Daily Requirements
With a few exceptions, the body is unable to make vitamins therefore
they must be supplied in the daily diet or through supplements. One exception
is vitamin D, which can be produced in the skin when the skin is exposed
to sunlight. Another vitamin, vitamin K, is not made by the human body
but is formed by micro-organisms that normally flourish in the intestinal
tract only when green, leafy vegetables and vegetable oils are ingested.
The body's vitamin requirements are expressed in terms of recommended
dietary allowances, or RDA. These allowances are the amount of essential
nutrients that, if acquired daily, are considered to be sufficient to
meet the known nutritional needs of most healthy persons. In the United
States, the RDA values are established by the Food and Nutrition Board
of the National Academy of Sciences/National Research Council (NAS/NRC).
In addition, two agencies of the United Nations, the Food and Agriculture
Organisation and the World Health Organisation develop RDA for different,
world-wide population groups.
In the past, the strength of a vitamin or the amount of the vitamin necessary
to produce a certain effect in the body was often expressed in terms of
international units, abbreviated IU. The unit corresponds to a weight
of the purified vitamin, and its value differs from one vitamin to another.
However today, the strength of a vitamin is generally expressed by its
metric equivalent either in micrograms or milligrams.
How Vitamins Work
In the body, proteins, carbohydrates, and fats combine with other substances
to yield energy and build tissues. These chemical reactions are catalysed,
or accelerated, by enzymes produced from specific vitamins, and they take
place in specific parts of the body.
The B vitamins act as coenzymes, compounds that unite with a protein component
to form an active enzyme. The enzyme then acts as a catalyst in the chemical
reactions that transfer energy from the basic food elements to the body.
It is not known whether vitamin C acts as a coenzyme.
When a person takes in more water-soluble vitamins than are needed, small
amounts are stored in body tissue, but most of the excess is excreted
in urine. Because water-soluble vitamins are not stored in the body in
appreciable amounts, a daily supply is essential to prevent depletion.
Fat-soluble vitamins seem to have highly specialised functions. The intestine
absorbs fat-soluble vitamins, and the lymph system carries these vitamins
to the different parts of the body. Fat-soluble vitamins are involved
in maintaining the structure of cell membranes. It is also believed that
fat-soluble vitamins are responsible for the synthesis of certain enzymes.
The body can store larger amounts of fat-soluble vitamins than of water-soluble
vitamins. The liver provides the chief storage tissue for vitamins A and
D, while vitamin E is stored in body fat and to a lesser extent in reproductive
organs. Relatively little vitamin K is stored. Excessive intake of fat-soluble
vitamins, particularly vitamins A and D, can lead to toxic levels in the
body.
Many vitamins work together to regulate several processes within the body.
A lack of vitamins or a diet that does not provide adequate amounts of
certain vitamins can upset the body's internal balance or block one or
more metabolic reactions.
Further Reading : -
Daily Requirements and How Vitamins
Work
Sources of vitamins and the Recommended
Daily Allowances (RDA)
Types of vitamins - Vitamin A, Vitamin
B Complex, Vitamin B1, or thiamine,
Vitamin B2, Niacin,
Vitamin B6, Vitamin
B12, Folic acid, Pantothenic
acid, Biotin, Vitamin
C, Vitamin D, Vitamin
E, Vitamin K