Although it's difficult to answer the thousands of questions we receive each year, here are the answers to a few of the most pressing.

Q. Do you burn fewer calories if you do the same exercise over and over again?

A. "Rarely," says William Haskell, deputy director of Stanford Prevention Research Center at Stanford School of Medicine. "For most activities, if a person performs them regularly at the same intensity (e.g., speed) and for the same duration, and the person's weight remains constant, energy expenditure will not change significantly over time. However, it could decrease if the person's skill increases significantly." One of the few instances where that happens is with beginning swimmers who become more efficient over time and therefore burn fewer calories during the same exercise session, Haskell says.

Q. Is couscous one of the better whole grains?

A. No. It's actually a refined (not whole) grain, made from coarsely ground semolina pasta, and even though it's often seen as a healthier alternative to rice, it's really no different. Look for whole-grain couscous.

Q. Is it more nutritious to eat fresh fruits and vegetables than frozen or canned?

A. No. Frozen and canned fruits and vegetables can be just as nutritious as fresh. They are often packaged immediately after picking, and this helps preserve the nutrients. Canned produce does have a downside — the vegetables can be high in sodium, and the fruits packed in sugary syrups.

Q. Can caffeine be lethal?

A. Caffeine overdose is extremely unlikely to be fatal. Terry D. Blumenthal, a professor of psychology at Wake Forest University, Winston-Salem, N.C., says, "You would have to ingest more than 30 cups of coffee in one sitting. It is, however, possible to swallow enough caffeine pills to cause fatal convulsions and respiratory failure."

Q. True or false: If you have an appetite, it means you're hungry.

A. False. "It is possible to be hungry without having an appetite, although this usually occurs only when someone is sick, or perhaps depressed or very stressed. The body sends signals that food is needed — hunger — but food might be unappealing, and consequently there is no appetite," says Hollie Raynor, assistant professor of psychiatry at Brown Medical School.

Q. True or false: Buttermilk is the most fattening of all milk.

A. False. In fact, buttermilk is usually made from skim milk, so it's actually lower in calories than whole milk, at 100 calories per cup vs. 150 in whole milk. Buttermilk is also low in cholesterol and fat.

Q. Are there any differences between Pilates and yoga?

A. Yes. Pilates and yoga are often compared because they both integrate mind and body and strengthen core muscles. However, according to Brooke Siler, a New York City Pilates instructor and author of "Your Ultimate Pilates Body Challenge" (Broadway, 2005), "Pilates is a system of exercise utilizing the body itself along with specifically designed apparatus as resistance tools to stretch, strengthen and tone. Unlike yoga, Pilates doesn't segregate body parts or focus on tranquil, meditative posses."

When it comes to yoga, in addition to the thousands of asanas or poses, there are a lot of other things going on.

"Yoga is a practice that integrates the whole person; it not only provides a challenging workout but also releases mental stress and tension and creates a feeling of emotional stability," says Baron Baptiste, a power-yoga expert and author of "My Daddy is a Pretzel." Yoga also has incorporated into its core a 5,000-year-old tradition, and is part of a more complicated, larger physical, philosophical and spiritual practice, making it much broader, says Dayna Macy, communications director of the Yoga Journal.

According to Mieke Scripps, an orthopedic physical therapist for the Miami City Ballet, "Injuries often occur from poor postural habits and muscular imbalance. Pilates and yoga are both great forms of exercise because they encourage the use of muscles that we don't use in daily life. And yoga and Pilates help correct these muscular imbalances for overall body health."

Q. Should you warm up or stretch before or after a workout?

A. Michelle Olson, a professor of exercise science at Auburn University in Montgomery, Ala., says many studies have found that stretching before exercise does not prevent injuries. Not only that, but Olson warns that "static stretching can decrease performance because it decreases the ability of the muscles to put out as much power as they normally would for up to one hour." Olson believes that stretching after you exercise is best. "Your muscles have more blood flow at that time and less friction force."

She does, however, recommend doing a warmup. Basically, this should consist of doing an activity very similar to the one you're about to do, but slower and at a lower intensity that builds over a 10- to 15-minute period.

Charles Stuart Platkin is a nutrition and public-health advocate. Write to info@thedietdetective.com.