HAVE YOU SEEN the TV commercial in which the members of a family appear on camera and vie for supremacy in the strange, new, but obviously challenging area of who is consuming the most fiber each day? One after the other they hold up their cereal boxes and claim that theirs has the highest fiber content. It looks like a promising pilot for a sitcom, "Meet the McFiber Family": Mother, father and children sit around the table every week arguing through a little Stonehenge of cereal boxes over who's getting the greatest benefit from his breakfast.

Come to think of it, it may be too close to reality for a sitcom. The national consciousness of what we eat and how it affects the burblings and churnings of our various internal systems is very high right now, with consequences that are pobably as healthy for the body as they are deadening for conversation and the appreciation of food.

There are, to be honest about it, some topics that are not inexhaustible. Among them are niacin, thiamine, riboflavin, minimum daily adult requirements of everything, and the entire alphabetic archipelago of vitamins. Moreover, there is something faintly unappetizing about viewing a banana solely as a potassium unit and spending half your lunch hour worrying about whether the cottage cheese has enough calcium.

Perhaps least inspiring, however, is the subject of fiber. It is dull, functional stuff and often tastes like sawdust. We know, it's good for you, and has achieved something approaching star status in the edible galaxy. "Oat bran is currently in the spotlight," said a column in The Post's food section last week -- "and deservedly." Pardon our intrusion into the sacred realm of drama criticism, but we don't think bran deserves the spotlight. When bran doesn't have bananas on it, it is mere mulch, and we can't think of a faster way to clear the theater than to confuse it with Madonna.