Q. Is it true there are differences in both the number and size o

A. The answers are not all in on this subject. Understanding more about fat cell number and size and the factors affecting cellular development is crucial to expanding our knowledge of obesity.

But from a practical point of view, for the parents of young children, what is important is to avoid overfeeding and to encourage physical activity.

An estimate of cell number is calculated by measuring total body fat and size of fat cells. These reckonings are at best inexact. There can be considerable error in estimating total body fat and great variability in determining fat cell size. Fat cells can vary in size from one part of the body to another and even within a sample from the same area. Moreover, those cells that contain no fat are not counted, so it is impossible to reach firm conclusions.

It is thought that infancy and early adolescence are the periods most sensitive to increasing cell number by consuming more calories than burned. Evidence suggests that during infancy there is a parallel increase in cell size in both obese and nonobese children. But after that, a significant increase in cell size occurs among those who are obese.

This difference remains throughout early and middle childhood but is no longer significant during adolescence. As the difference in cell size diminishes, there is a significant rise in cell number among obese children.

Q. Does watermelon have diuretic properties?

A. Watermelon has a number of good points. It is low in calories, containing only 40 in a cup of cubes. It also has some iron and some B vitamins, as well as generous amounts of both vitamins A and C. In addition, it is a good source of potassium. And beyond all that, few things are more refreshing on a hot summer's day than a cool slice of rosy-red watermelon.

But despite popular claims to the contrary, neither watermelon, celery, asparagus, nor any other fruit or vegetable has been shown to have diuretic properties.

Q. Is there is a reasonably easy way to estimate how many calories a day I burn?

A. Yes, although it will furnish only a rough estimate. First, divide your weight in pounds by 2.2 to convert it to kilograms. Then determine your resting metabolic rate (RMR), or the energy required to maintain body functions and digest and absorb food. For young adults, RMR is about 1 calorie per kilogram of body weight per hour for males, and 0.9 for females. For individuals over 50, the figure drops to 0.9 and 0.8 for men and women, respectively. Thus for a 28-year-old man weighing 185 pounds (84 kg.), RMR would be 1x84x24, or 2,016 calories.

Next you will need to decide on a general classification for your activity level. Very light activity burns 0.6 calories per kg. per waking hour; light activity, 0.8; moderate activity, 1.1; strenuous, 2.4. Then multiply your waking hours by the selected activity level and by your kilogram weight. Thus if the 84 kg. man slept eight hours and considered his activity level to be light, he would burn an additional 0.8x84x16, or 1,075 calories. His total caloric requirement for the day would then be just over 3,000.

This type of estimate is vulnerable to error. In fact, all estimates of caloric expenditure are difficult to make. However, if you are interested in a more accurate assessment, you will have to keep a diary of typical daily activities and then consult calorie tables, which are readily available in introductory nutrition textbooks. One such volume is "Nutrition: Principles, Issues and Applications" by Eleanor R. Williams and Mary Alice Caliendo (McGraw Hill, 1984, $22.45).