Is it true that celery is poor food for dieters because it causes fluid retention?

That idea has been around as long as we can remember. It is linked to the fact that celery is one of several vegetables that are relatively high in sodium. Others in the group include artichokes, beets, carrots, white turnips, spinach and other dark-green leafy vegetables. This characteristic is of little practical importance, except in rare cases where sodium intake must be sharply limited. In those situations, the amount of all these vegetables is restricted. But for dieters who like it, celery is perfectly acceptable.

Please examine the enclosed label from a nutritional supplement. On the advice of a friend I have been taking it for about a year, and I wonder whether the use of such large doses of nutrients has any serious side effects.

Fortunately, the supplement you are taking does not provide amounts of any single nutrient at levels likely to be toxic. Nonetheless, there are three reasons why you are wasting your money.

First, the main ingredient in the product is soy-protein isolate. In all likelihood, you are getting all the protein you need and then some. Beyond that, soy protein does not contain enough of all the essential amino acids necessary for the growth and repair of body tissues.

Second, there is no physiologic reason to take a supplement providing anywhere from 160 percent to more than 800 percent of the U.S. Recommended Daily Allowances for essential nutrients. Remember that these levels are already set to include a generous margin of safety.

Finally, we question why the manufacturer has added a smorgasbord of compounds, among them papain, inositol and rutin, for which it clearly states that "the value in human nutrition has not been established."

You have not told us how much you pay for this supplement. Our guess is that if you are looking for nutritional insurance, you can get it a lot more cheaply somewhere else. Simply choose the least-expensive supplement you can find that contains 100 percent of the RDA for essential vitamins and minerals, and count on your diet to provide all the protein you need.

Is it true that soft-drink consumption in this country is still on the rise?

As a nation, we seem to have an insatiable thirst for beverages that contain almost nothing but calories. Figures recently released by the U.S. Dept. of Agriculture indicate that Americans drank 45.6 gallons of soft drinks per capita in 1985, or 92 percent more than they did 15 years earlier. Indeed, that figure was up 5.6 gallons over two years before, and represents no less than 486 12-ounce cans of soda for every person in the country. Juice consumption has climbed 59 percent since 1970, but still adds up to just a little over 7 gallons per person.

The consumption of coffee has followed a reverse trend, down from 33.4 gallons in 1970 to 25.9 gallons in 1985, a decline of 22.5 percent.