Why: No supplement can compensate fully for a poor diet or lifestyle, but these supporting actors (or "cofactors") can help make the processes in your body work optimally. These nutrients are also in increased demand whenever your body's physiological processes are stressed, such as in response to intense training, dieting, periods of growth, or immune system attacks, just to name a few.

What: An intelligently designed multi should provide active ingredients shown to significantly elevate circulating levels of the biologically active forms of the following vitamins and minerals:

Start reading about multis, and you'll sometimes hear that you shouldn't consume this or that vitamin or mineral in combination with others. Calcium decreases the absorption of iron, for example, and zinc can decrease absorption of chromium and magnesium. However, such statements are just marketing-driven spin to justify why ingredients that take up a lot of space in a capsule or tablet haven't been included in a particular supplement.

Why? Because nearly every one of the nutrients can decrease the absorption of one nutrient…and increase the absorption of something else. Calcium increases vitamin B12, to return to our earlier example, and zinc increases the absorption of selenium. Also, having low circulating zinc levels can decrease the absorption and utilization of vitamin A and folate, and decrease the amount of vitamin E in the body.

The point is that the human body has a very large number of examples where competition for absorption occurs. It doesn't mean that absorption doesn't occur at all, though; just that it can be less efficient. The human body has synergistic feed-forward mechanisms whereby circulating levels of one vitamin or mineral will affect the bioavailability of others. Therefore, take a multi for what it is: a good back-up for your diet. Nothing more.

How: Take half a dose with breakfast and another half-dose with another meal later in the day. Even better, split a full day's dose across three or four servings. If you take a high-potency multi on an empty stomach, be forewarned that not only will the absorption of many of the nutrients be reduced, there's also a very good chance you'll begin to feel nauseated within about 30 minutes of taking the supplement.

Don’t just buy some store-brand multi and hope for the best. Use this guide to buy the best product for you, and to take it right!