(FORTUNE Magazine) – Stress is a popular topic. So's prostate cancer. But the thing most of my patients want to talk about is diet. They've all got friends who've lost 20 pounds in two weeks on a steak and bacon-and-eggs diet, and they want to know why they can't join the high-protein, low-carbohydrate bandwagon.

The fad diets--Atkins (unlimited fat, few carbohydrates), the Zone ("Eating fat does not make you fat"), Sugar Busters (avoid all sugar and the carbohydrates that raise blood sugar), and Protein Power (surprisingly, the amount of protein recommended is close to what most people already consume)--are variations on the same theme. Namely, that the bad guy is carbohydrates.

For years nutritionists have been urging people to eat low-fat, high-carbohydrate diets. What gives?

Maybe some people took the low-fat, high-carb thing too far. After all, if you cut your fat intake but eat double portions of pasta, unlimited amounts of low-fat chips, and tons of low-fat-but-high-sugar yogurt, soda, and candy, you're going to consume more calories. No matter what anyone says, calories still count. And if you haven't lost weight on a "healthy" diet, it's convenient to point to carbohydrates as the culprit.

Enter the current diets, most of which are based on a simple rationale: Carbohydrates increase blood sugar and insulin levels, and insulin promotes weight gain. Therefore, goes the theory, if you cut back on carbohydrates, your insulin levels will drop and you'll lose weight.

Trouble is, the reasoning is backward. Carbohydrates do increase blood sugar and insulin levels, but only short term. That being the case, it's the increase in total calories--not just the carbohydrates--that leads to increased body weight and, in turn, to an increase in blood sugar and insulin levels.

Some people on high-protein, low-carb diets lose weight, because almost all initial weight loss is water, and we're talking some people, not all. We're bombarded by the success stories, but we rarely hear about the thousands who gave up or wound up heavier than when they started. Also, if you virtually eliminate carbohydrates from your diet, you're almost certain to take in fewer calories. I'll say it again: Calories count.

If people can't stay on a fad diet, and if they regain the weight they've lost once they end it, they might as well not have tried it in the first place. Then there's the health aspect. In addition to carbohydrates, many of today's popular diets say no to fruits and vegetables. That's strange, given that (1) an estimated one-third of all cancers are related to diet, and (2) the good guys in the cancer-fighting equation are the very foods the fad diets rule out. Fruits and vegetables contain vitamin C, folic acid, fiber, and many other nutrients that researchers believe protect against cancer. It's also pretty well agreed that other plant chemicals known as phytochemicals--lycopene in tomatoes, isothiocyanates in broccoli, genestein in soybeans, and allyl sulfides in garlic and onions, to name a few--also play a protective role. Supplements are fine, but we'll never be able to pack all those nutrients into a pill.

Which is precisely why the American Cancer Society's No. 1 dietary guideline is to "eat more plant products," including fruits, vegetables, and whole-grain carbohydrates, a recommendation supported by thousands of scientific studies. By contrast, the creators of the latest fad diets have yet to publish a single peer-reviewed study documenting that their approach is safe, or even that it works for most.

All of which suggests that while some people may temporarily lose weight on a high-protein, low-carbohydrate diet, their long-term health may suffer. And isn't long-term health what we're all shooting for?

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