Myth No. 3: Pasta makes you fat.

The Theory: When you eat carbohydrates, your body turns them into sugars, which are then stored as fat.

The Reality: Carbohydrates per se don’t make you fat; extra calories do, whether you eat them in the form of carbs, fats, or protein. Besides, carbohydrates include vegetables, fruits, and whole grains, which are important parts of a healthy diet. In short, the problem isn’t pasta but the sheer volume consumed. “Americans tend to eat too much carbohydrates, fat, and protein. But they overeat carbs most of all,” says Barbara Moore, Ph.D., a nutritionist in Clyde Park, Montana, and a spokesperson for the American Society for Nutrition. “You go to a restaurant and you’re served three cups of pasta with lots of sauce.” Those three cups of pasta can pack up to 600 calories without the sauce.

The Best Advice: Pasta in moderation is fine. Dietitians recommend two or three ounces of uncooked noodles per person — or half of a one-pound box to serve a family of four. This may look like a puny amount, but try thinking of “pasta as an ingredient, rather than as the basis of a dish,” says Mark Bittman, author of How to Cook Everything Vegetarian (Wiley, $35, www.amazon.com). “Start with a lot of grilled or sautéed vegetables and maybe a tomato-based sauce. Then add some pasta, sparingly.” If you want protein, add beans, chicken, or shellfish. (For some delicious pasta dishes, see Healthy Pasta Recipes. Make your pasta — or bread or rice or cereal — whole-grain, which has more vitamins and minerals than white pasta. You’ll also be getting fiber, which helps you feel full.

Myth No. 4: Coffee can help you lose weight.

The Theory: The caffeine in coffee acts as an appetite suppressant and a metabolism booster.

The Reality: While coffee may temporarily squelch your appetite, drinking a couple of cups a day won’t have enough of an effect to help you lose weight. Besides, pouring too much coffee into your system — drinking, say, four to seven cups a day — may lead to anxiety, sleeplessness, and an increase in heart rate and blood pressure.

The Best Advice: Enjoy a cup or two of coffee (or tea) every day, if you please. Just be sure that if you add anything to the brew — like cream, sugar, or cocoa powder — you take those calories into account. For example, a 16-ounce Starbucks café mocha can contain a whopping 330 calories (60 more than some chocolate bars). What’s more, those calories might not make you feel as full as the same number of calories eaten in solid form.

Another coffee concern: sleep disruption, which new evidence reveals is linked to weight control. “Every time people feel tired, they think, I have to have a latte,” says Liz Applegate, Ph.D., director of sports nutrition at the University of California, Davis. “They become addicted to caffeine on a higher level, and it takes four to six hours to clear out of the system. Sleep is not as good, and you’re tired the next day.” And probably hungrier, too. At least two studies have shown that when people are sleep-deprived, they produce more of the hormone ghrelin, an appetite stimulant, and less leptin, an appetite suppressant. Not to mention that your resistance to the doughnut’s siren song is a whole lot lower when you’re pooped.