Units of measurement
There are many ways of measuring exercise dose and the the best unit of aerobic exercise is the Metabolic Equivalent or MET. This is a measure of the intensity of exercise or physical activity, not total dose. It reflects the rate at which oxygen used, or calories burned, during that exercise.
The unit is standardized so it can apply to people of varying body weight and allow comparisons between different individuals. A person who weighs more will burn more calories per minute.
One MET is the rate at which oxygen is used in the sedentary state. This is approximately 3.5 ml of Oxygen, or one kilo-calorie, per kilogram of body weight per minute. To convert this into total work done it must be multiplied by the time taken which gives the number of MET minutes or MET hours of physical activity performed.
MET levels of different activities
The MET can define the intensity of physical activity:
- Light-intensity activities: under 3 METs
- Moderate-intensity aerobic physical activities: 3 to 6 METs.
- Vigorous-intensity aerobic physical activities: over 6 METs.
Why measure the dose of exercise or physical activity?
You might wonder why it might be helpful to regard exercise as a “dose” let alone measure it. Two reasons pop into my mind:
- Because of its widespread health benefits exercise can be equated to medication. How big a dose is needed to help weight control, reduce risk of diabetes or lengthen life? What is the dose response – ie the ratio between exercise dose and the extent of any of these and other benefits? What would be the dose required to produce the same life prolonging effect as say taking a cholesterol-lowering drug like simvastatin at say 40mg daily? The answer is never straightforward. For instance the intensity with which the exercise is performed changes the effect on health. An hour’s brisk walking may require the same exercise dose as walking about slowly all day but the effects will be very different.
- Exercise is on one side of the equation which decides weight change. Comparing the calorie content of our food with the calorie output of our exercise allows us to see why we are getting fatter – and what might be helpful to reverse this. For this purpose the intensity of the exercise should not be important – only the total dose.
Intensity of exercise versus total energy expenditure
There are several ways of expressing the quantity of exercise taken and all are related to the oxygen cost of the activity. Apologies for some repetition of what I have written above.
Rate of exertion: This is the measure of exercise intensity which tells us the rate of oxygen consumption. It can be expressed as an absolute rate ie litres of oxygen per minute (L/min) or relative to the exerciser’s weight as millilitres of oxygen per minute per kilogram body weight (ml/min/kg). The weight-related figure is divided by 3.5 to give the rate of exertion in METs.
An example would be the rate of oxygen consumption required by a 70kg man walking on a treadmill at say 4 mph – this takes approximately 17ml/min/kg or 4.9 METs and equates to about 1.2 litres of oxygen per minute. The heavier you are the more effort you need to move yourself along so for a 100kg man the equivalent figures would still be 17ml/min/kg or 4.9 METs but this would equate to about 1.7 litres oxygen per minute. No wonder fat people get more out of breath with exercise than thin people.
Total energy expended: This tells us the total oxygen consumed, or calories expended, by a particular period of exercise and is the actual dose of exercise taken. In the example given above the 70kg man walks at 4mph for ten minutes he will have expended consumed 12 litres of oxygen while the fat man will have consumed 17 litres. These figures can be converted into calories, or the metric equivalent, joules.
Kilocalories and calories
Unfortunately common usage has made the calorie a more complicated unit than it need be – sorry about this but you do need to understand what a calorie means. A calorie is the amount of energy (in the form of heat) needed to increase the temperature of 1ml of water by 1 degree centigrade. Since this is such a small amount of energy, nearly everybody works in kilocalories (kcals or Calories – note the capital C), calories multiplied by 1,000.
Confusingly many people call the kcal a calorie! So if you read figures measured in calories, particularly in relation to food, the writer probably means kcals!! It would be a lot simpler if we converted to the metric equivalent, joules, but calories are deeply ingrained in our language and culture so that is never going to happen. One little calorie is equivalent to about 4.2 joules and one big Calorie or kcal is equivalent to about 4.2 kilojoules (kjoules).
In human physiology, each litre of oxygen used is converted into about 5 Calories of energy or about 21 kjoules. In the cases of our walking men above, in ten minutes the thinner man has used 12×5 = 60 Calories or 12×21=252 kjoules. The fatter man has used 85 Calories or 357 kjoules – just work it out.
Next week I will discuss the energy expenditure of different physical tasks.
My old friend, Professor Richard Moxon, emeritus professor of paediatrics at Oxford, has written a fascinating book called Brain Fever. This describes the horrors of meningitis and the development of vaccines to prevent a number of different forms of the disease. Richard himself was the brains behind much of this work.
Richard and his publishers are holding a virtual launch of the book at 5pm on July 29th. You can take part and learn more by clicking on this link