When it comes to weight control, appetite isn't everything. Our relative leanness—or lack thereof—also depends on how our bodies balance the storage and burning of dietary fat. And what we eat can make a big difference. Fat, it turns out, carries instructions about how it should be used. Saturated fats, the type found in meat and dairy products, are hard for cells to break down, so they tend to get tucked away. Unsaturated fats, found in olive oil and other plant oils, are readily consumed for energy. What's more, they direct the body to burn more fat.
Having too much fat lying around is bad because it can trigger insulin resistance—the first step on the path to diabetes. Fat encourages muscle to reject glucose as an energy source. To keep this sugar from building up in the blood, the pancreas produces extra insulin. Over time, however, the pancreas becomes overworked and can no longer compensate. So blood sugar rises and diabetes develops.
Exercise improves the situation, because the stretching of muscle fibers provokes cells to take up glucose, removing it from the blood. Unfortunately, more weight usually goes hand in hand with less exercise—even in mice. Rodents treated to a "Western" diet, rich in fat, grow pudgy and sluggish, sitting more often than they scurry. In humans, a high-fat diet coupled with poor exercise leads to syndrome X, a metabolic disorder characterized by insulin resistance, high blood pressure, and heart disease.