March 28, 2025, 8:39 pm | Read time: 7 minutes

Intermittent fasting is popular, especially with people who want to lose weight. But what if you want to lose pounds but also want to have toned muscles and, therefore, train hard? How effective is intermittent fasting in this case? A recent study got to the bottom of precisely this question — FITBOOK Editorial Director Melanie Hoffmann explains the results.

Meta-Analysis Examines the Combination of Intermittent Fasting and Various Training Regimens

For the current study, researchers from the University of Mississippi and Texas Tech University joined forces and analyzed 15 randomized trials. The study included a total of 338 healthy adults who exercised regularly. The subjects who had done intermittent fasting had mostly used the 16:8 method (16 hours of fasting, eight hours of eating).⁵

In order to find out how interval fasting measurably influences body composition in exercisers, the scientists focused on the following key figures in their analysis:

• fat mass
• body fat percentage
• fat-free mass (muscle mass)

How the Study Was Carried Out

The researchers only included studies in their analysis that contained a sports component (a type of training) in addition to interval fasting and in which the test subjects had been examined over a period of at least four weeks. All studies had to provide pre- and post-intervention data on fat mass, body fat percentage, and fat-free mass, as well as information on training.

The control groups also exercised, but unlike the study groups, they ate without time restrictions, i.e., without specific fasting phases.

Evaluated Studies

The studies analyzed were published between 2016 and 2023, mostly conducted in the USA, and included various forms of training:

• 4 studies with endurance training
• 6 studies with strength training
• 5 studies with a combined approach

The intervention duration was mostly four to eight weeks; only one study lasted longer than twelve weeks. In most cases, the eating window was between 12 noon and 8 pm. The training took place within the eating window. Compliance with intermittent fasting was monitored via dietary protocols and, in some cases, through discussions with nutrition experts.

Evaluation Method

The statistical evaluation was carried out using randomized effect models. They are suitable for determining the effects of variables both individually and in combination (here, variable 1: intermittent fasting, variable 2: training). It should be emphasized that random effects can be evaluated in this way but not causalities.

Moderator analyses were also carried out. This is a statistical study that can determine the extent to which a third variable could have influenced the two variables examined (in this case, intermittent fasting and training). In this meta-analysis, the method was used to evaluate influencing factors such as type of training, BMI, or energy intake.

Intermittent Fasting Combined with Training Reduces Fat and Maintains Muscle Mass

In their meta-analysis, the researchers came to the conclusion that athletes could actually benefit from combining their training with interval fasting. They were able to statistically determine the following effects: Test subjects who exercised and ate according to the principle of interval fasting …

• lost an average of 2.9 pounds of fat mass
• experienced an average reduction of approximately 1.3 percent in body fat compared to the beginning of the study (a significant reduction)
• did not show a significant reduction in muscle mass (meaning they maintained their muscle mass)

In practice, this means that the body fat percentage decreases while the muscles remain intact — at least in comparison to the group without time-eating restrictions. Neither the age of the participants nor the type of training (strength, endurance, or mixed), duration of the intervention, or calorie intake fully explained the effect. However, the results showed a high degree of scatter (heterogeneity), which indicates large differences between the individual studies.

Significance of the Analysis Results

The analysis demonstrated that combining intermittent fasting with regular exercise has a moderate yet measurable impact on body composition. Fat loss is slightly higher than with pure training without a meal window, while muscle mass is maintained. For many fitness enthusiasts, this is a decisive advantage, as their focus is often precisely on fat loss without muscle loss.

However, the differences are relatively small — around 1.3 kilograms less fat after an average of eight weeks. Whether the additional effort of a strict eating schedule is worthwhile, therefore, depends heavily on your personal goals. The effects might be more pronounced for those new to exercise or individuals with a higher body fat percentage, although this was not confirmed in the analysis.

It is noteworthy that potential physiological benefits, such as enhanced insulin sensitivity or increased fat oxidation in muscles, are associated with time-restricted eating (TRE); however, these mechanisms were not examined in this study. It also remained unclear whether there are differences between early (e.g., 8 am – 4 pm) and late (e.g., 2 pm – 10 pm) TRE, as there was a lack of relevant comparative data.

Classification of the Study and Possible Limitations

The study is compelling because of its robust methodological approach and the extensive data derived from 15 high-quality individual studies. Nevertheless, there are limitations: Most of the studies had a short duration (four to eight weeks), and only one exceeded the 12-week mark. Statements on long-term effects are, therefore, not possible.

No Explanation of the Effects Possible

The studies analyzed did not deal with physical processes that could underlie the effects of interval fasting in combination with training — e.g., the effect on insulin sensitivity or fat burning in the muscle. The analysis was also unable to determine whether the time of the fasting window (8 am to 4 pm vs. 2 pm to 10 pm) had an influence. As all exercisers performed their exercise during the eating window, it is not clear whether this was the secret of the effects on body fat and muscle mass, i.e., whether the results would have been different if they had exercised during the fasting phases. Further research would be needed to be able to make a clear recommendation in this regard, for example: Training during interval fasting is best only during the eating phases. Also not considered in detail are possible differences in effects due to different training methods.

Limited Selection of Test Subjects

The participants were predominantly healthy adults of normal weight who were active in sports. People who were severely overweight or had chronic illnesses were rarely examined. Women were also underrepresented: Only three studies exclusively examined female participants. Consequently, the applicability of the findings is limited.

Uncertainty of Information

Another problem lies in the self-reported dietary information used in all studies. This is known to underestimate actual calorie consumption. In addition, the studies analyzed used different measurement methods to determine body composition, which limits comparability. The high heterogeneity of the results also indicates individual differences in effectiveness. This could, for example, have been due to the fact that the test subjects used their eating window differently (lower-fat meals vs. high-fat meals, more or fewer calories, more or less protein, etc.). The analysis also lacks a differentiated evaluation in this respect.

Conflict of Interest

Last but not least, there is a potential conflict of interest: co-author Grant M. Tinsley is the inventor of a patent in the field of intermittent fasting. However, as studies carried out by others were evaluated, this is unlikely to have had any influence on the statistical results.

More on the topic

Study Provides Findings Which Type of Training is Best for Heart Patients

Study on mice Certain forms of intermittent fasting can prolong life

Optimal Results What You Should Do Before Strength Training to Improve Muscle Building

Conclusion

Despite the aforementioned limitations, the current meta-analysis provides interesting evidence that anyone who exercises regularly and aims to lose body fat while maintaining muscle can safely rely on intermittent fasting as a nutritional strategy.

Our assessment is that intermittent fasting, for both the general population and athletes, can only yield the desired outcomes if accompanied by a balanced diet during the eating windows. This is because the right nutrients, sufficient protein (for the muscles), and a calorie deficit (to be able to lose weight) are particularly important for reducing body fat and building muscle.