Should You Try Fasted Exercise?

Picure from Boycemode

I rode to Kukup, a small fishing village in Johor, Malaysia on Good Friday. My bicycle speedometer showed (pictured below) that I rode 160 km when I got home. I had to lie down on the floor after setting my bike aside. I was super tired and was cramping everywhere. Took me quite a while before I could shower.

160 km

There must have been about 40 riders in that group. We stopped for a drink at a small coffee shop in Kukup and I overhead another rider saying he fasted for the ride. I don't know how he did that.

Fasted training means one abstains from consumption of food for 8-12 hours in advance of a training session. Most people do an overnight fast so training is done in the morning before breakfast. It has become popular among people interested in fat loss, metabolic health and for performance or adaptation reasons (especially endurance athletes).

Why would anyone do fasted training? There are lots of videos and reels online suggesting that when you train in a fasted state, your insulin levels are lower, so your body is more likely to use your fat stores for fuel (lipolysis) and better fat utilization.

There are endurance athletes who use fasted training to encourage adaptations like increased mitochondrial density. This is to allow their cells to make more new mitochondria and increase existing mitchondria.  Our cells often increase mitchondria biogenesis in response to increased energy demands triggered by physical activity. With more mitochondria in our cells we become stronger.

With fasted training, endurance athletes are training for better fat utilization to spare their glycogen stores so they can have better endurance. This may work if you're exercising at lower intensities, not when you are going fast at higher intensities since carbohydrate/ glycogen is still very much needed. With fasted training, there will be a severe restriction on the ability to do anaerobic work.

Moreover, over the course of a day, the net fat loss may not be that much different from fed training.

Fasted training may improve insulin sensitivity and glucose tolerance especially in sedentary or overweight individuals.

I also know people who feel better when  they exercise or train on an empty stomach and most find it logistically easier to exercise right after waking up.

Although fasted training can help metabolic adaptations described above, it can also potentially lead to a deficit in total daily calorie intake and this has been shown in active males.

If this reduction in daily calorie intake is not the intended outcome of fasted training it can potentially lead to a state of low energy availability, which when repeated frequently may lead to stress fractures and poor bone health (Raleigh et al, 2024). 

The incidence of bone injury was 1.61 times higher in those who currently use fasted training compared to those who never used it.

Fasted training is not ideal for everyone since you may feel sluggish, lift less (if you're weight training) and perform worse in high intensity workouts. In prolonged fasted states, there is a higher chance that your muscles will be broken down to be used for fuel with intense workouts.

In summary, weigh the pros and cons of fasted training, be aware of how your body responds to it and ensure that it works for your body in the long run.

References

Raleigh C, Madigan S, Sinnott-O'Connor C et al (2024).Prevalence Of Reducing Carbohydrate Intake And Fasted Training In Elite Endurance Athletes And association With Bone Injury. Eur J Sp Sci. 24(9): 1341-1349. DOI: 10.1002/ejsc.12170

Zouhal H, Saedi A, Salhi A et al (2020). Exercise Training And Fasting: Current Insights. Open Access J Sp Med. 21(11): 1-28. DOI: 10.2147/OAJSM.S224919