Story time! It’s the best way to talk about the hot topic of fasted/low-carbohydrate training…
A prelude. Carbohydrate is needed to perform high-intensity efforts. However, the body only has so much carbohydrate stored which gets to be a problem because carbohydrates are also used during low-intensity efforts. As a result, an athlete can use up their carbohydrate stores before the end of a race when carbohydrates are needed the most (i.e. during a sprint stage). Conserving these carbohydrate stores could therefore theoretically improve performance.
Now, onto the story… It’s an overcrowded and poorly lit lab space and a researcher is thinking about how to help athletes perform better. Or maybe it’s an athlete pedaling along during a long sweltering day at the Tour. Just like we’ll never know how a rainbow is made, the origin of this story is unknown.
So, what’s the big idea? Train the body to use more fat so that carbohydrates will be conserved for when they’re needed most. To do this, all you’ve got to do is train while eating a low carbohydrate diet or in a fasted state (to keep it simple, I’ll just say fasted training – in other words, training before breakfast and without eating food). Training in this way will use up whatever carbohydrate stores you have and then require the body to use fat so that you can keep exercising (even if you’re skinny, you’ve still got ample fat stores).
Not only does this idea make sense physiologically, evidence supports the idea that fasted training improves the body’s ability to use fat as an energy source. Research has found fasted training increases the reliance on fat metabolism as well as increases a number of cellular markers associated with fat metabolism1,2,3.
So far, so good. But here’s where the story takes a turn. You may have heard that the person with the highest VO2max doesn’t always win the race. In other words, lab findings don’t always translate to race day performances. This initial research focused on a variety of markers for fat metabolism but failed to look at actual athletic performance. When actual performance was measured following fasted training, researchers have found no improvements in performance or even worse performance compared to training with a traditional high-carbohydrate diet2,3,4.
As you might imagine, this is when the townsfolk began to think about overthrowing science. But physiology is complicated. Yes, fasted training improves fat metabolism. However, it has some other unintended consequences. Maintaining training intensity and even completing planned training sessions is difficult when fasted1,2. The immune system is highly reliant on carbohydrate5, so the ability to fight infection may also decrease with fasted or low carbohydrate training1.
But one of the main pitfalls of fasted training is that the body’s ability to use carbohydrate gets worse6. When training fasted, the body isn’t using carbohydrate so it can down-regulate the machinery needed for using carbohydrate (if you don’t use it, you lose it). And remember, carbohydrate metabolism is needed for those race-winning, high-intensity efforts. As a result, athletic performance suffers.
In an attempt to maintain carbohydrate metabolism machinery, some have suggested athletes train with some sessions fasted and other sessions fed (a “periodized” style of training)2,7,8. There have been a handful of these studies with somewhat conflicting findings. For example, a couple of studies found periodized fasted training resulted in greater performance benefits compared to a traditional high carbohydrate diet7,8. On the other hand, another study found no differences2. Why the difference in findings? One reason may be that the studies included participants of different ability levels and elite athletes may not respond as well to fasted training.
Now, before you go thinking that this story has a sad ending, there are some cases where fasted training can be beneficial. Ultra-length endurance athletes who compete at a consistent lower intensity may potentially benefit from fasted training. Fasted training might also be a way to introduce some novelty to what can otherwise be boring winter base miles (just don’t do it near target races).
For this story, a lot has been simplified. If you’re interested in learning more, let us know! But for now, I’ll end story time. I hope you enjoyed it while eating (or not eating) a high-carbohydrate snack… Thanks for reading!