The Effects of Exercise on β-Hydroxybutyrate Concentrations over a 36-h Fast: A Randomized Crossover Study

Purpose: This study assessed beta-hydroxybutyrate (BHB) concentration during a short-term fast and the degree to which an initial bout of exercise influences the rate of ketogenesis. Methods: Twenty subjects (11 male, 9 female) completed two 36-h fasts, with one protocol requiring the subject to complete a treadmill exercise session at the beginning of the fast. BHB levels were assessed via portable meter every 2 h, along with mood and hunger ratings. Venipuncture was performed every 12 h. Results: The mean (SD) areas under the curve for BHB concentration were 19.19 (2.59) mmol.L-1 (nonexercised) and 27.49 (2.59) mmol.L-1 (exercised), yielding a difference of 8.30 mmol.L-1 between conditions (95% posterior probability interval (PPI), 1.94 to 14.82 mmol.L-1; posterior probability (PP) = 0.99). The mean (SD) times to BHB concentration of 0.5 mmol.L-1 were 21.07 (2.95) h (nonexercised) and 17.5 (1.69) h (exercised), a 3.57-h difference (95% PPI, -2.11 to 10.87 h; PP = 0.89). The differences in area under the curve between conditions were 5.07 mu U.mL(-1) (95% PPI, -21.64 to 36.18 mu U.mL(-1); PP = 0.67) for insulin, 97.13 pg.mL(-1) (95% PPI, 34.08 to 354.21 pg.mL(-1); PP = 0.98) for glucagon, and 20.83 (95% PPI, 4.70 to 24.22; PP = 0.99) for the insulin/glucagon ratio. Conclusions: Completing aerobic exercise at the beginning of a fast accelerates the production of BHB throughout the fast without altering subjective feelings of hunger, thirst, stomach discomfort, or mood. Insulin and the insulin/glucagon ratio experience a marked reduction within the first 12 h of fasting and was not altered with exercise. Thus, exercising at the beginning of a fast may improve the metabolic outcomes of fasting.

Automated summary

This study found that completing aerobic exercise at the beginning of a fast can accelerate the production of BHB without altering subjective feelings; insulin and the insulin/glucagon ratio experience a marked reduction within the first 12 hours, and exercise does not change this.