The immune system defends the body against invading microbes such as bacteria and viruses. Many supplement manufacturers claim their products support immunity. But so far, there is not enough scientific evidence to back up the claims. The reason, according Michael N. Starnbach, Ph.D., the Harvard Medical School expert who edited The Truth About Your Immune System, is that science has not yet determined what level of immune system cells will best help the body resist disease. While some proponents of vitamins and supplements claim that boosting the number of immune cells improves immunity, that link has yet to be established. The variety of immune cells is vast, and their interactions remain largely unknown.

The Truth About Your Immune System explains what scientists do know about human immunity and also describes areas under investigation, including the development of new vaccines and the influence of lifestyle on immunity. So far, the report says, the lifestyle factors that appear to help keep the immune system in shape are the same ones that promote overall health, such as these:

avoiding tobacco eating a diet high in fruits, vegetables, and whole grains, and low in saturated fat exercising regularly maintaining a healthy weight controlling your blood pressure drinking only in moderation (if at all) getting adequate sleep taking steps to avoid infection, such as washing your hands frequently and preparing foods safely.

Dr. Starnbach, the editor of the 43-page report, is a professor of microbiology and molecular genetics at Harvard Medical School.

The Truth About Your Immune System: What you need to know is available for $16 from Harvard Health Publications, the publishing division of Harvard Medical School. Order it online at health.harvard/E or by calling 1-877-649-9457 (toll free).

health.harvard

The responses across all three years were similar except for three: Americans were less likely to pay more for healthier foods, less knowledgeable about healthy menu items and more likely to consider healthy items bland-tasting in 2006 than in 2004, said Borradaile.

On a scale of one to seven, respondents in 2006 rated the following as the most appealing incentives for making healthier meal choices: better-tasting health foods, lower prices for healthy foods, more convenient availability of healthy foods and greater availability of healthy foods.

In recent years, restaurants have made efforts to increase the number and quality of healthy offerings. For example, McDonald's now offers grilled chicken sandwiches, fruit and yogurt parfaits and apple slices with low-fat caramel dip. Burger King pledged last month to offer kids healthier food options, such as apples cut to resemble french fries and a kids' meal with low-fat, flame-broiled chicken tenders, unsweetened applesauce and low-fat milk. The restaurant chain T.G.I. Friday's recently unveiled smaller portion sized meals at lower prices.

Restaurant chains should be commended for increasing the number of healthy offerings; continued efforts are needed to make healthy food attractive to the palate and pocket, Foster said.

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