Effects of eight weeks of time-restricted feeding (16/8) on basal metabolism, maximal strength, body composition, inflammation, and cardiovascular risk factors in resistance-trained males | Moro et al., 2016
Time-restricted feeding (or time-restricted eating) involves eating all calories in a day within a certain number of hours and abstaining from eating calories outside of these hours. For example, a “16/8” or 16:8 structure denotes a fasting window of 16 hours and a feeding window of 8 hours and a 20/4 structure denotes a fasting window of 20 hours and a feeding window of 4 hours. There’s not a ton of research on time-restricted eating in humans. But, up to this point, some interesting findings have been reported. I’m covering some of the interesting research in this series. To kick us off, I’m starting with this study from Moro et al. published in 2016 in the Journal of Translational Medicine.
-Type of Study: Single-Blind, Randomized, Experimental Trial
Researchers conducting the outcome assessments were blinded to group assignment. Subjects were randomly assigned to the time-restricted feeding (TRF) group or the normal diet (ND) group.
-Duration: 8 weeks
- 34 young men (25-33 years old) with at least 5 years of resistance training experience participated in this study.
-Design & Measurements
- The study lasted 8 weeks.
- 17 were assigned to the time-restricted feeding group (TRF) and the other 17 were assigned to a “normal diet” group (ND).
- The TRF group consumed food in an 8 hour window each day. Meals were consumed at 1 pm, 4 pm, and 8 pm in this group. 40% of daily calories were consumed at meal 1, 25% at meal 2, and 35% at meal 3.
- The ND group consumed food at 8 am, 1 pm, and 8 pm. 25% of daily calories were consumed at meal 1, 40% at meal 2, and 35% at meal 3.
- Dietary data was examined based on a 7-day food log prior to the study, and participants were instructed to maintain their habitual calorie intakes. No snacks were allowed between meals except a whey protein shake containing 20 grams of protein after each training session. Adherence to the protocol and intakes throughout the study were examined via weekly phone interviews with participants by a registered dietitian.
- Resting energy expenditure and respiratory exchange ratio were measured using open-circuit calorimetry to estimate the number of calories expended per day and the number of calories expended from the oxidation of glucose and fat at rest.
- Body composition was measured via a DEXA scan before and after the study.
- Arm and thigh muscle cross-sectional area was estimated from the use of a digital tape measure, skinfold measurements, and software called “FitNext”.
- Blood was collected in the fasted state before and after the study in the morning to examine several markers related to inflammation, metabolic health, and hormonal status.
- Participants performed the same resistance training program 3 days per week. Training sessions were performed on non-consecutive days and involved a “split routine” where the upper body was trained twice per week and the lower body once per week. Loads of ~85-90% 1RM were employed for each movement with 3 minutes of rest between sets and exercises with a target rep range of 6-8 reps per set. Training was performed between 4 and 6 PM.
- Bench press and leg press 1RM were assessed before and after the study.
- Calorie and macronutrient intake was not significantly different between groups.
- Subjects in the TRF group lost more body fat.
- Both groups appeared to maintain fat-free mass and muscle size.
- Both groups improved strength and there wasn’t a significant difference in this improvement between groups.
- Blood levels of adiponectin increased in the TRF group, but not the ND group.
- Blood levels of leptin, IL-6, TNF-ɑ, IL-1𝛃, total testosterone, and IGF-1 decreased in the TRF group.
- When normalized to bodyweight, the decrease in leptin observed in the TRF group was no longer significant.
- Blood glucose and insulin decreased in the TRF group, but not the ND group.
- T3 decreased in the TRF group.
- HDL-c increased in the TRF group and triglycerides decreased in the TRF group.
- The respiratory exchange ratio (RER) decreased in the TRF group from 0.83 to 0.81 but not the ND group (0.83 at PRE and POST), which suggests the TRF group was using slightly more fat for energy at rest.
The TRF group lost more body fat despite consuming similar calories and macronutrients each day throughout the study. However, diet data throughout the study was collected from interviews conducted with participants which I think is a notable limitation. That’s not to say we can’t trust the data at all, it’s just that asking participants about their intake via an interview and recording this data retrospectively is probably not as accurate as having them measure and log intake carefully in software after each meal with timestamps, or having meals prepared for participants like in other studies. So this should be interpreted with caution, and the authors mention this as a limitation of the study as well. In other words, it’s possible subjects in the TRF group consumed fewer calories and macronutrients and the physiological data are a consequence of eating less food rather than an effect of the compressed feeding window or the temporal nature of when food was consumed during the study.
With that said, if we accept the dietary data as accurate, some of the physiological data is intriguing. Blood adiponectin levels increased more in the TRF group. Adiponectin is a hormone produced and secreted by adipocytes (fat cells). Adiponectin is also categorized as an “adipokine”. If you’d like to read more about it, check this article out. An increase in adiponectin blood levels appears to be associated with improved sensitivity to insulin, lower blood glucose and triglycerides, and decreased vascular inflammation. In this study, the decrease in blood glucose, insulin, and triglycerides in the TRF group support this hypothetical model.
IL-6, IL-1𝛃, and TNF-ɑ decreased in the TRF group. “IL” stands for interleukin. IL-6, IL-1𝛃, and TNF-ɑ are “cytokines”. Cytokines are small, soluble proteins that “transduce signals in adjacent cells or transmit signals to distant organs” (Kang et al., 2020). These cytokines are involved in the inflammatory process and are categorized as “pro-inflammatory” cytokines. These cytokines are produced by immune cells during the “acute phase” response to an infection or in response to injury. The authors included these in the analysis as proxy markers of inflammation with higher levels indicating increased inflammation and lower levels indicating lower inflammation. Inflammation is a catch-all term related to the activity and abundance of immune cells. Hallmark signs of inflammation include redness, pain, heat, tenderness, and swelling in response to injury or allergic reaction, but “low-grade” inflammation is somewhat more discrete. Higher levels and activities of pro-inflammatory cytokines and immune cells are associated with obesity, autoimmune conditions, and various pathologies. Lower levels of these pro-inflammatory cytokines in the TRF group suggests that individuals in the TRF group decreased their inflammatory status. I don’t think the mechanism of action is clear, but it could be related to a decrease in the intake of foods that invoked an immune response, fewer pronounced blood glucose excursions that might stimulate immune cell activity and cytokine production, or I think it may be more related to the loss of body fat. Adipocytes can produce and secrete pro-inflammatory cytokines, and since the subjects in the TRF group lost more body fat, this could be related to a decreased production of adipocyte-originating pro-inflammatory cytokines.
Blood glucose and insulin decreased in the TRF group, but not the ND group. While it’s not entirely clear, this could be related to an increased sensitivity to insulin in the TRF participants. This is possibly related to lower inflammation—considering the reduced levels of pro-inflammatory cytokines discussed above—and increased levels of adiponectin. In short, insulin resistance has been associated with elevated levels/activity of the pro-inflammatory cytokines discussed above, and this may affect the binding of insulin to its receptor and disposal of glucose into cells. The aggregate effect of this could be relatively higher levels of glucose, insulin, and pro-inflammatory cytokines in the blood which were observed in the ND group. Additionally, increased levels of adiponectin have been associated with improved sensitivity to insulin as mentioned above and participants in the TRF group exhibited higher levels at the end of the study. Triglycerides also decreased in the TRF group which has also been associated with increases in adiponectin. Overall, the markers of metabolic health paint a pretty good picture for TRF compared to the ND group.
No significant changes in fat-free mass, arm, or thigh cross-sectional area were observed in either group which suggests both groups effectively maintained their muscle mass. Strength improved similarly in each group. Considering neither group was intentionally in a calorie deficit, I was surprised there wasn’t a significant increase in fat-free mass, or cross-sectional area, in either group. The training program appears to have under-dosed volume for these participants to maximize growth. Either way, both groups at least maintained their muscle size based on the measurements collected. One of the concerns with a TRF approach is that it may not facilitate maximal muscle growth or might run the risk of losing muscle mass since protein isn’t consumed as frequently. However, according to the results in this study, in young resistance-trained men it didn’t appear to result in a loss of muscle mass over the course of 8 weeks compared to a “normal diet”.
While the discussion so far paints a pretty positive picture for TRF, the observed decrease in total testosterone and T3 warrants consideration. Total testosterone decreased from 21.26 nmol/L to 16.86 nmol/L in the TRF group. Total testosterone didn’t change significantly in the ND group. Without knowing more about the amount and type of dietary fat, cholesterol, micronutrient intakes, sleep data, and other factors it’s difficult to explain this finding. More research can help uncover mechanisms at play related to this. It’s also relevant to consider that free testosterone (unbound) wasn’t measured and it’s possible despite total testosterone decreasing that free testosterone remained relatively stable. Since no deleterious effect was observed on fat-free mass or strength improvement from TRF, despite the decrease in total testosterone, this may have been the case.
T3 (triiodothyronine) is a hormone secreted by the thyroid gland. Blood levels decreased in the TRF group but didn’t significantly change in the ND group. Again, it’s not clear why from the study and the authors were careful not to speculate on the cause of this given a lack of assessment of potential factors. More research is necessary to uncover mechanisms at play. TSH (thyroid stimulating hormone) is a hormone secreted by the pituitary gland that signals for the thyroid to T3 and T4 (thyroxine). TSH levels were similar in each group at pre and post, so it doesn’t appear to be related to an upstream signaling difference. Resting energy expenditure was similar at pre and post in each group as well. Since the TRF group lost more body fat, and their metabolic rate seemed to remain stable, this change didn’t appear to significantly impair their ability to lose body fat in this study. Similar to total testosterone versus free testosterone, more insight could have been gathered from examining free T3 levels along with total and free T4 levels for a more comprehensive picture. I’m going to be digging into more research on this in the coming weeks, so stay tuned for that.