Proteins are the fundamental building blocks of life, present in every cell of the human body. They play a crucial role in repairing cells, making new ones, and supporting growth and development, especially in children, teenagers, and pregnant women. The basic structure of protein is a chain of amino acids.
Understanding Protein and Amino Acids
The protein in food is broken down into amino acids during digestion. The human body requires a sufficient amount of various amino acids to maintain optimal health. These amino acids are classified into three groups:
- Essential amino acids: These cannot be made by the body and must be obtained from food.
- Nonessential amino acids: The body can synthesize these.
- Conditional amino acids: These are needed during illness and stress.
Amino acids are derived from both animal and plant sources. Animal sources include meats, milk, fish, and eggs, while plant sources include soy, beans, legumes, nut butters, and some grains like wheat germ and quinoa. A balanced diet can provide all the necessary protein, even without animal products.
Dietary Protein Needs
The amount of protein needed varies based on individual calorie needs. For healthy adults, the recommended daily intake is 10% to 35% of total calorie needs. Since one gram of protein provides 4 calories, a person consuming a 2000-calorie diet could aim for 100 grams of protein, which would constitute 20% of their daily calories.
Typically, one ounce (30 grams) of most protein-rich foods contains approximately 7 grams of protein. This equates to:
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- 1 oz (30 g) of meat, fish, or poultry
- 1 large egg
- ¼ cup (60 milliliters) of tofu
- ½ cup (65 grams) of cooked beans or lentils
Low-fat dairy and whole grains are also excellent sources of protein, with whole grains offering more protein than refined products. Protein needs may vary for children and teens depending on their age.
Healthy Protein Sources
Animal Protein
- Turkey or chicken (skin removed)
- Lean cuts of beef or pork (round, top sirloin, or tenderloin, with visible fat trimmed)
- Fish or shellfish
- Bison (buffalo meat)
Other Protein Sources
- Pinto beans, black beans, kidney beans, lentils, split peas, or garbanzo beans
- Nuts and seeds (almonds, hazelnuts, mixed nuts, peanuts, peanut butter, sunflower seeds, or walnuts) - mindful of portion sizes due to high fat content
- Tofu, tempeh, and other soy protein products
- Low-fat dairy products
Nutrients: The Building Blocks of Life
A nutrient is a substance essential for an organism's survival, growth, and reproduction. This applies to animals, plants, fungi, and protists. Nutrients are incorporated into cells for metabolic processes or excreted to form non-cellular structures like hair, scales, or exoskeletons. Some nutrients, like carbohydrates, lipids, and proteins, can be metabolically converted into smaller molecules, releasing energy.
Essential Nutrients
Essential nutrients for animals include energy sources, certain amino acids, fatty acids, vitamins, and minerals. Plants require a wider range of minerals absorbed through their roots, along with carbon dioxide and oxygen absorbed through their leaves. Different organisms have different essential nutrient requirements. For example, ascorbic acid (vitamin C) is essential for humans and some animals, while most other animals and many plants can synthesize it.
Organic and Inorganic Nutrients
Nutrients can be organic or inorganic. Organic compounds primarily contain carbon, while all other chemicals are inorganic.
Macronutrients and Micronutrients
Nutrients are also classified into macronutrients and micronutrients based on the amount needed. Macronutrients, including carbohydrates, fats, proteins, and water, are consumed in large amounts (grams or ounces) and are used for energy or tissue growth and repair. Micronutrients, such as vitamins and minerals, are needed in smaller amounts (milligrams or micrograms) and play biochemical and physiological roles in cellular processes.
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Consequences of Nutrient Deficiency
Inadequate intake of essential nutrients or diseases that interfere with absorption can lead to deficiency states, compromising growth, survival, and reproduction. Consumer advisories like the United States Dietary Reference Intake provide guidelines for both lower and upper limits of macronutrient and micronutrient intake to prevent deficiencies.
Food Labeling
Many countries require food product labels to display information about the amounts of macronutrients and micronutrients present in significant quantities.
Macronutrients: Carbohydrates, Proteins, and Fats
The chemical compounds humans consume in the largest quantities for energy are carbohydrates, proteins, and fats.
- Carbohydrates: These are compounds made up of various types of sugar.
- Proteins: These are organic compounds consisting of amino acids joined by peptide bonds. The body cannot produce essential amino acids, so they must be obtained through diet.
- Fats: These consist of a glycerin molecule with three fatty acids attached. Fatty acids can be saturated (connected by single bonds) or unsaturated (connected by both double and single bonds). Fats are crucial for cell membrane construction and maintenance, maintaining body temperature, and sustaining healthy skin and hair.
Essential Minerals and Vitamins
Dietary minerals, such as potassium, sodium, and iron, are elements native to Earth and cannot be synthesized. They are required in microgram or milligram amounts. An essential amino acid is one that an organism needs but cannot produce on its own, necessitating its dietary intake.
Vitamins occur in various forms known as vitamers, which perform the functions of that vitamin and prevent deficiency symptoms. Vitamins are essential organic molecules that are not classified as amino acids or fatty acids. They commonly function as enzymatic cofactors, metabolic regulators, or antioxidants. Humans require thirteen vitamins in their diet: vitamins A, C, D, E, K, thiamine (B1), riboflavin (B2), niacin (B3), pantothenic acid (B5), pyridoxine (B6), biotin (B7), folate (B9), and cobalamin (B12).
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Minerals are exogenous chemical elements indispensable for life. While carbon, hydrogen, oxygen, and nitrogen are essential, they are so abundant in food and drink that they are not considered nutrients with recommended intakes. The need for nitrogen is addressed by protein requirements, which are composed of nitrogen-containing amino acids.
The essential nutrient trace elements for humans, in order of recommended dietary allowance, are potassium, chloride, sodium, calcium, phosphorus, magnesium, iron, zinc, manganese, copper, iodine, chromium, molybdenum, and selenium. Cobalt is a component of vitamin B12, which is also essential.
Choline: An Essential Nutrient
Choline is an essential nutrient and a family of water-soluble quaternary ammonium compounds. Healthy humans fed choline-deficient diets develop fatty liver, liver damage, and muscle damage.
Conditionally Essential and Non-Essential Nutrients
Conditionally essential nutrients are organic molecules that can normally be synthesized by an organism but are insufficient under certain conditions. Non-essential nutrients are substances within foods that can have a significant impact on health.
Ethanol
Ethanol (C2H5OH) is not an essential nutrient but supplies approximately 29 kilojoules (7 kilocalories) of food energy per gram.
Nutrient Deficiencies and Recommended Intakes
An inadequate amount of a nutrient is a deficiency. Estimated Average Requirements (EARs) and Recommended Dietary Allowances (RDAs) for vitamins and minerals, and Population Reference Intakes (PRIs) for the European Union, are based on the amount required to prevent deficiency. RDAs are set higher than EARs to cover people with higher-than-average needs. Adequate Intakes (AIs) are set when there is insufficient information to establish EARs and RDAs.
Plant Nutrients
Plants absorb carbon, hydrogen, and oxygen from air and soil as carbon dioxide and water. Other nutrients are absorbed from soil. There are 17 important nutrients for plants:
- Macronutrients: nitrogen (N), phosphorus (P), potassium (K), calcium (Ca), sulfur (S), magnesium (Mg), carbon (C), oxygen (O), and hydrogen (H)
- Micronutrients: iron (Fe), boron (B), chlorine (Cl), manganese (Mn), zinc (Zn), copper (Cu), molybdenum (Mo), and nickel (Ni)
Amino Acids: The Building Blocks of Protein
Both animal and plant proteins are made up of about 20 common amino acids. The proportion of these amino acids varies as a characteristic of a given protein, but all food proteins-with the exception of gelatin-contain some of each. Amino nitrogen accounts for approximately 16% of the weight of proteins. Amino acids are required for the synthesis of body protein and other important nitrogen-containing compounds, such as creatine, peptide hormones, and some neurotransmitters.
Proteins and other nitrogenous compounds are continuously degraded and resynthesized. Several times more protein is turned over daily within the body than is ordinarily consumed, indicating that reutilization of amino acids is a major feature of the economy of protein metabolism. This recapture process is not completely efficient, and some amino acids are lost by oxidative catabolism.
A continuous supply of dietary amino acids is required to replace these losses, even after growth has ceased. Amino acids consumed in excess of the amounts needed for the synthesis of nitrogenous tissue constituents are not stored but are degraded; the nitrogen is excreted as urea, and the keto acids left after removal of the amino groups are either utilized directly as sources of energy or are converted to carbohydrate or fat.
Nine amino acids-histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan, and valine-are not synthesized by mammals and are therefore dietarily essential or indispensable nutrients. These are commonly called the essential amino acids. Histidine is an essential amino acid for infants.
Under special circumstances (e.g., in premature infants or in people with liver damage), amino acids such as cystine and tyrosine, not normally essential, may become so because of impaired conversion from their precursors.
Protein Deficiency
Protein deficiency rarely occurs as an isolated condition. It usually accompanies a deficiency of dietary energy and other nutrients resulting from insufficient food intake. The symptoms are most commonly seen in deprived children in poor countries. Where protein intake is exceptionally low, there are physical signs-stunting, poor musculature, edema, thin and fragile hair, skin lesions-and biochemical changes that include low serum albumin and hormonal imbalances. Edema and loss of muscle mass and hair are the prominent signs in adults. Deficiency of this severity is very rare in the United States, except as a consequence of pathologic conditions and poor medical management of the acutely ill.
Estimating Protein Requirements
At submaintenance levels of protein intake, a diminished turnover of tissue protein is accompanied by a reduced catabolic rate for the amino acids liberated by protein breakdown. Similarly, turnover rate is increased with increased intake. In this way, the tissue protein pool can, within limits, enter a new steady state appropriate for the diminished or increased protein intake from food.
Under the experimental conditions of a protein-free diet, protein synthesis and breakdown continue by reutilizing amino acids. This process becomes very efficient, but some amino acids are still catabolized and the nitrogen is excreted. This lower limit, termed the obligatory nitrogen loss, has been extensively studied in adults fed protein-free diets. Values are remarkably uniform.
In the past, a factorial method was used as a basis for predicting the protein requirements of various age groups. For adults, the requirement for dietary protein was considered to be the amount needed to replace the obligatory nitrogen loss after adjustment for inefficiency in utilization of dietary protein and for the quality of the dietary protein consumed (i.e., its digestibility and amino acid composition). For children and pregnant and lactating women, an additional amount of protein for tissue growth or milk formation was incorporated into this factorial estimate of requirements. Because of the assumptions required, the validity of the factorial approach has been questioned.
Protein synthesis and breakdown are energy-dependent and thus are sensitive to dietary energy deprivation. Consequently, the body's energy balance becomes an important factor in determining nitrogen balance and influences the apparent utilization of dietary protein. Protein requirements are determined and allowances established for conditions of adequate energy intake and balance.
Nitrogen Balance
Nitrogen balance is the difference between nitrogen intake and the amount excreted in urine, feces, and sweat, together with minor losses occurring by other routes. To estimate the protein requirement, levels of dietary protein below and near predicted adequate intake are fed and nitrogen balance is measured at each level. The requirement is estimated by interpolating or extrapolating the nitrogen balance data to the zero balance point (nitrogen equilibrium) for adults or to a defined level of positive balance (to allow for growth) for children.
Other Criteria of Adequacy
Most studies of protein requirements have been short. In the few long-term studies that have been reported, investigators have explored the usefulness of various biochemical indices (e.g., serum aspartate and alanine amino transferase activities), but no agreement on a sensitive and reliable marker has been reached.
Because the human body can adapt to low and high intakes of nitrogen, there is a substantial difference between intakes barely sufficient to compensate for losses or to permit growth and intakes that may be associated with harmful effects. Since there are few criteria by which to evaluate the significance of the rate of protein turnover and pool size, value judgments must be made as to what is desirable in adults. For children, the protein required for growth is relatively small compared to that needed for maintenance. Nevertheless, satisfactory growth is a sensitive indicator of protein nutritional status.
The Requirement for Amino Acids
In determining the requirement for protein, the subcommittee first considered requirements for the essential amino acids. The required amounts of the nine essential amino acids must be provided in the diet, but because cystine can replace approximately 30% of the requirement for methionine, and tyrosine about 50% of the requirement for phenylalanine, these amino acids must also be considered.
The relatively low requirements estimated for adults have been confirmed by Inoue et al. (1988) using the nitrogen balance method. Studies of whole body lysine, leucine, valine, and threonine oxidation rates suggest that adult requirements for these essential amino acids have been underestimated.
Studies on requirements for individual essential amino acids in the elderly are inconsistent. Some suggest that requirements are increased in the elderly; others indicate that they are decreased. The pattern of requirement for essential amino acids in the elderly is accepted to be the same as for younger adults.
Recommended Allowances for Protein
In establishing an RDA for protein, three steps were followed: (1) The subcommittee first estimated the average requirement for reference proteins (i.e., highly digestible, high-quality protein such as egg, meat, milk, or fish) according to sex, age, and reproductive status of women. (2) The standard deviation of requirement was determined and average requirement values were increased accordingly to compute the recommended allowance of reference protein. (3) Amino acid scoring patterns were tabulated. These were based on requirements of various age groups for essential amino acids and for total protein.
Protein: Quality vs. Quantity
Protein is an essential macronutrient, but not all food sources of protein are created equal, and you may not need as much as you think. Protein is found throughout the body-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.
The National Academy of Medicine recommends that adults get a minimum of 0.8 grams of protein for every kilogram of body weight per day, or just over 7 grams for every 20 pounds of body weight. The National Academy of Medicine also sets a wide range for acceptable protein intake-anywhere from 10% to 35% of calories each day. Beyond that, there’s relatively little solid information on the ideal amount of protein in the diet or the healthiest target for calories contributed by protein. Individual needs will vary based on factors such as age, exercise level, health conditions, and overall dietary pattern.
Complete vs. Incomplete Proteins
Some proteins found in food are “complete,” meaning they contain all twenty-plus types of amino acids needed to make new protein in the body. Others are incomplete, lacking one or more of the nine essential amino acids, which our bodies can’t make from scratch or from other amino acids.
Animal-based foods (meat, poultry, fish, eggs, and dairy foods) tend to be good sources of complete protein, while plant-based foods (fruits, vegetables, grains, nuts, and seeds) often lack one or more essential amino acid. It’s important to note that millions of people worldwide, especially young children, don’t get enough protein due to food insecurity.
The Protein "Package"
When we eat foods for protein, we also eat everything that comes alongside it: the different fats, fiber, sodium, and more. A 4-ounce broiled sirloin steak is a great source of protein-about 33 grams worth. 4 ounces of grilled sockeye salmon has about 30 grams of protein, naturally low in sodium, and contains just over 1 gram of saturated fat.
Protein powders can come from a variety of sources, including eggs, milk (e.g., casein, whey), and plants (e.g., soybeans, peas, hemp). Some protein powders contain protein from multiple sources; for instance, a vegan option might include protein derived from peas, pumpkin seeds, sunflower seeds, and alfalfa. They can often contain non-protein ingredients, including vitamins and minerals, thickeners, added sugars, non-caloric sweeteners, and artificial flavoring.
Available evidence indicates that it’s the source of protein (or, the protein “package”), rather than the amount of protein, that likely makes a difference for our health.
Health Implications of Protein Sources
Research conducted at the Harvard Chan School of Public Health has found that eating even small amounts of red meat-especially processed red meat-on a regular basis is linked to an increased risk of heart disease and stroke, and the risk of dying from cardiovascular disease or any other cause. Conversely, replacing red and processed red meat with healthy protein sources such as beans, soy foods, nuts, fish, or poultry seems to reduce these risks.
One of the reasons why plant sources of protein are related to lower risk of cardiovascular disease compared to protein from red meat and dairy is because of the different types of fat in these protein packages. Plant-based protein sources are more unsaturated, which lowers LDL cholesterol-an established risk factor for heart disease. Also, plant sources contain no cholesterol.
Plant-Based Protein and Heart Health
A 20-year prospective study of over 80,000 women found that those who ate low-carbohydrate diets that were high in plant-based sources of fat and protein had a 30% lower risk of heart disease compared with women who ate high-carbohydrate, low-fat diets. A healthy diet that replaced some carbohydrate with healthy protein (or healthy fat) did a better job of lowering blood pressure and harmful low-density lipoprotein (LDL) cholesterol than a higher carbohydrate diet.
Protein and Diabetes Risk
The source of protein matters more than protein quantity when it comes to diabetes risk. A 2011 study found that people who ate diets high in red meat, especially processed red meat, had a higher risk of type 2 diabetes than those who rarely ate red or processed meat. For each additional serving a day of red meat or processed red meat that study participants ate, their risk of diabetes rose 12% and 32%, respectively.
How meat is cooked may also affect type 2 diabetes risk. In a study that tracked the health of over 289,000 men and women, researchers found that individuals who most frequently ate red meats and chicken cooked at high temperatures were 1.5 times more likely to develop type 2 diabetes, compared to those who ate the least.