MEASURING EXERCISE DOSE

 Why measure exercise dose?

You might wonder why it is helpful to regard exercise as a “dose” let alone measure it. Two reasons pop into my mind:

1. 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.
2. 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.

How to measure exercise dose

Exercise dose can be expressed as either intensity – the rate of energy expenditure – or as total amount of exercise taken – total energy expended.

Rate of energy expended:  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 usually divided by 3.5 to give the rate of exertion in metabolic equivalents or METs (see my last Blog, Exercise and Oxygen). 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 17 ml/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 17 ml/min/kg or 4.9 METs but this would equate to about 1.7 litres of oxygen per minute. No wonder fat people get more out of breath with exercise than thin people.

2. Total energy expended: This tells us the total amount of oxygen consumed 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 consumed 12 litres of oxygen while the fat man will have consumed 17 litres. These figures can be converted into Calories. Each litre of oxygen used is converted into about 5 Calories of energy. In the cases of our walking men above, in ten minutes the thinner man has used 12×5 = 60 Calories. The fatter man has used 85 Calories.

The trouble with 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 with a big C), calories multiplied by 1,000. Confusingly many people call the kcal a calorie with a small c! So if you read figures measured in calories, particularly in relation to food, the writer usually 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.

Food and its Exercise equivalent

Knowing the calorie values of both exercise and food is useful for assessing the dose of exercise needed to burn off what you have eaten.  I regret to tell you that it takes a disappointingly small quantity of food to fuel enormous efforts! One Mars Bar will provide enough energy for 40 minutes of brisk walking at 4mph for the 70 kg man. An often quoted index of high calorie food is its equivalence in teaspoonfuls of sugar. A teaspoonful is just slightly over 5ml which converts to about 4 grams of sugar which is worth about 20 Calories – about enough to fuel about 3 minutes of brisk walking.

Using METs

Total exercise dose can also be calculated as MET minutes or MET hours.  As explained above, one MET, or metabolic equivalent, is the rate at which energy is used by the body at rest. It is expressed in relation to body weight and is taken as 3.5ml of oxygen per minute per kilogram. A 70kg man walking at 4 mph will be exercising at about 5 METs. If he maintains this pace for an hour he will have expended 5×60 = 300 MET minutes or 5 MET hours.

It is not too difficult to calculate the total number of MET minutes of exercise you expend in a week – though it may demand a degree of obsessiveness to do so. Just add up the MET minutes of all the exercise you have done, but remember that the actual energy cost of each exercise is only an approximation. If you are anything like the rest of humanity this will be an overestimate!
It is then possible to convert the exercise dose from MET minutes into Calories using this formula:
Total MET minutes x 3.5 x body weight in kilograms ÷ 200 = number of Calories used.

For example the 70kg man walking briskly for an hour will have used:
5 x 60 x 3.5 x 70 ÷ 200 = 367.5 Calories

Next week we will look at the energy costs of different activities.

PS

Two more developments in the footballer/dementia story this week. A study of  352 amateur footballers found that those with a particular genetic characteristic, APOE e4, were at greater risk of damage to memory than those who did not possess this gene. The authors are quick to state that it is too early to recommend genetic testing of all footballers to see whether it is safe for them to be heading the ball.

The second study was of American football players. The more concussion symptoms former National Football League players had during their careers, the more likely they were to develop erectile dysfunction later in life. Oh, dear. I don’t think that this possibility has been looked at for soccer players but it certainly gives pause for thought.