Of course, the mere suggestion that the Atkins diet and others like it are worthy of scientific attention still makes many experts bristle. Yet it is also clear that the low-fat paradigm has developed some cracks in its façade. It turns out that not all fats are bad for you. Those found in fish, nuts and certain vegetables may actually increase your chances of living a good long life. By the same token, not all diets that are low in fat are necessarily healthy — as anyone who has ever truly considered the difference between a low-fat banana cream pie and a banana could tell you. About one thing, however, there is no dispute. The developed world is clearly in a state of nutritional crisis and in need of radical remedies.

The statistics are sobering. After 30 years of seemingly solid advice aimed at lowering dietary fat, Europeans have grown collectively fatter than ever. Today more than 50% of adults in the European Union are classified as overweight or obese. So many children have become heavy that pediatricians are now facing an epidemic of Type 2 diabetes and hypertension — diseases that are closely associated with being overweight and were unheard of among youngsters just a generation ago. (See following story.) On one level, there is no mystery about why we as a society are fat: because we consume too many calories and expend too few. Though it is true that the proportion of fat in our diet has fallen from 40% in 1990 to roughly 36.5% today, the calories available in the food we consume have gone up, from 3,187 calories per capita per day in the 1970s to 3,400 in the 1990s, according to the U.S. Department of Agriculture (USDA). Even the traditionally slender French are succumbing to the trend. A recent study by the French National Institute of Medical Research found that the proportion of obese people rose from 9.6% in 2000 to 11.3% in 2003.

But there is a deeper question — one that has plagued anyone who has ever struggled to take off more than a few pounds. And that is: How do some folks manage to live in the same "toxic environment" and never gain weight? The quest to answer this double-sided question is in its earliest stages. Already, however, a series of fascinating insights into the biology of obesity has emerged. Behind our broadening behinds and widening waistlines, scientists say, lies a complex array of genes that, directly and indirectly, links our gut to our brain.

These genes, honed by millions of years of evolution, appear to have betrayed many of us in the 21st century world. Scientists have long suspected that human beings come into the world equipped with dozens, perhaps hundreds, of genes and associated hormones that regulate what scientists call the energy-balance equation. On one side of the equation are the calories we consume; on the other side are the calories we burn — through physical activity as well as whatever is needed just to keep the body in good working order. Anything left over gets converted to body fat. With the notable exception of insulin, which helps the body process sugars from carbohydrates, the identity of most of the major players in this biochemical balancing act could for years only be guessed at.

Scientists thought they had a major breakthrough in 1995, when Jeffrey Friedman, a molecular geneticist at Rockefeller University, stunned the scientific world by announcing that he and his colleagues had discovered a hormone, called leptin, that caused fat to melt away, at least in laboratory mice. Genetically engineered mice that lacked the gene for making this hormone developed ravenous appetites and became grossly obese. When these same mice were injected with the missing hormone they shrugged off one-third of the weight they had gained.

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