Lose weight! Feel great! Live longer!

Every diet guru makes the same claims. But can so many different plans get the same results? Can you achieve the same positive responses from the low-carb, high-saturated-fat Atkins regimen as you do from the abstemiously low-fat, almost vegetarian approach at the other extreme?

This may come as a surprise, but the answer is probably yes. While these regimens seem very different, they share a significant common theme: They restrict your calories and cause weight loss.

Scientists are finding there may be a more straightforward way to not only lose weight, but to avoid heart disease, cancer, diabetes and to even live longer. The secret is to focus sharply on reducing calories.

A growing body of scientific evidence shows that any time you restrict calories you will receive significant health benefits that may not only help you lose weight, but effect a series of biological mechanisms that may prolong your life. (It is important to note that once you go off these diets and if you gain weight, all of the positive benefits are lost.)

It works in animals. Rats fed calorie-restricted diets live 50 percent longer than their normally fed counterparts. Their quality of life is superior, too. They're healthier, more active, their hormones are at more youthful levels and their immune function is superior. The same is true with fish, flies and worms. There have been preliminary positive results with rhesus monkeys and even humans.

The results of the first human study on calorie restriction, performed at the Washington University School of Medicine in St. Louis, are interesting.

"People on severe calorie restriction have a reduced risk of developing stroke, heart attack, arteriosclerosis and diabetes," said Luigi Fontana, instructor of medicine at the university's division of geriatrics and nutritional science and lead author of the study in the Proceedings of the National Academy of Sciences published last month.

"Since 40 percent of Americans die of these causes, these health improvements would increase many peoples' life expectancy," said Fontana. "Calorie restriction would also help reduce the 300,000 cases of preventable death due to obesity."

The kind of calorie restriction being studied shouldn't be confused with malnutrition (which occurs with starvation or semi-starvation when food is scarce) or in disordered eating. The important difference is while the calorie restrictors cut calories by 25 percent, they eat dietitian-designed, doctor-supervised diets containing all of their essential nutrients. They eat about 20 percent to 26 percent of their calories from protein, 28 percent from fat and the rest from high-volume, low-calorie, nutrient-dense carbohydrates such as fruits, vegetables, whole grains and legumes.

As pure science, calorie restriction makes a compelling case. Anecdotally, we've all seen how reducing calories restores health, energy and well-being in our friends and colleagues who are on diets and losing weight. But most don't continue on those regimens, and, therefore, the benefits don't last. As a practical matter, long-term severe calorie restriction is probably unworkable for most Americans, and it still hasn't been proven that it's healthy or safe for humans in the long run.

"Calorie restriction studies are provocative; but you may end up with deficiencies dangerous for your health," says Fontana. "Chronic calorie restrictors are taking risks."

But the research is finding that there are certain aspects of aging that we can change. There are several theories as to why and how calorie restriction prolongs your life. The reasons it reduces disease risk factors and prolongs life are probably varied:

* It lowers your metabolic rate. "The higher your metabolism, the more oxygen your body burns," says Eric Ravussin, a professor at the Pennington Biomedical Research Center in Baton Rouge, La. "This produces 'reactive oxygen species,' which are byproducts of metabolism and are harmful."

ROS damage the building blocks of life, including protein, lipids and genetic material, DNA. This leads to abnormal genetic signals, causing cancers and disregulation of cells, leading to organ damage, skin deterioration, and maybe even gray hair -- all signs of aging.

* Calorie restriction reduces inflammation, which is one cause of many diseases, such as heart disease and arthritis. Also, calorie restriction reduces some "growth factor" hormones.

"When you eat extra calories, your body gets a signal that you're growing and growth factor hormones promote cell proliferation, which may increase the risk of cancers, among other dangers," says Fontana.

* Calorie restrictors also have lower hormone levels, body temperatures, plasma insulin levels, and higher levels of the hormone, DHEA. The same is also true about people who live longer, according to the Baltimore Longitudinal Study on Aging, an ongoing study funded by the National Institutes of Health.

* Calorie restriction reduces body fat, which research shows has many benefits.

"We used to think body fat was inert, But it isn't," says Fontana. "Fat tissue produces hormones, pro-inflammatory chemicals, which regulate metabolism, the immune system, inflammation and the progression of artery hardening, so that when you have less body fat, you get many biological benefits."

There are many advantages to calorie restriction. But there are downsides to severe calorie restriction, too. In fact, scientists are not recommending it as a key way to stay healthy, since the research is not complete. Abnormally low metabolism can cause irritability or depression in some people or may backfire and lead to an eating disorder. If you eat too few calories and hormone levels lower too much, this may lead to infertility problems or increase chances of osteoporosis in women.

I believe that one of the real benefits of the low-calorie approach is that it places important emphasis on the central issue: reducing calories, rather than getting diverted into seemingly important, but in fact, peripheral matters, such as carb and fat counting.

Katherine Tallmadge is a Washington nutritionist and author of "Diet Simple" (Lifeline Press, 2004). Send e-mails to her at food@washpost.com.