The following is a quote from page 43 of "The Glucose Revolution" by Jennie Brand-Miller, Thomas M. S Wolever, Stephen Colagiuri and Kaye Foster-Powell.
"THE POWERFUL EFFECT OF ACID ON G.I. (VINEGAR, LEMON JUICE, SOURDOUGH BREADS)
Within the last few years, several reports in the scientific literature have indicated that a realistic amount of vinegar or lemon juice in the form of a salad dressing consumed with a mixed meal had significant blood sugar lowering effects.
As little as 4 teaspoons of vinegar in a vinaigrette dressing (4 teaspoons vinegar and 2 teaspoons oil) taken with an average meal lowered the blood sugar by as much as 30 percent. These findings have important implications for people with diabetes or individuals at risk of diabetes, coronary heart disease or Syndrome X (impaired glucose tolerance, hypertension and high blood lipid levels).
The effect appears to be related to the acidity because other organic acids (such as lactic acid and propionic acid) also have a blood sugar lowering effect but the degree of reduction varies with the type of acid. Our findings show that among the various types of vinegar, red wine vinegar was the best. And lemon juice was just as powerful. It is well known that acidity in food puts the brake on stomach emptying, slowing the delivery of food to the small intestine. Digestion of the carbohydrate in the food is therefore slowed and the final result is that blood sugar levels are significantly lower. Good news for people with diabetes! The take home message is that a side salad with your meal, especially a high G.I. meal, will help to keep blood sugar levels under control.
SOURDOUGH BREADS, in which lactic acid and propionic acid are produced by the natural fermentation of starch and sugars by the yeast starter culture, also produce reduced levels of blood sugar and insulin compared with normal bread. The area under the plasma insulin curve was 22 percent lower with the sourdough product. In addition, there was higher satiety associated with breads having decreased rates of digestion and absorption. Thus there is significant potential to lower blood sugar and insulin and increase satiety with sourdough bread formulation.