(FORTUNE Magazine) – Starve a cold, feed a fever. A lot of upper-airway secretions are swallowed when you have a cold, and this can decrease appetite by filling you up. It's also true that a fever will raise your metabolic rate a little, but it's pretty much temporary and not enough to warrant eating more. Colds, fevers--the rule is the same: Drink a lot of fluids and eat nutritious, easily digestible foods, such as rice, cooked vegetables, and soup.

Eating carrots helps you see at night. The first sign of vitamin A deficiency is difficulty with night vision. Carrots are high in beta-carotene, which is converted to vitamin A. But if you're not deficient in vitamin A, consuming more won't improve your night vision. It's like adding gas to a car with a full tank--the engine won't run any better.

Cracking knuckles leads to arthritis. Mayo's orthopedists swear this isn't true. Knuckle-cracking may bother your mother, but it won't damage your joints.

Vegetarians don't get enough protein. First, most Americans eat twice as much protein as they need. Second, while individual plants don't contain all the required amino acids (the building blocks of protein), anyone eating a variety of plant foods can be pretty sure the protein mix will be greater than the sum of its parts.

Excess weight is the result of a low metabolism. We used to measure metabolic rate regularly in our nutrition clinic, but we stopped for a simple reason. Almost without exception, the metabolism readings we got weren't low. Excess pounds come from taking in more calories than you burn, and people burn what they should. The excess caloric intake is the problem.

High blood pressure causes symptoms such as headaches. Many people believe they can tell when their blood pressure is elevated. If someone is in a stressful situation, it may raise their blood pressure, but they would be aware of it because of the situation, not because of any symptoms. High blood pressure rarely causes symptoms, which is why it's so important to check your pressure periodically--more often if it's on the high side.

Never go swimming after eating. Even the American Red Cross says there are no good reasons not to swim after eating. For a strenuous swim, you might want to let your food settle first.

Chicken soup will help when you're ill. No myth here. Studies have shown that chicken soup really boosts immune function. On this one, Mom was right.

For more information on this topic: Go to mayoclinic.com. Mayo Clinic offers Executive Health programs at Mayo Clinics in Jacksonville, Fla.; Rochester, Minn.; and Scottsdale, Ariz.