Human nutrition

Protein intake

The Body’s Fuel Sources
This recommendation was based on the food-combining rule to separate protein and starch meals, but the rule doesn't apply on a starch-free raw diet. The American Journal of Clinical Nutrition. The omega designations also referred to as n-3 and n-6 indicate the location of the first double bond from the methyl end of the fatty acid. This spares carbohydrates for more active energy use, and spares protein from being broken down for energy, thus allowing better tissue growth and maintenance. The human body regulates blood glucose levels so that they are neither too high nor too low. There are many types of sugars namely:

Blood sugar and cells

Difference Between Glucose and Glycogen

Of particular importance for humans are the carbon polyunsaturated fatty acids alpha-linolenic acid an omega-3 fatty acid and linoleic acid an omega-6 fatty acid ; these are known as essential fatty acids because they are required in small amounts in the diet.

The omega designations also referred to as n-3 and n-6 indicate the location of the first double bond from the methyl end of the fatty acid. Other fatty acids can be synthesized in the body and are therefore not essential in the diet. About a tablespoon daily of an ordinary vegetable oil such as safflower or corn oil or a varied diet that includes grains, nuts , seeds , and vegetables can fulfill the essential fatty acid requirement.

Essential fatty acids are needed for the formation of cell membranes and the synthesis of hormone -like compounds called eicosanoids e. The consumption of fish once or twice a week provides an additional source of omega-3 fatty acids that appears to be healthful. A fat consisting largely of saturated fatty acids, especially long-chain fatty acids, tends to be solid at room temperature; if unsaturated fatty acids predominate, the fat is liquid at room temperature.

Fats and oils usually contain mixtures of fatty acids, although the type of fatty acid in greatest concentration typically gives the food its characteristics.

Butter and other animal fats are primarily saturated; olive and canola oils, monounsaturated; and fish, corn , safflower , soybean, and sunflower oils, polyunsaturated. Although plant oils tend to be largely unsaturated, there are notable exceptions, such as coconut fat , which is highly saturated but nevertheless semiliquid at room temperature because its fatty acids are of medium chain length 8 to 14 carbons long.

Saturated fats tend to be more stable than unsaturated ones. The food industry takes advantage of this property during hydrogenation , in which hydrogen molecules are added to a point of unsaturation, thereby making the fatty acid more stable and resistant to rancidity oxidation as well as more solid and spreadable as in margarine.

However, a result of the hydrogenation process is a change in the shape of some unsaturated fatty acids from a configuration known as cis to that known as trans. Trans -fatty acids, which behave more like saturated fatty acids, may also have undesirable health consequences. A phospholipid is similar to a triglyceride except that it contains a phosphate group and a nitrogen -containing compound such as choline instead of one of the fatty acids.

In food, phospholipids are natural emulsifiers , allowing fat and water to mix, and they are used as food additives for this purpose. In the body, phospholipids allow fats to be suspended in fluids such as blood , and they enable lipids to move across cell membranes from one watery compartment to another. The phospholipid lecithin is plentiful in foods such as egg yolks, liver, wheat germ, and peanuts. However, the liver is able to synthesize all the lecithin the body needs if sufficient choline is present in the diet.

Sterols are unique among lipids in that they have a multiple-ring structure. The well-known sterol cholesterol is found only in foods of animal origin—meat, egg yolk , fish , poultry , and dairy products. There are a number of sterols in shellfish but not as much cholesterol as was once thought. Cholesterol is essential to the structure of cell membranes and is also used to make other important sterols in the body, among them the sex hormones, adrenal hormones , bile acids, and vitamin D.

However, cholesterol can be synthesized in the liver , so there is no need to consume it in the diet.

Cholesterol-containing deposits may build up in the walls of arteries, leading to a condition known as atherosclerosis , which contributes to myocardial infarction heart attack and stroke. Furthermore, because elevated levels of blood cholesterol, especially the form known as low-density lipoprotein LDL cholesterol, have been associated with an increased risk of cardiovascular disease , a limited intake of saturated fat—particularly medium-chain saturated fatty acids, which act to raise LDL cholesterol levels—is advised.

Trans-fatty acids also raise LDL cholesterol, while monounsaturated and polyunsaturated cis fats tend to lower LDL cholesterol levels. The complex relationships between various dietary lipids and blood cholesterol levels, as well as the possible health consequences of different dietary lipid patterns, are discussed in the article nutritional disease. Proteins , like carbohydrates and fats, contain carbon, hydrogen, and oxygen, but they also contain nitrogen , a component of the amino chemical group NH 2 , and in some cases sulfur.

Proteins serve as the basic structural material of the body as well as being biochemical catalysts and regulators of genes. Aside from water, protein constitutes the major part of muscles, bones, internal organs, and the skin , nails , and hair. Protein is also an important part of cell membranes and blood e. Enzymes , which catalyze chemical reactions in the body, are also protein, as are antibodies , collagen in connective tissue, and many hormones, such as insulin.

Tissue proteins are in a dynamic equilibrium with proteins in the blood, with input from proteins in the diet and losses through urine , feces , and skin. In a healthy adult, adjustments are made so that the amount of protein lost is in balance with the amount of protein ingested. However, during periods of rapid growth, pregnancy and lactation , or recuperation after illness or depletion, the body is in positive nitrogen balance, as more protein is being retained than excreted.

The opposite is true during illness or wasting, when there is negative nitrogen balance as more tissue is being broken down than synthesized. Each gene makes one or more proteins, each with a unique sequence of amino acids and precise three-dimensional configuration. Amino acids are also required for the synthesis of other important nonprotein compounds, such as peptide hormones, some neurotransmitters , and creatine.

Food contains approximately 20 common amino acids, 9 of which are considered essential, or indispensable, for humans; i. The essential amino acids for humans are histidine , isoleucine , leucine , lysine , methionine , phenylalanine , threonine , tryptophan , and valine.

Conditionally indispensable amino acids include arginine , cysteine , and tyrosine , which may need to be provided under special circumstances, such as in premature infants or in people with liver disease, because of impaired conversion from precursors. The relative proportions of different amino acids vary from food to food see table. Foods of animal origin— meat , fish , eggs , and dairy products —are sources of good quality, or complete, protein; i.

Gelatin , which lacks the amino acid tryptophan , is an exception. Individual foods of plant origin, with the exception of soybeans , are lower quality, or incomplete, protein sources. Lysine , methionine , and tryptophan are the primary limiting amino acids; i. However, a varied vegetarian diet can readily fulfill human protein requirements if the protein-containing foods are balanced such that their essential amino acids complement each other.

For example, legumes such as beans are high in lysine and low in methionine, while grains have complementary strengths and weaknesses. Thus, if beans and rice are eaten over the course of a day, their joint amino acid patterns will supplement each other and provide a higher quality protein than would either food alone.

Traditional food patterns in native cultures have made good use of protein complementarity. However, careful balancing of plant proteins is necessary only for those whose protein intake is marginal or inadequate. In affluent populations, where protein intake is greatly in excess of needs, obtaining sufficient good quality protein is usually only a concern for young children who are not provided with animal proteins.

The World Health Organization recommends a daily intake of 0. Thus, a kg pound man would need This recommendation, based on nitrogen balance studies, assumes an adequate energy intake.

Infants, children, and pregnant and lactating women have additional protein needs to support synthesis of new tissue or milk production. Protein requirements of endurance athletes and bodybuilders may be slightly higher than those of sedentary individuals, but this has no practical significance because athletes typically consume much more protein than they need.

During conditions of fasting , starvation , or insufficient dietary intake of protein, lean tissue is broken down to supply amino acids for vital body functions. Persistent protein inadequacy results in suboptimal metabolic function with increased risk of infection and disease.

Vitamins are organic compounds found in very small amounts in food and required for normal functioning—indeed, for survival. Humans are able to synthesize certain vitamins to some extent. For example, vitamin D is produced when the skin is exposed to sunlight ; niacin can be synthesized from the amino acid tryptophan; and vitamin K and biotin are synthesized by bacteria living in the gut.

However, in general, humans depend on their diet to supply vitamins. When a vitamin is in short supply or is not able to be utilized properly, a specific deficiency syndrome results.

When the deficient vitamin is resupplied before irreversible damage occurs, the signs and symptoms are reversed. The amounts of vitamins in foods and the amounts required on a daily basis are measured in milligrams and micrograms. Unlike the macronutrients, vitamins do not serve as an energy source for the body or provide raw materials for tissue building.

Rather, they assist in energy-yielding reactions and facilitate metabolic and physiologic processes throughout the body. Vitamin A , for example, is required for embryonic development, growth, reproduction, proper immune function, and the integrity of epithelial cells, in addition to its role in vision. The B vitamins function as coenzymes that assist in energy metabolism; folic acid folate , one of the B vitamins, helps protect against birth defects in the early stages of pregnancy.

Vitamin C plays a role in building connective tissue as well as being an antioxidant that helps protect against damage by reactive molecules free radicals. Now considered to be a hormone , vitamin D is involved in calcium and phosphorus homeostasis and bone metabolism. Vitamin E , another antioxidant, protects against free radical damage in lipid systems, and vitamin K plays a key role in blood clotting. Although vitamins are often discussed individually, many of their functions are interrelated, and a deficiency of one can influence the function of another.

Vitamin nomenclature is somewhat complex, with chemical names gradually replacing the original letter designations created in the era of vitamin discovery during the first half of the 20th century. Nomenclature is further complicated by the recognition that vitamins are parts of families with, in some cases, multiple active forms.

Some vitamins are found in foods in precursor forms that must be activated in the body before they can properly fulfill their function. The 13 vitamins known to be required by human beings are categorized into two groups according to their solubility.

The four fat-soluble vitamins soluble in nonpolar solvents are vitamins A, D, E, and K. Although now known to behave as a hormone, the activated form of vitamin D, vitamin D hormone calcitriol , is still grouped with the vitamins as well.

The nine water-soluble vitamins soluble in polar solvents are vitamin C and the eight B-complex vitamins: Choline is a vitamin-like dietary component that is clearly required for normal metabolism but that can be synthesized by the body. Although choline may be necessary in the diet of premature infants and possibly of those with certain medical conditions, it has not been established as essential in the human diet throughout life.

Different vitamins are more or less susceptible to destruction by environmental conditions and chemical agents. For example, thiamin is especially vulnerable to prolonged heating, riboflavin to ultraviolet or fluorescent light, and vitamin C to oxidation as when a piece of fruit is cut open and the vitamin is exposed to air.

In general, water-soluble vitamins are more easily destroyed during cooking than are fat-soluble vitamins.

The solubility of a vitamin influences the way it is absorbed, transported, stored, and excreted by the body as well as where it is found in foods. With the exception of vitamin B 12 , which is supplied by only foods of animal origin, the water-soluble vitamins are synthesized by plants and found in both plant and animal foods.

Strict vegetarians vegans , who eat no foods of animal origin, are therefore at risk of vitamin B 12 deficiency. Fat-soluble vitamins, on the other hand, are found in association with fats and oils in foods and in the body and typically require protein carriers for transport through the water-filled compartments of the body. Water-soluble vitamins are not appreciably stored in the body except for vitamin B 12 and thus must be consumed regularly in the diet.

If taken in excess they are readily excreted in the urine, although there is potential toxicity even with water-soluble vitamins; especially noteworthy in this regard is vitamin B 6. However, the fact that these vitamins can be stored increases the possibility of toxicity if very large doses are taken.

This is particularly of concern with vitamins A and D, which can be toxic if taken in excess. Niacin , for example, is used to lower blood cholesterol levels; vitamin D is used to treat psoriasis ; and pharmacological derivatives of vitamin A are used to treat acne and other skin conditions as well as to diminish skin wrinkling.

However, consumption of vitamins or other dietary supplements in amounts significantly in excess of recommended levels is not advised without medical supervision. Vitamins synthesized in the laboratory are the same molecules as those extracted from food, and they cannot be distinguished by the body. However, various forms of a vitamin are not necessarily equivalent.

Vitamins in food have a distinct advantage over vitamins in supplement form because they come associated with other substances that may be beneficial , and there is also less potential for toxicity. Nutritional supplements cannot substitute for a healthful diet.

Unlike the complex organic compounds carbohydrates, lipids, proteins, vitamins discussed in previous sections, minerals are simple inorganic elements—often in the form of salts in the body—that are not themselves metabolized, nor are they a source of energy.

Minerals constitute about 4 to 6 percent of body weight—about one-half as calcium and one-quarter as phosphorus phosphates , the remainder being made up of the other essential minerals that must be derived from the diet. Minerals not only impart hardness to bones and teeth but also function broadly in metabolism—e. As nutrients, minerals are traditionally divided into two groups according to the amounts present in and needed by the body.

The major minerals macrominerals —those required in amounts of milligrams or more per day—are calcium, phosphorus phosphates , magnesium , sulfur, sodium , chloride , and potassium.

The trace elements microminerals or trace minerals , required in much smaller amounts of about 15 milligrams per day or less, include iron , zinc , copper , manganese , iodine iodide , selenium , fluoride, molybdenum , chromium , and cobalt as part of the vitamin B 12 molecule. Fluoride is considered a beneficial nutrient because of its role in protecting against dental caries , although an essential function in the strict sense has not been established in human nutrition.

The term ultratrace elements is sometimes used to describe minerals that are found in the diet in extremely small quantities micrograms each day and are present in human tissue as well; these include arsenic , boron , nickel , silicon , and vanadium. Despite demonstrated roles in experimental animals, the exact function of these and other ultratrace elements e. Minerals have diverse functions, including muscle contraction, nerve transmission, blood clotting, immunity, the maintenance of blood pressure, and growth and development.

The major minerals, with the exception of sulfur, typically occur in the body in ionic charged form: Mineral salts dissolved in body fluids help regulate fluid balance , osmotic pressure, and acid-base balance. Other mineral elements that are constituents of organic compounds include iron , which is part of hemoglobin the oxygen-carrying protein in red blood cells , and iodine , a component of thyroid hormones, which help regulate body metabolism. Additionally, phosphate groups are found in many organic molecules, such as phospholipids in cell membranes, genetic material DNA and RNA , and the high-energy molecule adenosine triphosphate ATP.

The levels of different minerals in foods are influenced by growing conditions e. Minerals are not destroyed during food preparation; in fact, a food can be burned completely and the minerals ash will remain unchanged. However, minerals can be lost by leaching into cooking water that is subsequently discarded.

Many factors influence mineral absorption and thus availability to the body. In general, minerals are better absorbed from animal foods than from plant foods. The latter contain fibre and other substances that interfere with absorption. Phytic acid , found principally in cereal grains and legumes , can form complexes with some minerals and make them insoluble and thereby indigestible.

Only a small percentage of the calcium in spinach is absorbed because spinach also contains large amounts of oxalic acid , which binds calcium. Some minerals, particularly those of a similar size and charge, compete with each other for absorption. For example, iron supplementation may reduce zinc absorption, while excessive intakes of zinc can interfere with copper absorption. On the other hand, the absorption of iron from plants nonheme iron is enhanced when vitamin C is simultaneously present in the diet, and calcium absorption is improved by adequate amounts of vitamin D.

Another key factor that influences mineral absorption is the physiological need for the mineral at the time. Unlike many vitamins, which have a broader safety range, minerals can be toxic if taken in doses not far above recommended levels. This is particularly true for the trace elements, such as iron and copper. Accidental ingestion of iron supplements has been a major cause of fatal poisoning in young children.

Although often overlooked as a nutrient, water H 2 O is actually the most critical nutrient of all. Humans can survive weeks without food but only a matter of days without water. Water provides the medium in which nutrients and waste products are transported throughout the body and the myriad biochemical reactions of metabolism occur.

Water allows for temperature regulation, the maintenance of blood pressure and blood volume, the structure of large molecules, and the rigidity of body tissues. It also acts as a solvent, a lubricant as in joints , and a protective cushion as inside the eyes and in spinal fluid and amniotic fluid. The flow of water in and out of cells is precisely controlled by shifting electrolyte concentrations on either side of the cell membrane.

Potassium, magnesium, phosphate, and sulfate are primarily intracellular electrolytes; sodium and chloride are major extracellular ones. Water makes up about 50 to 70 percent of body weight , approximately 60 percent in healthy adults and an even higher percentage in children.

Because lean tissue is about three-quarters water, and fatty tissue is only about one-fifth water, body composition—the amount of fat in particular—determines the percentage of body water. In general, men have more lean tissue than women, and therefore a higher percentage of their body weight is water. Water is consumed not only as water itself and as a constituent of other beverages but also as a major component of many foods, particularly fruits and vegetables, which may contain from 85 to 95 percent water.

Water also is manufactured in the body as an end product of metabolism. Because water requirements vary with climate, level of activity, dietary composition, and other factors, there is no one recommendation for daily water intake. However, adults typically need at least 2 litres 8 cups of water a day, from all sources. Thirst is not reliable as a register for dehydration , which typically occurs before the body is prompted to replace fluid. Therefore, water intake is advised throughout the day, especially with increased sweat loss in hot climates or during vigorous physical activity, during illness, or in a dehydrating situation such as an airplane flight.

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Our editors will review what you've submitted, and if it meets our criteria, we'll add it to the article. Please note that our editors may make some formatting changes or correct spelling or grammatical errors, and may also contact you if any clarifications are needed. The energy value and nutrient content of some common foods food energy kcal carbohydrate g protein g fat g water g Source: Approximate energy expenditure for activity levels activity category energy as multiple of resting energy expenditure REE kilocalories per minute Source: Dietary Reference Intakes for selected nutrients for adults recommended daily intake Recommended Dietary Allowance or Adequate Intake women men macronutrients 1 Based on a recommended intake of 0.

As potential fuel sources, the carbohydrate, fat, and protein in the foods that you eat follow different metabolic paths in the body, but they all ultimately yield water, carbon dioxide, and a chemical energy called adenosine triphosphate ATP.

Think of ATP molecules as high-energy compounds or batteries that store energy. Anytime you need energy—to breathe, to tie your shoes, or to cycle miles km —your body uses ATP molecules.

ATP, in fact, is the only molecule able to provide energy to muscle fibers to power muscle contractions. Our daily food choices resupply the potential energy, or fuel, that the body requires to continue to function normally.

This energy takes three forms: The body can store some of these fuels in a form that offers muscles an immediate source of energy. Glucose can be used immediately as fuel, or can be sent to the liver and muscles and stored as glycogen. During exercise, muscle glycogen is converted back into glucose, which only the muscle fibers can use as fuel.

During exercise, your muscles pick up some of this glucose and use it in addition to their own private glycogen stores. Blood glucose also serves as the most significant source of energy for the brain, both at rest and during exercise. The body constantly uses and replenishes its glycogen stores. The carbohydrate content of your diet and the type and amount of training that you undertake influence the size of your glycogen stores.

The capacity of your body to store muscle and liver glycogen, however, is limited to approximately 1, to 2, calories worth of energy, or enough fuel for 90 to minutes of continuous, vigorous activity. To keep up with this greatly elevated demand for glucose, liver glycogen stores become rapidly depleted.

Foods that you eat or drink during exercise that supply carbohydrate can help delay the depletion of muscle glycogen and prevent hypoglycemia. During exercise, stored fat in the body in the form of triglycerides in adipose or fat tissue is broken down into fatty acids. These fatty acids are transported through the blood to muscles for fuel.

This process occurs relatively slowly as compared with the mobilization of carbohydrate for fuel. Fat is also stored within muscle fibers, where it can be more easily accessed during exercise. Unlike your glycogen stores, which are limited, body fat is a virtually unlimited source of energy for athletes. Even those who are lean and mean have enough fat stored in muscle fibers and fat cells to supply up to , calories—enough for over hours of marathon running!

Fat is a more efficient fuel per unit of weight than carbohydrate. Carbohydrate must be stored along with water. Our weight would double if we stored the same amount of energy as glycogen plus the water that glycogen holds that we store as body fat.

Most of us have sufficient energy stores of fat adipose tissue or body fat , plus the body readily converts and stores excess calories from any source fat, carbohydrate, or protein as body fat. In order for fat to fuel exercise, however, sufficient oxygen must be simultaneously consumed. Rather, protein is used to build, maintain, and repair body tissues, as well as to synthesize important enzymes and hormones.

In some situations, however, such as when we eat too few calories daily or not enough carbohydrate, as well as during latter stages of endurance exercise, when glycogen reserves are depleted, skeletal muscle is broken down and used as fuel. This sacrifice is necessary to access certain amino acids the building blocks of protein that can be converted into glucose.

Remember, your brain also needs a constant, steady supply of glucose to function optimally. Get the latest news, special offers, and updates on authors and products. We use cookies so we can provide you with the best online experience. You can change your cookie settings at any time.

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Utilization of food by the body