Let’s face it; dieting sucks and everyone is looking for a solution to make life easier when on a diet. The newest trend is intermittent fasting, in which periods of fasting are combined with periods of re-feeding. Although there are different forms of intermittent fasting, a common theme is that each form of intermittent fasting utilizes repeated periods of fasting that extend beyond the duration of a typical overnight fast and typically lead to reductions in energy intake.
Intermittent fasting has been suggested to be ideal for losing body fat, but there is little research about gains in muscle mass. One form of intermittent fasting is time-restricted feeding which utilizes a daily schedule allowing a limited number of hours as a feeding window and the remaining hours as a fasting window. A previous study investigating a time-restricted feeding for eight weeks (20 hours of fasting and 4 hours of feeding per day) consuming three meals per day. At the end of eight weeks, fat mass was significantly lower after eight weeks of time-restricted feeding. Interestingly, lean mass was greater by an average 1.5 kg after time-restricted feeding as well.
Researchers had subject’s placed on a time restricted feeding consisting or a regular diet. The time restricted feeding group consumed all calories within a four-hour period for four days per week, but included no limitations on quantities or types of foods consumed during this four-hour period. So eat anything you want when you are re-feeding during this 4 hour period.
Additionally, the subject’s consumed a resistance training program performed three days per week and consisted of alternating upper and lower body workouts. For each exercise, four sets leading to muscular failure between 8 and 12 repetitions. On the three days per week that participants performed resistance training, they were allowed unrestricted food intake. On non-workout days (four days per week), participants were required to consume all calories in any four-hour window between 4 p.m. and midnight.
At the end of the study, the time-restricted feeding group with no limitations on food consumed on exercise days while restricted feeding on non-training days were able to reduce energy intake in young exercising males by approximately 650 kcal per day of modified fasting. Despite the reduction in calories of 650 kcal, this did not translate into group body composition changes over the course of 8 weeks. One concerning result of the time-restricted feeding group was the loss of lean muscle. Subject’s performing resistance training and a normal diet gained 2.3 kg of lean muscle mass on average, as opposed to −0.2 kg in strength training -time-restricted feeding group, indicating that time-restricted feeding group may have hindered hypertrophic adaptations to resistance training.
So why did the time-restricted feeding group lose muscle? The time-restricted feeding group-resistance training consumed less protein. When calculated relative to body weight, the average daily protein intake in the resistance training-normal diet group was 1.4 g/kg body weight/day, whereas the intake in the resistance, training-time-restricted feeding group was on average 1.0 g/kg. This is well below the threshold of Dr. Phillips recommendation of recently stated protein intake of 1.3–1.8g/kg, or perhaps higher, can promote lean mass retention during low-calorie diets.
Key Points: If the time-restricted feeding group had increased protein in their diet, they may have been able to maintain lean muscle mass. More research needs to be conducted, but the current study found that time restricted feeding with protein at 1.0 kg/bw was not enough to maintain lean muscle mass but strength levels were maintained.