I do a fair amount of cycling over what some would consider long distance - ~40 mile rides every 2-3 weeks and shorter rides more frequently.

I have to consume a fair quantity of high GI carbs while on the bike or I will most certainly Bonk somewhere round the 120 minute mark.

What I would like to understand is how this consumption affects fat burning. How does the body "select" what energy sources to draw on while exercising. Does the exercise trigger the body to continually burn fat (and everything else it can find), or is low blood glucose needed to force this?

Please note this is NOT a generic fat loss question. I'm not looking for "how to maximise fat loss". I'm looking to understand the science.

From what I've read the heart rate during exercises is linked to the proportion of calories drawn from glycogen and fat. I've seen significant disagreement about weather this means fat burning drops off at high heart rates or not. But lets presume for the sake of argument that I'm exercising at a rate which does burn significant quantities of fat.

It's also understood that taking in carbs during very long rides will increase endurance by allowing your body to burn the carbs you're eating / drinking before burning the glycogen and thus increase the time before glycogen is depleted.

What I'm really asking here is whether the carbs I eat during the ride affect fat burned and glycogen burned evenly (leaving the proportions unchanged). Or does or is this affect asymmetric? Does it significantly affect one more than the other and thus change the proportions?

  • Just as an aside - if you are "bonking" at the 60-90 minute mark, either you have some metabolic problem, or you are exercising at near 100% intensity. You should have plenty of available glycogen stores to go longer than 60-90 minutes at submaximal effort levels. For an example, here are two examples of true bonking (Completely running out of glycogen stores, things begin to shut down): youtube.com/watch?v=Zs4lbNPiat0 and youtube.com/watch?v=LKhkvSRQZYo
    – JohnP
    Commented Aug 27, 2014 at 21:18
  • That will also adapt, and you'll be able to go longer on less.
    – JohnP
    Commented Aug 27, 2014 at 21:29
  • Make that a separate question, that's a good training question. I don't think it's been asked on here yet.
    – JohnP
    Commented Aug 27, 2014 at 21:44

3 Answers 3


The body "selects" the substrate for fuel on the amount of time that it has to provide energy to sustain the exercise, and the amount of oxygen available to use in the conversion process. At lower intensities, fat (both stored and circulating ffa {free fatty acids}) is the preferred source, with carbohydrates in minimal proportions.

As you get higher in intensity and heart rate, the proportions start to switch, until at all out maximal levels (anaerobic), you are getting 100% from stored glycogen.

Everyone want's to know what their "fat burning zone" is, and the answer is that it's all of them. Exercise for 30 minutes at X level and you will get 75% from fat. However, if you exercise for the same amount of time at Z level, you will get 45% from fat, but because of the oddities of percentages and training and calories, you may burn slightly more fat calories with the second.

The bottom line, however, is that fat reduction is a function of calorie intake compared to calorie burn, and quality calorie intake (Rather than junk food). Study after study has proven that it's not the specific type of diet (paleo, atkins, etc) that makes as much difference as simple calorie reduction.

It also depends some on the type of exercise. For example, running at a 10 met level will burn more calories than cycling at a 10 met level (MET = metabolic equivalent, it's used to figure out energy burned during exercise).

So if you want to maximize fat loss, figure out your goal weight, eat a clean diet and burn more calories than you consume. Lather rinse repeat.


The short answer is that the science is not completely understood, as every individual has a unique physiology and no 2 people respond to fat/carb intake the same way as far as what their body is burning for fuel.

The rough estimate that works for most people is that somewhere around the max aerobic heart rate (180 - your age is a pretty good estimate) your body switches proportions from mostly fat to mostly carbs. As far as I know there are no concrete data showing just how much "mostly fat" and "mostly carbs" are, again mostly likely because the body's choice of fuel at any given time is not fully understood. But you can probably figure that if you're well below that heart rate your body will be burning mostly fat stores, and if you're well above it your body will be burning glycogen.

So if you have access to some form of heart rate monitoring while you're out riding, it may well be that if you're bonking at 60-90 minutes, your heart rate is higher than optimal and you're burning through your glycogen stores.

  • 2
    This is true from the standpoint of muscle physiology. However, one more thing should be noted. Ingestion of carbs stimulates the release of insulin, which in term inhibits adipolysis. I.e. it makes it harder to lose weight. In light of this, if the goal is fat loss, I would suggest minimizing carb intake during and immediately before exercise. There are numerous studies showing that carb loading is most efficient the day before, and immediately after a finished workout. Commented Jul 28, 2014 at 23:56
  • True. I generally prefer to work out at an intensity such that I'm burning primarily fats (I try and keep my heart rate below 150) and drastically limit simple carbs when I'm in weight loss mode.
    – alesplin
    Commented Jul 29, 2014 at 3:17
  • @couling - mmm...sort of. Energy from fat is a higher percentage at lower efforts. The higher you go on the effort scale, the more you start getting it from glycogen, both stored and circulating. Even at the extreme ends of the scale, you're still burning both in some percentage. Where it gets funky is the amount burned to sustain the exercise.
    – JohnP
    Commented Aug 27, 2014 at 21:23

Fat burning zones are foolish. Sprinters never train in the fat burning zone and are ripped. Higher intensity exercise can raise your metabolism long after you are done training. The shortest answer is, eating carbs when training stops burning fats, as eating carbs generally stops burning fat.

  • Bad answer based on bro-science and "seems right".
    – JohnP
    Commented Dec 1, 2014 at 15:13

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