Cutting the calories from sugar-spiked soda is a no-brainer: At 150 calories a can, those can add up to serious poundage. But diet soda packs on the pounds as well-it's just more passive-aggressive about it. "Artificial sweeteners affect our sense of satiety," says Isabel Smith, RD, of Isabel Smith Nutrition. "Our bodies have evolutionarily developed to expect a large amount of calories when we take in something exceedingly sweet, and those artificial sweeteners are from 400 times to 8,000 times sweeter than sugar. It causes a couple things to happen: The muscles in your stomach relax so you can take in food, and hormones are released. With artificial sweeteners, your body says, 'Wait a minute, you told me you were going to give me all this high-calorie food.' It can actually send some people searching for more food, out of lack of satisfaction."

"Even though diet drinks are calorie-free, they cause insulin to be released in your gut because their artificial sweeteners are sweet like sugar, and that actually prevents weight loss," says Miriam Jacobson, RD. "Insulin is your body's primary fat-storage hormone," she explains, adding, "Trying to lose weight by trading a Coke for a Diet Coke is doing the body just as much harm, because of all the chemicals in the calorie-free version." While you're weaning yourself off of your soda habit, add these fat burning foods to your diet, some of which actually block adipogenesis.

3. You'll Get Sick Less

The acidity in soda is bad news for your digestive system, eroding tooth enamel and worsening acid reflux. But diet sodas are especially treacherous for your gut-and the far-reaching bodily systems it affects. "Researchers are finding that artificial sweeteners may affect our healthy gut bacteria, which can affect everything from blood-sugar control to weight management to disease-how our immune system works and how our body responds to infection," says Smith.