Intermittent fasting (IF) is an eating approach that involves extended periods of fasting to reduce calorie intake, aiming to enhance body composition and improve metabolic health. (1) While it’s well-known for aiding fat loss, could it also impact muscle growth? Dr. Layne Norton highlighted a 2025 study in the Journal of the International Society of Sports Nutrition investigating this phenomenon.
The study explored the effects of IF versus continuous meal feeding during an eight-week resistance training program. (2) Unlike much of the previous research, participants in this study were placed in a 10% calorie surplus while consuming an adequate protein intake of 2.2 grams per kilogram of body weight. Both men and women engaged in resistance training four times a week, following an evidence-based training plan.
The study measured key parameters, including training volume, performance, energy levels, and changes in lean and fat mass. Participants in the intermittent fasting group consumed all their daily calories within an eight-hour window, starting at least an hour after completing their workouts, meaning they trained in a fasted state.
The continuous feeding group spread their meals throughout the day. This design offered insight into how different feeding strategies impact resistance training outcomes in a calorie-surplus environment.
Results
Both groups experienced comparable increases in lean mass, with no statistically significant differences observed. However, the researchers noted the following distinctions:
- The intermittent fasting group reported lower energy levels and underperformed in their lifting sessions. Overall, they completed fewer reps during training and showed smaller improvements in their 1RM squat.
- The groups had no significant differences regarding other lifts, particularly upper-body exercises.
- On average, the continuous feeding group experienced a four-kilogram greater increase in squat performance than the intermittent fasting group.
The group practicing IF gained less body fat than those following a continuous feeding schedule. This result puzzled Norton, as both groups were supposed to consume the same number of calories. To investigate further, he consulted members of the research team.
They concluded that while the study aimed to equate calorie intake between the groups, it was conducted in a free-living setting, making it likely that the intermittent fasting group naturally consumed fewer calories overall.
Meals were not provided to the participants. It’s unlikely the individuals on the intermittent fasting diet just ate less.
—Dr. Layne Norton
A larger calorie surplus does not necessarily lead to greater gains in lean mass. Only a modest surplus is needed to build lean mass while exceeding this threshold, primarily resulting in increased fat gain. This explains why the group following continuous feeding consumed more calories overall and gained slightly more body fat.
Dr. Norton supports this conclusion, noting that studies comparing intermittent fasting to constant feeding consistently show no significant differences in fat mass.
Wrapping Up
This study supports existing research suggesting that IF may hinder strength gains, particularly in the lower body, when measured through strength metrics. It also shows a reduction in total work or reps performed. However, more extensive studies with larger sample sizes and longer durations are needed to gain deeper insights.
“It was a well-done study. The data is interesting and not surprising based on the information we have from previous studies on time-restricted eating.” Dr. Norton concluded.
More In Research
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- Mini Cuts — The Short-Term Diet for Long-Term Muscle Gain
- Does Dietary Fiber Combat Hypertension?
References
- Patterson, R. E., Laughlin, G. A., LaCroix, A. Z., Hartman, S. J., Natarajan, L., Senger, C. M., Martínez, M. E., Villaseñor, A., Sears, D. D., Marinac, C. R., & Gallo, L. C. (2015). Intermittent Fasting and Human Metabolic Health. Journal of the Academy of Nutrition and Dietetics, 115(8), 1203–1212. https://doi.org/10.1016/j.jand.2015.02.018
- Blake, D. T., Hamane, C., Pacheco, C., Henselmans, M., Tinsley, G. M., Costa, P., Coburn, J. W., Campidell, T., & Galpin, A. J. (2025). Hypercaloric 16:8 time-restricted eating during 8 weeks of resistance exercise in well-trained men and women. Journal of the International Society of Sports Nutrition, 22(1), 2492184. https://doi.org/10.1080/15502783.2025.2492184
Featured image via Shutterstock/PeopleImages.com – Yuri A
