When you eat well you limit the excursions in your blood glucose levels, generally making it easier to control your weight, your pimples and your diabetes, should they be problems. It has been known for some time that adding a tablespoon of vinegar to a meal (eg vinegar in your salad dressing) can reduce the blood glucose response to the meal by 30%.
The theory is that adding vinegar to a meal slows down the rate at which food empties from the stomach and enters the small intestine for digestion, therefore slowing the absorption of glucose.
A recent paper (Eur J Clinical Nutrition 2010; 64: 727-732) indicates that the vinegar effect is more pronounced after high GI meals and not the low GI meals we suggest for good health (and less zits). They used a wine vinegar. I assume the effect is still present with a flavoursome 4-clover leaf balsamic.