Fasting vs continuous energy restriction for body composition and strength during resistance training Original paper

Reductions in lean body mass tend to follow fat loss when attempting to lose weight. Changes in protein turnover during continuous energy restriction (CER) probably contribute, but the underlying mechanisms are still not completely understood. Does intermittent fasting (IF) during resistance training maintain more lean body mass and strength — without reducing weight loss — when compared to CER?

This 12-week randomized trial recruited 34 untrained participants (average age of 24 years; 50% male) who completed three sessions of resistance training per week with either 20% daily energy restriction (CER group) or 5:2-style intermittent fasting with about 70% energy restriction on two days per week (IF group).

Both groups were prescribed ≥1.4 grams of protein per kilogram of body weight per day (e.g., about 100 grams of protein per day for a 70-kilogram individual). Basal energy requirements were calculated using the Mifflin St. Jeor equation. The participants completed two supervised resistance and one unsupervised bodyweight (aerobic/resistance) training combination sessions per week.

The IF group consumed protein shakes and high-protein soups to get as close as possible to the prescribed protein goal on fasting days. They also exercised on nonfasting days.

Body composition was measured with DXA. Thigh muscle size and quality were measured with ultrasonography. Surface area of muscle and intramuscular and subcutaneous fat were measured with quantitative computed tomography. Strength was measured with a 3-repetition maximum and strength endurance test for both bench and leg press. Dietary intake was assessed with 3-day food diaries.

The IF and CER groups did not differ with respect to increases in lean body mass (+3.7%) and decreases in body weight (−4.6%) and fat (−24.1%). Both interventions also improved thigh muscle size and quality and reduced intramuscular and subcutaneous fat.

However, the CER group demonstrated greater increases in muscle surface area when compared to the IF group over time.

The inclusion of supplements for the IF group, as well as the self-reported dietary data, suggest a bit of caution when interpreting results. Moreover, the authors themselves suggested that the sample size may have been too small to detect meaningful differences between groups.

Some differences in BMI, weight, lean body mass, and energy and carbohydrate intake based on sex differences were noted over time, as well as a difference in muscle surface area based on sex and group interactions, but it is difficult to interpret these results in the small sample and considering the study design.