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In English-speaking countries, surveys show, about 53 percent of the adult population takes one or more supplements, and about 46 percent of the adult population takes a multivitamin every day. In the United States and Canada, even junk food contains vitamins, because of the B vitamins (and iron) added to white flour and corn meal. There is no doubt that people who have bad diets, who don't have enough money for fruits and vegetables, or who have conditions like pernicious anemia, high blood pressure, diabetic neuropathy, obesity, or certain genetic variations that affect the way the body makes enzymes to process vitamins benefit from supplements, but not everybody benefits from multivitamins equally.
Let's start with the scientific evidence in favor of taking multivitamins:
- One study found that men who had taken multivitamins for at least three years were 26 percent less likely to die of cardiovascular disease over the next 20 years, even if they stopped taking the multivitamins for many years. The same study found that women who had taken multivitamins for three years were 44 percent less likely to die of cardiovascular disease over the next 20 years.
- Another study of women of ages between 50 and 80 who had invasive breast cancer were 30 percent less likely to die of the disease over the next seven years if they took daily multivitamins.
- Men who take Centrum Silver daily were found to have a 12 percent lower risk of being diagnosed with any form of cancer in one large survey study, but the study found no benefit for preventing prostate cancer.
- Men who take Centrum Silver were found to have a 9 percent lower risk of developing cataracts in a study completed in the 1990's. Centrum has changed the formula since the study, but since the changes were to add eye-healthy lutein and lycopene, probably the product is as effective today as it was 20 years ago.
Not every study has found that taking more vitamins is always a good thing. The Centrum Silver study, for example, found that taking a 500 mg capsule of vitamin C every day canceled out the benefits of taking Centrum Silver for eye health. And, famously, a Finnish study of using beta-carotene supplements for preventing lung cancer in smokers was stopped when the scientists noticed that users of the supplement were experiencing higher rates of cancer.
As a general rule, getting small doses of vitamins you may or may not need (that is, without a diagnosis of deficiency) is helpful, but taking large doses of vitamins for which you do not have a diagnosed deficiency tends to have side effects.
Some people need specific vitamins for specific reasons. Obese people tend to store vitamin D in body fat. They often need vitamin D supplements even if they get regular sun. People who have a mutation in the gene for making methylenetetrahydrofolate reductase need supplemental methylfolate, because their bodies can't use regular folate. People who have a condition called pernicious anemia need vitamin B12, initially by injection, and then as a supplement. People who have iron-deficiency anemia need iron, but iron deficiency is always something that should be diagnosed by a blood test.