Leibowitz now thinks that this serotonin signal is directly related to the bingeing behavior that is the sine qua non of bulimia. "Bulimics have a deficit in brain serotonin. The mechanism for stopping carbohydrate intake doesn't seem to be there."
Every meal, then, and the appetite for it, is differently regulated and presided over by a separate cocktail of neurochemicals. The neurochemically correct breakfast is a quick blast of carbohydrate right after awakening. Say, a glass of orange juice for speedy transport of sugar into the bloodstream to restore glycogen. Then a piece or two of toast, a more complex carbohydrate to deliver a more sustained supply of glucose over the morning.
For those who don't do it regularly, a breakfast of, say, eggs benedict--rich in protein and fat as well as carbohydrate--will send your neurochemicals spinning, throw off their normal rhythm of production, and affect many other neurotransmitters in the bargain. Ever wonder why you're just not sharp enough after an unusually rich breakfast?
The Big Switch
Not only are the neurochemicals of appetite active at different times over the course of a day, they are differently active over the course of development. Before puberty, Leibowitz finds, animals have no interest in eating fat. Children, too, have little appetite for fat, preferring carbohydrates for energy and protein for tissue growth. But that, like their bodies, changes.
In girls, the arrival of the first menstrual period is a milestone for appetite as well as for sexual maturation. It stimulates the first desire for fat in foods. And that, says Leibowitz, is when a great deal of confusion sets in for anorexics.
"We hit puberty and that turns galanin on." The female hormone estrogen primes the neurochemical pump for galanin.
There are other sex-based differences in nutrient preference. In studies of animals, young females tend to have higher levels of Neuropeptide Y and favor carbohydrates. Their preference for carbohydrates peaks at puberty. Males favor protein to build large muscles.
When puberty strikes up the taste for fats, males are inclined to mix theirs with protein--that sizzling porterhouse steak. Women, their already high levels of Neuropeptide Y joined by galanin, are set to crave high-calorie sweets--chocolate cake, say, or ice cream. It's bread-and-butter nutritional knowledge that carbohydrate makes fat palatable in the first place.
This neurochemical combo particularly sets women up for late-afternoon snacking, possibly bingeing. Late afternoon may be the time when those who skip breakfast are particularly likely to pay for it, and there in turn with exaggerated increases in their Neuropeptide Y levels leading them into late-day gorging.
Patterns of Preference
When Leibowitz allows animals to choose what they eat, they show marked individual preferences for nutrients. These nutrient preferences, in turn, create specific differences in feeding patterns. In this animals are just like people, and fall into one of three general categories.
In about 50 percent of the population, carbohydrate is the nutrient of choice. Such people naturally choose a diet in which about 60 percent of calories are derived from carbohydrate, and up to 30 percent come from fat. They are neurochemically in line with what nutritionists today are recommending as a healthy diet. High-carbohydrate animals consume smaller and more frequent meals, and they weigh significantly less, than other animals.
Some Like It Fat
A small number of people and animals are dedicated to protein. But 30 percent of us have a predilection for fat. And those who do take in 60 to 70 percent of their calories in straight fat, as opposed to the 30 percent considered appropriate to a lifestyle that's more sedentary than our ancestors'.
Not only is this not likely to sit well with arteries, but such preferences also correlate highly with body weight in animals. Those constituted to favor fat consume the most calories and weigh the most. And they seem to be particularly predisposed to food cravings late in the day.
Early indicators
What is perhaps most intriguing in all of this to Leibowitz is that individual taste preferences first show themselves when animals are very young, notably at the time of weaning, even before their neurochemical profiles are fully elaborated. The same is true of people. "We know early in family life what we are going to become," she contends.
The New York researcher believes that by sampling infants' tastes, it will be possible to predict eating and weight-control problems long before they occur. And, of course, if we choose, do something to prevent them from ever occurring.
More than Metabolism
At the time of weaning--21 days in rat pups, 1 1/2 to 2 years in human infants--taste preferences largely reflect differences in genetic makeup. And in those animals that prefer sucrose or fat--"you put it on an infant's tongue and watch how they react to it, whether they become active or not"--their appetite is strongly predictive of how much weight they will gain later on in life. And their neurochemical make-up.
"We believe there is strong appetitive component to pre-ordained weight gain," Leibowitz says. "We think there's more to it than just metabolism. We are on the verge of linking that early taste with later eating behavior and weight gain."
The Wages of Stress
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