Acetic acid is is the acid in vinegar and is found naturally in apples, cocoa, coffee, wine, aged cheeses, grapes, and other fruits. Acetic acid is used as an acidic flavoring agent for ketchup, pickles, mayonnaise, wine, and some cheeses.

Research shows that acetic acid, most likely due to its action as a weak acid, helps to improve insulin sensitivity and reduce glucose and insulin responses.

References

Ostman E, Granfeldt Y, Persson L, Bjorck I.Vinegar supplementation lowers glucose and insulin responses and increases satiety after a bread meal in healthy subjects.Eur J Clin Nutr. 2005 Sep;59(9):983-8.Johnston, C. S., Kim, C. M., Buller, A. J. Vinegar improves insulin sensitivity to a high-carbohydrate meal in subjects with insulin resistance or type 2 diabetes. Diabetes Care 2004 Jan;27(1):281-282.