A Matter of Fat

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By Michelle Andrews, Women's Health

Last Updated: 12/01/2006 09:18:26

Saddlebags, spare tires, muffin tops we've dreamed up as many names for body fat as Starbucks has for coffee. Even as otherwise sensible, well-adjusted adults, many of us spend a huge amount of time dwelling on our double chins, chubby ankles, or flabby arms. We're both fixated and repulsed: We hate our fat, but we can't stop thinking about it.

Well, stop obsessing about your love handles for a minute and ruminate on this: your fat doesn't just sit there glob-like and idle. Fat is actually a great big gland, churning out hormones and other chemical substances that are essential to many bodily functions. Depending on where it's located, fat may protect you from developing heart disease, high blood pressure, and diabetes. Much of this research is just emerging, but when you've heard the latest, you'll never look at that little pooch over your waistband the same way again.

Fat Fundamentals
Basically, fat is stored energy. When you eat, your body transforms carbohydrates, protein, and dietary fat into fatty acids (chains of molecules that are the building blocks of body fat), glucose (blood sugar), or amino acids. These provide energy that you either burn right away or pack up for later. Without body fat, you'd have to eat all the time just to keep your body functioning, your heart beating, your eyes moving across the page, and your hand traveling to your mouth to sip your latte.

Fat that isn't used right away gets stored in cells. If you looked at one of them under a microscope, you'd see standard cell equipment — a nucleus, mitochondria, that sort of thing dwarfed by a big fat droplet that makes up about 85 percent of the cell's volume. Fat cells typically start at 5 millionths of a meter in diameter, too tiny to be seen by the naked eye. But they're insanely elastic. each one can increase by 100 times in volume, to about the size of the period at the end of this sentence — if you keep stuffing pizza down your throat. For a cell, that's positively ginormous. For the longest time researchers thought that fat cells were kind of like height: you kept adding to them until you passed puberty, then stopped. But a 2004 study published in the American Journal of Physiology: Endocrinology and Metabolism found that people actually create new fat cells throughout their adult lives. When a fat cell is full to bursting (i.e., period size), it sends a chemical signal to surrounding tissue to create new ones. While an average nonoverweight adult has roughly 30 to 40 billion fat cells, someone who's very obese might have as many as 100 billion. And once you've got a fat cell, there's no way to get rid of it — even by losing weight — unless you suck it out.

Beefy Buffer Zone
In addition to serving as ingenious soft-sided storage bins for our body's energy, fat protects us in many ways. For one thing, it can act as a cushion to keep us from being injured. A recent study found that plump men were more likely than thin ones to survive car crashes. Unfortunately, the results didn't hold true for chubby women, perhaps because our fat is typically concentrated in the lower body, below most car accidents' point of impact.

Fat also provides a layer of insulation that keeps us warm. Newborns have pockets of a kind of fat called brown adipose tissue — as opposed to white adipose tissue, or regular fat in their neck and shoulders that actually generates heat. These pockets disappear by adulthood, but fat continues to be important to the survival of our species: If a woman's body fat drops below about 18 percent, she stops menstruating and can't reproduce. Nature decides she isn't a good candidate for nourishing an infant.

Most important, fat is insurance against starvation. Imagine for a moment that Kate Hudson and her chunky cousin (okay, we're not sure she has one) are stranded on a desert island with only water to drink. Kate, assuming she weighs 120 pounds and has 20 percent body fat, could live for 65 days on her fat stores. Her cousin, at 150 pounds and 30 percent body fat, could last for 105 days, significantly increasing her chances of rescue. What if the two tried to swim to safety? The heftier gal wins again. That's because fat floats; muscle, which is heavier, doesn't.

Busy Body
Fat doesn't just keep us safe and warm (and above water). It's a busy endocrine factory, secreting substances that play a role in everything from regulating weight to constricting blood vessels. It was the discovery of the hormone leptin in 1994 that woke researchers up to fat's active side. Fat produces leptin, which then travels to the hypothalamus, the part of the brain that controls appetite. There it binds to receptors that send messages signaling that the body is full.

All of that said (you knew this was coming), fat does have its downsides. In the past decade, scientists have uncovered more than 100 biochemical substances, called adipokines, created by our fat. Many cause inflammation in tissues and blood vessels, which can raise the risk of heart attack and stroke. Others increase blood pressure or cause clots. Several make our bodies resistant to the effects of insulin, which is what helps move glucose out of the bloodstream and into tissues and organs where it can be used immediately or stored for energy. About the only bright spot among the adipokines is a hormone called adiponectin, which actually improves the body's response to insulin and reduces the clogging effects of fat on arteries. Unlike the other adipokines, however, you actually make less of this hormone, as you get fatter, though no one knows why for sure.

Some fat, specifically, the fat that clusters around our internal organs also secretes a form of estrogen in both women and men. (The fat in hips and thighs doesn't.) That can be good and bad. If you're an overweight woman who hasn't yet hit menopause, the extra estrogen may reduce your risk of osteoporosis. On the flip side, it may increase your risk of breast cancer. Although researchers don't know a whole lot about the connection between fat and breast cancer, they suspect that the proximity of visceral fat to our breasts might account for the increased risk. Overweight men have cause to worry too, the high levels of estrogen generated by their excess fat can sometimes give them extra breast tissue and can even make them impotent.

But the relationship between fat and estrogen is even more involved than that. Our ovaries produce more estrogen than our fat does. When women reach menopause and the estrogen produced by our ovaries gradually tapers off, a funny thing happens to our fat distribution: It migrates from our hips and thighs to our waists. Researchers think that the lack of estrogen at menopause may play a role in driving our fat northward.

Location, Location, Location
Right about now you're thinking, okay, how much of this stuff do I have to worry about? The body of a nonoverweight woman in her 20s and 30s is about 20 to 25 percent fat. By the time she reaches her 50s and 60s, that number will increase to about 28 to 33 percent. (Men are generally leaner: Young men have just 15 to 20 percent body fat.) More research needs to be done, but scientists do know that "females are organized to store fat more efficiently than men," says Susan Fried, Ph.D., an obesity researcher at the University of Maryland medical school. We pack it away in bigger fat cells and release it very begrudgingly, thanks to higher levels of something called lipoprotein lipase. This enzyme controls the ability of fat cells to grab onto and store away the fat that's circulating in our blood after a meal.

No matter how your fat is distributed, though, about 80 percent of it is deposited in squishy pads just beneath the skin of your thighs, butt, stomach, and chest, with smaller pouches in your arms and lower legs. While you may be willing to wager that all of your body fat has made a home on your rear end, it's actually packed around your organs, too, and inside your muscles, like marbling on a steak.Where your fat is concentrated can make a big difference to your health. Women are more likely to carry fat in their hips, thighs, and buttocks. Researchers are still trying to figure out why precisely this is, but let's face it: you don't need a scientific study for proof. Just look down. But what you probably don't realize is that people who haul around a caboose "pears" in popular fruit lingo have higher HDL (good) cholesterol and lower triglycerides and insulin, putting them at lower risk for heart disease, high blood pressure, and diabetes. Turns out our hips and thighs provide a roomy storage area for fatty acids to collect instead of circulating through our bloodstream, where they could turn into potentially damaging unstable molecules called free radicals.

For reasons researchers don't yet know, men are much more likely than women to store fat in their midsections though plenty of women have this "apple" shape as well. We're not talking about that loose flab that makes you rethink leaving the house in your low-rise jeans. That just-below-the-skin, or subcutaneous, fat isn't harmful. The bad stomach fat, called visceral fat, is packed around your liver, heart, and other organs. It can clog your arteries and drain right into your liver, where it can impair the organ's vital functions, like converting food into nutrients and removing harmful substances, remember last night's lychee martinis?- from your system.

People who have more visceral fat are at higher risk for heart disease, cancer, and diabetes. In fact, many studies have found that waist circumference, rather than body mass index (BMI), is the more important factor in determining a person's risk for disease. The good news is that if you lose weight, you'll lose visceral fat more quickly than the kind around your hips and thighs because it's more easily broken down.

Blubber Bug
One of the most fascinating areas of fat research has to do with the ways that viruses can affect fat. Scientists have discovered that if they inject animals with one of several common viruses that typically cause colds or other minor ailments, the animals get fat. This leads to an intriguing question: Is it possible that we can actually catch fatness, just like we catch a stomach bug?

Nikhil Dhurandhar, Ph. D., an associate professor at Pennington Biomedical Research Center, thinks we can. Dr. Dhurandhar first made the fat/virus -connection while working in Bombay in the 1980s, when he noticed that chickens dying of a virus got fat instead of wasting away. After finding a lab in the United States to fund his research, he injected AD-36, an adenovirus (the most common type of virus around) that typically causes diarrhea, into monkeys, chickens, and mice. Lo and behold, they all porked up.

Then he tested the blood of 500 people, both fat and thin, figuring that those who had been exposed to the virus would have developed antibodies to it. Bingo. Thirty percent of the test subjects who were obese had antibodies to AD-36, compared with just 11 percent of nonobese subjects, indicating that they had probably been infected by the virus. Until further research is done, Dr. Dhurandhar can't be certain that the virus actually played a role in making those people fat. It's also unclear how they were exposed to the virus, though animals in Dr. Dhurandhar's lab became infected when they were put in a cage with others that carried the virus. There are about 50 adenoviruses. To date, researchers have identified nine pathogens, including four adenoviruses that cause obesity in animals. But so far, AD-36 is the only one that researchers have tested humans for.

Now, if you're looking down at your body and thinking, Did I catch this paunch from that enormous guy at the deli?!? Stop it right now. For one thing, scientists have no idea yet whether a human being can catch the fat virus from another person (though they're looking into it). For another, worry can actually make the problem worse. When you're stressed out, your adrenal glands which are located on top of your kidneys and produce hormones that control your heart rate and other bodily functions pump out the hormone cortisol. No one's sure why, but one theory is that cortisol triggers an increase in another hormone called 11 Beta HSD. High levels of this hormone in the blood cause your body to add more stores of visceral fat.

So chill. And think about the fact that fat is so much more interesting than you'd imagined. You're actually ready to embrace it, right? Fat chance. But knowing that at least some of it is helping keep you healthy may make you a little more forgiving of your own muffin top.