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| U N I V E R S E M A G A Z I N E - F A L L 1 9 9 7 | ON THE EDGE by Tim Steury Not so long ago, nutritionists happily calculated the body’s needs simply in terms of gross calories. Your body size and health depended on the right number of calories and vitamins, and that was about it. Like so many things in life, however, further investigation has shown that it’s not quite that simple. For one thing, says Sue Ritter, who studies appetite, the brain has very little energy reserve. “The brain,” she says, “lives on the edge.” Although it has a very high metabolism rate, it cannot use fat or protein as energy, but only glucose, because only glucose can pass through the blood-brain barrier, the capillary filter that protects the brain against toxins. In other words, Big Macs are not ideal brain food. The brain holds in reserve perhaps eight calories of energy, stored as glycogen, a product of glucose. Compare that to the 140,000 calories stored in the rest of the normal adult human’s body. Eight calories is enough to keep the brain operating for but a few moments if the blood supply is cut off. So the brain needs a way to monitor the availability of glucose. To a certain extent, says Ritter, that monitoring system has to be independent of other needs. Mapping the brain and locating those receptors have been the foci of her lab’s research over the past several years. The receptors that control one’s appetite for glucose appear to be localized in the most primitive part of the brainstem, probably in the medulla oblongata. Ritter and others have also determined that the receptors that stimulate one’s appetite for fat are located outside the brain. What all this means, she says, is that we control nutrient intake based at least in part on the needs of specific organ systems, the brain being particularly demanding. This might lead one to hope that one’s natural impulses are thus the best guide to nutrition. However, Ritter has performed experiments in which rats are given separate containers of pure carbohydrates, pure protein, and pure fat, all supplemented equally with vitamins and minerals. The animals naturally select a nutritionally balanced mixture. However, if the taste of one dish is enhanced, the rats will favor it. “I don’t know,” says Ritter, “if we do the best for our bodies just by listening to our natural appetites. There are too many distracters.” Big Macs, for example. While Ritter studies appetite, her husband, Bob, studies satiation. An article by him, detailing the findings of his lab, is featured in the Spring 1997 Universe. | U N I V E R S E M A G A Z I N E H O M E | |
