grams per kilogram. This increased intake supports muscle repair, growth, and recovery from exercise. Older adults, particularly those over 40-50, also benefit from higher protein intake, 1.0 to 1.2 grams per kilogram, to help counter age-related muscle loss known as sarcopenia.
Consequences of Imbalance
Both insufficient and excessive protein intake can have consequences for the body. A prolonged lack of adequate protein can lead to protein deficiency. Common signs include muscle wasting, as the body breaks down its own muscle tissue to meet protein needs, resulting in weakness and fatigue.
Protein deficiency can also impair immune function, making individuals more susceptible to infections. Other signs include changes to skin, hair, and nails, such as brittle hair and dry or flaky skin, because these tissues are largely composed of protein. In severe cases, particularly in children, undernutrition can lead to conditions like Kwashiorkor, characterized by swelling (edema) due to fluid retention.
Conversely, consuming excessive amounts of protein, particularly over 2 grams per kilogram of body weight daily for extended periods, can pose concerns for some individuals. For healthy individuals, moderate excess protein is well-tolerated. However, concerns include strain on the kidneys, especially for those with pre-existing kidney conditions, as kidneys work harder to filter waste products from protein metabolism.
High protein intake, especially from animal sources, can also lead to increased calcium loss through urine, associated with bone health concerns. Digestive issues like constipation, bloating, or diarrhea may also occur, particularly if a high-protein diet lacks sufficient fiber from fruits, vegetables, and whole grains. Maintaining a balanced diet with a variety of macronutrients is important to avoid nutrient displacement and support overall health.