For the past three decades, both experts and laymen have accepted as gospel that exercise is the key to weight loss. People believed that it did so directly, by burning calories, and indirectly, by motivating people to eat more healthfully and reinforcing a feeling of well-being, even moral self-satisfaction. However, over the past decade or so, one study after another has put chinks in that theory, and we’ve learned that exercise alone doesn’t much affect weight loss. In fact, studies have shown that working out, by increasing the production of ghrelin, a hormone that stimulates appetite, often makes you hungrier.

The average person who’s overweight is going to be hard-pressed to expend enough energy and control diet sufficiently using exercise alone. For most people it’s not an effective tool for weight loss. However, recent studies hint that exercise of a certain form and duration has the capacity to change the way our body interacts with food in complex ways that give our appetite the stop sign.

Human appetite is an extremely complex mechanism, driven by signals from our brain, fat cells, glands, genes and nervous system. Over the past 15 years, science has uncovered numerous hormones, such as ghrelin, leptin (the long-term appetite-control hormone), the appetite suppressors PYY and POP, obestatin, CCK and glucose insulinotropic peptide (GIP), as well as some newly discovered hormones whose roles remain a mystery.

Each of these appetite-related hormones, such as ghrelin, plays a number of intricate roles in the body. It does far more than stimulate hunger and, as such, it is difficult to assume one particular effect—such as hunger stimulation—when it can also sequester growth hormone, promote anti-inflammatory processes, enhance memory and learning, and affect sleep and anxiety.

When people run at high intensity and burn more calories, the researchers thought that they’d compensate by eating more, but they ended up with a negative energy balance. The mode of exercise may also affect appetite. Swimming seems to make people very hungry, running not so much and cycling’s effect is somewhere in the middle. The effects of swimming may be due to changes in body temperature from being in and out of the water—and those changes may affect appetite. The brain is the most powerful regulator of metabolism. “You can ‘fool’ the brain into using less energy by dieting (which often ends up causing weight gain), or you can alter brain function with exercise which, over time, boosts your metabolism. The bottom line is that to keep your brain functioning optimally, you should never go on a diet. As people become better trained, they may need to more finely regulate their food intake.

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