Cliff Harvey PhD
For a long time, people have been told to train in the morning on an empty stomach to increase fat-loss.
So, what does the scientific research say?
A quick review of the scientific literature shows us some interesting things:
Training in a fasted state doesn’t always seem to have an effect on fat vs. carbohydrate use during exercise,(1) even though we see a decrease in exercise-induced glycogen breakdown and an increase in some of the proteins involved in fat handling after fasting training,(2) and fasted training reduces the amount of blood glucose ‘drop’ during training.(3)
Interval training in the fed or fasted state is equally effective for reducing body-fat,(4) and body composition changes associated with aerobic exercise in conjunction with a calorie-restricted diet are similar, regardless of whether someone is fasted or fed prior to training.(5) And for those fasting for relatively long periods (e.g. greater than 12 hours, such as during the Ramadan fast) there is likely to be no difference in body fat or muscle changes whether training in a fasted or fed state.(6) Furthermore, there doesn’t appear to be any significant increase in thermogenesis as a result of fasted training, over non-fasted.(7)
So, it seems from that, that the case is closed and there is no benefit to training fasted…
But wait! There’s more…
When people are not eating a calorie-restricted diet, the results of fasted training might be quite different. A couple of studies have looked at diets that aren’t calorie-restricted. In these diets training in a fasted state-controlled body weight, enhanced full-body glucose tolerance and improved insulin sensitivity along with some underlying benefits for fat metabolism.(8, 9) A 12-hour overnight fast also increases the use of fat-for-fuel during lower intensity aerobic bouts,(10) and In a fasted state glucose use is lower and fat use is higher.(11) And for those of you who are following lower-carb, high-fat and ketogenic diets, fasted state exercise increases ketogenesis (production of ketones) in nutritional ketosis(12, 13) and so, might help you to get into and stay in ketosis (the unique state of fuel use and fat-burning that comes in a very low carb and high-fat diet).
For those following a ‘targeted’ ketogenic approach or doing ‘carb back-loading’ and eating a higher carbohydrate meal after fasted training there appears to be an enhanced glycogen replenishment effect after fasted training.(14) (Although this study used fairly long training sessions of two hours).
Because of the somewhat mixed results in studies, it has been suggested that a mixed-approach is likely to be best and that doing at least some training sessions fasted is beneficial.(15)
But aside from what the research tells us, there are some interesting clinical observations too. I have worked over the years with a lot of emergency responders, special forces soldiers and others that may not always have the luxury of eating to fuel performance. With these people, it has been interesting to see that as they become more used to training, competing and performing in a fasted state, they become much better at it, to the point where many see no real difference between performance whether fed or even in a prolonged fasting situation. So, in this way, training fasted can improve your overall resilience. Think about it this way; if you get up early to go to the gym and you currently feel like you ‘need’ to eat before you train, wouldn’t it be easier to just wake up and go instead?
Based on the evidence there are a few takeaway points that can help you decide whether fasted training is beneficial for you.
There is no need to eat before training
If you are currently dieting (restricting calories heavily) there may not be any additional benefit from fasted training, but it’s also not going to hinder your results
But, if you are not currently dieting, fasted training can help to reduce or eliminate unwanted fat gain
If you are following an LCHF diet, fasted training might help you to become even more fat- and keto-adapted
You can ‘learn’ to perform while fasted and this could save you time and energy!
- Stannard SR, Buckley AJ, Edge JA, Thompson MW. Adaptations to skeletal muscle with endurance exercise training in the acutely fed versus overnight-fasted state. Journal of Science and Medicine in Sport. 2010;13(4):465-9.
- De Bock K, Derave W, Eijnde BO, Hesselink MK, Koninckx E, Rose AJ, et al. Effect of training in the fasted state on metabolic responses during exercise with carbohydrate intake. Journal of Applied Physiology. 2008;104(4):1045-55.
- Van Proeyen K, Szlufcik K, Nielens H, Ramaekers M, Hespel P. Beneficial metabolic adaptations due to endurance exercise training in the fasted state. Journal of Applied Physiology. 2011;110(1):236-45.
- Gillen JB, Percival ME, Ludzki A, Tarnopolsky MA, Gibala MJ. Interval training in the fed or fasted state improves body composition and muscle oxidative capacity in overweight women. Obesity. 2013;21(11):2249-55.
- Schoenfeld BJ, Aragon AA, Wilborn CD, Krieger JW, Sonmez GT. Body composition changes associated with fasted versus non-fasted aerobic exercise. Journal of the International Society of Sports Nutrition. 2014;11(1):54.
- Trabelsi K, Stannard SR, Ghlissi Z, Maughan RJ, Kallel C, Jamoussi K, et al. Effect of fed- versus fasted state resistance training during Ramadan on body composition and selected metabolic parameters in bodybuilders. Journal of the International Society of Sports Nutrition. 2013;10(1):23.
- Pacy PJ, Barton N, Webster JD, Garrow JS. The energy cost of aerobic exercise in fed and fasted normal subjects. The American Journal of Clinical Nutrition. 1985;42(5):764-8.
- Van Proeyen K, Szlufcik K, Nielens H, Pelgrim K, Deldicque L, Hesselink M, et al. Training in the fasted state improves glucose tolerance during fat-rich diet. The Journal of Physiology. 2010;588(21):4289-302.
- Van Proeyen K, Deldicque L, Nielens H, Szlufcik K, Francaux M, Ramaekers M, et al. Effects Of Training In The Fasted State In Conjunction With Fat-rich diet On Muscle Metabolism. Medicine and Science in Sports and Exercise. 2010;42(5):42 (May).
- Bergman BC, Brooks GA. Respiratory gas-exchange ratios during graded exercise in fed and fasted trained and untrained men. Journal of Applied Physiology. 1999;86(2):479-87.
- Massicotte D, Péronnet F, Brisson G, Boivin L, Hillaire-Marcel C. Oxidation of Exogenous Carbohydrate During Prolonged Exercise in Fed and Fasted Conditions*. Int J Sports Med. 1990;11(04):253-8.
- Fery F, Balasse EO. Ketone body turnover during and after exercise in overnight-fasted and starved humans. American Journal of Physiology - Endocrinology And Metabolism. 1983;245(4):E318-E25.
- Fery F, Balasse EO. Response of ketone body metabolism to exercise during transition from postabsorptive to fasted state. American Journal of Physiology - Endocrinology And Metabolism. 1986;250(5):E495-E501.
- De Bock K, Richter EA, Russell AP, Eijnde BO, Derave W, Ramaekers M, et al. Exercise in the fasted state facilitates fibre type-specific intramyocellular lipid breakdown and stimulates glycogen resynthesis in humans. The Journal of Physiology. 2005;564(2):649-60.
- Bartlett JD, Hawley JA, Morton JP. Carbohydrate availability and exercise training adaptation: Too much of a good thing? European Journal of Sport Science. 2015;15(1):3-12.