If you're diabetic or have another chronic disease, or are at high risk for one, you should talk with a nutritionist about special tweaks you may want to make to your diet—being sure you avoid saturated fat or excess sodium, for example, or, in the case of diabetes, picking foods that keep your blood sugar steady. (That's what the Nutrisystem D plan does; it's a low-calorie diet, but its foods have a low glycemic index, meaning they more slowly affect blood sugar levels, so dieters can manage their disease better.)
People with the metabolic syndrome are generally advised to lose weight by limiting calories and following a healthful diet rich in fruits and vegetables and low in unhealthful fats, rather than with any special carb-restriction regimen, says Maria Collazo-Clavell, an endocrinologist at the Mayo Clinic. (Dietitian Kress says she has unpublished data on nearly 4,000 patients showing that her diet for those with "Metabolism B," as she calls metabolic syndrome, works better than the standard low-fat, calorie-controlled diet advice.)
If you're generally healthy but just need to drop some weight, you might do some experimenting. Cassandra Forsythe, author of the Women ' s Health Perfect Body Diet, recommends eating a high-carb breakfast of a bagel and sugar-free jam one day and monitoring your mood, energy levels, and hunger throughout the morning. Then try a low-carb breakfast of scrambled eggs with feta, plus a small apple, the next. (Repeat to confirm the results.) She puts forth two different meal plans depending on which breakfast makes you feel better: one for the carb-energized, which includes about two more servings of carbs per day than the other, and one for the carb-sluggish, which substitutes about two servings of fat. Even if you don't want to go to the trouble of a formal experiment, just pay attention to how you feel after different foods. Do you feel satisfied? Can you go about your day without obsessing over your next meal? "Think about what you do best with. Maybe you feel fuller after lentils and rice rather than chicken or fish," says Forsythe.
Notice she didn't mention choosing between four different types of Lunchables.Remember, whatever ratio of carbs, proteins, and fat you find works best for you, your diet should primarily be comprised of healthful whole foods. Sounds boring. Won't sell many books. But at this point, it's still the simplest and best route to success.
Clarified on 10/02/09: An earlier version of this story gave an imprecise title for Ken Kornman. He is founder and chief scientific officer of Interleukin Genetics, which makes Inherent Health brand tests.