You can drink diet soda while fasting because it typically contains zero or very few calories and no sugar, so it does not technically break a fast from a caloric perspective. However, diet sodas contain artificial sweeteners like aspartame or sucralose, which may trigger insulin responses or affect gut microbiome health in some individuals. This can interfere with some of the benefits of fasting, such as appetite control, metabolic health, and autophagy. If the main goal of fasting is weight loss through calorie restriction, diet soda is generally considered acceptable but should be consumed with caution as it may increase cravings for sweet or sugary foods. For fasting focused on gut health, blood sugar regulation, or cellular repair, it is better to avoid diet soda during fasting periods. The safest beverages during fasting are water, black coffee, and unsweetened tea. In summary, diet soda won't break a fast in the classic sense but may have subtle negative effects depending on individual responses and fasting goals.