Vitamins are one of two groups of substances classified as micronutrients
(the other is minerals) that are essential in human nutrition. Vitamins
are organic, meaning they contain the element carbon and are found in plant
and animal substances in small amounts. We obtain them by eating the
plants and animals that make them. Most vitamins cannot be manufactured
in our bodies; exceptions are some of the B vitamins, which can be made by
our intestinal bacteria, and occasional biochemical conversions from a
precursor to the active form of the required vitamin, as when beta-carotene
is converted to vitamin A. Vitamins, however, are not sources of energy
(calories). We obtain our energy from the macronutrients— carbohydrates,
proteins, and fats. In fact, vitamins help to convert these macronutrients to
more bioavailable, or metabolically useful, forms. Vitamins function
principally as coenzymes (in collaboration with enzymes) for a variety of
metabolic reactions and biochemical mechanisms within our many bodily
systems. Each enzyme is specific to one biochemical reaction. Enzymes are
catalysts—that is, they speed up specific chemical reactions that would
proceed very slowly, if at all, without vitamins.
Vitamins themselves are not part of our body tissues; they are not building
blocks but helpers in metabolism. We cannot live on vitamins but need food
that provides energy and helps form the actual tissues of our body. In fact,
we need food and certain minerals to best absorb any vitamin supplements
we take. Vitamins are essential for growth, vitality, and health and are
helpful in digestion, elimination, and resistance to disease. Depletions or
deficiencies can lead to a variety of both specific nutritional disorders and
general health problems, according to what vitamin is lacking in the diet.
Historically, vitamins were discovered primarily through the deficiency
diseases caused by their absence in the diet. Many vitamins were discovered
in naturally occurring human experiments and in laboratory studies using
rats, mice, or birds fed specific diets lacking certain essential foods. It was
not until the early 1900s, however, that Sir Frederick Galen Hopkins
published a paper suggesting that there were some “accessory nutrients”
needed in the human diet in small quantities to maintain good health. In
1911, the first vitamin was isolated from rice polishings (this vitamin was
thiamin). It was found to be an amine (containing nitrogen) and thus was
termed vitamine—that is, “amine essential for life.” Other vitamins were
ultimately discovered that were not amines, and the term was converted to
its current form, vitamin.
Back in the 1500s, records were first made of nature’s vitamin
experiments and vitamin deficiency disease. The British Royal Navy during
Queen Elizabeth’s reign was fed a diet of dried or salted meat or fish,
biscuits, butter, cheese, and beer. Many men developed a serious and painful
disease, and more than 10,000 died during a 20-year period. The disease,
named scurvy, was associated with rotten gums and tooth loss, painful jaws,
swollen legs, aches and pains, and easy bruising. When the men were fed
raw potatoes, watercress, raisins, or scurvy grass—all sources of vitamin C—
their pains and other symptoms of the disease reversed rapidly. Likewise,
beriberi, now known to be a thiamin deficiency, was in the late 1800s a
debilitating and potentially deadly disease among Japanese sailors who ate a
diet deficient in whole-grain foods; contributing particularly to this disease
was the processing that removed the outer covering of rice. Dietary
adjustment was helpful in curing this disease. The simple addition of wheat
aboard sailing vessels was enough to shift the prevalence of this problem on
sailing vessels, although it took much longer for broad cultural trends to
shift. When rats were experimentally fed only proteins, starches, sugars, and
fats—without milk products or vegetables—their eyes were affected and they
failed to grow. Vitamin A was then isolated from milk, butter, liver, and egg
yolks, and this substance was shown to reverse the eye problem and promote
growth in the rats. There are many more stories regarding experiments and
discoveries of specific vitamins. Various experiments have determined the
U.S. government’s development of the commonly quoted recommended
dietary allowances (RDAs), which are discussed in this chapter.
Vitamins are usually classified into two categories: those that are water
soluble and those that are fat soluble. They are further categorized by letters,
groups, and individual chemical names.
Water-soluble vitamins include mainly the many B vitamins and vitamin
C. They are most likely to be lost when foods are cooked, because they are
especially sensitive to heat. But water-soluble vitamins can also be lost from
raw foods because they can also be sensitive to air, light, and the passage of
time. The more time that passes between harvest and eating, the more likely
the loss of water-soluble vitamins, so it is good to consume produce that is
fresh and locally grown. Commonly found in the vegetable foods, these
vitamins are contained less so in most animal sources. The water-soluble
vitamins are not stored in the body to a very large degree, so they are needed
regularly in our diets; this makes them much less potentially toxic than the
fat-soluble vitamins, which are stored when taken in higher dosages. Most of
the water-soluble vitamins act in the body as coenzymes in combination with
an inactive protein to make an active enzyme.
Fat-soluble vitamins are vitamins A, D, E, and K, which are found in the
lipid component of both vegetable-and animal-source foods (the source
varies for each vitamin). Grains, seeds, and nuts contain vitamin E, and
many vegetables contain vitamin A or, more commonly, its precursor betacarotene. These fat-soluble vitamins can be stored in the body tissues, so we
can function for longer periods of time without obtaining them from the diet
than we can without the water-soluble ones. For this reason, toxic levels can
occur more easily from regular increased intake of these vitamins, especially
vitamin A; vitamins D and K can also cause problems when taken in high
dosages. Toxicity is less likely with vitamin E, because it is used readily by
the body as an antioxidant to help protect against the harmful by-products of
metabolism and against outside pollutants. Vitamin A adds cellular
protection as well as resistance to infection, while vitamin D aids in
absorption of calcium from the gut and thus is important to skeletal health.
Vitamin K helps make factors crucial to blood clotting to prevent bleeding.