Protein is a macro nutrient that is essential to building muscle mass. It is commonly found in animal products, though is also present in other sources, such as nuts and legumes.
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How much protein?
The Institute of Medicine recommends that 10 to 35 percent of daily calories come from protein. How that equates to grams of protein depends on the caloric needs of the individual. According to the U.S. Department of Agriculture, the amount of protein foods a person should eat depends on age, sex, and level of physical activity. Most Americans eat enough food from this group, but need to make leaner and more varied selections of these foods.
“A safe level of protein ranges from 0.8 grams of protein per kilogram of body weight [2.2 lbs.], up to 2 grams of protein per kilogram for very active athletes,” said Crandall. “But most Americans truly need to be eating about 1 to 1.2 grams of protein per kilogram of body weight.”
What do proteins do?
Proteins play a role in nearly every biological process, and their functions vary widely.
The main functions of proteins in the body are to build, strengthen and repair or replace things, such as tissue.
They can be structural, like collagen, they can be hormonal, like insulin, they can be carriers, for example, hemoglobin, or they can be enzymes, such as amylase. All of these are proteins.
Keratin is a structural protein that strengthens protective coverings, such as hair. Collagen and elastin, too, have a structural function, and they also provide support for connective tissue.
Enzymes are catalysts. This means they speed up chemical reactions. They are needed for respiration in human cells, for example, or photosynthesis in plants.
Protein foods provide amino acids that our bodies use to synthesize proteins.
When people eat foods that contain amino acids, these amino acids make it possible for the body to create, or synthesize, proteins. If we do not consume some amino acids, we will not synthesize enough proteins for our bodies to function correctly.
There are also nine essential amino acids that the human body does not synthesize, so they must come from the diet.
All food proteins contain some of each amino acid, but in different proportions. Gelatin is special in that it contains a high proportion of some amino acids, but not the whole range.
The nine essential acids that the human body does not synthesize are: Histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan, and valine.
Foods that contain these nine essential acids in roughly equal proportions are called complete proteins. Complete proteins mainly come from animal sources, such as milk, meat, and eggs.
Soy and quinoa are vegetable sources of complete protein. Combining red beans or lentils with wholegrain rice or peanut butter with wholemeal bread also provides complete protein. Research shows that the body does not require all essential amino acids at each meal because it can utilize amino acids from recent meals to form complete proteins.
So, the recommended nutrient is protein, but what we really need is amino acids.
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Protein deficiency is unusual as an isolated condition. If a person lacks protein, they normally have a wider lack of nutrients and energy, due to low food intake. This may be due to poverty or illness.
Very low protein intake can lead to weak muscle tone, edema, or swelling, thin and brittle hair, and skin lesions, and, in children, stunted growth. Biochemical tests may show low serum albumin and hormone imbalances.
Eating more protein may boost muscle strength and encourage a lean, fat-burning physique. This, of course, depends on total food intake and activity levels.
Protein deficiency is rare in the United States, unless it results from a disorder, such as anorexia nervosa or the later stages of cancer.
The body's need for dietary protein is tied up with nitrogen levels in the body. Not having enough of the right levels of the right amino acids can lead to a nitrogen imbalance.
Exactly how much protein a person should consume remains a matter of debate.
Past studies have shown that a breastfed American infant aged up to 3 months will develop satisfactorily with a protein intake of 1.68 grams per kilogram of body weight per day, but experts have suggested that 1.1 grams per kilogram is probably enough.
The current minimum recommended amount of protein per day is 0.8 gramsper kilogram for an average adult to stay healthy, according to Harvard Health.
However, recommending exact amounts is difficult, because a range of factors, such as age, gender, activity level, and status, for example, pregnancy, play a role.
Other variables include the proportion of amino acids available in specific protein foods, and the digestibility of individual amino acids. It also remains unclear how protein metabolism affects the need for protein intake.
According to the U.S Department of Agriculture (USDA), the following foodswill provide about 7 grams of protein per serving listed below:
1 ounce lean meat, poultry, seafood
1 Tablespoon peanut butter
Half an ounce nuts or seeds
A quarter cup cooked beans or peas
Protein and calories
Protein provides calories. One gram of protein or carbohydrate contains 4 calories. One gram of fat has 9 calories.
Consuming 0.8 grams per kilogram of protein each day would provide 10 percent of an average person's calories.
The average American consumes around 16 percent of their calories from protein, whether of animal or plant origin.
It has been suggested that Americans obtain too many calories from protein, but now some experts are calling this a "misperception."
The 2015 to 2020 Dietary Guidelines for Americans recommend that for people over the age of 4 years, between 10 percent and 35 percent of their calories should come from protein, depending on their age and gender.
Derek Pendick, former editor of Harvard Men's Health, comments in an article on protein requirements:
"Research on the optimal amount of protein to eat for good health is ongoing and is far from settled. The value of high-protein diets for weight loss or cardiovascular health, for example, remains controversial."
He adds that consuming more protein does not necessarily mean eating more steak, and it may mean eating less of something else, say, carbohydrates, to maintain a healthy weight.
Some diets recommend eating more protein in order to lose weight.
However, nutritionists stress that there is insufficient evidence that adding protein will guarantee weight loss, and that people should consider their overall consumption and dietary consequences when making this kind of change.
If eating more protein results in a lower fiber intake, for example, this may not be beneficial.