Q. I was under the impression that cereals contained little or no fat and presented no problem for anyone following a cholesterol-lowering diet. To my dismay, I noticed on the nutritional label of one of my favorite cereals that an ounce, or one-fourth cup, contains 5 grams of fat. I usually eat a half-cup serving, which I assume is too much. Can you tell me which cereals to avoid?

A. WHile many popular cereals should be bypassed simply because they contain excessive amounts of sugar, fortunately very contain significant amounts of fat. And they're all in one category: the relatively new, so-called "natural cereals."

All but three of the regular cereals we checked (including approximately 50 different products) contained anywhere from zero to 2 grams of fat per ounce. Three ahd 3 grams of fat per serving, and only one, made with peanut butter, contained 4 grams of fat. These last might well be avoided since they contained too much sugar, anyway.

But the natural cereals present a different picture. Of the varieties examined, only one had as little as 2 grams of fat per ounce. And as you point out, a serving of one of these cereals, which are quite heavy, is likely to be more than an ounce.

Perhaps the most important point, however, is that while some of the fat in these cereals comes from the whole grains and nuts, most of the cereals are made with added oil. Where the type of oil was sepcified, it was invariably coconut oil, which is highly saturated.

So if you do enjoy natural-type cereals, choose those lowest in fat. Or, try making your own. You cutting down on the amount of sugar and honey. We think you'll find it won't compromise the flavor a bit.

Q. I was surprised to read in your column that carbohydrates have an essential function in the body, and I don't really understand what it is. Could you explain?

A. Carbohydrates - that is, sugars and starches - are an important energy source, and there are good reasons why we should rely on them more than most of us have in the past. First, they are far less expensive than proteins. Indeed, using animal protein for energy is wasteful in terms of the dollar cost to consumers and in terms of the natural resources required to produce it. Second, to cut our total fat consumption, we need to depend increasingly on carbohydrates for energy.

Contrary to popular belief, carbohydrates are no more "fattening" than protein. Moreover, they contain less than half as many calories as an equal amount of fat. A gram of carbohydrates or protein provides 4 calories, while a gram of fat carries 9 calories.

Incidentally, the brain is the one organ that cannot, under normal circumstances, burn anything but glucose. It needs anywhere from 500 to 600 calories of glucose a day, which it gets from the breakdown of dietary sugars and starches.

Remember, too, that carbohydrate foods are important carriers of a number of essential vitamins and minerals. For example, how would you meet your need for ascorbic acid without fruits and vegetables?

Unfortunately, concerning carbohydrates, we have come to equate "inexperience" with "inferior," and this is just not so. Only the empty-calorie food, such as white flour and refined sugars, truly deserve the poor reputation that all carbohydrates seem to have.