Energy expenditure, health and fitness

Another recap

Total exercise dose can be calculated as MET minutes¹ or MET hours. And 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.

Walking

A 70kg man or woman walking at 4 mph will be exercising at about 5 METs. If he/she maintains this pace for an hour he/she will have expended 5 MET hours or 5×60 = 300 MET minutes. 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 person walking briskly for an hour will have used:
5 x 60 x 3.5 x 70 ÷ 200 = 367.5 Calories

The exercise which most people do most of is walking – and very good exercise it is too. Brisk walking is all you need to satisfy the recommendations of the health gurus so here is a table giving you the exercise intensities of walking at different speeds.  For most people brisk walking means travelling at between 3 and 4 mph.The figures given are for walking on the flat with neither a following nor a head wind. The MET values are the same for everyone irrespective of weight. The Calories consumed column is for a 70kg (11 stone) individual. Lighter people will expend fewer calories and heavier people will expend more, so one advantage of being overweight is that the heavier you are the easier it should be to lose weight:

Speed mph	Mins per mile	METs	VO2 ml/min/kg	Cals/hour
2.0	30	2.5	8.9	176
2.5	24	2.9	10.2	205
3.0	20	3.3	11.5	233
3.5	17	3.7	12.9	261
3.75	16	4.4	15.3	310
4.0	15	4.9	17.1	346

The relationship between walking speed and oxygen need is a straight line at lower speeds but above 3mph it becomes harder to increase speed – and each increment in speed becomes more costly in terms of oxygen demand.

Jogging and running

Jogging/running is one of the most efficient exercises for expending calories or getting fitter. It is quick to do, needs minimal equipment and cost and does not require an opponent – and even so it can be a most satisfying social occasion. Here are the exercise intensities of running at different speeds. Again the figures presume the unlikely scenario of no hills and no wind. The calorie expenditure is that of an individual weighing 70kg (11 stone):

Speed mph	Mins per mile	METs	VO2 ml/min/kg	Cals/hour
3.5	17min	6.4	22.3	451
4.0	15min	7.1	24.9	501
4.5	13min 20sec	7.9	27.6	558
5.0	12min	8.7	30.3	614
5.5	10m 54sec	9.4	33	663
6.0	10min	10.2	35.7	720
6.5	9m 13sec	11	38.3	776
7.0	8m 34sec	11.7	41.0	826
7.5	8min	12.5	43.7	882
8.0	7min 30sec	13.3	46.4	939
8.5	7min 04sec	14	49.1	988
9.0	6min 40sec	14.8	51.7	1045
9.5	6min 19sec	15.5	54.4	1094
10.0	6min	16.3	57.1	1150

 

You will see that running at low speeds is less efficient than walking. If you wish to go at 4mph, walk don’t run!

Cycling

Cycling is another exercise for which it is possible to calculate energy expenditure. Here are the very approximate figures for the MET value, oxygen cost and calorie expenditure for cycling on the flat without wind for a 70kg (11 stone) individual. Real life conditions means that the energy costs out there are usually much higher:

Speed in MPH	METs	VO2	Cals/hour
10	5	17.5	470
12	7	24.5	514
14	9	31.5	661
16	11.5	40.2	845
18	14	49.0	1200

National guidelines

For health benefits for adults, the national guidelines suggest using exercise to burn about 900 Calories per week, roughly 150 minutes brisk walking.  For weight loss and even greater health benefits you need an expenditure of 1,800 Calories per week or five hours walking – about 25 MET hours.

Unfortunately it is not easy to measure exercise intensity or dose however we choose to do it or to express it. Very few exercises are sufficiently independent of the effort we put into them or our skill in their performance to use an amount of energy which is more or less the same for everyone. Those which are predictable include walking, cycling and running – on the flat without a wind. For most other exercise we can only give approximations of energy expenditure. For instance it is possible to give rough estimates of the energy costs of playing tennis or various gym activities but they will be very dependent on the amount of effort put into the activity by the performer.

Last week I gave a table of energy expenditure for a variety of different activities – from sleeping to very vigorous. I expressed the expenditure in METs, oxygen consumption and Calories. The MET expenditure is as given but both the oxygen consumption and Calories expended depend on body weight. My apologies for not saying that the figures in the table referred to a 70kg (11 stone) individual. Lighter people would use less oxygen and Calories while heavier people would use more.

Using modern accelerometer devices (Fitbits and the like) should allow more accurate estimates of the amount of exercise taken but would rely on heart rate responses and would need to be calibrated to the individual to produce anything like accurate measures of energy expenditure.

PS

Don’t forget:

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

register for the event.