Q: A friend and I were discussing diets. We both vaguely recalled a long-ago fad that involved excessive chewing. Can you refresh my memory?

A: You are probably thinking of Fletcherizing, a turn-of-the-century fad that swept this country and many European cities as well. It took its name from Horace Fletcher, the man who popularized the idea.

Although a successful businessman, at the age of 40 Fletcher found himself in poor health, complaining of a variety of symptoms. He had been turned down by his insurance company, probably because he was an estimated 80 pounds overweight. That is a hefty amount of extra poundage, especially since the man stood only about 5 feet 6 inches tall. Many of Fletcher's other health problems may have been traceable to his obesity.

He decided to retire to Venice and try to regain his health. In the process of winding up his affairs, he happened upon some writing that deeply impressed him. The British statesman Gladstone suggested that man should chew each bite of food 32 times -- one chew for each tooth in his head.

Partly drawing on that notion, Fletcher came up with his own new eating "system." He recommended holding one's face down and chewing each bite 50 to 60 times until every last iota of flavor was extracted. The "food gate" at the back of the mouth would then open and the food would automatically disappear.

Such excessive chewing takes a very long time and can be fatiguing, so it may make people eat less. Fletcher himself lost 65 pounds. He then went on to capture the support of many physicians and scientists. The topic was discussed in both scientific journals and lay magazines. Fletcher wrote popular books on the miracle of Fletcherizing, which promised to do much more than just help one lose weight.

Over the years, Fletcher's own diet became increasingly restrictive. He finally died in Battle Creek, Mich., at Dr. Kellogg's sanitarium, where a large sign in the main dining room advised everyone to "FLETCHERIZE."

To learn more about Fletcher and other faddists who came before and after him, we suggest a book called, "The Nuts Among the Berries," by Ronald Deustch (Ballantine Books Inc., 1967).

Q: What is the difference between corn grits and cornmeal?

A: Like other grains, the basic elements of a kernel of corn include the hull, the germ and the endosperm. Corn grits and cornmeal both come from the endosperm.

In refining corn, the kernels are first steamed to loosen the hull. Then they are split, and the hull and the germ removed.

The remaining endosperm is passed through heavy steel rollers that break it into granules. The granules are screened to separate the particles of different size. The largest granules are the grits, the medium are cornmeal, and the finest are used for corn flour. Corn grits and cornmeal come from the harder portion of the endosperm and corn flour from the softer section.

This is the general process. But there can be variations. For example, "bolted" cornmeal still contains some of the hull and the entire germ. One can also buy unbolted cornmeal, which is the corn counterpart to whole-wheat flour. And hominy, another corn food that is particulatly popular in the South, is the entire endosperm, free of the bran and the germ.

Q: I noticed that the label on the peanut butter I use says that it contains less sugar than other brands. Is there really a significant difference between brands?

A: No. Peanut butter is one of the foods for which there is a Standard of Identity, a legally defined recipe. Under that standard, at least 90 percent of the weight of the finished product must be peanuts. That leaves a maximum of 10 percent for all the other optional ingredients a manufacturer may use. These can include stabilizers such as hydrogenated or partially hydrogenated vegetable oils and "suitable seasoning." Considering that a tablespoon of peanut butter weighs 16 grams, this means that the total weight of all these substances can be 1.6 grams at most, or a little more than 1/4 teaspoon. Obviously, brand to brand differences would have to be small.

It is interesting that the standard for peanut butter specifies that while suitable seasonings are allowed, artificial flavorings or sweeteners, chemical preservatives, vitamins and coloring agents cannot be used in making peanut butter.

Q: I am considering buying a microwave oven. Before I do, I would like to know if this heating method is more destructive of vitamins than conventional cooking.

A: An important advance in the design of microwave ovens, called the variable-power feature, allows greater control of the rate of heating -- and thereby aids nutrient retention. A review of recent studies reported in the Journal of the American Dietetic Association found that low-power microwave techniques resulted in equal or better retention of several nutrients in foods prepared in microwave ovens, when compared with the same food cooked by conventional methods.

More specifically, studies of the retention of thiamin, a B vitamin particularly vulnerable to heat, have generally found that the cooking method did not affect nutrient retention, either when meat was roasted from the raw state or reheated.

Folacin is another B vitamin especially vulnerable to destruction during preparation and cooking. Again, studies of a variety of vegetables found nutrient retention to be at least as high as with conventionally prepared vegetables. And when cooked for a short time with little or no water, folacin was retained extremely well.

Finally, studies comparing the effects of heat on ascorbic-acid retention found that the combined effect of reduced cooking time and less water provided an edge in retention. In other tests, results of various cooking methods were comparable.

Perhaps the most important message from these and other studies of vitamin retention in foods cooked in microwave ovens is that in no case does microwave cooking seem to have a significant negative effect. When the best techniques of preparation are used, it may even offer an advantage.

Moreover, it is expected that improved technology will lead to even better nutrient retention in foods cooked or reheated in microwave ovens.