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Converts to low-carb diets don't need scientists to tell them they can lose weight eating bunless bacon cheeseburgers for a couple of weeks. The bigger issue is the long-term health effects of protein-heavy diets. Very little data is available, but many researchers are worried that such diets can lead to kidney and liver problems. Research also suggests that too much protein can leach calcium out of the body, increasing the risk of osteoporosis. Dieters still need to be concerned about the risks of shedding pounds while slurping eggs Benedict and lobster thermidor. The bottom line is the same: calories matter, and so does a balanced diet.

You can lose weight on any calorie-restricted diet. It can be pickles, pie or cabbage soup. Eat fewer calories than you burn, and pounds melt away. The monumental problem dieters face is making the shift from quick weight-loss schemes to healthy eating. It's a tall order, and the vast majority of dieters fail. After a few months of abstinence, most revert to old habits and gain back everything — and often more. The secret of dieters who keep pounds off for good is that they skip gimmicks altogether and focus from the very beginning on healthy eating habits they can sustain for a lifetime.

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One thing scientists do know is that much of the yummy stuff in low-carb diets — think filet mignon with bearnaise sauce — comes loaded with artery-clogging saturated fats. Low-carb mania has not upended the scientific consensus that saturated fats are the enemy: a major risk factor for heart disease and stroke.

That's the bad news. The good news is that there are good carbs and bad. All foods can be divided into combinations of three different nutritional categories based on their chemical components: carbohydrates, proteins and fats. Carbs are broken down by the body into sugars that course through the bloodstream and serve as the body's key source of energy. White bread, pasta and potatoes earn a bad rap because they are simple carbs that are very quickly broken down into sugar in the body. Most excess sugar is stored as fat. Some fruits, vegetables, beans and whole grains, on the other hand, are also carbs, but they are complex ones that break down slowly and are rich in vitamins, minerals and especially fiber. A diet rich in fiber can lower cholesterol and reduce the risk of cardiovascular disease.

So what's a healthy diet that people can stick to long term? Listen to Dr. David Katz of the Yale School of Public Health: "Diets rich in fiber and complex carbohydrates, found in fruits, vegetables, beans and whole grains, have been shown in a wide array of studies," says Katz, taking a deep breath, "to be associated with longevity, lasting weight control, reduced risk of cancer, reduced risk of cardiovascular disease, reduced risk of diabetes, reduced risk of gastrointestinal disorders and overall health promotion. In other words, the notion of cutting carbs is a step in the opposite direction from everything we know about healthful eating."

Bear in mind too that carbs are linked to the regulation of a key neurotransmitter called serotonin, which, says M.I.T. brain researcher Judith Wurtman, "is essential for feelings of fullness as well as a good mood." And again, complex, not simple, carbs are the best way to maintain an even keel when it comes to both appetite and mood.

So what's the optimal mix of carbs, fat and protein? Experts disagree as to exact numbers, but a middle-of-the-road menu calls for 25% of calories from healthy oils, 20% from lean protein and 55% from complex carbs. So if the low-carb mantra has made you cut simple sugars and refined carbohydrates from your diet, great. But if you are skimping on produce and whole grains and instead shoveling animal and dairy fats into your body, you are short-changing your health. "The diet-industrial-complex is now pushing low carbs full steam ahead," says Wurtman. "It may take a long time, but 10 years from now, people are going to look back on this and say, 'Boy, were we really stupid.'"

• David Bjerklie