EATING FOR BIOLOGICAL REASONS
by: Christine Gorman
Our desire for food----and lots of it.
is hardwired into our cells.
Do our bodies want to be fat?
THERE’S NO QUESTION THAT SOME PRETTY STRONG social, emotional and behavioral forces play a part in determining what, when and how much we eat.
But if you really want to know why some people are fat and others aren’t, you have to take a good look at biology as well. Mother Nature simply can’t afford to leave anything so important to human survival as eating to the whims of cultural fashion. Ten years after the discovery of the first obesity gene, scientists are only beginning to understand just how hard-wired our desire for food—and lots of it—truly is.
What they are finding is an exquisitely fine-tuned system of chemical and neurological checks and balances that regulates both what we eat and how much our bodies store as fat, The average American consumes about 1 million calories a year —and, under normal circumstances, burns almost exactly that amount. The body achieves that balance by automatically increasing or decreasing the efficiency with which it performs various tasks, thus consuming fewer or more calories. (Most of the calories we expend are used to breathe, maintain body temperature, keep the brain chugging along, etc. Depending on how much you move, physical activity typically accounts for 15% to 30% of the total.) If you pack on a couple of pounds over the course of the year, your body’s error rate is still less than 1%.
Accomplishing that feat requires a lot of communication and coordination among the fat cells, the liver the muscles, the brain, the stomach and the gastrointestinal tract. Sometimes the signal is a molecule. Other signals are actually conducted along nerve paths. There are even mechanical signals, like the stretching of the stomach, which is one way the body says, “I’m full.”
As if all that weren’t complicated enough, the body must also regulate its food intake and manage its weight over time. “There are short-term signals and long- term signals,” says Judith Korner, an endocrinologist at Columbia University in New York City. “Some signals are both short term and long term, and then there are medium-term signals.” As you might expect, the short-term signals are involved mostly with the initiation and completion of meals. Ghrelin, a hormone produced by the stomach, tells the brain, “It’s time to eat!” When enough food leaves the stomach and reaches the small intestine, another hormone, called cholecystokinin, signals that the meal is over—and triggers the release of enzymes in the gallbladder and the pancreas.
The hormones leptin and insulin are longer-term signals. Produced by fat cells, leptin helps manage just how much fat you store around your organs and under your skin through a complex feedback loop. If your fat deposits start to shrink—for example, when you lose weight—the amount of leptin in your body falls, a situation that the brain interprets as a result of starvation. The whole system of chemicals and neurological impulses shifts in an attempt to get the body to burn fewer calories so that it can regain the weight. The greater the weight loss, the stronger the signals to eat more and replenish fat stores.
There are many other factors that affect this delicate balance. For example, lab- oratory evidence suggests that a diet that boosts your triglycerides—typically, one high in fatty, fried or highly refined foods—may interfere with both leptin’s and insulin’s actions on the brain, leading to an erroneous signal that the body is in danger of starving. The same receptors in the brain that are responsible for a marijuana high also boost appetite, which is why pot smokers get the munchies.
The more scientists learn about these biochemical, neurological and dietary factors, the more they marvel that anyone in our culture manages to stay thin, given the abundance and easy availability of food. If there’s some kind of biological mechanism that protects certain people against weight gain, researchers haven’t discovered it. By contrast, the evidence in favor of one that protects against weight loss is increasingly strong. Genetic variations clearly push some people toward bigger appetites, slower metabolisms and greater weight gain than others. “There are genes in the population that predispose to obesity,” says Dr. Jeffrey Friedman, a molecular geneticist at the Rockefeller University in New York. “Obviously, there’s an environmental contribution, but no one questions that genes are involved.”
So the next time you stare in judgment at a fat person on the bus or bemoan your physique in the mirror, remember that nature has stacked the deck against weight loss. Trimming 25 lbs. from your figure may not be that difficult. But try shedding 100 lbs., and your body is going to scream. Whether willpower, exercise, drugs or even surgery is enough to quiet the body’s basic need for fat is still an open question.
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