Q: What's the best way to lose belly fat: diet or exercise?
A: You really need both, and here's why: Losing fat (all kinds of fat, not just the belly fat that covers your lower abs) requires an energy deficit. At the end of the day, you need your body to realize that it didn't take in enough caloric currency to pay for your daily efforts, forcing it to tap into its energy savings account (stored fat). The greater this deficit, the more weight you will lose.
Diet and exercise can both help you create an energy deficit and lose weight, but I'm assuming you don't just want what works, you want what works best. What gives you the biggest return for the time you invest
Why Just Dieting Doesn't Work in the Long Run
There are numerous hormones that impact weight loss, but for the sake of brevity we're going to focus on two: leptin and ghrelin. Leptin is a hormone released by your fat cells that signals you to eat less and burn more calories. Scientists have called leptin a "fat loss thermostat." The more active leptin in your system, the more fat you are going to burn. Drastically cutting calories decreases leptin release and its ability to signal the brain, making weight loss harder and eventually slower. This forces you to keep eating fewer and fewer calories to get the same weight-loss results.
If cutting calories is your only weight-loss strategy, you'll need to keep your calories low for a very long time to ensure that you don't gain the weight back. Many people justify this, telling themselves "it's worth it and I'll just adjust to eating less." Unfortunately, research shows that you don't adjust to eating less and that even after a year of calorie restriction, one of your hunger hormones, grehlin, is as high as ever, making you as hungry as the first day you set out on your lean body journey.
Why Just Exercising Doesn't Work in the Long Run
The health benefits of regular exercise are too many to count. But as anyone who has lost a lot of weight can attest, burning off your extra flab at the gym is a long and arduous process. And if you aren't using a modestly calorie-restricted diet to support your efforts, it's even longer. This is primarily because eating extra calories takes no time at all (I'll just have one more heaping tablespoon of peanut butter), while burning off those calories can take more than an hour, depending on your preferred mode of exercise. There just isn't enough time in the day! What's more, numerous studies show that exercise (especially aerobic exercise) alone does not cause significant and long-term weight loss.
The combination of diet and exercise works best for weight loss. Why? Because optimal weight loss isn't just about creating an energy deficit. Optimal weight loss is about creating an energy deficit and optimizing weight-loss hormones that will allow for sustained and permanent weight loss.