Fuelling for endurance

When it’s OK if energy in <= energy out

Update 22/07/16: coming back to this post, I feel like it should come with a simple, but loud caveat. It’s okay to burn fat for fuel and not replace it if you need to lose fat. if you’re already a healthy weight, however, be very careful about exercise with a continuous deficit. It is vital for us to store enough fat around the body and very low body fat and long-term energy deficits can lead to health problems. If you want to fuel well and ride all day without losing body fat over time, be sure to replenish it when you’re not riding or exercising to stay fit and healthy.

With the winter base-training season upon us and long-steady rides and runs being the order of the day, getting fuelling right so you can keep the aerobic engine ticking over while avoiding big fluctuations in your body composition (one way or the other) is really important. I love riding my bike and if I could, I think I’d ride it all day, as often as possible! The wonderful thing is that, theoretically, at the right intensity that’s perfectly feasible! Yay! It’s just about balancing the energy in, the energy out and how your body exchanges one for the other. So this means you just need to match energy in with energy out, right?

It’s actually better than that. Not as simple, but better for those long runs, days in the saddle or multi-sport events. The body is more efficient than first meets the eye. Let me explain why you may not need as much fuel as you think. This might be something you intuitively know if you’re a keen long-distance person, but sometimes it’s helpful to understand why.

If you’ve looked at how much energy you can absorb from carbohydrates per hour and compared to how much energy you burn per hour and wondered about how to make up the deficit once you’ve used up the glycogen stores in your muscles, you can heave a sigh of relief! If this idea is new to you, let me first explain what I mean about the rate of carbohydrate absorption. As with any body function, our digestive system has limits. Even if you consume pure glucose, the simple sugar molecule that our body uses to transport energy around the body, there’s only so much that can pass into the bloodstream at a time. So what are the limiting factors? Contrary to what you might guess, the amount of carbohydrates we can absorb is not dependent on bodyweight, but our digestive capabilities as well as the type of carbohydrates.

Homemade dried fruit and nut balls

Dates, nuts, seeds, coconut and goji berries providing a mixture of energy sources

A review of research1 concludes that the figure is a maximum of 90g per hour for highly trained athletes consuming ‘multiple transportable carbohydrates’ i.e. a mix of glucose or maltodextrin and another carbohydrate molecule like fructose, which speeds up the rate absorption. For most people a significantly smaller dosage is recommended, but as 1g of carbohydrate contains 4kcals of energy, the maximum intake of 90g per hour only provides 360kcals of energy. To put that in context, let’s say you’re burning 600kcals per hour for your run. That means for in the second or third hour of an endurance training session or race, it would be almost impossible to restock all the energy you burned on the go from the carbohydrates you take in, especially given that most of us could only ingest 30-60g or 120-240kcals. But, depending on the exercise intensity, this is not a problem: if you’re working at an intensity that generates about half its energy from glycogen and half from fat, the most you need to replace is the half from glycogen. When blood sugar levels are low, the body responds by transporting fat from fat cells to use in the citric-acid cycle which recycles waste products to create more energy, as long as sufficient oxygen is available.

What this means for you

If you’re doing a lot of endurance sessions which are hours long at a time, don’t panic if you find that energy in is often less than energy out. If you’re using an app like MyFitnessPal plus one working out the calories burned on your run or ride, don’t worry about making up small deficits as you don’t necessarily need the total for a given day to break-even. The longer you were out, the lower the intensity would have needed to be and therefore the more energy would have come from oxidising fat. The balance can be in the region of 70% fat/30% carbs at low intensities and the balance shifts all the way up to 0% fat and 100% carbs at very high intensities. If you’re running on 100% carbs you won’t last that long because you won’t be able to replenish it quickly enough. If you’re well trained though, you could burn most of your glycogen stores plus most of the energy you’ve consumed on a ride, then ease off, let your heart rate come down, have another small snack (if it’s not too soon since the last one) and settle back into an easier zone where you start burning fat again. In fact, this is what usually happens if your ride has a hill climb or your mates decide to practise time-trialling when you’re still a long way from home. If you want to work this out for your own body with more precision, I highly recommend the introduction in Feed Zone Portables2 which will help you calculate this.

Kick-starting the fat burning process

The fat oxidisation process happens when the adrenal glands respond to a drop in insulin (which happens when blood sugar levels drop) by releasing a hormone to start moving fat to the muscle cells to be converted to energy. This means that even if you train at low intensity, if you always aim to maintain high blood sugar levels, you won’t trigger this process. Apart from the fact that most of us would like to burn some fat during exercise, some studies shows that training this process can make it happen more readily and therefore allow you go further with less fuel intake when you need to.

The warning label

A word of caution: your brain can only run on glucose, it cannot metabolise fat like the muscles – so it is vital to always keep a little in reserve. What’s more, you need to have some carbohydrates present to convert the fatty acids to energy, another reason to keep the bananas, dates, rice-cakes or (if you must!) gels (OK, and cake!) within reach.

I hope you find this useful, it is such a big topic so in such a short article (or series of articles) it’s inevitable that some questions are answered but lots of other questions are raised! I’d love to hear what questions this raises for you or if there’s anything you think I should elaborate on. Feel free to leave a comment below or if you prefer, drop me an email. Happy winter training!

Ride to Cold Christmas


  1. Jeukendrup, A. (2014). A step towards personalized sports nutrition: carbohydrate intake during exercise. Sports Med. 44:S25-S33. http://www.ncbi.nlm.nih.gov/pmc/articles/PMC4008807 Accessed on 6/11/2015
  2. Thomas, B. (2013). Feed zone portables: A cookbook of on-the-go food for athletes. Boulder, Colorado: Velo Press

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