DEAR DR. BLONZ: How is it that people with similar caloric intakes can end up with such different weights? Three of my best friends and I have been exercising together on and off for about 10 years. We went through school together, went out for the same sports, and at the time we had similar body types with similar heights and weights. On an agreement made this past new year, we began to measure our caloric intakes, checking online food composition tables. To our surprise, we found that we eat just about the same number of calories every day. Despite all this, our weights and body shapes have changed, in some cases rather dramatically. It was frustrating for me, as I have one of the larger waistlines in the group. - F.S., Chicago, Ill.

DEAR F.D.: Food-composition tables are helpful because they provide useful information about what's in the foods we eat. The calories they list, however, are nothing more than calculations of potential energy. The efficiency with which a fixed number of calories is utilized, however, varies from body to body. It's similar to the way gas mileage varies among different types of automobiles.

One's basal metabolism - the amount of calories used while the body is at rest - rises, for example, as the amount of muscle in the body goes up. This is because muscle is "active tissue." Fat tissue, by contrast, is not very active. Again using an automobile analogy, this would be similar to how a car with an eight-cylinder engine burns more gas at idle than one with a four-cylinder engine.

As a general rule, other factors that help to explain individual differences include age, sex, genetic makeup, level of physical activity and the efficiency with which one digests and absorbs food. All these can affect the way a given number of calories will work in a particular body. The key contribution of exercise is that it not only burns calories, but it also can increase the amount of muscle in the body. The additional muscle mass burns more calories even when you are at rest.

As is true with most aspects of life, some people have to work harder to accomplish the same goals. Where weight is concerned, though, don't get bogged down by comparisons to others. Rather, focus on doing the best you can with what you've got.

DEAR DR. BLONZ: It was pointed out by my daughter that the dried apricots I have been enjoying for a number of years have sulfur-dioxide added. She has been urging me to avoid this compound. What is sulfur dioxide and what purpose does it serve? What other products contain sulfur dioxide? - C.R., Arlington Heights, Ill.

DEAR C.R.: Sulfur dioxide belongs to the group of preservatives known as sulfites. Other members of this group include sodium sulfite, potassium sulfite, sodium metabisulfite, potassium metabisulfite and sulfur dioxide. All can be used to help keep fruits and vegetables looking fresh. They can prevent discoloration as well as the growth of unwanted bacteria, molds and yeasts. Sulfur compounds are also used to help sanitize containers that are used in fermented beverages. In addition, sulfites are used routinely in wines and in dried fruits, such as apricots, to allow a longer shelf life. Bacteria and molds need small amounts of water to grow. If your apricots did not contain sulfites or a similar type of preservative, they would have to be dried down to a leather-like consistency to achieve a comparable shelf life.

Some people experience adverse reactions to these preservatives, including headache, hives and shortness of breath. An estimate by the Food and Drug Administration (available at tinyurl.com/zlywh) reveals that under 1 percent of the general public will react to them, but this number goes as high as 5 percent among those with asthma. The FDA has banned the use of sulfites on fresh fruits and vegetables (except potatoes), and it has to be indicated on the label whenever sulfite preservatives are present. You have been enjoying your dried apricots, so this would not appear to be an issue. There is always the option of sampling your fruit alongside apricots dried without sulfites to see which you prefer.

Send questions to: "On Nutrition," Ed Blonz, c/o Newspaper Enterprise Association, 200 Madison Ave., New York, NY 10016. For e-mail, address inquiries to: ed@blonz.com. Due to the volume of mail, personal replies cannot be provided.

Ed Blonz, Ph.D., is a nutrition scientist and the author of "Power Nutrition" (Signet, 1998) and the "Your Personal Nutritionist" book series (Signet, 1996).