DEAR DR. BLONZ: I am on a weight-loss program that involves counting calories. I remember reading that the calories in carbohydrates should count for less, because they require energy to be used. If this is the case, why treat all calories as equal? Also, what is the difference between "aerobic" and "anaerobic" when it comes to food, versus exercise? -- S.F., San Jose, California

DEAR S.F.: A calorie is actually a unit of heat energy, but it can be thought of as representing our food's fuel/energy potential. Everything that happens in the body requires energy; it's needed 24/7, but we only eat at distinct times. So at mealtime, we take in more energy than we need at the moment. Any excess is changed into long chains of fat -- the most concentrated source of calories -- and stashed away for later use.

Changing dietary fat to stored fat doesn't require much energy: The process extracts about a 3 percent "tariff" from the ingested calories. In contrast, changing carbohydrate calories to stored fat is a multistep process that can use up about 23 percent of the ingested calories. From this, it might appear as though carbohydrate calories contribute less than fat calories, but it's a bit more complicated than that.

Our body doesn't like to waste energy, so when carbs are available, the body doesn't waste energy changing them to fat. It uses them right away to fuel muscular work and biosynthetic processes. Carbs also replenish our glycogen stores, glycogen being a prime source of stored carbohydrate found in the liver and muscles. Only after these process have been taken care of will the excess carbohydrates be changed into fat.

Bottom line: On a diet that counts calories, it is best to consider carbohydrate and fat calories as equal. One study actually examined this issue, shifting people between equal-calorie diets based on high-fat or high-carbohydrate foods. The study found no difference in how the diets affected body composition of the volunteers.

The characterization of "aerobic" (needs oxygen) and "anaerobic" (doesn't need oxygen) fuels relates to the body's energy-producing chemical reactions. Carbohydrate is an anaerobic fuel because any oxygen it needs is integrated into its structure; it can release its energy without needing oxygen from the bloodstream. Oxygen is, however, needed for muscles to burn (aerobic) fatty acids for fuel. This is why our breathing rate picks up when we become more physically active, even when it is something as routine as walking up a flight of stairs. As soon as the needed oxygen becomes available, the muscles shift gears to the more efficient fat-burning mode.

As far as exercise, "anaerobic" usually indicates an activity with quick bursts of higher-intensity muscular work, such as weightlifting and track events. "Aerobic" exercise, such as walking, jogging and cycling, are usually lower in intensity but longer in duration. This type of exercise typically includes a warm-up period that increases the rate of breathing and allows the body to mobilize its fats.

Send questions to: "On Nutrition," Ed Blonz, c/o Universal Uclick, 1130 Walnut St., Kansas City, MO, 64106. Send email inquiries to questions@blonz.com. Due to the volume of mail, personal replies cannot be provided.

DEAR DR. BLONZ: I have been reading about our bodies' need for enzymes. While the companies selling enzyme supplements certainly have an interest in promoting their importance, I'm concerned that there is something to their claims. Does cooking really destroy enzymes, and do we stop producing them as we age? -- E.I., Oakland, California

DEAR E.I.: A defining characteristic of an enzyme is that it effects change in another substance while remaining unchanged itself. The enzymes in our body are specialized proteins that not only help us make use of the energy in foods, but also play a role in the building and taking apart of complex substances.

There are situations where it might be reasonable to take an enzyme supplement. For example: Many adults no longer produce a sufficient amount of lactase, the enzyme that breaks down the lactose carbohydrate found in milk. The symptoms of "lactose intolerance" can include intestinal gas, cramping and diarrhea. Taking lactase supplements with milk products can reduce the annoying side effects. Another popular enzyme supplement (Beano) is based on alpha-galactosidase, an enzyme that helps digest a type of carbohydrate found in beans and some other vegetables.

There are also medical situations in which dietary enzymes might be prescribed. The pancreas produces many of our digestive enzymes. If a medical problem develops that affects the pancreas's ability to either produce or deliver its enzymes to the digestive tract, the body will be unable to digest and absorb a variety of essential nutrients.

Problems with enzymes can also be experienced by those who have undergone a gastric bypass. Those with such issues would have a legitimate reason to take specialized enzyme supplements.

But what about those who don't have an enzyme-related malady?

There is an unsubstantiated theory that our lifespan is closely intertwined with our ability to produce enzymes. It is based on the dubious idea that we can only manufacture a fixed amount of enzymes in our life; when we run out, the game, so to speak, is over. The theory posits that fresh fruits and vegetables contain "live enzymes." By eating a diet rich in raw, whole foods (because cooking kills the enzymes, they say), or by taking enzyme supplements, we effectively spare the "drain" on our bodies' resources and help extend our lives.

There is no foundation for this enzyme theory, especially the part about the body having a fixed amount of enzymes. Fresh foods are healthful not because they contain enzymes, but because they are our richest source of nutrients and phytochemical substances. Given that enzymes are protein substances, those in foods will be inactivated in the stomach's acid environment and disassembled by the protein-digesting enzymes in our digestive tract.

By all means, eat and enjoy fresh, whole plant foods -- but don't hold them in any special esteem for their enzymatic powers.

Send questions to: "On Nutrition," Ed Blonz, c/o Universal Uclick, 1130 Walnut St., Kansas City, MO, 64106. Send email inquiries to questions@blonz.com. Due to the volume of mail, personal replies cannot be provided.

DEAR DR. BLONZ: Our school year is now underway, and I am again being pressured by my sons to include too-sugary snacks in their lunch boxes. I have volunteered in the lunchroom, and have noticed that those who don't get what they want tend to trade items with their classmates. My boys are 8 and 10, are active and usually eat well. I try to only buy snacks that claim to be made with fruit, but I am skeptical of their nutrition. -- F.K., Los Angeles

DEAR F.K.: The idea of the lunchtime snack represents a gold mine for food manufacturers, and your plight is by no means unique. All one needs to do is check the advertisements in magazines, the commercials during kids' TV shows, the ever-present online ads and the kids-eye-level shelves at the supermarket. Although there have been some changes, most "fruit snack" and "fruit roll" products still offer little more than a sweet taste and a lot of packaging. Many are just pseudo-fruit doodads aimed at the young consumer.

The manufacturers' goal is to cajole parents into choosing their products as a convenient way to add fruit to their children's diet. Though the packages boast that they are "made with real fruit," they're usually nothing more than gummy sugar concoctions with little more than a hint of the genuine article. Check the Nutrition Facts label and the ingredients list to compare the amount of added sugar to actual fruit.

A better way to add good taste and nutritional value to the lunch box is to pack real, dried fruits. There are many options in that category, too, so choose ones with the least amount of added sugar (if any). These will offer the sweetness kids seek, but with more varied flavors -- not to mention, much more impressive nutritional content.

For example, delicious dried blueberries contain 10 grams of fiber per half-cup serving; figs and dates contain nine grams; dried apricots, eight; prunes, seven; and raisins, five grams of fiber per serving. By comparison, a slice of whole-wheat bread or a half-cup of broccoli each have about two grams of dietary fiber. (The typical American consumes about 12 grams of dietary fiber each day -- far short of the recommended 25 to 30 grams.)

In addition to fiber, dried fruits are good sources of B vitamins and other nutrients. Figs are one of the richest nondairy sources of calcium: One serving of figs contains 144 milligrams of calcium, which is higher than milk on an ounce-by-ounce basis. Figs also are a good source of iron, vitamin B-6, magnesium and copper. Apricots are a standout because one serving provides one-fourth of the U.S. RDA for iron, and enough beta carotene to satisfy most of the daily value for vitamin A.

Nuts and seeds are also healthy snack options, and they go well with dried fruit. You can also try a few pretzels, low-fat chips or a bit of granola. I recommend working with your children to make their own personal trail mix -- a healthful combination individually tailored to each child's tastes.

Send questions to: "On Nutrition," Ed Blonz, c/o Universal Uclick, 1130 Walnut St., Kansas City, MO, 64106. Send email inquiries to questions@blonz.com. Due to the volume of mail, personal replies cannot be provided.