Protein in nutrition

Protein in the human body

Take away the water and about 75 percent of your weight is protein. This chemical family is found throughout the body. It's in muscle, bone, skin, hair, and virtually every other body part or tissue. It makes up the enzymes that power many chemical reactions and the hemoglobin that carries oxygen in your blood. At least 10,000 different proteins make you what you are and keep you that way.
Twenty or so basic building blocks, called amino acids, provide the raw material for all proteins. Following genetic instructions, the body strings together amino acids. Some genes call for short chains, others are blueprints for long chains that fold, origami-like, into intricate, three-dimensional structures.
Because the body doesn't store amino acids, as it does fats or carbohydrates, it needs a daily supply of amino acids to make new protein.

Source: Harvard School of Public Health

Amino acids, the body's building blocks, are instrumental in forming cells, repairing tissue, making antibodies (to combat invading bacteria and viruses), building nucleoproteins (RNA/DNA), carrying oxygen throughout the body, assisting muscle activity, as well as being part of the enzyme and hormonal system. The human body requires approximately 20 amino acids in order to synthesize proteins. About half of the amino acids are made by the body and so don't need to be in the diet - these are known as the 'nonessential' amino acids (not essential in the diet). The remaining amino acids (actually 9 for adults, 10 for the young) are not made in the body, so are obtained only from food - these are the 'essential' amino acids (essential in the diet).

Appropriate Dietary Protein Intake

Protein is not stored in the body as such, unlike fat (in fat cells) and glucose (in muscle or liver). Because muscles, for example, are built from protein, we need to consume, and synthesize, enough protein to maintain healthy, hard-working muscles.

What about excessive protein consumption?

Be wary of high-protein diets, which may also be high in fat and may lead to high cholesterol, heart disease or other diseases, such as gout. A high-protein diet may put additional strain on the kidneys when extra waste matter (the end product of protein metabolism) is excreted in the urine.

What about insufficient protein consumption?

Lack of protein on the other hand, can cause growth failure, loss of muscle mass, decreased immunity, weakening of the heart and respiratory system, and death. Protein malnutrition leads to the condition known as kwashiorkor, suffered especially by people in regions of the world where protein is not available.

Adequate protein consumption

A nutritionally balanced diet provides adequate protein, and a vegetarian diet can provide the proper combination of plant proteins to achieve this. Two to three servings of protein-rich food supplies the daily needs of most adults, depending upon age, medical conditions, and the type of diet employed. Select lean meat, poultry without skin, fish, dry beans, lentils, and legumes often, as these are the protein choices lowest in fat content.

Food Sources of Protein

If the protein in a food supplies enough of the essential amino acids, it is called a complete protein; if not, it is an incomplete protein. The protein content of cooked meat and dairy products is from 15% to 40%, and that of cooked cereals, beans, lentils, and peas only from 3% to 10%.

Animal dietary protein sources

All meat and other animal products are sources of complete protein. For example:

• beef, lamb, pork
• poultry, eggs
• fish, shellfish
• milk and milk products

Vegetable dietary protein sources

Plant foods contain the same amino acids as animal foods, but in differing amounts. Protein in foods such as most grains, fruits, and vegetables are considered incomplete proteins, being either low in, or lacking, one of the essential amino acids. Plant protein sources can be combined with other plant or animal products to form a complete protein; eg rice and beans, milk and wheat cereal, corn and beans. Plant foods considered complete proteins:

• Nuts
• Soy foods (tofu, tempeh, miso, and soy milk)
• Sprouted seeds (each type of sprout differs in nutrient proportions, so eat a variety)
• Grains (especially amaranth and quinoa, highest in protein)
• Beans and legumes (especially when eaten raw)
• Spirulina and chorella or blue-green algae (over 60% protein)

Recommended Dietary Allowance (RDA) for Protein

Daily Protein Intake

A minimum daily intake of protein is about ¼ ounce per lb or 0.8 grams per kilogram of body weight, while excess protein is defined as anything more than twice that amount. For an average-build 155 lb/70 kg man in good health, the RDA amounts to 2oz/56g as a minimum, but less than 4oz/112g per day. A percentage of the population, however - growing children, pregnant and lactating women, the elderly, anyone undergoing severe stress (trauma, hospitalization, surgery), disease or disability - need more protein than the RDA. Also anyone doing endurance training (not resistance training which builds muscle and uses protein more efficiently) requires higher dietary protein - from ¼ to ½ as much again per day.

Protein and Food Servings

Common serving sizes for a healthy adult consuming 2 to 3 servings per day to provide adequate protein:

• 2-3 ounces/56-85g of cooked lean meat, poultry, and fish
• ½ cup of cooked dry beans, lentils, or legumes
• 1 egg or 2 tablespoons of peanut butter (equivalent to 1 ounce/28g of lean meat).

For example, cereal with milk for breakfast, a peanut butter and jelly sandwich for lunch, and a piece of fish with a side of beans for dinner, totals about 70 grams of protein for the day. 1 gram of protein is equal to 4 calories, so a food serving containing 20 g or 0.7oz of protein equates to 80 calories.

Conclusion

As a rule of thumb then, three times each day you should consume about the amount of protein source in a food serving which could be held in the palm of your hand — about the size of a chicken breast. This constitutes about 30% of your calories. Have whatever fat comes associated with that protein, and add the equivalent of three handfuls of high-fiber vegetables.