The problem is that when you rely on exercise alone, it often backfires, for a couple of reasons. This is partly because of exercise’s effects on the hunger and appetite hormones, which make you feel noticeably hungrier after exercise. “If you walk briskly for an hour and burn 400 kcal,” says Klein, “and then have a beer and a slice of pizza afterwards because the exercise made you feel hungry…you will eat more calories than you have burned.” It may not always be beer and pizza, but people do tend to naturally compensate for the calories they expend.

The size of your plates and bowls may also have a lot to do with portion control. If you have large dinner plates, the temptation is there to fill them up. Try dining from a side plate instead, and eat slowly so your body has a chance to register that you're no longer hungry. Measuring cups are also useful. If you want a snack, allow yourself half a cup. Measure it into a bowl and don't refill. It's a lot easier to manage portions when you measure them.
One study showed that older women who lost at least 5% of their body weight lowered their chances of breast cancer by 12%. There’s no such clear proof that losing weight protects you from other types, but some changes that happen when you shed pounds hint that it might. For example, overweight people who slim down have lower levels of some hormones linked to cancer, like estrogens, insulin, and androgens.