I’ve read some research about how fasting during exercise can help you feel clearer, and give you more energy. But, I also have read from experts in the field, that if your goal is performance, you should be having some type of carbohydrate load, or protein recovery, in order to reach peak performance levels. Is it possible that both could be true? Let’s take a deeper look.
fasting
Some common fasting schedules are:
- fasting for 16 hours, leaving a 8 hour window to eat
- fasting for 18 hours, leaving a 6 hour window to window
- alternate day fasting typically done 2-3 days out of the week where you would only drink water, then eat your regular amount of food on eating days (4-5 days out of the week)
Some benefits of practicing fasting are: reduced body weight + body fat, lower cholesterol, and lower triglycerides (1). There has also been research with findings of improved blood sugar (3), and improvements in brain function and structure (4). It is worth noting, that many of these studies had either human participant or mice follow a fasting style diet for many weeks, and sometimes many months.
For more information on fasting, and a deeper dive into the benefits it has on body inflammation, check out our article here.
eating
We EAT so we can SURVIVE! Food gives us energy, nutrients, and even happiness. I certainly am a fan of eating.
exercise
The CDC lists off many reasons why it is beneficial to exercise. From brain health, to string bones, to reduced disease risk, exercise is an important part of health that should be incorporated into your daily life.
fasting during exercise
I’ve met people who fast for exercise, or happen to be fasting the entire day and incorporate exercise.
Benefits:
In a study looking at obese adults, participants who followed a 14 hour fast overnight with a snack @ hour 12 had more meaningful weight loss and improvements in fasting blood glucose levels than those who followed a 12 hour time restricted feeding pattern (13). Which is interesting, since technically the individuals in the first group “broke” their fast at the same time of the other group, though the snack consisted of a keto-style food product. The researchers believe this helped to curb intense cravings and increase overall feelings of fullness. Could this have a ripple effect throughout the day and mitigate the urge to “over-eat” once the fast is broken?
There have been reported benefits of generally more fat oxidation* during aerobic** exercise performed in the fasted state (6). Depending on if you are exercising fasted during the day time after previously eating, or overnight-fasted does yield some different responses in muscle glycogen breakdown. You may need to wait up to 6 hours after a carbohydrate rich meal to exercise and be in a state where more fat oxidation can occur (7).
When looking at protein ingestion before exercise in an energy and carbohydrate restricted state, there were no effects on muscle protein synthesis (10), but it is possible it might reduce muscle protein breakdown during exercise.
One study by Kotarsky et al. even found that overweight and obese adults who exercised fasted had a decrease in fat mass, and an increase in lean mass (11)!
*the process of breaking down fatty acids; the ability to burn fat
**requiring oxygen; the body producing energy with the use of oxygen; swimming, walking, cycling, rowing, running, jumping rope
Drawbacks:
With any type of restricted eating pattern, like fasting, there is the potential for under-fueling, where hormonal and immune imbalances occur, and the potential for injury goes up (8).
If your goal is not performance and you’re exercising for shorter durations, fasted exercise may be perfect for you. Most studies, as shown in this review, indicated no improvement in performance, or negative effects on performance in the fasted state (15).
fueled by food
Drawbacks:
It is likely you can guess some of the drawbacks of eating before exercise, like feeling sluggish, or GI upset. But, that can all be manipulated by meal timing, and composition of macronutrients. For more tips on eating for exercise, check out the Mayo Clinic’s guide.
bottom line
Whether you exercise fasted or not is completely up to you. We’ve explored the many benefits, like reduced fat mass, and better blood glucose control, as well as improved lipid profiles (12, 14). While there can also be some drawbacks, like increased risk for injury if under-fueling.
Fueling properly with food to aid in exercise, and proper recovery, is optimal for an athlete (5), ideally consuming carbohydrates when training times are less than 8 hours apart. If there is 24 hours between exercise times, athletes can generally get away with more personalisation in food timing. Typically, simple carbohydrates are best for a readily available source of glucose, but being mindful of whole food sources to also reap the benefits of micronutrients and other macronutrients can be beneficial.
Adequate fueling also helps reduce risk of injury, and ensures you have enough replenishment for proper hormone and immune function.
As far if it is healthy or not, I say go for it, depending on your goals and the style of exercise you like.
References:
(1) Tinsley, Grant M., and Paul M. La Bounty. “Effects of intermittent fasting on body composition and clinical health markers in humans.” Nutrition reviews 73.10 (2015): 661-674.
(2) Maughan, R. J., J. Fallah, and E. F. Coyle. “The effects of fasting on metabolism and performance.” British journal of sports medicine 44.7 (2010): 490-494.
(3) Arnason, Terra G., Matthew W. Bowen, and Kerry D. Mansell. “Effects of intermittent fasting on health markers in those with type 2 diabetes: A pilot study.” World journal of diabetes 8.4 (2017): 154.
(4) Li, Liaoliao, Zhi Wang, and Zhiyi Zuo. “Chronic intermittent fasting improves cognitive functions and brain structures in mice.” PloS one 8.6 (2013): e66069.
(5) Burke, Louise M., Bente Kiens, and John L. Ivy. “Carbohydrates and fat for training and recovery.” Food, Nutrition and Sports Performance II (2004): 24-49.
(6) Vieira, Alexandra Ferreira, et al. “Effects of aerobic exercise performed in fasted v. fed state on fat and carbohydrate metabolism in adults: a systematic review and meta-analysis.” British Journal of Nutrition 116.7 (2016): 1153-1164.
(7) Rothschild, Jeffrey A., Andrew E. Kilding, and Daniel J. Plews. “What should I eat before exercise? Pre-exercise nutrition and the response to endurance exercise: Current prospective and future directions.” Nutrients 12.11 (2020): 3473.
(8) Tenforde, Adam S., et al. “Parallels with the female athlete triad in male athletes.” Sports Medicine 46 (2016): 171-182.
(9) Aird, Thomas P., Robert W. Davies, and Brian P. Carson. “Effects of fasted vs fed‐state exercise on performance and post‐exercise metabolism: A systematic review and meta‐analysis.” Scandinavian journal of medicine & science in sports 28.5 (2018): 1476-1493.
(10) Larsen, Mads S., et al. “Effects of protein intake prior to carbohydrate-restricted endurance exercise: a randomized crossover trial.” Journal of the International Society of Sports Nutrition 17.1 (2020): 7.
(11) Kotarsky, Christopher J., et al. “Time‐restricted eating and concurrent exercise training reduces fat mass and increases lean mass in overweight and obese adults.” Physiological reports 9.10 (2021): e14868.
(12) Cho, A-Ra, et al. “Effects of alternate day fasting and exercise on cholesterol metabolism in overweight or obese adults: A pilot randomized controlled trial.” Metabolism 93 (2019): 52-60.
(13) Peeke, Pamela M., et al. “Effect of time restricted eating on body weight and fasting glucose in participants with obesity: results of a randomized, controlled, virtual clinical trial.” Nutrition & Diabetes 11.1 (2021): 6.
(14) Moro, Tatiana, et al. “Twelve Months of Time-restricted Eating and Resistance Training Improve Inflammatory Markers and Cardiometabolic Risk Factors.” Medicine and science in sports and exercise (2021).
(15) Zouhal, Hassane, et al. “Exercise training and fasting: current insights.” Open access journal of sports medicine (2020): 1-28.
