About two out of five Americans take a vitamin or mineral supplement regularly. But is this money well spent? Are you already getting the nutrients you need in your diet? Is it enough to take a daily multivitamin, or should you consider adding certain vitamins or minerals and perhaps cutting down on others? Do you know the right amount you need based on your age and gender? Could you be courting health troubles by consistently getting too little — or too much — of certain nutrients?

Many people still lack sufficient levels of certain vitamins and minerals. Over time, this contributes to chronic illnesses such as heart disease, osteoporosis, and cancer. At the opposite end of the spectrum, entirely avoidable ailments are cropping up in some health-conscious people who take dietary supplements in excess.

Understanding your body’s requirements can be confusing. Some people, particularly older adults, are deficient in vitamin B₁₂ because they have trouble absorbing this vitamin from food. A lack of B₁₂ can cause memory loss, dementia, and numbness of the arms and legs. Large amounts of supplemental vitamin A can actually be harmful to bones and increase the risk of hip fracture. And the recommended allowance of vitamin D (which can reduce the number of non-spinal fractures) for an individual 71 years of age is triple the amount recommended for a person at age 50. But long-term studies and new recommendations from the Institute of Medicine can shed light on these questions.

Take, for example, zinc, which helps form many enzymes and proteins, creates new cells and is necessary for a healthy immune system and wound healing. The recommended dietary allowance (RDA, also known as adequate intake, or AI) for men is 11 milligrams per day, and for women is 8 milligrams per day. Good sources of zinc include red meat, poultry, oysters and some other seafood, fortified cereals, beans, and nuts. Because vegetarians absorb less zinc, experts suggest that they get twice the recommended requirement of zinc from plant foods. When taken with certain antioxidants, zinc may delay the progression of age-related macular degeneration.

It’s clear that our knowledge of nutritional needs is evolving, not definitive. New studies can shake the foundation of what researchers once thought was a healthy choice. For example, at one time research indicated that taking vitamin E supplements might boost your brainpower, reduce your risk of heart disease, and protect you against cancer. However, the results from clinical trials have been disappointing. The most recent studies have concluded that taking vitamin E supplements probably won’t hurt, but don’t expect much benefit. On the other hand, there is greater evidence for the benefit of vitamin D supplementation, yet people may not be getting as much of this vitamin as they need. It takes some effort to stay on top of the latest nutrition science, but the payoff can be better health.