Vinegar is a product of fermentation. This is a process in which sugars in a food are broken down by bacteria and yeast. In the first stage of fermentation, the sugars are turned into alcohol. Then, if the alcohol ferments further, you get vinegar. The word comes from the French, meaning "sour wine." While vinegar can be made from all sorts of things -- like many fruits, vegetables, and grains -- apple cider vinegar comes from pulverized apples. In order to complete the apple cider vinegar diet, you must take one, two, or three teaspoons of apple vinegar before each meal. While it can be quite difficult to get three teaspoons down initially, after a couple of days most people have no problem working up to it. Any more than three is most likely beyond what your stomach can handle anyway, due to the strongly acidic nature of the vinegar. That said, there are now apple cider vinegar diet pills which are probably far easier to consume if you are set on trying this diet.
The main ingredient of apple cider vinegar, or any vinegar, is acetic acid. However, vinegars also have other acids, vitamins, mineral salts, and amino acids. Like many other fad diets , it's not clear as to whether there are any properties in the vinegar that will help you lose weight. In fact, vinegar has been around as a "diet miracle" since Lord Byron popularized it in the 1820's according to the American Dietetic Association's Fad Diet Timeline. The primary reason that this diet would work is that you are told to eat moderate portions, watch the nutritional composition of the food you eat, and get exercise. Just doing those alone is often enough to stimulate your body to maintain a healthy weight, if not lose weight.