Posted Sunday, June 29, 2003; 14:08BST
Since they're having trouble changing our sedentary lifestyles, health professionals are focusing on making us at least eat more healthily. The British Medical Association is discussing levying a fat tax on high-fat foods like cakes, biscuits and processed meals. In another attack on unhealthy eating, a group of lawyers in Boston — led by tobacco-litigation veteran John Banzhaf — recently met to discuss ways of suing the fast food industry. Banzhaf has written to the bosses of McDonald's, Burger King, Wendy's, KFC, Taco Bell and Pizza Hut warning of growing evidence to suggest that fast food "can act on the brain in the same ways as alcohol or heroin." He intends to demand that they put up signs to that effect in their restaurants.

Why, then, do we like these foods so much? For answers, researchers are once again turning to laboratory animals, which exhibit many of the same dietary proclivities we do. Rats, for example, will labor mightily to obtain a sugar pellet even after they have dined on rat chow and aren't particularly hungry. The reason, thinks Allen Levine, director of the University of Minnesota's obesity center, has a lot to do with sugar's impact on mood-enhancing circuits in the brain. Sugar gives rats — and by extension humans — a buzz.

There is also reason to think that our penchant for making unhealthy choices may be enhanced by the abundance of particular foods. Consider the results of an experiment recently conducted at Philadelphia's Monell Chemical Senses Center. Rats given one cup each of fat, protein and carbohydrates were found to make balanced diet choices, eating a tad more protein than carbohydrates and a tad more carbohydrates than fats. But rats presented with more cups of fat and carbohydrates than protein dramatically increased their consumption of the former while sharply curtailing their intake of the latter — to the point, in some cases, that the rats became protein deficient. Why would rats do this? Perhaps because their brains are hard-wired to take advantage of sudden windfalls of food, and in the wild, of course, such windfalls do not occur every day.

They do, however, in restaurants. Soft drinks are now delivered in one-liter cups, observes Judith Stern, vice president of the American Obesity Association. The venerable Joy of Cooking has capitulated to the trend; the same recipes that used to provide meals for six now feed only four. And even in Paris, home of cuisine minceur, super-size pains au chocolat are now available from the patisserie. So cheap are carbohydrates and fats that making extra-large sizes costs the food industry next to nothing.

One of the reasons that fast foods are so attractive is their high fat content. Fat is energy dense, with twice as many calories per gram as carbohydrates or protein. "We are pretty much programmed to go for energy- dense foods," says Dr. Hannah Theobald of the British Nutrition Foundation. "So fat's attractive. It makes flavors melt in the mouth." The trouble is, those energy-dense foods pile on the pounds. "It's very easy to eat a little bit more at each meal," says Dr. John Wilding, a consultant physician who runs an obesity clinic in Liverpool. "An extra 100 calories a day and you will be 7 kg heavier at the end of the year."

Adults are free agents. They can overeat and gain weight if they want to. But perhaps what is most disturbing about their overeating is that they are unwittingly, and in myriad ways, passing on that tendency to children. It's not just that children pick up their parents' bad habits. There is growing evidence that what you eat early in life can permanently boost your body's desire for food. A 2001 study by German pediatrics professor Berthold Koletzko found that breast-feeding children for at least six months gave them a 40% lower risk of becoming obese in later life. "Infants fed formula are more likely to become obese," he says. "The higher protein content compared with breast milk could be a causal factor."

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