It's not just the empty calories you need to worry about when it comes to junk food; it's how these foods affect your insulin. Jason Fung, MD, a nephrologist, explained that processed carbs and sugary foods such as doughnuts and white bread spike your insulin levels, which can cause weight gain. Keeping your insulin levels low will help you reduce your overall body fat percentage, and that means belly fat.
Aside from being high in carbs, these foods also tend to be high in calories. That's not the only problem, though — eating them doesn't offer your body any nutrition, so you end up feeling hungry, which makes you eat more. Eating sugary foods also increases your cravings for them, again, making you eat more. If you cut down on these types of food, you'll definitely notice a difference in how you feel and how your belly looks.