Protein is one of the six major nutrients and serves a diverse set of functions. It is present in many foods, and is not typically a nutrient that Americans are deficient. However in underdeveloped nations, protein deficiency exists. Protein is often associated with healthfulness or building muscle, which sometimes results in increased consumption.
Identification
Protein is made up of amino acids, which are building blocks of proteins. Proteins consist of about 20 amino acids. Of these 20, the body does not produce 9 of them, so they're called essential amino acids. Getting protein from food is necessary to supply these essential amino acids. Foods that contain the essential amino acids are referred to as high biological value proteins.
Significance
Protein provides the basic structural material of all cells, and performs a dynamic set of functions that are necessary for human life. Protein is responsible for growth and maintenance of body tissues, and also acts as enzymes, hormones and antibodies. Protein also serves as a regulator of fluid balance, a buffer to help balance the acid base of the body and a transporter of nutrients (such as fats, vitamins and minerals) and oxygen around the body. Protein also provides energy for the body, in the form of 4 calories per 1 g of protein in food.
Animal and Plant Protein Foods
Protein exists in many foods, and can be from plant or animal sources. Animal sources include foods from animals such as meats, fish, poultry, milk and butter. Plant sources include grains, beans, legumes and nuts. Vegetables contain protein, but in small amounts, and they lack many essential amino acids. Plant sources of protein are usually lacking in one or more essential amino acids, depending on the plant. This does not mean that plant sources of protein are insufficient, because if plant sources of protein are combined they can provide all of the essential amino acids together. This is referred to as complementing, or combining, proteins. An example of combining proteins is a tortilla with beans, or legumes and rice.
Recommended Amounts
According to the Dietary Reference Intakes of the United States Department of Agriculture, the protein recommendation for adults is 10 to 35 percent of total calories. The table indicates 56 g a day for men, and 46 g a day for women (non-pregnant or non-lactating), based on .8 g per kg body weight for adults. Most Americans meet protein requirements, and often exceed them. For example, 1 oz. of meat, poultry or fish contains about 7 g of protein, and 1 cup of milk (whole or fat-free) contains about 8 g of protein. So if you ate 3 oz. of meat, and 1 cup of milk at a meal, you consumed 29 g of protein, more than half of the daily requirement.
Warnings
Too much protein can be unhealthy because it is not stored in the body, so it has to get broken down. Excess protein produces extra nitrogen, which needs to be excreted by the kidneys via urine as urea or ammonia, which can burden the kidneys. Often, people eat high-protein, low-carbohydrate diets to help lose weight, or athletes and bodybuilders might consume extra protein or protein supplements for better performance or to build more muscle. The American Heart Association does not recommend high protein diets for weight loss. The Physicians Committee for Responsible Medicine also cautions that excess protein can be associated with other health risks such as impaired kidney function, cancer, osteoporosis, heart disease and can increase incidence of kidney stones.
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