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How Does Your Body Really Deal With Appetite, Hunger and Physical Activity?


Our Relationship With Food
Think 11
The Hunger Hormone: Ghrelin
The Fat Hormone: Leptin
Exercise vs. Drugs
Common Sense


Somewhere in your brain, there's a chocolate circuit. how it works is not entirely clear, and you couldn't see it even if you knew where to look. But it's there all the same and it's a powerful thing. You didn't pop out of the womb prewired for chocolate, but long ago, early in your babyhood, you got your first taste of one, and instantly a series of sensory, metabolic and neurochemical fireworks went off.

The mesolimbic region in the center of your brain the area that processes pleasure lit up. The vagus nerve flashed signals to the stomach, which began to secrete digestive acids. The pancreas began churning out insulin. The liver cranked up to refine the body's chemistry to accommodate the sugar and fat and starch that were coming in. As all those complex processes were unfolding, your midbrain filed away a simple, primal, unconscious idea: Chocolate is good. A lifetime love affair perhaps pleasant, perhaps tortured began.


Our Relationship With Food


Human beings have always had a complicated relationship with food. Staying alive from day to day requires our bodies to keep a lot of systems running just so, but most of them circulatory, respiratory, neurological, endocrine operate automatically. Eating is different. Like sex, it's a voluntary thing. And like sex, it's an essential element to keep the species going. So nature cleverly rigs the game, making sure we pursue them both by making sure we can't resist them. In the case of food, that has lately spelled trouble. Human history has usually been characterized by too little to eat rather than too much. Nature never planned for what could happen when unchecked appetites were suddenly matched by unchecked resources. But we're seeing it now.

Postindustrial humans as any trip to an all-you-can-eat buffet will tell you have become a soft, sedentary, overfed lot. It's not just that almost 70% of the U.S. population is either overweight or obese, people are getting fatter in all parts of the world. The fact is, when faced with an abundance, we know that we're full, but seem helpless to control ourselves. We lose weight and routinely regain it; we vow to eat healthfully and almost always lapse. Our doctors warn us about our rising blood pressure and creeping cholesterol, and we get briefly spooked until we're offered the next helping of cheesecake or curly fries, our appetite shouts down our reason and before we know it, we're at it again.

Just why is our appetite such a powerful a driver of our behavior, and, more important, how can we bring it to heel? If that question has long defied easy answers, it's no wonder. Understanding a single biological unit the heart, the lungs is hard enough. Understanding a process as complex as appetite one that involves taste, smell, sight, texture, brain chemistry, gut chemistry, metabolism, evolution and, most confounding of all, psychology is exponentially harder. But science is trying to explain it.

Researchers in labs and institutes around the world are peering into the brain to understand the regions where appetite is perceived and satisfied, and pinpointing the receptors on cell surfaces that keep us hungry or get us sated. They're studying the neural wiring of the stomach and intestines, as well as the operation of the genes that drive our appetite, to track how satiety signals are sent and determine why they sometimes get lost. And they're peering back into human history to understand better how we were booby-trapped for overeating from the start and how we might be able, so many eons later, to cut the trip wire at last. "The scourge of body-weight disregulation has become a leading cause of death worldwide," says Dr. David Cummings, an associate professor of medicine at the University of Washington. "Understanding it is perhaps the most compelling agenda in the field of medical research."

If you're among the millions who have ballooned past their target weight, you can take some consolation from the fact that your early ancestors would be very proud of you. Human beings emerged into a world in which food often was scarce, often spoiled and when we learned the art of hunting sometimes bit back, making the idea of eating a lot when you could both sensible and necessary. If you never knew when you were going to have dinner again, it was best to gorge when the food was there.

"We were hardwired to eat and eat and particularly eat fatty foods because we didn't get them often," says Sharman Apt Russell, author of Hunger: An Unnatural History. We're programmed not only to overeat but also to fail to recognize immediately just when we've reached that point. Mothers tell kids not to wolf their food because it's harder to enjoy it that way and also because even after you've had enough, it can take a while for your brain to get the message. By the time it does, you're not just full; you're stuffed. "The people who didn't immediately lose their appetites, who could gorge themselves and keep going, those people would survive longer during the next famine," says Dr. Jeffrey Flier, obesity scientist and professor at Harvard Medical School.

That's not to say that your body doesn't work hard to keep itself balanced. Over the course of a year, the average adult male consumes about 900,000 calories, yet his weight may not rise or fall by more than a pound. Since a pound equals about 4,000 calories, that means his annual intake is just 0.4% or 11 calories a day above or below precisely what he needs to keep going. "You are within a potato chip a day of matching your intake with expenditure," says Randy Seeley, professor of psychiatry and associate director of the Obesity Research Center at the University of Cincinnati.


Think 11

That's why exercise is so critical to the equation. The burning of fuel is essential to our total energy requirement. If for example, our total energy requirement is equal to 2000 calories per day and we've ingested 2011 calories per day. We need to have a plan to get rid of those extra 11 calories, since we've ingested more than we've expended. Regardless of what types of foods these extra 11 calories consist of, whether it be a liquid or solid, fat or protein, fruit or chocolate bar, salad or french fries, our body is inevitably programmed to turn these extra calories into fat. We will then store this fat until it is needed for energy.

In theory, if we continue to maintain this calorie surplus daily, we would end up storing an additional 84 calories per week, 336 calories per month and 4368 calories per year (that's over 1 pound of fat). Keep in mind that this calculation is based on exceeding your daily energy expenditure by only 11 calories. That's equivalent to just over 1 cup of lettuce, 2 cups of plain black coffee, 1 M&M peanut or 1 glass of Kool-Aid. Since most people exceed their calorie expenditure by far more than 11 calories per day, it's no surprise that obesity is becoming the number one health problem in developed nations.

It takes a lot to maintain such a precisely balanced cycle of fueling and burning, and in most cases, it all starts with the clock. Like other animals, we are creatures of dietary habit. Feed us at 8 a.m., 1 p.m. and 7 p.m., and we learn to get hungry as those hours approach. Throw in a snack at 3:30 or before bedtime, and we get the itch then too. At all these moments, what's fueling the feeling is a substance called ghrelin.


The Hunger Hormone: Ghrelin


Identified in 1999, ghrelin is often called the hunger hormone because that precisely captures what it does. Ghrelin is produced in the gut in response to meal schedules and, according to some theories, the mere sight or smell of food and is designed to give rise to the empty feeling we recognize as wanting to eat. When ghrelin hits the brain, it heads straight for three areas: the hindbrain, which controls the body's automatic, unconscious processes; the hypothalamus, which governs metabolism; and the mesolimbic reward center in the midbrain, where feelings of pleasure and satisfaction are processed. That's a neural triple play that guarantees that when ghrelin talks, the brain will listen.

Cummings has conducted studies in which he measured the hormone in people's blood every 20 min. and found that levels reliably spike as mealtimes approach. Add or subtract a daily meal, and you soon gain or lose a surge. "Grazing animals have little spikes of ghrelin all day long 20 to 30 in the case of a rat," Cummings says. One of the reasons gastric-bypass surgery can work in severely obese people apart from the fact that it reduces the carrying capacity of the stomach is that it also appears to turn down the ghrelin spigot. An Italian study even looked at ghrelin in anorexics and found that levels of the hormone were chronically high a chemical alarm that the self-starvers trained themselves to ignore. All this research confirmed ghrelin's role in driving appetite, both when we really need to eat and when we merely expect to.

If ghrelin were all there was to it, we and the rats would eat ourselves to death. But even as one system is gunning our hunger higher, another is standing by to slow things down. The first step in that appetite-taming process occurs in the stomach and upper intestine, where nerves that sense stretching and distension eventually alert the brain that we're getting full. That message is reinforced by three substances that travel northward from the gut. The first, a peptide released by the upper intestine called cholecystokinin (CCK), is the most fleeting of the three, reaching the brain and increasing the feeling of heavy satisfaction that prods you to push away from the table. But CCK does not last long, certainly not long enough to prevent you from eating again well before your body needs more fuel.

Racing in after CCK are two hormones, GLP-1 and PYY, that really slam on the brakes. Produced in the lower gut, they not only tell your brain you've had enough but also tell your stomach to stop what it's doing and not move anything further along into the intestines where the real business of digestion takes place until what's there has been broken down some. If you've ever finished a heavy meal at 8:30 p.m. and realized that you still feel stuffed when you climb into bed at 11, that's why. What's more, GLP-1 adjusts blood chemistry, stimulating the pancreas to release more insulin, which soaks up sugars released into the blood by the in rushing food and stores them in the body's fat deposits. "These two hormones go beyond meal intake and regulate overall energy balance," says Hans-Rudolf Berthoud, head of the neurobiology and nutrition laboratory at the Pennington Biomedical Research Center in Baton Rouge, La.


The Fat Hormone: Leptin


If despite all those obstacles in the path of overeating you still pack in too much food and as a result pack on too much fat the body has one other, much bigger gun it can roll out: leptin. An appetite-suppressing hormone discovered in 1994, leptin is produced by body fat itself, usually in direct proportion to how much of the tissue you're carrying. The fatter you are, the more leptin you produce. Once in the bloodstream, the hormone travels to the hypothalamus, one of the same brain regions targeted by ghrelin, seeks out a pair of neuropeptides known to stimulate appetite and partly muffles their signals. The result is, or should be, that fatter people want to eat less. Not surprisingly, the discovery of leptin was huge news in the diet community. Maybe obese people were simply suffering from a shortage of leptin; supplement the hormone with periodic injections, and the fat would dissolve away.

As it turned out, things weren't so easy. For one thing, there are hundreds of millions of obese people in the world, but even after 13 years of study, researchers have found only a handful on the order of 10 to 20 with a congenital deficit in leptin production or function. In fact, the leptin system in most overweight people works precisely the way it's supposed to, with hormone levels climbing more or less in lockstep with weight. The problem is, at some point the stuff simply stops working or at least stops keeping pace with the numbers on the scale. When the few people born with a leptin deficit are given supplemental injections, they respond to the treatment. But in other obese people whose systems have been overexposed to the hormone over the years and thus grown resistant to it the treatments do no good at all. Once again that's where exercise comes to the rescue. Studies have shown that leptin sensitivity can be improved by exercising and losing body fat. Exercise will lower the levels of leptinin the body - no matter how fat a person is. A team at the Harvard School of Public Health found that, fat or thin, men who exercised the most had lower levels of leptin in their blood.

If we get more exercise, we can lower your leptin levels, even if our BMI stays the same. BMI stands for body mass index, and although not an accurate measurement of health, it is used by some researchers to calculate obesity. Scientists have been intrigued since the hormone leptin was discovered, and became excited when they found that injecting leptin into rats could cause them to lose weight. Sadly, the same is not true for humans. They found that the human metabolism was most effective at reducing the amount of leptin in our blood by decreasing body fat through exercise and a good diet.


Exercise vs. Drugs


If we haven't yet figured out how to tame our need to eat, one reason may be that ghrelin, leptin and the handful of other gut chemicals are only the big dogs of the appetite-control system. Researchers have discovered at least two dozen other hormones and peptides that play a role too. Adjusting the levels of just the few we know best is a little like upgrading the quality of the gas in your car and thinking that is going to boost it from 20 m.p.g. to 75 m.p.g. You may notice some improvement, but if you really want a better, more efficient machine, you have to open the hood and retool things in a much more fundamental way...exercise.

Many health experts predict that most future studies will inevitably conclude that exercise will do more and better than any drug, hormone or chemical therapy ever can to tweak our metabolism. Between researching appetite-controlling receptor sites, genetic variances, mutations, blocking pathways and other physiological entities that control our metabolism, it's obvious that drug therapy is the underlying goal for most studies which are funded by drug companies. The problem is, with each drug that is developed to tweak a specific chemical in the body, there is always an unwanted side-effect from its very specific action. It's kind of like trying to fix specific components of an engine, when the real problem is the gas itself.

The body has an innate ability to fine tune every chemical process in unison to effectively control our energy-burning motor, something a drug has never been able to do without serious side-effects. It's just the way our body is designed. Artificial chemical intervention has consistently created more problems than benefits, and probably always will.

Exercise time and time again is proving to control our health better than any drug. We know that exercise alone thwarts pre-diabetic syndrome and prevents diabetes by improving markers for glucose and fat metabolism. A large randomized clinical trial proved that lifestyle modification is better than drugs. Exercise also helps control and reduce dangerous levels of cholesterol and high blood pressure by directly preventing these diseases. There is also mounting evidence that vigorous exercise and bursts of exercise are extremely valuable for long-term weight loss and controlling visceral fat which also influences appetite. The benefits to exercise are endless and postively impact the human body and our appetite naturally without chemical intervention, without side-effects and with all the enhancements we need to propel our health where we need it most.


Common Sense


So besides exercise, what can we do on the diet front? At the moment, some of the research in the kitchen involves trying to find a more precise way to balance the glucose loads various foods deliver to the body. That's important, since the bigger the glucose hit, the greater the sense of satiation, but only for a little while. Afterward, hunger returns stronger than ever. "High glycemic foods" like refined breads and sugars push the body to refuel," says nutrition scientist Marlene Most, head of the metabolic kitchen. "In low glycemic foods, there is a constant flow of glucose and insulin, so we don't need to refuel as much."

It's all about common sense. Barbara Rolls, a professor of nutritional sciences at Pennsylvania State University, advocates another way to attack hunger even more aggressively. Rolls currently tops the best-seller lists with a book about what she calls the "volumetrics" eating plan the kind of prefab word that cries out diet fad but in this case describes a sensible idea, provided that it's followed in moderation. The key to volumetrics, Rolls explains, is to consume foods that are high in volume but not in calories in order to stimulate the digestive system's distension nerves. It's the difference between, say, a large, filling salad with a low-calorie load and a small, unfilling brownie with a high one.

"This whole idea of eating smaller portions I'm really fed up with it," Rolls says. "It's not big portions that make you eat more. It's big portions of calories. If you eat big portions of fruits and vegetables, they displace other foods." Rolls stresses that it's important to eat a variety of tastes and textures. If you overload on one thing say, the heavy dose of meats that the low-carbohydrate Atkins plan recommends you're going to crave the sweet or crunchy or doughy experience of the fruits and breads you're forbidden. "It's called sensory-specific satiety," she says, and it's one of the reasons we still have the appetite for a sweet dessert even after we stuff ourselves with a heavy dinner.

The very discordance between a mouthful term like sensory-specific satiety and the uncomplicated joy of a crème brûlée at the end of a meal speaks to the puzzle that is the human appetite. We may always be pleasure-seeking creatures, intoxicated by the very experience of food with its colors and textures and notes of flavor but that doesn't mean our ancient impulse to eat whenever we can must always yield to our modern ability to satisfy that urge. The same human brain that invented the food court and the supermarket must now develop ways to control how and when we use them. Physical activity and balanced nutrition are key to our survival. Just as when we were learning to hunt and eat on the savanna, our health was at stake then, just as it is today.


 

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