There are seven major classes of nutrients: carbohydrates, fats, fiber, minerals, protein, vitamin,
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) and energy. Some of the structural material can be used to generate energy internally,
and in either case it is measured in joules or calories (sometimes called "kilocalories" and on
other rare occasions 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 simplemonosaccharides (glucose, fructose, galactose) to
complex polysaccharides (starch). Fats are triglycerides, made of assortedfatty
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
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 le