Nutrition is the provision of essential nutrients necessary to support life and health. In general, people can survive for about two weeks without food, depending on stored body fat and muscle mass.
There are seven major classes of nutrients: carbohydrates, fats, dietary fiber, minerals, proteins, vitamins, and water.
These nutrient classes can be categorized as either macronutrients (needed in relatively large amounts) or micronutrients (needed in smaller amounts). The macronutrients are carbohydrates, fats, fiber, proteins, and water. The micronutrients are minerals and vitamins.
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) and energy.
Carbohydrates and proteins provide approximately 4 kcal of energy per gram, while fats provide 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.
Carbohydrates are found in foods such as rice, noodles, bread, and other grain-based products.
Carbohydrates are classified as monosaccharides, disaccharides, or polysaccharides depending on the number of monomer (sugar) units they contain.
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 longer to digest and absorb since their sugar units must be separated from the chain before absorption.
Fats - classified as saturated or unsaturated depending on the detailed structure of the fatty acids involved.
Unsaturated fats may be further classified as monounsaturated or polyunsaturated. Trans fats are a type of unsaturated fat; these are rare in nature and in foods from natural sources; they are typically created in an industrial process called (partial) hydrogenation.
Many studies have shown that unsaturated fats, particularly monounsaturated fats, are best in the human diet. Saturated fats, typically from animal sources, are next, 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, but have properties useful in the food processing industry.
Dietary fiber is a carbohydrate (or a polysaccharide) that is incompletely absorbed in humans 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.
Proteins are the basis of many body structures (e.g. muscles, skin, and hair). They also form the enyzmes which catalyse chemical reactions throughout the body. Each molecule is composed of amino acids which are characterized by containing 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). Amino acids are soluble in the digestive juices within the small intestine, where they are absorbed into the blood. Once absorbed they cannot be stored in the body, so they are either metabolised as required or excreted in the urine.
Twenty two amino acids can be 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 a common misconception that a vegetarian diet will be insufficient in essential proteins!
Both vegetarians and vegans of any age and gender, with a healthy diet, can flourish throughout all stages of life, although the latter group typically need to pay more attention to their nutrition than the former.
Minerals: 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 an old one, 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. Minerals are often artificially added to the diet as supplements; the most famous being iodine in iodized salt which prevents goitre.
- Magnesium, required for processing ATP (AdenosineTriPhosphate) and related reactions (builds bone, causes strong peristalsis, increases flexibility, increases alkalinity). Approximately 50% is in bone, the remaining 50% is almost all inside body cells, with only about 1% located in extracellular fluid. Food sources include oats, buckwheat, tofu, nuts, caviar, green leafy vegetables, legumes, and chocolate.
- Phosphorus, required component of bones; essential for energy processing. Approximately 80% is found in inorganic portion of bones and teeth. Phosphorus is a component of every cell, as well as important metabolites, including DNA, RNA, ATP, and phospholipids. Food sources include cheese, egg yolk, milk, meat, fish, poultry, whole-grain cereals, and many others.
- Potassium, a very common electrolyte (heart and nerve health). With sodium, potassium is involved in maintaining normal water balance, osmotic equilibrium, and acid-base balance. In addition to calcium, it is important in the regulation of neuromuscular activity. Food sources include bananas, avocados, vegetables, potatoes, legumes, and mushrooms.
- 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
Vitamins: 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 UV radiation.) Vitamin deficiencies may result in disease conditions: goitre, scurvy, osteoporosis, impaired immune system, disorders of cell metabolism, certain forms of cancer, symptoms of premature ageing, 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. Both deficiency, as well as excess, of minerals can also have serious health consequences. For those who have healthy kidneys, it is somewhat difficult to drink too much water, 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.