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Today, 07 November 2009

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Question

When you exercise your heart rate decreases as you get fitter. Why? When you exercise you increase your metabolism by increasing your heart rate, so as your heart rate slows when you are fitter doesn't your metabolism slow as well? Is this how these diet pills which increase your metabolism work? My heart rate is slow compared to my friends and I go to gym and run regularly. Does this mean that my metabolism is slow?

Answer

To start with question number one - I am actually not sure why your resting heart rate decreases as you get fitter. It probably has to do with the complex physics of stroke volume and cardiac output which control heart rate and which will change as the heart muscle gets stronger and more efficient.

It is not your heart rate increasing which increases metabolism and it is certainly not true to say that as you get fitter and your heart rate slows, that your metabolism slows as well. The rate of metabolism is controlled by many complex factors, including activity levels and what you are eating. If you exercise hard then you increase your metabolism not only during the time over which you are exercising, but in the hour after exercise as well - during which time your heart rate will have returned to normal. The harder you exercise, the greater this effect, so you actually burn more total fat and calories by exercising hard than by exercising moderately - contrary to what you may have gathered from some literature.

Any diet pill which claims to increase metabolism is almost certainly not doing what it claims, unless it contains substances such as thyroxine, which are very dangerous when taken by someone with normal thyroid function. If a diet pill increases your heart rate, all it contains are substances which act on the cardiac system to increase heart rate - they are not increasing your metabolism at the same time. Most of the appetite suppressants do this as a side effect.

If you have a lower heart rate than most of your friends, it could be that you are a lot fitter than they are, or that you simply have a relatively low resting heart rate - this varies between people anyway.