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Why We're Fat


So why is obesity happening? The obvious, clichéd-but-true answer is that we eat too much high-calorie food and don't burn it off with enough exercise. If only we had more willpower, the problem would go away. But it isn't that easy.

When warned about the dangers of overeating, we get briefly spooked and try to do better. Then we're offered a plateful of pancakes smothered in maple syrup, our appetite overpowers our reason, and before we know it, we're at it again. Just why is appetite such a powerful driver of behavior, and, more important, how can we tame it?

Within the past few years, science has linked our ravenous appetites to genes and hormones. Among the hormones that fuel these urges are ghrelin and leptin, known as the "hunger hormones." Ghrelin is produced mostly by cells in the stomach lining. Its job is to make you feel hungry by affecting the hypothalamus, which governs metabolism. Ghrelin levels rise in dieters who lose weight and then try to keep it off. It's almost as if their bodies are trying to regain the lost fat. This is one reason why it's hard to lose weight and maintain the loss.

Leptin turns your appetite off and is made by fat cells. Low leptin levels increase your appetite and signal your body to store more fat. High leptin levels relay the opposite signal. Many obese people have developed a resistance to the appetite-suppressing effects of leptin and never feel satisfied, no matter how much they eat. Basically, your body uses these hormones to help you stay at your weight and keep you from losing fat — which is another reason why dieting can be so difficult.

Lack of sleep promotes obesity by messing with these hunger hormones. If you skimp on sleep, ghrelin levels rise, making you hungry, and leptin levels dip, which signals a need for calories. During my internship, I was chronically sleep-deprived because I had to be at the hospital and stay up all night every third night. I gained a lot of weight that year; now I know why. Years later, after I gave birth to my son Alex, I put on a lot of pounds, too — twenty pounds left over from the pregnancy, and twenty pounds from being up all night with him. He had colic (fussiness) and never slept more than twenty minutes at a time the first six months of his life. My weight started coming off more easily when I got more sleep.

Besides ghrelin and leptin, many other hormones play a role in appetite. Scientists have been looking for ways to control all these hunger hormones, but so far nothing usable has panned out.

Hunger isn't the only reason people eat and eat and eat. Stress, depression, boredom, loneliness, and even joy all come into play. And some of us may just be inclined to gain weight while others are not, due to genetics. Scientists are unclear as to how much of a role genetics plays in how chubby you are, but estimates range from as low as 20 percent to as high as 90 percent. In studies of twins, researchers have found that siblings wind up with similar body weights, whether or not they are raised in different families, and that adopted children are much more likely to grow to the size of their natural parents than their adoptive ones. Scientists speculate that part of the reason so many of us are susceptible to layering on fat is due to the "thrifty gene." Supposedly, it's a gene passed down from our prehistoric ancestors who could eat a lot and build up reserves of fat to survive frequent famines. Genetically, this made sense when you didn't know if your next meal would be tomorrow or a week from now, but when your next meal is whenever you drive by a fast-food restaurant, that's a problem.

Another cause of obesity, one that really fascinates me, has to do with a virus called the adenovirus-36. It comes from the family of common viruses that causes colds, pneumonia, diarrhea, and pinkeye and is present in 30 percent of obese people and 5 percent of non-obese people. Scientists found that when human stem cells are exposed to adenovirus-36, they turn into fat cells. This discovery is more evidence that our expanding waistlines are due to factors other than weak willpower and, theoretically, could lead to the development of a vaccine to prevent obesity.

Often, hard-to-budge weight is a symptom of a hidden medical problem. You could have hypothyroidism (sluggish thyroid function), a condition that slows down your metabolism and makes it tough to lose weight, or Cushing's disease, characterized by high levels of the hormone cortisol in the blood. A glut of cortisol triggers weight gain, mostly in the trunk and face. Some women have polycystic ovary syndrome (PCOS), a treatable condition that involves a hormone imbalance. And about 30 percent of people who are obese have bingeeating disorders. Sufferers don't just munch on a few potato chips, they inhale the whole bag, and they do this kind of thing habitually. Even certain medications, some of which may be sitting in your medicine cabinet right now, can put on pounds. Common offenders include some antidepressants like Paxil and Zoloft; mood stabilizers; diabetes drugs; blood pressure agents; steroids; antiseizure drugs; certain hormones; and antihistamines.

So perhaps there is a medical issue that spells weight trouble, or our genes and hormones may be conspiring to makes us eat more, or maybe we're infected with a virus that makes us fat. Should we throw up our hands and say, "Well, I can't do anything about it, so I'll just stay fat"? No, because obesity will affect your health, and it may kill you eventually. You will just have to work a little harder to get your weight under control.


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