Nutrition (also called nourishment or aliment) is the provision, to
cells and
organisms, of the materials necessary (in the form of food) to support
life. Many common health problems can be prevented or alleviated with a
healthy diet.
The
diet of an organism is what it eats, and is largely determined by the perceived
palatability of foods.
Dietitians are
health professionals who specialize in human nutrition, meal planning, economics, and preparation. They are trained to provide safe, evidence-based dietary advice and management to individuals (in health and disease), as well as to institutions.
A poor diet can have an injurious impact on health, causing deficiency diseases such as
scurvy,
beriberi, and
kwashiorkor; health-threatening conditions like
obesity and
metabolic syndrome, and such common chronic systemic diseases as
cardiovascular disease,
diabetes, and
osteoporosis.
Animal nutrition
Overview
Nutritional science investigates the
metabolic and physiological responses of the body to diet. With advances in the fields of
molecular biology,
biochemistry, and
genetics, the study of nutrition is increasingly concerned with metabolism and
metabolic pathways: the sequences of biochemical steps through which substances in living things change from one form to another.
A
carnivore and
herbivore diet is contrasting, with basic
nitrogen and
carbon proportions being at varying levels in particular foods. Carnivore nutrition has more nitrogen than carbon and herbivore nutrition contains less nitrogen than carbon, when an equal quantity is measured. Nitrogen is the predominant source of energy for organisms, through the
nitrogen cycle, with various
minerals and
vitamins being equally vital. Nitrogen is also the primary energy for regular metabolic rates, this necessitates greater sums of plants for herbivore digestion, to absorb significant energy for sustenance and stamina.
The
human body contains
chemical compounds, such as
water,
carbohydrates (sugar, starch, and
fiber),
amino acids (in
proteins),
fatty acids (in
lipids), and
nucleic acids (
DNA and
RNA). These compounds in turn consist of
elements such as
carbon,
hydrogen,
oxygen,
nitrogen,
phosphorus,
calcium,
iron,
zinc,
magnesium,
manganese, and so on. All of these chemical compounds and elements occur in various forms and combinations (e.g.
hormones,
vitamins,
phospholipids,
hydroxyapatite), both in the
human body and in the plant and animal organisms that humans eat.
The human body consists of elements and compounds ingested, digested, absorbed, and circulated through the
bloodstream to feed the
cells of the body. Except in the unborn fetus, the
digestive system is the first system involved[
vague]. In a typical adult, about seven liters of digestive juices enter the
lumen of the digestive tract.[
citation needed][
clarification needed] These break
chemical bonds in ingested molecules, and modulate their
conformations and energy states. Though some molecules are absorbed into the bloodstream unchanged, digestive processes release them from the matrix of foods. Unabsorbed matter, along with some waste products of
metabolism, is eliminated from the body in the
feces.
Studies of nutritional status must take into account the state of the body before and after experiments, as well as the
chemical composition of the whole diet and of all material
excreted and eliminated from the body (in
urine and feces). Comparing the food to the waste can help determine the specific compounds and elements absorbed and metabolized in the body. The effects of nutrients may only be discernible over an extended period, during which all food and waste must be analyzed. The number of
variables involved in such
experiments is high, making nutritional studies time-consuming and expensive, which explains why the science of human nutrition is still slowly evolving.
In general, eating a wide variety of fresh, whole (unprocessed), foods has proven favorable compared to monotonous diets based on processed foods. In particular, the consumption of whole-plant foods slows digestion and allows better absorption, and a more favorable balance of essential nutrients per
Calorie, resulting in better management of cell growth, maintenance, and
mitosis (cell division), as well as better regulation of appetite and blood sugar[
citation needed]. Regularly scheduled meals (every few hours) have also proven more wholesome than infrequent or haphazard ones.[
citation needed]
Nutrients
Main article:
Nutrient
There are six major classes of nutrients:
carbohydrates,
fats,
minerals,
protein,
vitamin, and
water.
These nutrient classes can be categorized as either
macronutrients (needed in relatively large amounts) or
micronutrients (needed in smaller quantities). The macronutrients are carbohydrates, fats, fiber, proteins, and water. The micronutrients are minerals and vitamins.
The macronutrients (excluding fiber and water) provide structural material (amino acids from which proteins are built, and lipids from which cell membranes and some signaling molecules are built),
energy. Some of the structural material can be used to generate energy internally, and in either case it is measured in
Joules or
kilocalories (often called "Calories" and written with a capital C to distinguish them from little 'c' calories). Carbohydrates and proteins provide 17 kJ approximately (4 kcal) of energy per gram, while fats provide 37 kJ (9 kcal) per gram., though the net energy from either depends on such factors as absorption and digestive effort, which vary substantially from instance to instance. Vitamins, minerals, fiber, and water do not provide energy, but are required for other reasons. A third class dietary material, fiber (i.e., non-digestible material such as cellulose), seems also to be required, for both mechanical and biochemical reasons, though the exact reasons remain unclear.
Molecules of carbohydrates and fats consist of carbon, hydrogen, and oxygen atoms. Carbohydrates range from simple
monosaccharides (glucose, fructose, galactose) to complex
polysaccharides (starch). Fats are
triglycerides, made of assorted
fatty acid monomers bound to
glycerol backbone. Some fatty acids, but not all, are
essential in the diet: they cannot be synthesized in the body. Protein molecules contain nitrogen atoms in addition to carbon, oxygen, and hydrogen. The fundamental components of protein are nitrogen-containing
amino acids, some of which are
essential in the sense that humans cannot make them internally. Some of the amino acids are convertible (with the expenditure of energy) to glucose and can be used for energy production just as ordinary glucose. By breaking down existing protein, some glucose can be produced internally; the remaining amino acids are discarded, primarily as urea in urine. This occurs normally only during prolonged starvation.
Other micronutrients include
antioxidants and
phytochemicals which are said to influence (or protect) some body systems. Their necessity is not as well established as in the case of, for instance, vitamins.
Most foods contain a mix of some or all of the nutrient classes, together with other substances such as toxins or various sorts. Some nutrients can be stored internally (e.g., the fat soluble vitamins), while others are required more or less continuously. Poor health can be caused by a lack of required nutrients or, in extreme cases, too much of a required nutrient. For example, both salt and water (both absolutely required) will cause illness or even death in too large amounts.
Carbohydrates
Main article:
Carbohydrate
Toasted bread is a cheap, high calorie nutrient (usually unbalanced, i.e., deficient in essential minerals and vitamins, largely because of removal of both germ and bran during processing) food source.
Carbohydrates may be classified as monosaccharides, disaccharides, or polysaccharides depending on the number of monomer (sugar) units they contain. They constitute a large part of foods such as
rice,
noodles,
bread, and other
grain-based products. Monosaccharides contain one sugar unit, disaccharides two, and polysaccharides three or more. Polysaccharides are often referred to as complex carbohydrates because they are typically long multiple branched chains of sugar units. The difference is that complex carbohydrates take slightly longer to digest and absorb since their sugar units must be separated from the chain before absorption. The spike in blood glucose levels after ingestion of simple sugars is thought to be related to some of the heart and vascular diseases which have become more frequent in recent times. Simple sugars form a greater part of modern diets than formerly, perhaps leading to more cardiovascular disease. The degree of causation is still not clear, however.
Simple carbohydrates are absorbed quickly, and therefore raise blood-sugar levels more rapidly than other nutrients. However, the most important plant carbohydrate nutrient, starch, varies in
its absorption. Gelatinized starch (starch heated for a few minutes in the presence of water) is far more digestible than plain starch. And starch which has been divided into fine particles is also more absorbable during digestion. The increased effort and decreased availability reduces the available energy from starchy foods substantially and can be seen experimentally in rats and anecdotally in humans. Additionally, up to a third of dietary starch may be unavailable due to mechanical or chemical difficulty.
Carbohydrates are not essential to the human diet, as they are relatively low in vitamins and minerals, and energy can be provided from excess fats and proteins in the diet.
Fiber
Main article:
Dietary fiber
Dietary fiber is a
carbohydrate (or a polysaccharide) that is incompletely absorbed in humans and in some animals. Like all carbohydrates, when it is metabolized it can produce four Calories (kilocalories) of energy per gram. But in most circumstances it accounts for less than that because of its limited absorption and digestibility. Dietary fiber consists mainly of
cellulose, a large carbohydrate polymer that is indigestible because humans do not have the required enzymes to disassemble it. There are two subcategories: soluble and insoluble fiber. Whole grains, fruits (especially
plums,
prunes, and
figs), and vegetables are good sources of dietary fiber. There are many health benefits of a high-fiber diet. Dietary fiber helps reduce the chance of gastrointestinal problems such as
constipation and
diarrhea by increasing the weight and size of stool and softening it. Insoluble fiber, found in whole-wheat flour, nuts and vegetables,
especially stimulates
peristalsis -- the rhythmic muscular contractions of the intestines which move digesta along the digestive tract. Soluble fiber, found in oats, peas, beans, and many fruits, dissolves in water in the intestinal tract to produce a gel which slows the movement of food through the intestines. This may help lower blood glucose levels because it can slow the absorption of sugar. Additionally, fiber, perhaps especially that from whole grains,is thought to possibly help lessen insulin spikes, and therefore reduce the risk of type 2 diabetes. The link between increased fiber consumption and a decreased risk of colorectal cancer is still uncertain
Fat
Main article:
Fat
A molecule of dietary fat typically consists of several
fatty acids (containing long chains of carbon and hydrogen atoms), bonded to a
glycerol. They are typically found as
triglycerides (three fatty acids attached to one glycerol backbone). Fats may be classified as
saturated or
unsaturated depending on the detailed structure of the fatty acids involved. Saturated fats have all of the carbon atoms in their fatty acid chains bonded to hydrogen atoms, whereas unsaturated fats have some of these carbon atoms
double-bonded, so their molecules have relatively fewer hydrogen atoms than a saturated fatty acid of the same length. Unsaturated fats may be further classified as monounsaturated (one double-bond) or polyunsaturated (many double-bonds). Furthermore, depending on the location of the double-bond in the fatty acid chain, unsaturated fatty acids are classified as
omega-3 or
omega-6 fatty acids.
Trans fats are a type of unsaturated fat with trans-isomer bonds; these are rare in nature and in foods from natural sources; they are typically created in an industrial process called (partial)
hydrogenation. There are eight kilocalories in each gram of fat.
Saturated fats (typically from animal sources) have been a staple in many world cultures for millennia. Unsaturated fats (e. g., vegetable oil) are considered healthier, while trans fats are to be avoided. Saturated and some trans fats are typically solid at room temperature (such as
butter or
lard), while unsaturated fats are typically liquids (such as
olive oil or
flaxseed oil). Trans fats are very rare in nature, and have been shown to be highly detrimental to human health, but have properties useful in the
food processing industry, such as rancidity resistance.[
citation needed]
Essential fatty acids
Main article:
Essential fatty acids
Most fatty acids are non-essential, meaning the body can produce them as needed, generally from other fatty acids and always by expending energy to do so. However, in humans at least two fatty acids are
essential and must be included in the diet. An appropriate balance of essential fatty acids—
omega-3 and
omega-6 fatty acids—seems also important for health, though definitive experimental demonstration has been elusive. Both of these "omega" long-chain
polyunsaturated fatty acids are
substrates for a class of
eicosanoids known as
prostaglandins, which have roles throughout the human body. They are
hormones, in some respects. The omega-3
eicosapentaenoic acid (EPA), which can be made in the human body from the omega-3 essential fatty acid
alpha-linolenic acid (LNA), or taken in through marine food sources, serves as a building block for series 3 prostaglandins (e.g. weakly
inflammatory PGE3). The omega-6 dihomo-gamma-linolenic acid (DGLA) serves as a building block for series 1 prostaglandins (e.g. anti-inflammatory PGE1), whereas arachidonic acid (AA) serves as a building block for series 2 prostaglandins (e.g. pro-inflammatory PGE 2). Both DGLA and AA can be made from the omega-6
linoleic acid (LA) in the human body, or can be taken in directly through food. An appropriately balanced intake of omega-3 and omega-6 partly determines the relative production of different prostaglandins: one reason a balance between omega-3 and omega-6 is believed important for cardiovascular health. In industrialized societies, people typically consume large amounts of processed vegetable oils, which have reduced amounts of the essential fatty acids along with too much of omega-6 fatty acids relative to omega-3 fatty acids.
The conversion rate of omega-6 DGLA to AA largely determines the production of the prostaglandins PGE1 and PGE2. Omega-3 EPA prevents AA from being released from membranes, thereby skewing prostaglandin balance away from pro-inflammatory PGE2 (made from AA) toward anti-inflammatory PGE1 (made from DGLA). Moreover, the conversion (desaturation) of DGLA to AA is controlled by the enzyme
delta-5-desaturase, which in turn is controlled by hormones such as
insulin (up-regulation) and
glucagon (down-regulation). The amount and type of carbohydrates consumed, along with some types of amino acid, can influence processes involving insulin, glucagon, and other hormones; therefore the ratio of omega-3 versus omega-6 has wide effects on general health, and specific effects on immune function and
inflammation, and
mitosis (i.e. cell division).
Protein
Most
meats such as
chicken contain all the
essential amino acids needed for humans
Main article:
Protein in nutrition
Proteins are the basis of many animal body structures (e.g. muscles, skin, and hair). They also form the enzymes which control chemical reactions throughout the body. Each molecule is composed of
amino acids which are characterized by inclusion of nitrogen and sometimes sulphur (these components are responsible for the distinctive smell of burning protein, such as the keratin in hair). The body requires amino acids to produce new proteins (protein retention) and to replace damaged proteins (maintenance). As there is no protein or amino acid storage provision, amino acids must be present in the diet. Excess amino acids are discarded, typically in the urine. For all animals, some amino acids are
essential (an animal cannot produce them internally) and some are
non-essential (the animal can produce them from other nitrogen-containing compounds). About twenty amino acids are found in the human body, and about ten of these are essential, and therefore must be included in the diet. A diet that contains adequate amounts of amino acids (especially those that are essential) is particularly important in some situations: during early development and maturation, pregnancy, lactation, or injury (a burn, for instance). A complete protein
source contains all the essential amino acids; an incomplete protein source lacks one or more of the essential amino acids.
It is possible to combine two incomplete protein sources (e.g. rice and beans) to make a complete protein source, and characteristic combinations are the basis of distinct cultural cooking traditions. Sources of dietary protein include
meats,
tofu and other
soy-products,
eggs,
legumes, and
dairy products such as
milk and
cheese. Excess amino acids from protein can be converted into glucose and used for fuel through a process called
gluconeogenesis. The amino acids remaining after such conversion are discarded.
Minerals
Main article:
Dietary mineral
Dietary minerals are the
chemical elements required by living organisms, other than the four elements
carbon,
hydrogen,
nitrogen, and
oxygen that are present in nearly all
organic molecules. The term "mineral" is archaic, since the intent is to describe simply the less common elements in the diet. Some are heavier than the four just mentioned—including several
metals, which often occur as ions in the body. Some dietitians recommend that these be supplied from foods in which they occur naturally, or at least as complex compounds, or sometimes even from natural inorganic sources (such as
calcium carbonate from ground
oyster shells). Some are absorbed much more readily in the ionic forms found in such sources. On the other hand, minerals are often artificially added to the diet as supplements; the most famous is likely iodine in
iodized salt which prevents
goiter.
Macrominerals
Many elements are essential in relative quantity; they are usually called "bulk minerals". Some are structural, but many play a role as
electrolytes.Elements with recommended dietary allowance (
RDA) greater than 200 mg/day are, in alphabetical order (with informal or folk-medicine perspectives in parentheses):
Calcium, a common electrolyte, but also needed structurally (for muscle and digestive system health, bones, some forms neutralizes acidity, may help clear toxins, and provide signaling ions for nerve and membrane functions)
Chlorine as
chloride ions; very common electrolyte; see sodium, below
Magnesium, required for processing
ATP and related reactions (builds bone, causes strong peristalsis, increases flexibility, increases alkalinity)
Phosphorus, required component of bones; essential for energy processing
Potassium, a very common electrolyte (heart and nerve health)
Sodium, a very common electrolyte; not generally found in dietary supplements, despite being needed in large quantities, because the ion is very common in food: typically as
sodium chloride, or common salt. Excessive sodium consumption can deplete
calcium and
magnesium, leading to high blood pressure and osteoporosis.
Sulfur for three essential amino acids and therefore many proteins (skin, hair, nails, liver, and pancreas). Sulfur is not consumed alone, but in the form of sulfur-containing amino acids
Trace minerals
Many elements are required in trace amounts, usually because they play a
catalytic role in
enzymes. Some trace mineral elements (RDA < title="Cobalt" href="http://en.wikipedia.org/wiki/Cobalt">Cobalt required for biosynthesis of
vitamin B12 family of
coenzymes. Animals cannot biosynthesize B12, and must obtain this cobalt-containing vitamin in the diet
Copper required component of many redox enzymes, including
cytochrome c oxidase
Chromium required for sugar metabolism
Iodine required not only for the biosynthesis of
thyroxin, but probably, for other important organs as breast, stomach, salivary glands, thymus etc. (see Extrathyroidal
iodine); for this reason iodine is needed in larger quantities than others in this list, and sometimes classified with the macrominerals
Iron required for many enzymes, and for
hemoglobin and some other pr
oteins
Manganese (processing of oxygen)
Molybdenum required for
xanthine oxidase and related oxidases
Nickel present in
urease
Selenium required for
peroxidase (antioxidant proteins)
Vanadium (Speculative: there is no established RDA for vanadium. No specific biochemical function has been identified for it in humans, although vanadium is required for some lower organisms.)
Zinc required for several enzymes such as
carboxypeptidase,
liver alcohol dehydrogenase,
carbonic anhydrase
Vitamins
Main article:
Vitamin
As with the minerals discussed above, some vitamins are recognized as essential nutrients, necessary in the diet for good health. (
Vitamin D is the exception: it can alternatively be synthesized in the skin, in the presence of
UVB radiation.) Certain vitamin-like compounds that are recommended in the diet, such as
carnitine, are thought useful for survival and health, but these are not "essential" dietary nutrients because the human body has some capacity to produce them from other compounds. Moreover, thousands of different
phytochemicals have recently been discovered in food (particularly in fresh vegetables), which may have desirable properties including
antioxidant activity
(see below); experimental demonstration has been suggestive but inconclusive. Other essential nutrients not classed as vitamins include
essential amino acids (see
above),
choline,
essential fatty acids (see
above), and the minerals discussed in the preceding section.
Vitamin deficiencies may result in disease conditions:
goitre,
scurvy,
osteoporosis, impaired
immune system, disorders of cell
metabolism, certain forms of cancer, symptoms of premature
aging, and poor
psychological health (including
eating disorders), among many others.Excess of some vitamins is also dangerous to health (notably
vitamin A), and for at least one vitamin, B6, toxicity begins at levels not far above the required amount. Deficiency or excess of minerals can also have serious health consequences.
Water
Main article:
Drinking water
A manual
water pump in
China
It is not fully clear how much water intake is needed by healthy people, although some assert that 6–8 glasses of water daily is the minimum to maintain proper hydration. The notion that a person should consume eight glasses of water per day cannot be traced to a credible scientific source.The effect of, greater or lesser, water intake on weight loss and on constipation is also still unclear. The original water intake recommendation in 1945 by the
Food and Nutrition Board of the
National Research Council read: "An ordinary standard for diverse persons is 1 milliliter for each calorie of food. Most of this quantity is contained in prepared foods." The latest dietary reference intake report by the
United States National Research Council recommended, generally, (including food sources): 2.7 liters of water total for women and 3.7 liters for men. Specifically,
pregnant and
breastfeeding women need additional fluids to stay hydrated. According to the
Institute of Medicine—who recommend that, on average, women consume 2.2 litres and men 3.0 litres—this is recommended to be 2.4 litres (approx. 9 cups) for pregnant women and 3 litres (approx. 12.5 cups) for breastfeeding women since an especially large
amount of fluid is lost during nursing.
For those who have healthy kidneys, it is somewhat difficult to drink too much water,[
citation needed] but (especially in warm humid weather and while exercising) it is dangerous to drink too little. People can drink far more water than necessary while exercising, however, putting them at risk of
water intoxication, which can be fatal. In particular large amounts of de-ionized water are dangerous.
Normally, about 20 percent of water intake comes in food, while the rest comes from drinking water and assorted beverages (
caffeinated included). Water is excreted from the body in multiple forms; including
urine and
feces,
sweating, and by
water vapor in the exhaled breath
