High blood levels of homocysteine, an amino acid (one of protein’s building blocks), have been linked to an array of health problems. Certain B vitamins—notably B12 and folic acid, also called folate—lower homocysteine.

Claims, purported benefits: Prevents cardiovascular disease, cognitive problems, and other disorders.

Evidence: While early research had linked high blood levels of homocysteine to heart disease and strokes, more recent studies have undercut the proposed cardiovascular benefits of reducing homocysteine. Recently, scientists have been focusing increasingly on the possible role of elevated homocysteine in age-related "cognitive impairment" and dementia, including Alzheimer’s disease. In 2007 at least four studies on this subject have yielded promising results. Some have emphasized the potentially adverse effect of homocysteine, while others have focused more on the potentially protective role of folic acid and/or vitamin B12.

Bottom line: Eat foods rich in these B vitamins, which promote health in many ways. If you eat at least five (better yet, nine) vegetables and fruits a day, as well as fortified grain products and small servings of lean meat, poultry, and fish, you’ll get lots of B’s. On top of that, a basic multivitamin will provide extra folic acid and B12. Because people over 50 tend to have reduced absorption of B12, they should consume at least 6 to 15 micrograms daily (the RDA is just 2.4 micrograms) from foods and supplements. Most multis marketed for seniors have 25 or 30 micrograms.

The new studies suggest that in people who have adequate blood levels of B12, taking additional folic acid may help protect against cognitive decline—but that in those with low B12 stores, extra folic acid may actually be detrimental. So if you are over 50, talk to your doctor about having your B12 level measured.