Fat’s Bad Rap
Sure, nobody likes body fat especially when it beats you through the door by five or six seconds. But despite potentially serious consequences, fat, by nature, is good. (That’s not a typo.) Besides helping Santa hopefuls land December jobs, it also helps your cells function and provides insulation. Most of your fat is stored in a reservoir throughout your body. You have drums and drums of it sitting passively, just waiting to be burned. But you have another kind of fat, too. It’s called brown fat and is usually found on the back of your neck and around your arteries (and has absolutely nothing to do with how much chocolate you eat). This increases in outdoor workers during cold spells to protect them from the weather; it insulates our vital organs. Though you have a fairly small percentage of brown fat as an adult about one-third of fat in babies is brown fat and it’s used primarily to keep them warm. What makes brown fat different? YOU-reka! Brown fat is alive. It has nerve fibers, like any organ, and it also has leptin receptors. When the level of this hormone goes up, it turns on energy consumption in the brown fat and burns it. This is important because it shows that the right leptin levels can signal you to immediately get rid of this fat. And it’s also symbolic of the inherent goodness of body fat-when it’s found in the right amounts.
That sign? Satisfaction.
As you change from always thinking about diet to never thinking about it, you will be reprogramming your body so that it’s not your eyes, tongue, or overzealous utensils that will guide you.
YOU-reka! Instead, it will be the chemicals in your brain and body.
By tuning in to your body’s signals, you’ll allow your anatomy to work the way it’s supposed to: so that you’ll never be famished, you’ll never pop a button at the table, and you’ll never bounce between hunger extremes. Instead, you’ll get a little hungry, you’ll eat, you’ll stop. Satisfied.
The Anatomy of Appetite
You’d think that the first place we’d start to talk about how appetite influences fat would be the spot that’s covered by an XXXXL shirt. But to understand appetite, you have to navigate farther north—to the place that may hold the least fat. In your brain, you’ll find the hypothalamus, a key command center for your body. Among the biological functions it controls are your temperature, your metabolism, and your sex drive. Located in the center of your brain, the hypothalamus (see Figure 2.1) also coordinates your behaviors that involve appetite—not just for food but also for thirst and even for sex. So while it may appear that call-to-duty signals come from your stomach growling or your loins tingling like a static shock, it’s actually your brain that’s sending out the signals that you crave either a quiche or a quickie. (At least one person we know helped curb an eating problem by having regular, monogamous, healthy sex. When the appetite function for sex was satisfied, the appetite function for food was diverted.)
FACTOID
As you get older, you have fewer leptin receptors in your hypothalamus-meaning that you have fewer satiety signals, which makes you more prone to gaining weight.
Hidden in your hypothalamus, you have a satiety center that regulates your appetite. It is controlled by two counterbalancing chemicals that are located side by side (see Figure 2.2).
The satiety chemicals led by CART (the C stands for cocaine and the A for amphetamine, since these drugs put this chemical into overdrive). CART stimulates the surrounding hypothalamus to increase metabolism, reduce appetite, and increase insulin to deliver energy to muscle cells rather than be stored as fat.
The eating chemicals driven by NPY (a protein called neuropeptide Y). NPY has the opposite effect on the hypothalamus; it decreases metabolism and increases appetite.
Think of these two command chemicals as any game or sport that involves offense and defense, like soccer, checkers, or even dating. The offense is always trying to make advances, trying to score points, and trying to attack, while the defense protects its territory.
Your eating chemicals play offense. They want as many points as possible, so they fire off those signals for your body to score: eat, eat, eat, calories, calories, calories, chimichanga, chimichanga, chimichanga. The biological message: Prevent starvation by eating. Meanwhile, your satiety chemicals play defense, like a goalkeeper, the back row of checkers, or a protective parent. They send the messages to your brain that you’re full, to shield you from steadily pumping bacon-wrapped scallops down your gullet. How do we know these centers work this way? For one, by looking at extremes and seeing what happens when the feeding system is turned completely on or off. When we study animal models, we see that if a rat’s eating center is destroyed, for instance, it forever forgets to eat. The resulting severe anorexia starves the body of all energy and nutrients so that it withers away to the approximate width of an envelope. In rats whose eating center is overstimulated, though, food is always on the radar screen. And those rats eat themselves to death—literally—by increasing their fat-induced diseases like diabetes, hypertension, and arthritis.
Figure 2.1 Food Fight In your hypothalamus, you have hunger and satiety chemicals. The hormone leptin goes to the satiety center to make you feel full and satisfied, while the signal from the hormone ghrelin makes you want to eat gorge, and slobber over your every feast.
Figure 2.2 Chemical Reaction If we look closely at the hypothalamus, we see that a small nucleus at the bottom houses NPY and CART, which fight the yin-yang battle to control the brain biochemistry of hunger. Each chemical readily travels to other nuclei in the hypothalamus. NPY causes our temperature to drop and our metabolism to decrease as we feel hungry. CART stimulates the opposite influence. The nearby mammillary body (literally shaped like a nipple) is part of our limbic system, where we store memories and emotions—just the right combination to create a craving for a favorite food. The thalamus is the body’s relay station and rapidly transmits orders throughout the brain based on the desires of the eating center.
FACTOID
CART (cocaine-amphetamine-regulatory transcript for those scoring at home) is the reason why cocaine addicts don’t gain weight. Cocaine and amphetamine stimulate this chemical, giving you a double brain bat to help you control appetite and increase metabolism. It’s unclear whether CART will be the basis for effective weight-loss treatments, but researchers are studying the neurological effects these drugs have on appetite to see if they could lead to long-term pharmaceutical solutions to weight loss (without of course, the dangerous side effects of illicit drugs). Marijuana, by the way, has its own receptors that overwhelm leptin, which is one big reason why pot smokers get the munchies. It’s also an area that’s a promising new approach to weight-loss drugs. By figuring out how the drug turns off the gene that produces leptin, we’ll be able to figure out how to flick it on-to keep leptin (and thus satiety levels) high. The prototype drug has done great in trials and symbolizes a new generation of smart weight-loss medications that work hormonally.
In a perfect system, your offense and defense complement each other; you get the foods you need and stop when you’ve had enough. Unfortunately for everyone except elastic-waistband manufacturers, a lot of things can mess up those systems (many of which we’ll discuss in a moment). But these obstacles aren’t insurmountable. You can take comfort (and find motivation) in the fact that your body wants you to reach