Protein, along with fats and carbohydrates, is one of the three macronutrients, which are the nutrients that provide energy (as calories). Protein contains approximately 4 calories per gram. The composition of a high-protein diet can be determined as an absolute amount of the protein, usually in grams per day; as a percentage of a person's total energy needs per day; or as the amount of protein ingested per kg of a person's body weight. In general, the recommended dietary allowance (RDA) for protein is 10%–35% of a person's daily calories, or about 0.8 grams (g) per kilogram (kg) of body weight for a healthy adult. A high-protein diet would be one based on the upper levels of these recommendations or would surpass them.
High-protein diets have been popular off and on since the 1960s. In the 1960s, Dr. Maxwell Stillman of the Stillman Diet was one of the first to advocate a high-protein, no-carbohydrate, low-fat diet for fast weight loss. In the 1990s, diet books promoting high-protein diets began to appear on bestseller lists. The most popular of these was the Atkins Diet. Other high-protein diets include the Zone Diet, Protein Power, and Sugar Busters. These diets were slightly modified from the Stillman Diet to include some carbohydrates and encouraged high-protein diets for weight loss and/or for bodybuilding.
All human protein is made from about 20 different small molecules called amino acids. Out of these 20 amino acids, 9 are essential in adults. They are considered essential because the body cannot make them (or make a sufficient amount) from other nutrients. If they are not obtained from the diet, the body will begin to break down its own protein to obtain them.
Both animals and plants provide sources of protein. Animal protein has a higher biological value than most plants because it is a complete protein. Complete proteins contain all nine essential amino acids. Animal proteins include meat, poultry, fish, egg whites, and dairy products.
Plant proteins typically have a lower biological value because most are incomplete proteins that do not contain all nine essential amino acids. Some plants are better sources of protein than others because they lack only one or two essential amino acids. Better plant proteins include dried beans and bean products such as tofu (made from soybeans), nuts, and grains such as corn and quinoa; quinoa is one of the few grains that serves as a complete protein. Incomplete proteins can be combined in one meal to form complete proteins, such as red beans with rice or corn tortillas with beans.
The requirements for protein in individuals requiring higher amounts of protein and calories varies. Protein intake is usually increased from 0.8 g of protein per kg (2.2 lb.) of body weight to 1–2 g per kg of body weight. Higher intakes are often used for children. High-protein diets for weight loss recommend that between 35% (Atkins) and 64% (Stillman) of daily calories come from protein. This is equivalent to about 2.2–4.4 g per kg (1–2 g per lb.).
Extra amino acids are not stored in the body. Instead, they are split apart by enzymes. The part containing nitrogen is excreted by the kidneys in urine, while the remainder is either converted into glucose (a simple sugar) and used for energy or stored as glycogen, a compound that can later be reconverted into glucose or converted to fat and stored for later use.
For people who want to lose weight, they take in higher quantities of protein but must restrict their total intake of calories, usually carbohydrates. This promotes a fast initial weight loss, but most of the loss comes from losing water weight. The reason for this is that the body is driven into a state called ketosis. The body prefers to break down carbohydrates into glucose and use that glucose for energy. When the body is starved of carbohydrates, it begins converting fat into glucose. The process of converting fat into glucose releases water molecules that then leave the body as urine.
Dieters wish to burn fat, but when fat is burned exclusively, molecules called ketones build up in the blood. Ketones are part of the body's defense against starvation. They suppress appetite and can also cause bad breath. If the body is deprived of carbohydrates for a long time, these ketones accumulate and can result in metabolic imbalances that cause serious harm to the kidneys and other organs.
Medical patients who are in a catabolic state may lose up to 40 g of nitrogen in their urine each day, equivalent to approximately 1 kg of muscle. Depletion of lean body mass occurs rapidly. Many other systems are affected in this hypermetabolic state, and if left untreated it can result in malnutrition or eventually death. A high-protein, high-calorie diet supports the body's need for more nitrogen in the form of protein and more calories to promote quick healing and strengthening of the immune system.
High-protein, low-calorie diets offer fast weight loss. The Stillman Diet claims that an individual can lose up to 30 lb. (13.5 kg) in 28 days. Popular high-protein diets also claim health benefits—the Zone diet claims to improve physical and mental performance, prevent chronic cardiovascular diseases, improve immune system functioning, decrease signs of aging, and increase longevity, but there is little scientific evidence that supports these claims. Slightly greater intial weight loss does occur following a high protein low-calorie diet but there is no evidence of any difference in the longer term (e.g. 12 months).There is some evidence that the high satiety effects of dietary protein may contribute to the success of high-protein diets for weight-loss.
High-protein diets can be high in saturated fats. Saturated fats are primarily found in animal fats, including fatty meats and full fat dairy. Diets rich in saturated fat contradict current healthy eating messages as they can raise the level of LDL cholesterol (“bad” cholesterol) in the blood. High LDL cholesterol levels have been associated with an increased risk of heart disease.
High-protein diets may also restrict calories by severely restricting carbohydrates. Whole-grain carbohydrates are a significant source of a number of nutrients including B-complex vitamins. An increasing body of science describes the ability of carbohydrate types to influence gut health, specifically the composition of bacteria living in the gut. The impact of high protein on the composition of this gut flora and longer term colonic health remains unknown. There are groups of people who require extra protein—rapidly growing adolescents, pregnant and nursing women, bodybuilders, endurance athletes, and some cancer patients—but protein should be increased as part of a well-balanced diet, without restricting other nutrients.
People with medical conditions or recent trauma or illness should not attempt a high-protein, high-calorie diet by themselves without the advice and monitoring of a medical professional, such as a doctor or registered dietitian.
Virtually all high-protein diets for weight loss recommend that dieters take some sort of vitamin or mineral supplement. Diets that severely restrict carbohydrates tend to be lower in vitamins, minerals, and dietary fiber. High-protein diets can also increase the amount of calcium excreted by the kidney and may lead to increased loss of calcium from bone or increases the risk of kidney stones, which are more likely to form when large amounts of calcium are present. Cholesterol levels can increase on high-protein diets that encourage high consumption of animal protein due to the resulting increased intake of saturated fats. Finally, if the body reaches a state of ketosis, the ketones that accumulate make the body more acidic, which can cause major damage to various organs.
The use of high-protein, high-calorie diets in the medical field is well accepted for certain medical conditions.
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Tish Davidson, AM
Revised by Anne P. Nugent, PhD