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.
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