Yes, you can have black coffee when fasting. Black coffee contains very few calories (typically less than 3-5 calories per cup) and does not significantly affect metabolism, insulin levels, or the fasting state. Therefore, it will not break your fast. In fact, black coffee may even help suppress appetite and enhance some benefits of fasting such as fat burning and improved brain function. However, adding any sugar, milk, cream, or other calorie-containing ingredients can break the fast. It is best to drink plain black coffee without additives during fasting periods. Additionally, be mindful of caffeine sensitivity and consumption timing to avoid side effects like jitteriness or sleep disturbances.