Apple Cider Vinegar (ACV)

Apple cider vinegar does not break a fast in the small amounts people typically use. A tablespoon contains only a few calories, not enough to trigger a meaningful metabolic response, so a splash in water during a fast leaves the fasted state essentially intact.

Its appeal during a fast is mostly about blood sugar. A meta-analysis of clinical trials found that vinegar taken with a meal can lower the post-meal rise in both glucose and insulin, an effect attributed partly to acetic acid slowing how quickly the stomach empties. That makes apple cider vinegar most useful around the meal that ends your fast, where it can blunt the glucose spike from breaking the fast, rather than as something that does much on its own during the fasting window.

Only a small amount is necessary (a tablespoon or so), since larger amounts add calories and can be hard on tooth enamel and the stomach. The glucose effect also varies between studies, and it is mainly relevant when a meal follows. As a fasting-window drink, apple cider vinegar is fast-compatible, and some people find it helps with appetite.

Source: Vinegar Consumption Can Attenuate Postprandial Glucose and Insulin Responses (Diabetes Research and Clinical Practice, 2017)