Intermittent fasting is a recent health trend touted for its proven and hypothetical benefits for weight loss, aging, cognitive health, and metabolism. Many studies — in animals and humans — have provided strong evidence that the risk for several diseases can be reduced through various forms of intermittent fasting.
One form of intermittent fasting, known as time-restricted eating (TRE) is probably even more popular because it is generally easy to adopt, less restrictive than other forms of fasting or dietary calorie restriction, and straightforward. With TRE, one consumes all of their daily calories within a predefined “eating window.” Most commonly, TRE takes the form of a 16:8 fast or 18:6 fast, where 16 or 18 hours are spent fasting each day, with the remaining 8 or 6 hours allocated for the eating window. Other than the timing, no “restrictions” are placed on the amount or types of food consumed.
Like intermittent fasting, TRE is also beneficial for numerous health outcomes. TRE is now popular among athletes who are looking to simultaneously lean out, build muscle, and improve performance. But improving body composition, strength, and aerobic fitness is also a relevant public health goal, as obesity levels rise and metabolic health declines throughout the world.
Physiological effects of fasting on numerous body tissues and organs. Source: de Cabo et al. 2019
In this case, whether TRE could be a useful dietary strategy to reduce body fat is an important question. Many traditional diets fail for various reasons, and TRE may represent a way to sidestep this barrier to adherence. Furthermore, very few studies have combined TRE with an exercise regimen to determine if this may further enhance weight/fat loss and help to promote lean muscle building, especially among metabolically unhealthy (i.e. overweight or obese) individuals.
A study published in Physiological Reports investigated the question of whether TRE may be an effective dietary strategy to reduce fat mass while preserving lean mass (fat-free mass) in inactive overweight and obese adults, and whether TRE + exercise would improve biomarkers of cardiometabolic health, hormone profiles, muscle performance, and alter dietary intake.
Brief study methods
21 physically inactive adults who were characterized as being overweight or obese (BMI between 25 and 34.9 kg/m2) completed this study. I’ll note here that 18 of the participants were female, while just 3 were male.
Participants were randomly assigned to one of two groups: a time-restricted eating (TRE) group — who were told to practice a 16:8 TRE regimen (eat between 12pm and 8pm each day) or a normal eating (NE) group — who were given no guidance on when to eat.
For 8 weeks, participants adhered to their prescribed eating regimen while also engaging in a progressive combined aerobic + resistance exercise training program. The program consisted of 3 days per week of strength training, which included various upper- and lower-body exercises, with at least 48 hours between each strength training session.
The aerobic exercise component involved gradually increasing the duration of exercise each week beginning with 75 minutes (week 1) and adding 75 minutes each week until week 5 (to hit a target goal of ~300 minutes per week.) The intensity of exercise was at >55% of heart-rate reserve (to calculate heart-rate reserve, take your maximum heart rate — resting heart rate.)
One notable aspect of the training was that participants never completed sessions in the fasted state — the TRE group trained between 1pm and 7pm and the NE group trained between 9am and 7pm. While not ideal (I would like to see everyone train at the same time of day), this removed the confounding factor of having some participants train fasted vs. fed.
Outcomes measures included dietary adherence and caloric intake (to observe effects of diet), body mass, body fat, and lean body mass (DEXA), blood pressure and heart rate, hormones (estradiol, progesterone, testosterone, DHEA, cortisol), and cardiometabolic biomarkers including total cholesterol, HDL, LDL, insulin, high-sensitivity C-reactive protein, hemoglobin A1c, and triglycerides.
A few things to note before going over the main findings. Overall, participants were very adherent to both the dietary regimens and the exercise training. The TRE group reported a daily eating window of ~7 hours (~17 hour fasting window) and the NE group reported a daily eating window of ~11 hours (~13 hour fasting window). Enough of a difference here to produce a likely difference in outcome measures, if they exist.
Both of the groups reduced their caloric intake throughout the study — the TRE group by 306 calories/day and the NE group by 253 calories per day. So, dietary restriction (inadvertently) occurred in both groups, despite no advice to participants to do so.
Let’s take a look at some outcomes
- Fat mass (tissue and region) was reduced in both the TRE and NE groups, however, the reduction in fat mass and total body mass was greater in the TRE group vs. the NE group. Specifically:
- The TRE group lost 3kg of body mass throughout the study
- BMI dropped by 0.36 kg/m2 in the TRE group
- Total fat mass dropped by 3% in the TRE group vs. 1% in the NE group
- Lower body (knee) strength and endurance improved in both groups
- Resting heart rate was reduced in both groups
- No changes in hormones or cardiometabolic biomarkers were observed (these were only analyzed in female participants)
These results provide some very relevant information for individuals who are interested in implementing TRE into their lifestyle. — whether looking to lose weight or not.
For one, it seems possible that lean mass can be increased if you combine strength training with TRE. This is a common argument against TRE, or fasting in general — that it’s not compatible with muscle building. If you add a consistent strength training regimen and adequate energy and protein intake to a pattern of short daily fasting, lean mass growth is possible.
Furthermore, not only can one improve lean mass through TRE, but also reduce their fat mass, which improves overall body composition (i.e. % body fat). Since fasting is a great way to get the body to use its own fat stores as an energy source, the benefits for reducing fat mass are to be expected.
Finally, at least from what we can tell from this study, TRE doesn’t necessarily impair fitness or muscle performance gains during strength and aerobic training. This study was in untrained individuals, so this puts a bit of a limitation on this conclusion — anyone is going to improve when the starting point is near zero. However, similar findings in regards to fitness and metabolic outcomes have been shown in more recreationally fit individuals and even some trained populations.
Of all the “flavors’’ of intermittent fasting out there, I think TRE is the one most likely to catch on and stick around for the long haul. Technically, we all do some form of TRE, just with varying lengths of fasting windows. Regimens such as 16:8 and 18:6 are trendy, but this doesn’t mean other variations may be any less effective, so I encourage you to try out a daily feeding:fasting window that works with your lifestyle and helps you feel and perform at your optimal.
I generally do a daily 16:8 fast, which sometimes creeps into the 18–20-hour fasting window on the weekend, if we have a later lunch. I find that this time window allows me to intake the energy I need while also (hopefully) gain some of the metabolic benefits of working out fasted.
While a simple clock is really all that you need to implement this practice, I have used the Zero Fasting app for years now and find it easy, fun, and even educational to use. I encourage you to download the app and try it for yourself.
Kotarsky CJ, Johnson NR, Mahoney SJ, et al. Time- restricted eating and concurrent exercise training reduces fat mass and increases lean mass in overweight and obese adults. Physiol Rep. 2021;9:e14868.