Protein is one of the three macronutrients found in food; the other two are fats and carbohydrates. Protein accounts for 20 percent of our body weight, but why do we need protein in our diet, and what foods are healthy sources of protein?
Why Do We Need protein?
Protein is important to many physiological functions in the body, and is a vital component of body tissues, enzymes, and immune cells. It helps to:
Proteins are complex molecules made up of amino acids. These amino acids link together in specific numbers and unique combinations to make each different protein. Therefore protein is an essential component of the diet, because it provides the amino acids that the body needs to synthesize its own proteins.
Proteins also interact with nutrients by binding with them and carrying certain vitamins and minerals including iron, copper, calcium, Vitamin A, and vitamin D. As a result, inadequate protein intake may impair the function of these nutrients.
Recent studies suggest that protein makes a meal more satiating, which in turn could help people maintain a healthy weight. A 2005 study published in the American Journal of Clinical Nutrition found that increasing protein from 15% to 30% of total calories, and reducing fat from 35% to 20% of calories - resulted in sustained weight loss .
What Are Healthy Sources of protein?
How healthy a protein-rich food is typically depends on what else it contains. For example:
Some foods contain all of the essential amino acids that the body needs; these are called complete proteins. There are other foods, however that provide some or none of the essential amino acids, and are referred to as incomplete proteins.
Eggs, dairy foods, meat, fish and poultry are typically considered to be complete proteins. Vegetarians or vegans often do not have a source of complete protein in their diets, but can easily obtain all of the essential amino acids by eating a variety of beans, grains, nuts, seeds, and vegetables.
 Weigle et al A high-protein diet induces sustained reductions in appetite, ad libitum caloric intake, and body weight despite compensatory changes in diurnal plasma leptin and ghrelin concentrations. Am J Clin Nutr 2005, pages 41-8.