Physiology Friday #133: Is Fasted Exercise Better for Improving Physical Fitness and Health?
The benefits of working out "on empty."
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This post is a “throwback” and was originally published on my blog in December 2020.
There’s probably no more trendy topic in health and nutrition right now than fasting. Intermittent fasting (IF), while an ancient practice, has gained newfound popularity for the supposed benefits it could have for weight loss, metabolic health, and even longevity.
An area where fasting has also gained attention is in relation to exercise. Many health “gurus” advocate fasted exercise. I even think that fasted exercise has a place in most training regimens, if used strategically.
Why? When you’re fasting, blood glucose, glycogen, and insulin decrease, and our body upregulates enzymes and pathways that oxidize fatty acids to produce energy. I.e. — we are in a “fat burning” state.
The theory goes that exercising while fasted should enhance your ability to burn fat and increase the actual amount of fat you burn — leading to metabolic adaptations and an improved body composition; more lean mass and less fat mass. There are also other potential benefits of fasted exercise including improved glucose tolerance and insulin sensitivity, increased mitochondrial adaptations, and exercise-induced ketone production.
I say theory because there hasn’t been much research on this topic. The research that has been done has surprisingly found little extra benefit of fed vs. fasted training.
As I said, the research in this area is pretty new and most of the studies have been done in younger adults. For that reason, more studies are being conducted to look at whether or not fasted training is really as effective as it is claimed to be. One of those studies is the focus of today’s newsletter.
The study, published in December of 2020, aimed to compare the effects of training in the fed (FED) vs. fasted (FASTED) state on outcomes including body composition, physical performance, and markers of metabolic health like cholesterol and blood glucose.
This study was novel in that they chose to study post-menopausal women (average age of participants was 60 years), giving us some insight into whether fasted training may be beneficial to health in the context of aging.
Two groups of women (FED group; n=12, FASTED group, n=13) were put through 12-weeks of a “multi-component” training regimen that involved a 90-minute exercise session completed twice per week. Exercise included stretching/coordination/balance, a strength circuit, aerobic activity (dancing and/or games), and relaxation/stretching/massage.
A side note here. This exercise regimen, to me, seems pretty unstructured and isn’t necessarily one that I would expect to improve performance or body composition. As we will see in the results, it didn’t, and I’ll attribute this to the pretty low-volume of training. Furthermore, the women recruited for this study were already physically active, and thus were probably performing a similar amount of exercise than their usual routine.
The FASTED group was required to have fasted for at least 12 hours before each training session. While no details were provided on the FED group status, they weren’t given any guidance on when or what to eat before their training.
Outcomes were measured before and after the 12-week training regimen. One other cool aspect of this study was the measurement of dietary intake in each group on three different days of the week — a training day, a weekday without training, and a weekend day without training. This might tell us how fasting affects appetite/food consumption during the 12 week intervention.
The only outcome to change in this study was waist circumference — which was reduced in both the FED and the FASTED group. This suggests that body composition was slightly improved.
Nothing else changed, nor were any changes different between both groups. This study found no change in physical fitness tests, stress levels, quality of life, energy intake, body mass, fat %, basal metabolic rate (BMR), blood glucose, cholesterol, or triglycerides.
Referring to my comment above — I don’t think this training regimen was enough of a stimulus to elicit any of the desired changes. At least 3 days a week would be needed, in my opinion.
One interesting find was that the FASTED group had a higher intake of carbohydrate on the weekend day and a higher intake of protein on the training day after the intervention. This might suggest they were “compensating” for nutrients on some days of the week. However, overall energy intake did not change before or after the intervention in this group or the FED group.
What does this study tell us? Given my concerns about the duration/volume of training, maybe not much. Since nothing really improved, it's hard to say that fasting is definitively no better than not fasting in regard to training, as neither showed benefits in this study.
I would like to see this study replicated but with a higher volume of training, and maybe using a program focusing on aerobic training like running or cycling or some more rigorous strength training.
Furthermore — it’s not clear whether the women in the FASTED group were required to fast throughout the intervention, or only on the training days, but it appears that the latter is true.
If only on the training days, this only equates to two “fasting” days per week. If it were the entire intervention, you might expect some changes even independent of exercise, since a lot of studies find benefits when people increase their daily fasting window to ~12-13 hours/day.
But these findings do corroborate other training studies that find no difference in outcomes when comparing fasted to fed exercise.
As far as this investigation goes — these findings don’t suggest that postmenopausal women should undergo fasted exercise, simply for the reason that it doesn’t provide any extra benefit, at least in the context of this study.
But, as far as personal experience goes, I definitely prefer to exercise fasted. I feel that my stomach and body feel much better this way, and many people feel the same. It all comes down to what works and feels best for you.
Thanks for reading. See you next Friday.
Rodrigues JAL, Cunha THA, Ferezin LP, Bueno-Júnior CR. Fasted condition in multicomponent training does not affect health parameters in physically active post-menopausal women. An Acad Bras Ciênc. 2020;92(4):e20200988. doi:10.1590/0001-3765202020200988