Vitamins: The Basics
Although vitamins are necessary to maintain good health, they are neither
sources of energy, nor significant contributors to the substance of the body.
They act principally as regulators of metabolic processes. In general, none of
them can be formed in the body - except niacin and vitamins A, D, and K. For
those who are in relatively good health, most health care providers believe that
adequate levels of the essential vitamins can be obtained through well-balanced
diets. The use of supplements to boost levels of these substances in the body
has been the focus of much research and debate.
Controversies surrounding vitamin supplements are related to many questions.
For example:
- Are supplements ever really needed?
- Will a supplement provide the same benefit when taken in an isolated form?
- What form(s) of the vitamin seems to provide the most benefits?
- How does a consumer truly know what he or she is taking?
- Are there levels that might have an adverse effect?
The body of evidence continues to grow to help sort out these issues. In the
meantime, helpful information is available through the Institute of Medicine's
Food and Nutrition Board. Essentially, they have developed a system to replace
the decades old Recommended Daily Allowances (RDAs). Dietary Reference Intakes (DRIs)
are values that can be more helpful in dietary planning and assessment. DRIs
differ from RDAs in the following ways
- First, where specific data on safety and efficacy exist, reduction in the
risk of chronic degenerative disease (e.g. osteoporosis) - rather than just
the absence of signs in a state of a deficiency (e.g. rickets) - is
incorporated in the recommendation.
- Second, where data is adequate, upper levels of nutrient intake are
established to prevent adverse effects.
- Finally, components of food that may not fit the traditional concept of an
essential nutrient, but are of possible health benefit, ultimately will be
evaluated to determine if reference intakes can be established.
Vitamin A: A fat-soluble vitamin formed from the yellow pigments of
plants (alpha, beta, and gamma carotene). It is essential for sound skin, tooth
and bone health and development; it is also important in immune system function
and the prevention of night blindness. It is found in animal fats, liver,
carrots, tubers, and green leafy vegetables. Recommended daily intakes for men
and women are 900 and 700 micrograms/day, respectively. The tolerable upper
intake level is 3000 micrograms/day, toxicity has been noted at levels of 15,000
micrograms/day for protracted periods. The role of beta-carotene as an
antioxidant in supplement form has been examined, but to date there is no
conclusive data to suggest a beneficial role in prevention of either
cardiovascular disease or macular degeneration.
B complex vitamins
Vitamin B1 (Thiamine): A water-soluble vitamin obtained in the diet
chiefly through plant sources. Deficiencies lead to impaired digestive function in mild cases and beriberi (cardiac or
nervous system dysfunction) in severe cases. In the USA, thiamine deficient
states are seen in alcoholism. Recommended daily intakes for men and women are
1.2 and 1.1 milligrams/day, respectively. Upper intake and toxic levels are not
established.
Vitamin B2 (Riboflavin): A water-soluble vitamin obtained in the diet
through dairy products, green vegetables and meats. Deficiencies lead to various
problems including skin atrophy and cataracts. Recommended daily intakes for men
and women are 1.3 and 1.1 milligrams/day, respectively. Upper intake and toxic
levels are not established.
Vitamin B6 (Pyridoxine): A water-soluble vitamin that is widely
distributed in all foods. It is involved in the metabolism of some amino acids
(building blocks of protein). In particular, it has been used to treat elevated
homocysteine levels in the blood - an emerging heart disease risk factor.
Deficiencies are rarely seen due to its ubiquitous nature; however, this can
occur in some genetic defects and medication side-effects (e.g. isoniazid used
to treat tuberculosis). Recommended daily intakes for men and women over 50
years of age are 1.7 and 1.5 milligrams/day, respectively. For all younger
adults 1.3 milligrams/day. Upper intake levels are established for all adults at
100 milligrams/day. Intake of 2000-5000 milligrams/day for months has caused
central nervous system dysfunction.
Vitamin B12 (Cyanocobalamin): A water-soluble vitamin that is largely
synthesized in the intestine, it is still required in the diet. It is important
in the formation of red blood cells and its deficiency leads to a disorder known
as pernicious anemia. Recommended daily intakes for adults are 2.4
micrograms/day. Upper intake and toxic levels are not established.
Niacin (Nicotinic Acid): A water-soluble vitamin that can be formed
from the essential amino acid tryptophan in the body. It is important in
metabolic processes. Its deficiency can lead to Pellagra, a chronic wasting
disease typically associated with dermatitis, diarrhea, and dementia.
Recommended daily intakes for men and women are 16 and 14 milligrams/day,
respectively. Upper intake levels for men and women are 35 mg/day. However,
niacin in higher doses is used to treat elevated cholesterol up to doses of 6000
mg/day. High doses can lead to liver damage, high blood sugar, and elevated uric
acid (seen in gout).
Folacin (Folate/Folic Acid): A water-soluble vitamin that is
important in the normal functioning of bone marrow to make blood cells. It can
be found in green leafy vegetables, liver, and yeast. Deficiencies can lead to
anemia and resultant neurologic deficits. Recommended daily intake for men and
women is 400 micrograms/day. Upper intake level is 1000 micrograms/day.
Biotin (Vitamin H): A water-soluble vitamin essential in the
metabolism of fats and carbohydrates. It is present to a large degree in some
organ meats and dairy products. Deficiency of biotin in children leads to
alopecia, mental and physical developmental problems, anemia and impaired
immunity. Recommended daily intake for men and women is 30 micrograms/day. Upper
intake and toxic levels are not established.
Pantothenic Acid: A water-soluble vitamin important in general
metabolism. Its deficiency has never been observed in humans. Recommended daily
intakes for men and women are 5 milligrams/day. Upper intake and toxic levels
are not established. Vitamin C (Ascorbic Acid): A water-soluble vitamin important in the
synthesis of collagen (necessary for wound healing) and an in the absorption of
iron in the diet. It is abundant in most fresh fruit and vegetables. Its
deficiency leads to lowered resistance to infections, dental disease, anemia and
scurvy in extreme cases. Recommended daily intakes for men and women are 90 and
75 milligrams/day, respectively. Upper intake levels are 2000 milligrams/day. No
toxic level has been established, but higher levels can lead to impaired
absorption of vitamin B12, increased estrogen levels in women taking
supplemental estrogen and cause formation of kidney stones. In general,
controlled studies have not been able to reliably demonstrate differences in
occurrence, severity or duration of colds when comparing it to a placebo.
Vitamin D: A fat-soluble vitamin that is essential in calcium and
phosphorous metabolism. It can be generated in the skin through ultraviolet
light exposure and can be found in fish, dairy products and yeasts. Deficiencies
of this vitamin in severe cases can cause rickets and osteomalacia (softening of
the bones). Recommended daily intake for men and women over age 50 is 10
micrograms/day (400 IUs). Younger adults otherwise require 5 micrograms/day (200
IUs). Total body sun exposure can generate 10000 IUs/day. Upper intake levels
are 50 micrograms/day (4000 IUs). Toxic levels occur at 500 micrograms/day
(40000 IUs).
Vitamin E (alpha tocopherol and other forms): A fat-soluble vitamin
that seems to serve as an antioxidant (its exact biochemical role is not fully
known) and is widely distributed in food. Deficiencies are very rare and are
usually only seen in cases of poor absorption from the bowel (e.g. some liver
diseases) and can lead to severe neurologic problems. Recommended daily intake
for men and women is 15 milligrams/day. Upper intake levels are 1000
milligrams/day. The body of evidence does not currently support dietary vitamin
E supplementation to decrease the risk of heart attack, stroke, cancer or
Alzheimer's.
Vitamin K: A fat-soluble vitamin that aids in blood coagulation. It is
found in fats, green vegetables, and various grains. Deficiencies are very
uncommon, but can lead to hemorrhaging. Currently, there are no dietary
recommendations.
Summary
In summary, taking vitamins in supplement form has not yet been
consistently shown to confer benefit over a well-balanced diet. However, it is felt
that benefits outweigh risks in multivitamin supplementation for patients who
are pregnant, nursing, have a chronic illness, or who have limited dietary
intakes. If you are taking supplements, review them with your
physician. Extensive information can be found through the website for the
National Academy of Sciences (www.nas.edu) or through your local library.
©
Copyright 1995-2005 The Cleveland Clinic Foundation. All rights reserved
index#9763
| 
