Vinegar is an acidic solution made from many different types of fermentable carbohydrates, such as grapes, apples, berries and grains. After the carbohydrate source ferments, a particular bacterium is added to the mixture to create acetic acid from the alcohol. Some proponents of vinegar for medicinal purposes claim that acetic and other organic acids present in vinegar can help break down your food and aid in digestion.
A 2010 article by Matthew Kadey, R.D. in "Vegetarian Times" magazine states that vinegar, specifically apple cider vinegar, can improve your digestion by helping to break down proteins. More unsubstantiated claims for vinegar are that it can treat conditions like rosacea by helping your body produce more stomach acid for food metabolism. It is unclear as of 2011 if vinegar actually has any net benefit on digestion.
Whether or not the acetic acid in vinegar helps lower post-meal blood glucose levels is an area of significant research. Registered dietitian Carol Johnston points out that well-controlled studies such as one she conducted in 2005 have found vinegar to reduce rises in post-meal glucose by 30 percent or more. One possible reason for this is that the vinegar suppresses stomach enzymes that break down starches, thus slowing the release of blood glucose from starchy foods.
Vinegar may also slow down the digestion of fats, meaning that protein is the only macronutrient in your diet that vinegar purportedly helps digest. A 2010 research review and study appearing in the journal "Lipids in Health and Disease" found that consumption of vinegar can decrease how well your body digests and absorbs fat. This may be because vinegar is rich in lipid-blocking compounds called polyphenols. Reduced fat absorption could benefit you by lowering your cholesterol and risk for heart disease.
Vinegar can have an acetic acid concentration between 4 and 7 percent depending on the type. Though vinegar is safe for human consumption, the acidic content can sometimes burn your esophagus and digestive tract if you do not neutralize it somehow. Professor of nutrition at Bastyr University Jennifer Adler states that dilution of vinegar with water in a ratio of 2 tbsp. vinegar to 1 cup of water can greatly reduce the acidity.