There are a number of questions that come up in relation to how fasting impacts on physical performance, both in athletes and in ordinary people. The first relates to how fasting impacts on your ability to perform everyday physical tasks, never mind hitting the gym or going for a run – or playing a sport.
Firstly, when your kilojoule intake has been greatly reduced whilst fasting, studies have shown that people have greater difficulty performing even mundane tasks.
A second factor is the possible effect fasting for weight loss has on overall muscle mass. Most weight loss causes greater loss of fat-free mass – i.e. your lean muscle, and everything else that’s not ‘fat’ – than fat mass.
How To Eat
Exercise physiologist and performance sports scientist Dr. Ross Tucker says that there are relatively few studies on fasting, and they often differ widely with respect to the methods and type of athlete studied.
Increasing protein intake on fast days can reduce some lean muscle loss. But exercise plays an important role too. Studies show that fat-free mass can be maintained by exercising through intermittent fasting, and studies on lab animals have suggested that intermittent and periodic fasting can actually increase fitness and resistance to injury.
Another consideration is the composition of what you eat before and after training. “Carbohydrates? are beneficial for performance, and I think that’s important for someone who is fasting too,” says Ross. “I’d suggest that anyone who tries fasting be mindful of this relatively obvious fact: that the training you’re going to do is being compromised when you are fasted.
And so, if you want to maintain your fitness or performance or muscle mass or whatever it is, you must make sure you’re providing your body with energy during training. Otherwise, what will happen is that you may well lose a bit of weight, but your training performance will be compromised, and whatever benefits you may gain in one area are cancelled out by the losses in other areas.
Train Less And Gain More
On the face of it, these seem contradictory: if fasting makes it harder to exercise, how can it possibly improve fitness? The answer might lie in how, and when, you exercise during fasting periods.
In one Ramadan fast study published in the International Journal of Sports Medicine in 2014, a group of young football players were assessed before and during the fast. For the duration of the study, some of the group continued with normal weight training over the Ramadan period, while others had their training volume reduced by 22%.
“What’s interesting is that they found that the group that had their training reduced actually ended up performing better as a result of the training programme,’ says Ross. “By the end of Ramadan their strength and power test results were better, and they were pretty much outperforming the higher training group across the board.”
Ross says this suggests “when you’re fasting, even in a Ramadan style where it’s more like a time restriction on when you can eat, you’re better off if you make adjustments to your training programme to train less.
“Part of the adaptation required to exercise and fast effectively, and get maximum benefit from both, is to take into account not just the intensity of your training but also the time(s) you eat and train relative to each other.
“I think the key is that if you fast, fine; but at the time that you exercise, or immediately after, you need to have energy available, or you’ll compromise the normal adaptations to training,” says Ross. “So you have to accept that fasting is sitting on one side of a see-saw, training on the other side, and if you fast then the other side must change too.”
Bottom line: reduce your training volume when fasting, and try to schedule your training so that it can ideally be preceded by a carb-containing meal beforehand (so you have energy) and protein-containing food after, for muscle mass and recovery.
What Do We Mean By Fasting?
- Continuous energy restriction – a typical diet
- Intermittent energy restriction – occasionally limiting kilojoules
- Intermittent fasting – as in (2), for extended periods
- Two-day fasting – 2 days in a row, 60-70% normal intake
- 5:2 fasting – 2 non-consecutive days a week, with energy intake 2 100-3 350 kilojoules
- Alternate-day fasting–Day1 fast (25%), Day 2 feast (125%)
- Time-restricted feeding – e.g. fast 16 hours, eat in 8hr period
- Periodic fasting – Scheduled fast, several days/weeks
- Fast-mimicking diets – Low-kJ diets, exclude e.g. fats, carbs
- Ramadan Intermittent Fasting – Between sunrise & sunset for a month