We’ve known about the antiglycemic effect of vinegar for more than 25 years, but researchers still aren't sure of the mechanism. Originally, it was thought that vinegar slowed stomach emptying, but even consuming vinegar outside of meals appears to help. For example, in one study type 2 diabetics consuming two tablespoons of apple cider vinegar at bedtime were found to wake up with better blood sugars in the morning. (Research suggests that consuming pickles or vinegar pills does not seem to have the same effect.)
However, I don't recommend drinking vinegar straight (as it can burn your esophagus) or in excess: a cup a day for six years (that’s 2,000 cups!) was discovered to be a bad idea.
Vinegar may also help with polycystic ovary syndrome (PCOS), improve arterial function, and help reduce body fat.
For example, a daily tablespoon of apple cider vinegar restored ovarian function within a few months in four out of seven women with PCOS. And a tablespoon of rice vinegar was found to acutely improve artery function in postmenopausal women. We’re not sure why, but the acetate from the acetic acid in vinegar may lead to improved nitric-oxide production. Such an effect would be expected to help with hypertension, and indeed there is a study purporting to show blood pressure benefits from a tablespoon of vinegar a day.
Despite folk wisdom to the contrary, vinegar does not appear to be an effective treatment for head lice, but it may help with weight loss. A double-blind, placebo-controlled (but vinegar-company-funded) study was performed, in which obese subjects consumed daily vinegar drinks with either one or two tablespoons of apple cider vinegar, or a placebo drink that tasted like vinegar but contained no acetic acid. Both vinegar groups lost significantly more weight than the control group. Though the effect was modest — about 4 pounds over a three-month period — CT scans showed the vinegar groups’ subjects lost a significant amount of their “visceral” fat, the abdominal fat that is particularly associated with chronic disease risk.
There are all sorts of flavorful, exotic vinegars to explore these days, including fig, peach, and pomegranate. I encourage you to experiment and find ways to incorporate some into your diet.