AUSTIN, Texas Low-carb diets like Atkins and South Beach have come in and out of fashion for years, but the latest reaches far back into human history for its inspiration.

"Paleo people joke that it's the fad diet with the longest life," said Melissa Joulwan, who transitioned to a Paleolithic diet a few years ago after a lifetime of struggling with her weight despite consistent exercise and following traditional low-fat, high-carb diets.

The Paleo concept is simple: Eat like our ancestors did in the millennia before agriculture, which means no grains, dairy, processed foods, sugar or legumes.

Before agriculture, hunters and gatherers ate meat and plants that didn't require processing to eat, but thousands of years later, grains and processed foods have become the foundation of the American diet.

Manufactured foods, especially those based on flour such as pasta and bread, are inexpensive, but anything made from grains, including corn, leads to a spike in blood sugar and can damage the insides of the intestines, say proponents of gluten-free diets, which are certainly not limited to people who fall in the Paleo camp.

"The evidence to support eating grains is underwhelming," said Dr. Michael Roussell, a Livestrong.com adviser, citing a recent Harvard study that found that simple sugars and refined grains are more detrimental to our health than animal fats. "Just about everybody can benefit from eating less carbohydrates because we're often not eating the best types."

Vegetables and fruit contain plenty of carbs and fiber, and the carbohydrates are easier to digest and don't raise your blood sugar like the starchy ones in white potatoes, rice, corn and other grains.

But when you make a switch from a majority-grain diet to one of mostly animal fats, proteins and vegetable carbs, how your body processes those fats changes, too.

"The less carbs you eat, the more saturated or animal fat you can eat," Roussell said. "It changes how your body metabolizes lipids."

Needing the fat

When Joulwan started looking into the diet, she had a hard time getting past the mind-set that fat is bad. "When I first heard that half my (calories) would be from fat, I almost had a heart attack," she said.

Our bodies need cholesterol, and the lean protein and nuts in a Paleo meal contain healthier unsaturated fats and low levels of saturated fats.

But as Joulwan slowly started changing her diet - before going fully Paleo she eliminated grains for a year and then took out dairy for another year - she started losing weight and realized that almost everything she thought she knew about food and nutrition was wrong.

"Your body doesn't see the difference between a potato, a slice of white bread or a Snickers bar because the sugar is perceived by your body in the same way," she said. "Any grain in its evil little heart is a sugar."

Like with any diet, cutting out sugar seems to be one of the hardest steps, and if you are particularly addicted to sweets, make sure you don't swap candy for candy-sweet fruits.

Joulwan blogs about her recipes and Paleo discoveries on The Clothes Make the Girl ( theclothesmakethegirl.com). She's persuaded her father and a close friend to switch to a Paleo diet, and they have lost more than 60 pounds each.

Over the past year, she's compiled more than 100 recipes for her first cookbook, "Well Fed: Paleo Recipes for People Who Love to Eat," which will be available in paperback on Amazon.com and as a PDF on her site in early December.

The drawbacks

Paleo critics are plentiful, from archaeologists who say modern humans have evolved so that they don't have to eat like hunter-gatherers to meat-free (or meat-light) environmentalists who say that too much animal consumption will destroy the planet. A major criticism is the cost.

Even within the Paleo community, some of those "no's" are negotiable. Some eat high fat yogurt and cheese, while others eschew New World fruits and vegetables, including those in the nightshade family such as tomatoes, peppers and eggplant. A recent New York Times story profiled Paleo eaters in New York who are taking the "caveman" approach to an extreme, often fasting and eating raw meat.

Roussell said that most of us, even those who have insulin resistance or heart disease, could benefit from a modified Paleo diet. Cutting out dairy, legumes and all grains is "unnecessarily restrictive," he said, but in general, replacing refined grains with vegetables and both lean and even some fatty cuts of meat - and making sure you're exercising regularly, of course - can improve your overall health. "More research is showing that with people who have insulin resistance, they lose more weight with more fruits, vegetables and less grains."

Making compromises

Most Paleo eaters prefer to eat a gluten-free diet that consists mainly of meat and vegetables, but tailored to their own dietary needs, likes, dislikes and cheats. During the holidays, special occasions and on vacations, Joulwan allows herself some foods that are off-limits. Joulwan says she's able to maintain her diet because she slowly transitioned into it.

"We think of it like making good health deposits in a bank that you can withdraw later," she said, having just returned from a trip to Europe where she and her Paleo husband, Dave, indulged in the best cheeses, breads, chocolates and wine they could find.