26 January 2010

Fueled by fat: fat burning 101

In the third installment of his series on exercise and weight-loss, FitnessDoc takes us through how exercise burns fat and the best way to go about this.

In the third installment of his series on exercise and weight-loss, FitnessDoc takes us through how exercise burns fat and the best way to go about this. 

So, what you're probably thinking is that theory that low intensity exercise is better if you want to burn fat is correct.  Well, think again.  It is true that at low intensity, when you walk, most of your energy comes from fat, and that as you increase the intensity, less and less comes from fat. 

But what is missing in this picture is the TOTAL amount of energy.  Let me phrase it this way:  Would you rather have 20% of the money in Jonathan's bank account, or 80% of the money in my bank account?

Your answer of course, should be that you don't know.  And what you should be asking is "How much money is in your bank accounts?".  The reason is, you'd be pretty annoyed if you took 80% of my money, because my bank account might be empty.  Jonathan, on the other hand, might be loaded, and 20% of his money sees you retiring at 40.

So, look at the graph again.  You should not be too pre-occupied with the percentage, but rather the total amount of fat that you are burning, because that 50% that comes from fat in the green zone might be more than the 20% that comes from fat in the pink zone.  For answer to that, we look at a study done many years ago by Romijn.

Below is a graph that I've redrawn with some calculated figures, based on Romijn's study.

So, you're looking at the same kind of graph as before, showing how much of your energy comes from fat and CHO at different intensities (I've further divided it into muscle, liver and adipose sources, but that detail is not vital right now.  Note, however that "Plasma FFA" refers to the fatty acids that originate from the adipose tissue - the triglyceride is broken down into free fatty acids which are used).  

This time though, there is also a measure of HOW MUCH total energy is being used, shown by the y-axis (in kCal/hour, for an 80kg man in this case).

So again, at low intensities (25% VO2max), you'll see again that most of the energy comes from fat (75% in this case), with only 25% from CHO.  Jump to 60% and the fat contribution falls to 48% and then at 85%, fat provides only 20% of the total energy.  This confirms what we saw in the first graph.

But, the key is that the TOTAL energy, shown here on the  Y-axis, rises as well, and so the 48% of energy from fat that you get at moderate intensity actually adds up to more TOTAL fat use than the 25% did at low intensity.  

Think of the Y-axis (total energy) as the size of the bank account, if you will.  The green boxes above ach bar show the total amount of fat burned in an hour - 24g, 37g and 23g per hour at the three intensities.

The moral of the story, which is shown further in the graph below, is that if your objective is to maximise fat burning, low intensity is not necessarily the best option.  Rather, moderate intensity burns more fat per hour, before a decline in fat use as the intensity rises beyond about 60%. 

The reasons for these shifts, incidentally, include increased sympathetic response to exercise, activation of different muscles as intensity increases, local regulation of metabolism in the muscle, and also the exponential nature of physiological responses to an increase in intensity.  That's a post of its own!

Remember also that these are graphs based on calculations and measurements for a typical person - the actual values may vary for different people depending on training, diet and sympathetic activity, as mentioned. 

One also has to make some assumptions, such as VO2max values, the RER during exercise and how it changes with intensity.  But I want to highlight the principle, not the values, so bear with those assumptions.

The role of time and putting it into perspective with diet

So, what this means is that if your goal is maximum fat use, then lower intensities may mean a greater relative contribution, but it ismoderate intensity that gives you greater fat use per unit time.

Time is key though, as many of you are no doubt thinking.  If you do a low intensity session, walking (25% VO2max in the above graphs), then while your total fat use per hour is not as high as at say 60%, you might be able to do this exercise for three hours, compared to only one hour at a moderate intensity. 

Three hours with 24 g per hour (see figure) beats one hour with 37g.  So duration plays an equally important role.  What I am emphasising is that for a given time, you will be better off at a moderate intensity somewhere between 50% and 65%.

The next point, before we get too carried away, is that the actual amounts are pretty small.  

Not to discourage you too much, because exercise is vital for weight management and health, but one rule of thumb is that you can estimate total energy use during running by saying that you burn 1 kcal per kilometre per kilogram.

So, taking our 80kg man, who runs 8km, he'll burn about 640 kcal.  It doesn't matter how fast he runs, this amount is the same (the difference, of course, will be how much time he spends burning it).  

Now, assume that half his energy comes from fat and half from carbohydrates.  That means 320 kCal from fat, corresponding to 35 g (this is about the same as in the graph, since I based that graph on this kind of calculation).

So, 35 g of fat in 8 km or 5 miles.  To burn 1kg of fat, he therefore has to run 230km (142 miles).  Not very encouraging.  

There is an additional 'after exercise' effect, which one can't ignore, but I want to make the point that actually burning fat is not as simple as we often assume.  In the words of Bengt Kayser, who pointed out this principle, it puts into perspective the resolution that says "I want to burn off 10kg of fat by running".

You can work out a similar thing for cycling.  If you ride for an hour at 200W, your energy use is probably going to be about 800 kcal.  

If 50% of this comes from fat, then you've got 400kCal, or about 44g of fat.  That is about the same rate of fat use as your 8km run gives you, and a lot of cycling to burn those 10kg of fat.

The importance of diet 

And this is where diet comes into the picture.  If you want to create the energy deficit in order to lose weight, and notwithstanding that this is an oversimplification as we described in Part 2B of this series, then the combination of diet and exercise the way to do it, because to actually burn energy directly is a lot more difficult than people think.  

This is why the combination, and the long-term approach is so important.  Things do not happen overnight in physiology, particularly when it comes to weight loss.

The progress can be slow, and invariably, when it is not, it's not sustainable.  Crash dieting, starvation might lead to rapid results, but they also lead to subsequent rebounds, and health problems that we won't go into now.

But for the purposes of this post, which I'll wrap up because it's been rather heavy on numbers, the key point is that burning fat during exercise happens right across the range, and the ideal intensity, if you are looking to make the most of your time, is a moderate intensity, and not the low intensities that you'll often hear recommended.

Next time, we'll look a little more at energy use during exercise and the role of diet.

Dr Ross Tucker, is Health24’s FitnessDoc and has a Ph.D. in Exercise Physiology from the University of Cape Town and a Post-Graduate degree in Sports Management from the UCT's Faculty of Commerce. He is currently employed at the University of Cape Town and Sports Science Institute of South Africa, and works as a consultant to various sporting teams, including South African Sevens, Canoeing, Rowing and Triathlon SA. He also blogs on


Read Health24’s Comments Policy

Comment on this story
Comments have been closed for this article.

Live healthier

Lifestyle »

E-cigarettes: Here are five things to know

E-cigarettes have become hugely popular in the past decade, but a rash of vaping-linked deaths and illnesses in the US is feeding caution about a product that's already banned in some places.

Allergy »

Ditch the itch: Researchers find new drug to fight hives

A new drug works by targeting an immune system antibody called immunoglobulin E, which is responsible for the allergic reaction that causes hives.