Why Protein Is Essential: Exploring Its Crucial Role in Our Health

We need protein to build and repair tissue, make enzymes and hormones, and support the immune system.

Roles

• Build and repair tissue: muscle, skin, hair.
• Enzymes and hormones.
• Immune system: antibodies.
• Transport: hemoglobin carries oxygen.
• Energy: 4 calories per gram.

Amino acids

Protein is made of 20 amino acids. Nine are essential and must come from food.

How much

The RDA is 0.8 g per kg of body weight.

Sources

Meat, fish, eggs, dairy, beans and nuts.

Functions in the body

Proteins are made of amino acids and carry out most of the work in cells. About 16% of body weight is protein.

• Structure: collagen, keratin and elastin form skin, hair, nails, tendons and bones.
• Enzymes: speed up chemical reactions, such as digestion (amylase, lipase).
• Hormones: insulin, growth hormone and others are proteins or peptides.
• Transport: hemoglobin carries oxygen, and lipoproteins carry fats.
• Immune defense: antibodies are proteins.
• Muscle contraction: actin and myosin.
• Fluid balance: albumin helps keep fluid in blood vessels.

Amino acids

There are 20 amino acids. Nine are essential, meaning the body cannot make them: histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan and valine. These must come from food.

Complete and incomplete proteins

Animal proteins (meat, eggs, dairy, fish) and soy and quinoa provide all essential amino acids. Most plant proteins are lower in one or more amino acids, but eating a variety (beans and rice) over the day provides all of them.

How much

The Recommended Dietary Allowance is 0.8 grams per kilogram of body weight per day, which is about 56 grams for a 70 kilogram man and 46 grams for a 57 kilogram woman. Older adults, athletes and pregnant women may need more. The Acceptable Macronutrient Distribution Range is 10% to 35% of calories.

Deficiency

Severe protein deficiency causes kwashiorkor, with swelling, muscle loss and a weakened immune system. It is rare in developed countries but affects millions of children globally.

Protein digestion

Stomach acid and enzymes break proteins into amino acids and peptides.

Excess

Healthy adults tolerate high protein, but people with kidney disease should get advice.

Dietary requirements and quality

The US Recommended Dietary Allowance for protein is 0.8 g per kilogram of body weight per day for healthy adults, about 56 g for an average man and 46 g for an average woman. Athletes and older adults are often advised 1.2 to 2.0 g per kilogram. The body needs nine essential amino acids that it cannot make: histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan and valine. Animal proteins and soy, quinoa and buckwheat are complete, while most other plant proteins can be combined across the day to cover all nine.

Sources and limits

One large egg has about 6 g of protein, 100 g of cooked chicken breast about 31 g, a cup of cooked lentils about 18 g and 100 g of firm tofu about 8 to 17 g depending on type. Severe deficiency causes kwashiorkor, which still occurs in regions of food insecurity. People with kidney disease may need to limit protein, so follow medical advice.

Based on data from the National Academies of Sciences and the US Department of Agriculture FoodData Central