Nutrition is a science that deals with nutrients and other food substances, and with how the body assimilates them. The extremely complex processes that nutrients undergo in the body—how they affect one another, how they are broken down and released as energy, and how they are transported and used to rebuild countless specialized tissues and sustain the overall health of the individual—are understand only approximately.
The word nutrient or food factor is used for specific dietary constituents such as proteins, vitamins and mineral. Dietetics is the practical application of the principles of nutrition; it includes the planning of meals for the healthy as well as the sick. While attention was concentrated on nutrition-deficiency diseases during the first few decades of the century; the science of nutrition was extending its influence into other fields—agriculture, animal husbandry, economics and sociology. This led to the ‘Green revolution’ and ‘White revolution’ in India and increased food production.
The science of human nutrition is mainly concerned with defining the nutritional requirements for the promotion, protection and maintenance of health in al groups of the population. Such knowledge is necessary in order to assess the nutritional adequacy of diets for growth of infants, children and adolescents, and for maintenance of health in adults of both sexes and during pregnancy and lactation in women. A balanced diet is defined as one which contains a variety of foods in such quantities and proportions that the need for energy, amino acids, vitamins, minerals, fats, carbohydrate and other nutrients is adequately met for maintaining health, vitality and general well-being and also makes a small provision for extra nutrition to withstand short duration of leanness.
In general, scientists recommend that a person should eat a variety of foods; maintain ideal weight; avoid too much fat, saturated fat, and cholesterol; eat foods with adequate starch and fiber; avoid too much sugar; avoid too much sodium; and drink alcohol only in moderation.
Foods can be roughly classified into breads and cereals; pulses, or legumes; tubers, or starchy roots; vegetables and fruits; meat, fish, and eggs; milk and milk products; fats and oils; and sugars.
Breads and cereals include wheat, rice, corn, and millet. They are high in starches and are easily procured sources of calories. Pulses, or legumes, include a wide variety of beans, peas, lentils, and grains, and even peanuts. All are rich in starch but might provide considerably more protein than do cereals or tubers.
Tubers and starch roots include various kings of potato, cassava, yam, and taro. They are rich in starch and relatively low in protein content, but provide a variety of minerals and vitamins. Vegetables and fruits are a direct source of many minerals and vitamins lacking in cereal diets, especially vitamin V from citrus fruits and vitamin A from the carotene of leafy vegetables and carrots.
Meat, fish, and eggs supply all the essential amino acids that the body needs to assemble its own proteins. Milk and milk products include whole milk, cheese, yoghurt, and ice cream, all of which are well known for their abundant protein, phosphorus, and especially calcium. Fats and oils include butter, lard, suet, and vegetable oils. They are all high in calories, but, apart from butter and such vegetable oils as red palm oil, they contain few nutrients. Sugars, preserves, and syrups are heavily consumed in more affluent countries, where they make up a large portion of the carbohydrate intake.
The science of nutrition is still far from explaining how foods affect certain individuals. Why some people can discontinue eating at a certain point and why others eat obsessively, is still a mystery. Researchers have recently found that shortly after ingestion, foods influence the release of important brain chemicals and that carbohydrate foods, in particular, trigger the release of serotonin, which, in turn, suppresses the desire for carbohydrates. Such a mechanism might have evolved to prevent people from glutting themselves on carbohydrates and failing to procure harder-to-find protein.
Until recent times, carbohydrate foods were far more accessible than protein. Serotonin is believed to work in complex relationships with insulin and several amino acids, especially tryptophan, all of which participate in monitoring the appetite for various food types. In the same area of research, nutrition experts are trying to unravel the relationship between diabetes and obesity and the role that sweets play for people with these conditions.