Your abs class leader is meticulous about your crunch form, and he authoritatively prescribes ten eggs a day to boost your energy. Then there's the yoga instructor who knows just the right stretches for your tender back and recommends you take ginseng for your high blood pressure. These experts can be so helpful in their area of expertise that it's tempting to follow their advice on all health matters. Knowing when to listen and when to plug your ears requires keeping a critical mind.

If you want to structure a workout, learn proper technique on the chest press, or increase your aerobic intensity safely, you can't go wrong with a personal trainer who's certified by one of these top organizations—American Council on Exercise, American College of Sports Medicine, or the National Council on Strength and Fitness. But if you're looking for dietary advice, a trainer isn't the ideal source—she may not know any more about proper eating than you do. Your doctor or a nutritionist is the person to ask.

And while yoga instructors can be empathetic, intuitive teachers, that doesn't give them license to diagnose your muscle or joint pain or prescribe herbs for your medical problems. Some instructors have certification (Yoga Alliance registers teacher training programs that meet their standards). But unless your yoga teacher also has extensive training in alternative medicine, take her advice on yoga, and be skeptical about her other tips.

There's no shortage of experts these days. A vitamin store clerk can sound very knowledgeable about antioxidants, but he's there to sell vitamins; it's unlikely that he has a degree in nutrition. Bottom line: Never take advice without considering the expertise of the person handing it out.