That's too good to be true? Well, wait until you've learned more about the methodology and results, but it is indeed cool that someone finally studied the effects of what the majority of people think of if you talk about "intermittent fasting" (that's in contrast to scientists who often think of alternative day fasting when they hear "intermittent fasting, the benefits of which I have discussed only recently, read more).
The subjects were recreationally active, but probably less active than the average SuppVersity reader (because they hadn't been following a consistent RT programme over the previous three months). The study was a randomized controlled 8-week trial that did or didn't involve time-restricted feeding (TRF, 4h eating, 20h fasting window) and identical resistance training (RT) programs:
"The RT programme for both groups consisted of three nonconsecutive days per week of training performed at the gym of the participant’s choice. Participants alternated between upper and lower body workouts. The upper body workout consisted of barbell bench press, seated row machine, dumbbell shoulder press, lat pulldown machine, dumbbell biceps curls, and triceps extension machine. The lower body workout consisted of barbell squat or hip sled machine, lunges with dumbbells, leg curl machine, leg extension machine, and calf raise machine.The variables the scientists tracked were, as previously mentioned, the subjects' individual nutrient intakes (as reported in a repeated four-day dietary record) and changes in body composition (X-ray absorptiometry using a "Hologic Discovery W" device for whole-body scans; muscle size was assessed by ultra-sound measurements) and muscular strength (assessed by obtaining the 1-repetition maximum (1-RM) using the hip sled and barbell bench press exercises).
Participants who were unfamiliar with the RT exercises were instructed regarding the proper execution of each exercise. Participants were also instructed to utilize a weight that elicited muscular failure after 8–12 repetitions and to adjust the weight as necessary to meet this criterion. Four sets of each exercise were performed and a 90- second rest period between sets was assigned (Tinsley. 2016; my emphasis).
How long did they fast? 20h? Isn't that too much? With a feeding window of 4 hours (at any time between 4PM and midnight | as in Kelly, 2007), the TRF(=IF) regimen in the study at hand is significantly shorter than that in the average IF-dieter who will probably use 6-8h fasting windows... whether the results would be fundamentally different for these shorter fasting windows of 16-18h would have significantly changed the outcome is questionable, also because the subjects fasted only on non-workout days and were allowed to eat ad-libitum on the three days on which they performed RT. Thus, further research is obviously warranted (see bottom line for suggestions).The reasons that the results are still relevant and worth reporting are (a) they are the first of their kind and (b) it is unlikely that the effect of intermittent fasting is different for people with different training status. What may difference is the baseline response to resistance training, but that's the same for both groups (you can also argue that the response will change over time which is why it is great that the study lasted eight, not just two weeks).
IF cuts energy intake, but not body fat? True, if we focus on statistically sign.
As you can see in Figure 1, the TRF reduced energy intake did, as SuppVersity readers will have expected the reduction in daily energy intake of the subjects by ∼650 kcal per day (see Figure in the bottom line for information on the "macros") - you can only eat so much in a given time-frame and four hours are not long. What may be more interesting than the reduction in energy intake you've expected based on previous research.
|Figure 1: Relative changes in body composition; none reached sign. inter-group diff., d-values indicate effect sizes which show a trend towards fat loss and lean & fat mass gains in the TRF and control group, respectively (Tinsley. 2016).|
Similar questions and that criticism of the methodology can be brought forward for the lack of significant difference in the increase in cross-sectional area of the biceps brachii and rectus femoris in the two groups. If we look closely at these values and take the effect size data (values over the bars) instead of the p-values for the absolute gains as our yardstick, there is evidence that the TRF regimen impaired the lean mass gains (+2.3 kg, d = 0.25) and upper and lower body muscular endurance increases (not shown in Figure 1) that were brought about by the standardized resistance training protocol both groups followed for 8 weeks.
- Kelly, Caleb J. "A controlled trial of reduced meal frequency without caloric restriction in healthy, normal-weight, middle-aged adults." The American journal of clinical nutrition 86.4 (2007): 1254-1255.
- Tinsley, Grant M., et al. "Time-restricted feeding in young men performing resistance training: A randomized controlled trial†." European Journal of Sport Science (2016): 1-8.