Overview and main messages

This post shows that if you run a fixed distance, then the number of calories you burn doesn’t depend much on your speed, but if you cycle a fixed distance then the faster you go the more calories you will burn. If you walk a fixed distance then going faster can actually lower the number of calories you burn, though walking super-fast can increase it.

I also explain that you can lose weight whilst eating what you want provided you exercise a lot, and that if you’re pushed for time, running is the best way to burn calories

Background

About six weeks ago I decided to lose some weight. I was 89 kg (14 stone) and at 5 ft 8 inches this put me about 14 kg (2.2 stone) over the recommended healthy ideal taken from a body mass index (BMI) calculation. BMI is only a guideline of course – some people can be healthy with a much higher weight than the charts suggest, but in my case my gut was getting too big and playing sport was much more of an effort than it used to be.

So I started exercising more and calorie counting for the first time in my life. I downloaded a free app (MyFitnessPal) to log everything I ate, and add up the calories, and all the exercise I did, which estimates how many calories you burn and deducts them. It’s not rocket science: if you burn more calories than you consume then you will lose weight.

I had no idea how well the app would work. It relies on estimates of (a) the number of calories you consume, (b) the number of calories you burn when exercising and (c) the number of calories you burn in a normal day without exercising at all (for a person who lay stationary all day this would be a minimum value known as the basal metabolic rate (BMR)). I know that BMR varies hugely from person to person (as much as 30% Wikipedia tells me) and was fairly sure the other estimates could vary a lot as well.

But six weeks on I’m pleased to say it’s working. I am now at 81 kg (12.7 stone). The tummy is smaller, clothes fit better and walking and running is getting easier. And I’ve never gone hungry. I quickly realised that provided I did a ton of exercise, I was able to eat what I wanted. (NB I have cut down on mid-week alcoholic drinks after work but not at weekends.)

Important observation about exercise and weight loss

Occasionally you will hear people say that if you want to lose weight then you must watch what you eat, and that exercise can help a bit but won’t really shift the pounds. In my experience this isn’t true but you do have to do a lot of exercise to eat what you want and lose weight. In my case I have been doing an average of 90 minutes vigorous exercise a day burning an average of 1,400 kcal due to exercise alone. This has allowed me to consume an average of 3,300 kcal per day which will sound like a huge amount to most dieters. If you can find an exercise you like and do plenty of it every day then that will be a more fun way to lose the weight than eating depressing bowls of salad and going to bed hungry.

Calories burned with time at various speeds

My preferred forms of exercise are cycling, swimming and five-a-side footy. I’m not so keen on running but recognise its benefits and provided I can find interesting circuits to run around with plenty of variety then I don’t mind it. Also, I don’t like public transport much so walk as much as possible. I used the app’s built in estimator to compute the calories I burned with each session. I compared several online estimators to see whether estimates differed by much. They do, by about 20%, which isn’t huge, and as I’ve said, they seem to work. One handy chart that seems to pitch its estimates in the mid-range is this here. The chart shows the calories burned per hour for various activities for four body weights. I place a degree of trust in these tables based on personal experience and the analysis that follows does rely on them being correct. You may be sceptical about how accurate they are, and indeed you should be – for now however, please put the judgement on hold and I’ll proceed:

Look at the cycling, walking and running figures. They contain values for different speeds. Though it doesn’t state so I’m pretty sure the values are for flat, hard ground in all cases. The data for a 180 lb person is summarised on the graphs below, with the speeds converted from mph to kmph

(NB I have chosen the 180 lb data as that is the closest to my weight. The general principles of the analyses that follow also apply to any weight thought the exact numbers vary, the most significant part being that the heavier you are the more calories you will burn.)

With each activity, the faster you go, the more calories you will burn in one hour. This is hardly surprising. Some observations:

Walking: walking at 2 km/h (a ponder-round-the-garden-with-your-gran pace) for 1 hour burns a mere 150 kcal, and the faster you go the more you’ll burn, up to the cracking pace of 8 km/h which, if you were fit enough to maintain it (I’m not), would burn 650 kcal in one hour.

Notice that the increase is not quite linear, i.e. the points aren’t in a straight line, and curve upwards a little. This will be important later.

8 km/h is the limit of a normal walking pace. Most of this can push ourselves along faster than this but we need to start running. (Professional speed walkers can actually go at about twice this pace but need to adopt the artficial funny-looking style of walking you may have seen on Olympic coverage.) While walking at 8km/h will seem absurdly fast to most of us, running at this speed will seem very slow. Both methods expend about the same amount of energy per hour however, even though the walking may seem to tax us more.

Running: increasing the running speed increases the energy used per hour linearly, with a speed of 17.5 km/h (attainable for one hour by a top level amateur) would burn a whopping 1,400 kcal. Most of our running speeds would fit somewhere along the line.

As stated, the points are linear this time; the straight line equation is displayed on the graph and will be used later.

Cycling: cycling also displays a linear rise, with a very casual ride at 14 km/h burning 300 kcal, up to the aggressive pace of 34 km/h burning 1,300 kcal. Once again, this top level is about the limit for an amateur cyclist and most of us pedal somewhere in the mid-range.

Advice if you want to lose weight by exercising and are pressed for time

If you have, say, just half an hour per day to exercise and want to maximise calorie expenditure during that time then going for a run is almost certainly the best way to do it. Running will definitely burn more calories than walking, and, unless you’re really bad at running and comparatively competent and quick on the bike your rate of energy expenditure will be lower if you pedal.

Calories burned with time over a fixed distance

While cycling around the park I got to thinking about whether one would burn more calories by going faster round a fixed distance, or slower. This isn’t obvious. Whether walking, cycling or running, going faster will increase the rate at which energy is consumed, but also reduce the time of the journey. So there is a trade off between the two factors.

I have investigated this by considering a 10km distance on the flat for walking, running and cycling using the data in the referenced table and summarised in figure 1. The calculation is simple enough: for a given speed, v (in km/h), the time taken to cover 10 km is 10/v hours and the energy used will be the this number multiplied by the energy per time at the particular speed. The results are summarised in the figure below:

As before, the same form of results hold for weights other than 180 lb. Also, the distance of 10km is arbitrary; the same type of results would hold whatever the distance.

As stated before the exact nature of the results was difficult to predict and the plots produced are perhaps surprising and appear to be completely different despite stemming from datasets that looked fairly similar to each other in figure 1. I’ll look at each activity in turn:

Advice for dieting walkers

For walking, covering 10km at 2.5 km/h (taking over 4 hours) burns 650 kcal. If you walk at a faster speed then you will burn less energy up to a speed of about 5.5 km/h (taking just under 2 hours) which burns just 550 kcal. This is a fairly respectable pace for most of us. Going faster than this will indeed burn more calories, up to 800 kcal if you are able to maintain the crazy 8 km/h pace and cover the distance in just over an hour.

How can the dieting walker use this information? Let’s say you have a walk you do regularly, say, a 1 km walk to the bus stop which will burn a few calories. You may feel that walking there faster will burn more calories than ambling. If you have no time pressure and can’t walk particularly quickly then you may be better off ambling. The fact is you really have to push the walking pace to quite a fast rate that would be uncomfortable for most people to expend more energy over the distance.

Advice for dieting runners

For running the amount of energy used seems to follow a bit of an odd zig-zag pattern. But notice the scale on the y-axis. It only varies between 810 and 880 kcal. The amount of uncertainty on the calories burned estimate is certainly greater than this percentage variation i.e. there is hardly any difference in the energy burned at all. This is a fantastic result! What it means is that when people of all abilities line up for a distance race on the flat, it doesn’t matter how quickly they do the race they will all expend the same amount of energy as the other people in their weight category. It also means that when training, if you have a standard flat circuit, it doesn’t matter how fast you do it, you’ll burn the same amount of calories. If you want to burn more calories you have to run for a longer time, not a longer distance.

(NB this analysis cannot be applied to interval training covering a fixed distance. The datasets used cover a speed range of 8 – 17 km/h and so we can only apply the analysis to running within these limits. Interval training involves sprinting above the range, walking below the range and repeated acceleration and decceleration. My guess is interval training is significantly more calorie intensive than simply jogging the same distance at a fixed pace; it is certainly more tiring.)

Advice for dieting cyclists

When pedalling a fixed distance going faster burns more calories. As the graph show, a slow pace over 10km burns 220 kcal, whereas the fastest pace burns about 370 kcal. So if you have a fixed circuit you like doing, pushing the pedals harder will indeed burn more energy. Notice that the slope is non-linear, with the gradient flattening with speed so going 1 km/h faster when travelling slowly will have more of a percentage difference than going 1 km/h faster when travelling quickly.

Coming soon and possible further investigation

This post has put across the main empirical results across that I wish to share for now. There is more to come that I will leave until later, partly for brevity (the post is already quite long) and partly as there are a few things I wish to think about a bit more before writing. Most notably:

(i) I will provide a physiological/biological explanation for why walking, running and cycling provide such different results. There are probably several reasons why but I have a key one in mind.

(ii) I will provide a simple mathematical reason for why the similar looking curves in figure 1 lead to such different squiggles in figure 2. It’s not too complicated to explain but it would clutter this post a little.

Vijay Tymms, August 2013