seems to stimulate NPY secretion, while the female sex hormone, estrogen, seems to have a varying effect depending on the stage of a woman’s cycle.
But our bodies aren’t always perfect, and leptin doesn’t always work the way it’s supposed to. In some research, when leptin was given to mice, their appetites decreased, as expected. When it was given to people, they initially got thin, but then something strange happened: They overcame the surge of leptin and stopped losing weight. This indicates that our bodies have the ability to override leptin’s message that our tank is full. How? When leptin tells your defense—the satiety chemicals—to kick in and protect you against stray bonbons, the pleasure center in your brain says, “Uh, yeah, three more this-a-way” That surge from the pleasure center, which we’ll discuss in more detail in part 3, can overrule leptin’s messages that you’re full. That’s called leptin resistance (there’s another form of leptin resistance as well, which happens when cells stop accepting leptin’s messages). Most obese people, by the way, have high leptin levels; it’s just that their bodies have the second form of leptin resistance—they don’t receive and respond to leptin signals.
That doesn’t mean leptin is always on the losing end of this chemical battle. YOU-reka! The challenge is to let leptin do its job so that the brain demands less food. One way to do it: Walk thirty minutes a day and build a little muscle (that’s part of our activity plan in part 4). When you lose some weight, your cells become more sensitive and responsive to leptin.
FACTOID
Scientists found how ghrelin works accidentally: in gastric bypass surgery, doctors cut out the part of the stomach that secretes ghrelin. They soon realized that it wasn’t just the smaller stomach but the reduced ghrelin production that helped surgery patients eat less food. The eat-everything signal was shut off, clearing the way for the satiety center to take care of its business.
Ghrelin Is the Gremlin: The Hormone of Hunger
Your stomach and intestines do more than hold food and produce Richter-worthy belches. When your stomach’s empty, they release a feisty little chemical called ghrelin. When your stomach’s growling, it’s this gremlin of a hormone that’s controlling your body’s offense; it sends desperate messages that you need more points, you need to score, you need to FedEx the chili dogs to the GI tract immediately. Ghrelin makes you want to eat—by stimulating NPY. YOU-reka! To make things worse, when you diet through deprivation, the increased ghrelin secretion sends even more signals to eat, overriding your willpower and causing chemical reactions that give you little choice but to line your tongue with bits of beef jerky.
Ghrelin also promotes eating by increasing the secretion of growth hormone (ghre is the Indo-European root word for “growth”). So when you increase ghrelin levels, you stimulate that growth hormone to kick in, and growth hormone builds you not only up but out as well.
Your stomach secretes ghrelin in pulses every half hour, sending subtle chemical impulses to your brain—almost like subliminal biological messages (carrot cake, carrot cake, carrot cake). When you’re really hungry or dieting, those messages come fast—every twenty minutes or so—and they’re also amplified. So you get more signals and stronger signals that your body wants food. After long periods, your body can’t ignore those messages. That’s why sugar cookies usually trump willpower, and that’s why deprivation dieting can never work: YOU-reka! It’s impossible to fight the biology of your body. The chemical vicious cycle stops when you eat; when your stomach fills is when you reduce your ghrelin levels, thus reducing your appetite. So if you think your job is to resist biology, you’re going to lose that battle time after time. But if you can re-program your body so that you keep those ghrelin gremlins from making too much noise, then you’ve got a chance to keep your tank feeling like it’s always topped off.
Food Fight: The Ghrelin Versus Leptin Grudge Match
So now let’s get back to that offense and defense. The natural state is for you to have a give-and-take relationship between your eating and satiety chemicals—between your ghrelin and leptin levels—to influence NPY and CART, respectively. It’s the relationship between the impulse that says, “I’ll take a large pepperoni with extra cheese,” and the one that says, “No more passengers, this belly is full.”
This battle over eating isn’t between your willpower and the Belgian waffles; it’s between your brain chemicals. The NPY is the villain—encouraging you to buffets, driving you to the pantry, pointing its chemical finger to the convenience foods, while CART is your dietary guardian angel, which encourages a cascade of allies to keep you full and satisfied and in no way interested in creamed anything. Think of the two substances—NPY and CART—competing for the same parking space, the one that will ultimately determine whether or not you eat (see Figure 2.3). They both arrive at the same time and want that space. Either more NPY or more CART sneaks into the spot, thus sending the all-important go or stop signal to your brain to influence the hormones that make you feel full or hungry.
Here’s how they all work together: Ghrelin works in the short term, sending out those hunger signals twice an hour. Leptin, on the other hand, works in the long term, so if you can get your leptin levels high, you’ll have a greater ability to keep your hunger and appetite in check. Isn’t that great? Leptin can outrank ghrelin—to keep you from feeling like feasting on anything short of fingernails every few minutes. If you focus on ways to influence your leptin levels, and, more important, leptin effects (through leptin sensitivity), your brain (through CART) will help control your hunger.
Sometimes, it may seem like we don’t have much control over the chemical reactions taking place within our arteries or inside our brains. But just as you can control things like cholesterol and blood pressure by changing the foods you eat or altering your behaviors, you can also control the satiety center of your brain. How? Through your choice of foods.
Figure 2.3 In a Jam The satiety center is waiting to be turned off by NPY or stimulated by CART. Whichever fills up the receptor docks first is what controls whether you want to eat more or not. In turn, these two proteins are influenced by lack of water, sleep, and even sex. They’re also influenced by ghrelin coming from your stomach, which stimulates NPY so you get hungry, and leptin from your fat, which is further stimulated by a chemical called CCK, released from your intestines after a meal.
At least as far as your body is concerned, foods are drugs; they’re foreign substances that come in and switch around all those natural chemical processes going about their business within your body. When your body receives foods, different chemical reactions take place, and messages get sent throughout your system—turning on some things, turning off others. While your body internally gives orders, you set the tone and direction of those orders through the food you’re feeding it. Eat the right foods (like nuts), and your hormones will keep you feeling satisfied. But eat the wrong foods (like simple sugars), and you’ll cause your body to go haywire hormonally, and that ends up with one result: the next notch in your belt.
A major gang leader against your body is fructose, found in high-fructose corn syrup (HFCS), a sweetener in many processed foods. Here’s how it works: YOU-reka! When you eat calories from
