There are four substances we refer to as food which can be used to produce energy: carbohydrates, fats, proteins and alcohol. Alcohol however is not considered a basic nutritional component of food.

1 gram of Carbohydrates contains 3.75 calories.
1 gram of fat contains 9 calories.
1 gram of protein contains 4 calories.
1 gram of alcohol contains 7 calories.

Carbohydrates

Carbohydrates (literally hydrates of carbon) are chemical compounds which act as the primary biological means of storing or consuming energy; other forms being via fat and protein. Relatively complex carboyhydrates are known as polysaccharides.The simplest carbohydrates are monosaccharides, which are small straight-chain aldehydes and ketones with many hydroxyl groups added, usually one on each carbon except the functional group. Other carbohydrates are composed of monosaccharide units, and break down under hydrolysis. These may be classified as disaccharides, oligosaccharides, or polysaccharides, depending on whether they have two, several, or many monosaccharide units.

Carbohydrates or saccharides (Greek sakcharon, "sugar") are simple molecules that are straight-chain aldehydes or ketones with many hydroxyl groups added, usually one on each carbon atom that is not part of the aldehyde or ketone functional group. Carbohydrates are the most abundant biological molecules, and fill numerous roles in living things, such as the storage and transport of energy (starch, glycogen) and structural components (cellulose in plants, chitin in animals). Additionally, carbohydrates and their derivatives play major roles in the functioning of the immune system, fertilization, pathogenesis, blood clotting, and development.

The basic carbohydrate units are called monosaccharides, such as glucose, galactose, and fructose. The general chemical formula of an unmodified monosaccharide is (C·H2O)n, where n is any number of three or greater. All carbohydrates have a hydrogen to oxygen ratio of 2:1.Monosaccharides can be linked together in almost limitless ways. Two joined monosaccharides are called disaccharides, such as sucrose and lactose. Carbohydrates containing between about three to six monosaccharide units are termed oligosaccharides; anything larger than this is a polysaccharide. Polysaccharides, such as starch, glycogen, or cellulose, can reach many thousands of units in length.

Fats

In biochemistry, fat is a generic term for a class of lipids. Fats are produced by organic processes in animals and plants. All fats are insoluble in water and have a density significantly below that of water (i.e. they float on water.) Fats that are liquid at room temperature are often referred to as oil. Most fats are composed primarily of triglycerides; some monoglycerides and diglycerides are mixed in, produced by incomplete esterification. These are extracted and used as an ingredient.

Products with a lot of saturated fats tend to be solid at room temperature, while products containing unsaturated fats, which include monounsaturated fats and polyunsaturated fats, tend to be liquid at room temperature. Predominantly saturated fats (solid at room temperature) include all animal fats (e.g. milk fat, lard, tallow), as well as palm oil, coconut oil, cocoa fat and hydrogenated vegetable oil (shortening). All other vegetable fats, such as those coming from olive, peanut, maize (corn oil), cottonseed, sunflower, safflower, and soybean, are predominantly unsaturated and remain liquid at room temperature. However, both vegetable and animal fats contain saturated and unsaturated fats. Some oils (such as olive oil) contain in majority monounsaturated fats, while others present quite a high percentage of polyunsaturated fats (sunflower, rape).

Proteins

A protein is a complex, high molecular weight organic compound that consists of amino acids joined by peptide bonds. Protein is essential to the structure and function of all living cells and viruses. Many proteins are enzymes or subunits of enzymes. Other proteins play structural or mechanical roles, such as those that form the struts and joints of the "cytoskeleton." Proteins are also nutrient sources for organisms that do not produce their own energy from sunlight. Proteins differ from carbohydrates chiefly in that they contain much nitrogen and a little bit of sulfur, besides carbon, oxygen and hydrogen. Proteins are a primary constituent of living things.

In carnivores protein is one of the largest component of the diet. The metabolism of proteins by the body releases ammonia, an extremely toxic substance. It is then converted in the liver into urea, a much less toxic chemical, which is excreted in urine. Some animals convert it into uric acid instead.

Protein nutrition in humans
In terms of human nutritional needs, proteins come in two forms: complete proteins contain all eight of the amino acids that humans cannot produce themselves, while incomplete proteins lack or contain only a very small proportion of one or more. Humans' bodies can make use of all the amino acids they extract from food for synthesizing new proteins, but the inessential ones themselves need not be supplied by the diet, because our cells can make them ourselves. When protein is listed on a nutrition label it only refers to the amount of complete proteins in the food, though the food may be very strong in a subset of the essential amino acids. Animal-derived foods contain all of those amino acids, while plants are typically stronger in some acids than others. Complete proteins can be made in an all vegan diet by eating a sufficient variety of foods and by getting enough calories. It was once thought that in order to get the complete proteins vegans needed to do protein combining by getting all amino acids in the same meal (the most common example is eating beans with rice) but nutritionists now know that the benefits of protein combining can be achieved over the longer period of the day. Ovo-lacto vegetarians usually do not have this problem, since egg's white and cow's milk contain all essential amino acids. Peanuts, soy milk, nuts, seeds, green peas, Legumes, the alga spirulina and some grains are some of the richest sources of plant protein.

All eight essential amino acids must be part of one diet in order to survive and are needed in a fixed ratio. A shortage on any one of these amino acids will constrain the body's ability to make the proteins it needs to function.

Different foods contain different ratios of the essential amino acids. By mixing foods that are rich in some amino acids with foods that are rich in others, one can acquire all the needed amino acids in sufficient quantities. Omnivores typically eat a sufficient variety of foods that this is not an issue, however, vegetarians and especially vegans should be careful to eat appropriate combinations of foods (e.g. nuts and green vegetables) so as to get all the essential amino acids in sufficient quantities that the body may produce all the proteins that it needs.

Protein deficiency can lead to symptoms such as fatigue, insulin resistance, hair loss, loss of hair pigment (hair that should be black becomes reddish), loss of muscle mass (proteins repair muscle tissue), low body temperature, and hormonal irregularities. Severe protein deficiency is fatal.

Excess protein can cause problems as well, such as causing the immune system to overreact, liver dysfunction from increased toxic residues, possibly bone loss due to increased acidity in the blood, and foundering (foot problems) in horses.

Proteins can often figure in allergies and allergic reactions to certain foods. This is because the structure of each form of protein is slightly different, and some may trigger a response from the immune system while others are perfectly safe. Many people are allergic to casein, the protein in milk; gluten, the protein in wheat and other grains; the particular proteins found in peanuts; or those in shellfish or other seafoods. It is extremely unusual for the same person to adversely react to more than two different types of proteins.