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Fasting can help rats exercise, but not humans


Several articles have appeared in sports magazines advocating fasting before athletic competitions. But fasting will markedly hinder performance by making you tire earlier.

Fasting before competition became popular as the result of an article that appeared in the Journal of Applied Physiology. It showed that rats that fasted for 24 hours could run farther than they could after they ate. How long you can exercise a muscle depends on how much sugar you can store in a muscle before you start to exercise. Fasting prior to exercising uses up a significant amount of stored muscle sugar and reduces the amount of time that the muscle can be exercised.

Evidently, fasting caused the rats' muscles to burn more fat and useless muscle sugar to make the sugar last longer. But this does not apply to humans. Fasting does not cause our muscles to use more fat; it just uses up the sugar already in the muscle.

My aerobics instructor talks a lot about maximum heart rate. Why is it important? Virtually every student in an exercise class learns that the fastest rate your heart can beat is determined by the formula 220 minus your age. But that formula applies only to out-of-shape older people. According to the formula, if you are 40 years old, the fastest your heart can beat is 220 minus 40, or 180. If you are 20 years old, the fastest your heart can beat is 220 minus 20, or 200 beats a minute. But very fit older athletes can have the same maximum heart rates as younger people.

The formula depends only on how strong your skeletal muscles are. Your heart rate is driven by how hard you exercise your muscles. When you start to exercise, your muscles alternately contract and relax. With each contraction, the muscles squeeze blood out of the veins near them. When the muscles relax, the veins near them fill up with blood. So exercise causes your muscles to pump far more blood to your heart. The heart responds to the increased return of blood by beating with more force at a faster rate.

Should I be taking aspirin to prevent heart attacks? Americans take more than 6 billion aspirin tablets each year, hoping that it prevents heart attacks. The vast majority will not benefit from the aspirin; some will be harmed.

A recent Harvard study showed that aspirin prevents heart attacks only in men who were older than 50 and those who were at increased risk of having a heart attack.

No studies show that aspirin prevents heart attacks in healthy men. The data show that aspirin helps to prevent heart attacks in men who have had a heart attack and those who already have heart disease.

Aspirin does not stop the formation of plaques; it prevents clots. So aspirin prevents heart attacks only in people who already have significant plaques in their arteries. One baby aspirin every other day gives you maximum protection against clotting. Taking doses higher than that does not benefit you further and can cause side effects, such as upset stomach and stomach bleeding.

Dr. Mirkin is a practicing physician in Silver Spring specializing in sports medicine and nutrition.


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