This article features a study that showed that ketosis could start earlier when high-intensity aerobic exercise is done at the start of fasting.
Ketosis is the state the ketone levels are high enough to be an energy source. In times of no caloric intake, glucose gets depleted, and the body starts to convert body fat into ketones.
Ketones burn “cleaner” because they do not produce as many free radicals as glucose. Free radicals cause oxidative damage and contribute to many inflammation-related diseases like cardiovascular disorders, diabetes, cancer, chronic pain, and epilepsy.
Ketosis has many health benefits. The figure below shows that diabetes, ketosis and cardiovascular risk, and more are improved with ketosis.[1]
When does ketosis start?
Fasting studies without exercise show that ketosis starts several hours after fasting. Haymond et al. showed that ketosis begins between 16 and 18 hours after fasting. Meanwhile, Browning and colleagues showed that beta-hydroxybutyrate (BHB) levels increase within 24-26 hours after a meal. BHB is technically not a ketone but is measured since it is co-produced with the other ketones, acetone, and acetoacetate.
In the featured study by Deru and colleagues, all from Brigham Young University, they added aerobic exercise at the beginning of intermittent fasting to see if ketosis would occur sooner.[2]
Study design
Twenty healthy adults (11 male and nine female) participated in a randomized cross-over study. In this cross-over study, all the participants fasted for 36 hours.
Participants were divided into exercise and non-exercise groups. After a minimum six-day washout between sessions, the two groups switched. Any variables present in individual characteristics are canceled by crossing over.
Exercise
Participants exercised on a treadmill at a grade and speed calculated to bring their estimated heart rate reserve (HRR) to 70%. The high intensity of the exercise was used to maximize glucose use.
The average Metabolic EquivalenT (MET) during the prescribed exercise for all participants was 9.14. A MET of 9.0 is like doing competitive football or cross-country skiing.
Fasting period
The 36-h water-only fast began at 8:00 PM and ended at 8:00 AM, 36 hours later. The participants were instructed to stay hydrated throughout the fast. (Being well-hydrated facilitates ketosis.)
During the waking hours of fasting, ketosis is tracked by measuring BHB levels every 2 hours.
Subjective effects of fasting
Ketosis after exercise and fasting is no good if the experience is unpleasant. To measure the subjective measures of the fasting experience, the participants rated their mood and energy levels using the Brunel Mood Scale (BRUMS) every two hours during fasting.
The 24-item BRUMS measures six identifiable mood states (tension, depression, anger, vigor, fatigue, and confusion).
Results
Vigorous aerobic exercise at the beginning of the 36-hour fast reduced the time to ketosis by 3.5 hours. Ketosis was defined in this study as a BHB concentration of 0.5 mmol·L¯¹.
There was also an increase in the concentration of BHB by 43%, compared with fasting alone. The effect was the same for males and females.
Hunger, thirst, stomach discomfort, or mood between exercise and non-exercise conditions during fasting were not any different.
Depression was reduced in the exercise condition.
In summary, vigorous exercise at the beginning of a 36-hour fast resulted in earlier and more pronounced ketosis without a greater degree of adverse effect.
The same authors made another study featured in Exercise Makes Fasting Easier.
Related anecdote
A friend of mine who fasts regularly played basketball after a 36-hour fast. He was surprised when he discovered that he played better and did not get tired during the game.
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References:
- Paoli A, et al. Beyond weight loss: a review of the therapeutic uses of very-low-carbohydrate (ketogenic) diets. Eur J Clin Nutr. 2013 Aug;67(8):789-96. doi: 10.1038/ejcn.2013.116. Epub 2013 Jun 26. Erratum in: Eur J Clin Nutr. 2014 May;68(5):641. PMID: 23801097; PMCID: PMC3826507.
- Deru LS, Bikman BT, Davidson LE, Tucker LA, Fellingham G, Bartholomew CL, Yuan HL, Bailey BW. The Effects of Exercise on β-Hydroxybutyrate Concentrations over a 36-h Fast: A Randomized Crossover Study. Med Sci Sports Exerc. 2021 Sep 1;53(9):1987-1998. doi: 10.1249/MSS.0000000000002655. PMID: 33731648.
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