All vinegars including apple cider vinegar work like a double edge sword. On one hand vinegar triggers AMPK- an important stress response pathway that increases fatty acid oxidation and inhibits fat gain (also activated by fasting and exercise) and on the other hand it acidifies the body. Acidification of the body can be a serious issue with people who already suffer from acidity related symptoms. Also long-term excessive use (ACV) could conceivably cause low potassium levels and lower bone density. Apple cider vinegar is highly acidic and this high acidic content may cause erosion of the teeth enamel. Apple cider vinegar contains chromium, which can affect a person’s insulin levels. This is a serious risk to diabetics who have a difficult time regulating their insulin levels.
Put it this way, in an alkaline diet rich in vegetables and fruits vinegar can be beneficial, but if the diet is predominately acid such due to high intake of protein and grain, vinegar can turn harmful.