Q. My grandson, age 2, is growing normally and is generally healthy. But he has persistent diarrhea for reasons which are not clear. I think it happens when he drinks a lot of apple juice. Before I suggest to my daughter that she try withholding it, can you tell me if my theory could be right?

A. An article in the American Journal of Diseases of Children suggests that apple juice even in reasonable amounts may be related to so-called chronic nonspecific diarrhea in children, a common problem without a clear-cut cause.

During a six-month period, the investigators saw five otherwise normal children between the ages of 13 months and 31 months whose parents reported that apple juice seemed to make their diarrhea worse. This represented 15 percent of those with chronic diarrhea seen in a pediatric gastroenterology clinic during that period.

Results of a lactose tolerance test to evaluate whether these children could digest milk sugar were normal. But when given seven to eight ounces of apple juice at room temperature after a 12-hour fast, breath hydrogen began to rise within as little as 30 minutes. In some case it continued to climb for two hours. This rise indicated that undigested carbohydate was reaching the bowel, where it was broken down by bacteria. Each of the children also developed diarrhea after drinking the apple juice.

The physicians who saw the children cannot completely explain this observation, but speculate that carbohydrate malabsorption may play a role in the condition. While it does not seem to involve sucrose or fructose found in the juice, it may be related to sorbitol, a sugar alcohol.

How commonly apple juice consumed even in normal amounts might contribute to chronic nonspecific diarrhea is unknown. Restricting intake in other patients with this condition had no effect. But in view of your observation, suggesting a brief trial of removing it from your grandchild's diet seems reasonable. But parents should rely on the child's pediatrician for the overall monitoring of the condition.

Q. Does it really cost more to buy lean ground beef? Since regular ground beef contains all that fat, by the time it is cooked are the losses so great that it costs as much as the more expensive beef?

A. To learn which is the best beef buy, you need to know the percentage of fat and lean in each type. In large supermarkets this information is now commonly printed on the package label. If not, ask the butcher. Then divide the cost per pound by the percent of lean to determine the actual cost of the lean meat you are getting.

We did this recently at a local supermarket with the lean from regular ground beef with a maximum of 25 percent fat selling for $1.59 per pound, lean ground beef with no more than 20 percent fat at $1.79 per pound, and extra lean beef containing not more than 16 percent fat at a cost of $1.99 per pound. They worked out to cost $2.12, $2.23, and $2.36, respectively. That gap was about the same as we found in a similar survey several years ago. At another market, however, we found that the premium for extra lean was considerably higher. So if budget is an important consideration, this financial exercise may be worthwhile.

In any case, if you want the best buy for your money, and wish to "eat lean," be sure to cook the meat by methods which get rid of excess fat.