Plain black coffee, without sugar, milk or cream, usually does not break an intermittent fast, since it contains almost no calories. For this reason, it is commonly allowed during the fasting window.
However, coffee is not completely neutral. Caffeine can temporarily reduce insulin sensitivity, although regular coffee consumption is associated with a lower risk of type 2 diabetes.
There is also no strong evidence that black coffee preserves every effect of fasting, such as autophagy. Adding milk, cream, sugar, syrup or butter does break the fast. Zero-calorie sweeteners do not add calories, but their effect on fasting is still unclear.
The type of fast also matters. Fasting before blood tests usually allows only water, while some religious fasts do not allow any drinks. Before surgery or other procedures, follow the specific instructions you received from your medical team.
On an empty stomach, coffee may cause heartburn, reflux, jitteriness or palpitations in some people. It is best not to exceed about 400 mg of caffeine per day, which is roughly 4 cups of coffee.
Anyone considering intermittent fasting should consult a registered dietitian to make sure it fits their needs. People with diabetes who use insulin or certain blood sugar-lowering medications should also talk to their doctor, as fasting can increase the risk of low blood sugar and may require medication adjustments.