Protein: Uses, sources, and requirements
Proteins are made of small compounds called amino acids. Hundreds of amino acids exist in nature, but the human body only uses 22 of them.
The body can produce all but nine of the amino acids it needs. These nine are called essential amino acids. They must come from food.
All foods contain differing combinations of amino acids. In general, animal proteins like meat, dairy, and eggs contain all the essential amino acids.
Plant-based proteins from foods like beans, grains, nuts, and soy are rich in some amino acids but may lack others. A well-balanced diet with a variety of foods can provide sufficient protein for the body's needs.
Here are some key points about protein. More detail is in the main article.
- Protein is important for growth and repair of body cells.
- Food sources of protein include meat, fish, dairy, lentils, beans, and tofu.
- Insufficient protein can lead to low growth and a weakened immune system.
- Excess protein may lead to weight gain and liver problems.
What does protein do?
Milk is a good source of protein.
Protein is the major building block of the human body. It builds and maintains tissue.
During periods of growth, such as infancy, childhood, and pregnancy, the body needs more protein.
Protein needs also increase for people who:
- have injuries
- have undergone surgery
- consistently break down muscle during exercise
Protein absorption: Is there an upper limit?
A common myth is that only about 20 or 30 grams (g) of protein at a meal can be absorbed and utilized, but there is no evidence to support this theory.
However, it can still be helpful for many people to reach their protein needs and improve their energy and blood sugar levels by spreading protein intake throughout the day.
A variety of common eating patterns that can help people reach their minimum protein goals.
Eating pattern 1
One is to eat a small amount of protein at breakfast, a moderate amount at lunch, and a large amount at dinner.
In a typical day, a person might eat:
- 10 g of protein or less at breakfast, for example, in oatmeal, nuts, and berries
- 25 g at lunch, for example, in a turkey sandwich with cheese
- 5 g in a snack, such as a granola bar
- 40 g at dinner, in chicken or beef and sides
This day would provide roughly 80 g of protein.
Eating pattern 2
Another common pattern is to eat a moderate amount of protein at all meals, breakfast, lunch, dinner, and snacks.
In a typical day, a person might eat:
- 20 g of protein at breakfast, for example, a 2-egg veggie omelet with a side of beans
- 15g in a morning snack of cottage cheese and fruit
- 25 g at lunch, for example, in a salad with a filet of fish on top
- 15g in a protein-rich snack, such as a protein shake
- 10 grams at dinner, in a lentil soup or meatless meal
This would also provide roughly 80 grams of protein.
How much do we need?
People can aim to consume a certain amount of protein to obtain maximum protein use, muscle generation, and recovery every time they eat.
According to the Institute of Medicine (IOM), the Recommended Dietary Allowance (RDA) for protein is 0.8 g per kilogram (kg) of body weight per day. The RDA is the minimum amount of protein needed for meeting nutritional requirements, not the maximum.
However, this amount depends on the person's body size and how active they are. A 6-foot, 250-pound man who strength trains five times a week can absorb and utilize more protein than a 5-foot female who does not exercise much.
- Endurance athletes may need from 1.0 to 1.6 g per kg of body weight, depending on the intensity of exercise.
- Recommendations for strength training or power athletes range from 1.6 to 2.0 g per kg of body weight.
The IOM suggest that between 10 and 35 percent of calories should come from protein each day.
It is not clear exactly how it will affect a person if they consume more than this, since the effect on long-term health and disease risk depends on the type of protein.
If a person does not consume enough protein, they may experience:
- lack of growth
- loss of muscle mass
- reduced immunity
- weakening of the heart
- respiratory problems
A protein deficiency can be fatal. In developing countries, some people develop kwashiorkor as a result of protein deficiency. It is a type of malnutrition, and it is common during a famine.
Early signs include swelling in the legs and possibly the face, due to edema, or fluid collecting under the skin. Other symptoms are a pot belly, fatigue, dry brittle hair, and cracked nails. The person will be more prone to infections.
In developed countries, those most at risk of a protein deficiency include people who do not eat properly, for example, due to a poorly managed weight loss diet, an eating disorder, or inability to cook their own food, for example, in older age.
Most Americans do not lack protein in their diet.
According to the United States Department of Agriculture (USDA), the following amounts of protein can be found in common sources of food:
Beans and lentils are a good source of protein for vegetarians and vegans.
- 3 ounces of chicken contains 20 g
- 3 ounces of ground beef contains 21 g
- 1 cup of milk contains 9 g
- 1 egg contains 6 g
- 1 cup of black beans contains 15 g
- 2 tablespoons of peanut butter contains 8 g
- Half a block of tofu contains 18 g
Some good sources of protein, for example, a broiled steak, can also contain high levels of fat and sodium. Other sources, such as salmon, are lower in saturated fat and sodium.
Beans, chickpeas, lentils, tofu, and low-fat dairy products are also good sources of protein, as well as many other health-promoting nutrients like antioxidants and fiber.
A diet that uses these at least sometimes instead of meat, especially red meat, is less likely to lead to weight gain and other health problems.
One study has shown that women who had high protein intake primarily from vegetable sources had a 30-percent lower risk of heart disease, compared with women who had a higher protein, lower carbohydrate intake, but primarily from animal sources.
Does protein encourage weight loss?
There is evidence that additional protein in the diet may contribute to some of the factors that encourage weight loss or weight control, especially in people with obesity.
However, researchers have not yet proven that consuming extra protein will lead to weight loss for most people.
In 2015, scientists concluded:
"Although greater satiety, weight loss, fat mass loss, and/or the preservation of lean mass are often observed with increased protein consumption in controlled feeding studies, the lack of dietary compliance with prescribed diets in free-living adults makes it challenging to confirm a sustained protein effect over the long term."
In 2016, findings were published of an investigation involving 40 young men who carried out "hard exercise" for a month, while consuming 40 percent less energy than would normally be required for this activity. Some also had a higher intake of protein than is normally recommended.
Those on a high protein diet lost more weight and more body fat than those on a low-protein diet.
The researchers caution, however, that this type of diet is not for everyone. The conditions were unusual, and the young men were supervised and monitored throughout this "tough" program.
In 2016, one small study found that women who followed a high-protein diet in order to lose weight, did not get the benefits of better insulin control that usually accompanies weight loss. The participants had obesity and were post-menopausal.
Concerns with a high protein diet for weight loss include:
- weight regain once protein intake is lowered
- missing out on valuable antioxidants, phytochemicals, and fiber found in plants
- the higher cost associated with a higher protein diet, which may make the diet unsustainable for many people.
Anyone who is considering a high-protein diet should first speak to a doctor.
Risks and precautions
Most studies support an intake of up to 2 g per kg of body weight without any negative or adverse effects.
Consuming more than 2.5 g of protein per kg of body weight (for example, over 225g protein for a person weighing 200 pounds) can increase the risk of:
- weight gain
- deficiencies of fiber, vitamins, minerals and antioxidants found in plants
- passing out more calcium in the urine
Eating more than 200 to 400 g of protein a day can make it hard for the liver to to convert excess nitrogen to a waste product called urea. This can lead to nausea, diarrhea, and other adverse side effects.
Some experts have warned that with the current craze for high-protein diets, including protein shakes, people may be consuming more protein than is healthy.
While protein supplements can assist those who have high protein needs reach their goals, it is usually best to for most protein to come from a well-balanced diet of whole foods.
A dietitian can help you find the right eating pattern, protein choices, and overall dietary approach to feel good and energized while reaching your protein needs.Written by Megan Ware RDN LD