Several studies have suggested that diet soda causes weight gain, not loss. In one research study in particular, which included 749 people who were 65 and older, those who had sugar-free sodas gained three times more weight around the waist – 3.2 inches – than those who didn’t. Some studies have shown that alcohol decreases fat burn in the stomach. That’s why nutritionists say that the fastest way to lose belly fat is by not drinking booze. Belly fat is closely linked to higher risk of heart diseases, inflammation and Type 2 diabetes.