Too much phosphorus has a negative effect on how well calcium is absorbed. Megadoses of fat-soluble vitamins A, D, and K can easily reach toxic levels. The NAS has recognized that vitamins and minerals are no longer just to prevent deficiencies but may actually have a role in disease prevention. The biggest news with vitamins and prevention is the finding that women who get 200-300mcg of Folate per day from their diet, plus take a supplement of Folic Acid, can reduce their risk by half of having babies with neural tube defects such as spina bifida. Fruits and greens are good sources of folate.
Grains are now fortified with folate.
Folate isn't just for the pregnant. Evidence continues to mount that the B vitamin can help prevent heart disease. The evidence is still preliminary but Folate, B6 and possibly B12 have been shown to lower levels of homocysteine, an amino acid in the blood that plays a role in narrowing crucial arteries that circulate blood to the brain and with the heart. Researchers believe that Americans would need to double their Folate intake from 200mcg to 400mcg to keep their homocysteine levels low enough to be beneficial. If you don't consume green vegetables like spinach, broccoli, asparagus, or dried beans or orange juice, then you may want to consider taking a supplement of 400mcg daily.