Modern diets fade in and out in a few short years -- think Atkins, South Beach or Jenny Craig -- but one of the best ways to lose weight and stay healthy may have existed since the dawn of man.

Move over, NutriSystem ( NTRI): It's called the Paleolithic diet, and the theory goes that our digestive systems have had roughly two million years to adapt to a hunter-gatherer lifestyle. In the mere 10,000 years since agriculture took hold, our guts haven't had a chance to adapt well to grains, legumes, refined sugar or other trappings of civilization, advocates say. Eliminate these foods, and many diseases of modernity, including heart disease, diabetes and obesity, could disappear along with them.

The idea gained medical recognition in 1985, when Drs. S. Boyd Eaton and Melvin Konner published "Paleolithic Nutrition: A Consideration of Its Nature and Current Implications," in the New England Journal of Medicine. It inspired Staffan Lindeberg of the University of Lund in Sweden, an advocate of Paleolithic-style eating, to continue to study evolutionary nutrition.

To find evidence that humans should eat like cavemen, Lindeberg took a trip to the Trobriand Islands in Papua New Guinea during the late 1980s and again in the 1990s. There, according to his Web site, he found one of the last societies to follow the Paleolithic diet strictly.

Among the 2,300 Trobrianders of the island of Kitava, there was a notable absence of sudden heart- and exertion-related death, and among the 6% of the population over 60, there were no signs of dementia or poor memory. According to the study, "Infections (primarily malaria), accidents, pregnancy complications, and old age were the dominant causes of death." Lindeberg felt it was possible that the Kitavan diet could contribute to such success.

Lindeberg says that Kitavans eat a lot of yams, tapioca, fruit, roots, vegetables and fish. He believes ancient humans likely also consumed a great deal of carbohydrates. Lindeberg points to humans' high levels of salivary amylase, an enzyme that breaks starch down into smaller molecules. This suggests "our ancestors at some stage ate a lot of starch." The carb prohibition of Atkins, therefore, does not hold for aspiring caveman dieters, so long as they confine their intake to fiber- and water-rich roots and fruit, while avoiding denser products like corn and wheat."

To test his nutritional hypotheses further, Lindeberg compared glucose-intolerant patients who spent three months on the Paleolithic diet with patients on a Mediterranean diet that permitted whole-grain cereals, low-fat dairy and other items. Lindeberg noted a vast improvement in glucose tolerance among the caveman dieters compared with that of the Mediterranean eaters.

Among other benefits of the diet, "it's a good way to lose weight," Lindeberg says. Rich in water- and fiber-heavy food, the Paleo diet offers a "way to restrict your calories without restricting the amount of food you eat." In an upcoming publication, Lindeberg says he demonstrates that the diet improves blood pressure and that there is indirect evidence that it prevents atherosclerosis, a cause of heart attacks.

The diet is also intuitive. "If patients buy a parrot, they might consider what a parrot is eating in a similar habitat." The Paleo diet takes the same approach with humans. It also offers the benefit of simplicity: Lindeberg doesn't count calories, he just urges people to hunt and gather at the grocery store.

Not everyone is jumping on the Flintstone-esque bandwagon, however. Keith Ayoob, an associate clinical professor of child pediatrics at the Albert Einstein College of Medicine, a noted enemy of fad diets and author of The Uncle Sam Diet: The 4-Week Eating Plan for a Thinner, Healthier America, has some objections. Of the diet's prohibition of grains, he says, "you're talking about leaving out a really important food group."

"Half the world lives on fish and rice," he says. "People say, 'Oh, it causes obesity and it causes a rise in blood sugar.' Only if you overdo it."

Moderation may be a better answer than a wholesale reworking of what people eat, Ayoob believes. "The idea that grains are a problem -- no. Excess is a problem." He thinks the diet's appeal lies in its broad prohibitions. "It's way easier to say to people, 'just don't eat it,' than say 'eat it moderately.'"

"If you look at people who lose weight and keep it off, they are physically active. ... If you really want to go Paleo, get rid of all the electronics. Paleo guy didn't have any wires coming out of his ears." Other factors, such as exercise, can make a huge difference in a person's overall health.

Another objection, raised by both Dr. Ayoob and JoAnn Carson, a registered dietician and professor of clinical nutrition from the University of Texas Southwestern Medical Center, concerns calcium.

A diet that bars dairy products could raise serious problems. Dr. Ayoob says: "Paleolithic man didn't have to worry about osteoporosis, because Paleolithic man wasn't around after age 30." Dr. Carson agrees, saying, "Dairy products, that would be a concern I would have, particularly if someone is at a stage of life when getting more calcium is an issue."

On the whole, though, Dr. Carson believes increasing fruit and vegetable intake is a good idea. And reducing sugar and salt intake, as the Paleo diet suggests, would be a boon for many Americans.

What the Paleolithic diet offers, then, is an intuitive series of restrictions that can be a step in the right direction, but may not be optimal for all comers. But starting with a few hard-and-fast rules on a path toward a disciplined diet that cuts down on harmful foods is likely to contribute to success.