Fasting can be defined as abstinence from ingesting food and caloric beverages for specified time periods, ranging from hours to several weeks [1
]. Recently, fasting has grown from its well-documented roots in religious practices to a common dietary strategy in both athletic and medical fields, often for the goal of improving body composition or cardiometabolic health markers [3
]. Currently, there are a multitude of fasting protocols categorized as intermittent fasting (IF) including alternate day fasting (ADF) and modified alternate day fasting (mADF), whole day fasting, and time-restricted feeding (TRF). While a comprehensive examination of the differences between the various forms of IF is beyond the scope of this report (for reviews, see Tinsley et al., (2015b) [5
] and Mattson et al., (2017) [6
]), it is important to highlight some commonalities. Each variant utilizes a defined period of abstinence from all calories that extends the overnight fast and often leads to reductions in overall caloric intake [7
], ranging from 25 to 38% [8
]. This has led many to question whether changes in the physiological outcomes (e.g., body composition and cardiometabolic health markers) often associated with fasting protocols are attributed to the unique benefits of fasting itself, or solely due to the resultant caloric restriction. While previous investigations have examined the potential impact of IF on body composition [9
] and metabolic health markers [9
], the majority of investigations have been carried out in overweight and obese populations and without the implementation of a structured exercise program [12
]. Furthermore, these investigations have primarily employed either ADF or mADF styles of fasting, as opposed to the more commonly practiced TRF.
TRF is traditionally characterized as daily caloric abstinence ranging from 12 to 20 h followed by ad libitum food and fluid consumption for the remaining hours of the 24-h period [5
]. Unfortunately, there is a dearth of scientific literature on this form of IF in general and a particularly noticeable lack of data in exercising populations, among whom TRF is often practiced. Two common TRF variations involve a daily 16-h fast with an 8-h feeding window (16/8; e.g., the Lean Gains Diet) or a daily 20-h fast with 4-h feeding window (20/4; e.g., the Warrior Diet) [5
]. To date, only three previous interventions have been carried out in resistance-trained populations, two of which employed the 16/8 protocol [21
], while the other examined a modified 20/4 regimen [23
]. First, the work by Tinsley et al. (2016) [23
] examined fasting on alternating days. Specifically, previously untrained men were asked to follow a fasting protocol for 8-weeks consisting of 4-h feeding windows four days per week, with ad libitum feeding the remaining three days per week, on which they also resistance trained. Despite participants in the TRF group reporting consumption of, on average, 667 kcals, 75 g of carbohydrates, 25 g of fat, and 30 g of protein (equating to 0.4 g/kg) less on fasting days than non-fasting days, no significant differences were noted for body mass (BM), lean soft tissue (LST), or fat mass (FM). Furthermore, muscular performance adaptations were not inhibited in the TRF group, with improvements noted for both lower body strength and endurance as well as upper body strength at the end of the 8-week investigation.
Similar findings were reported by Moro and colleagues [21
], who investigated a 16/8 TRF protocol in well-trained men. Unlike Tinsley et al. (2016), participants in the TRF group were required to fast everyday throughout the duration of the study instead of just on non-training days. Furthermore, both groups reported consuming roughly the same number of kcals (~3000 kcals/day) and protein (~2.0g/kg/day) spread out across three meals. The non-fasting group consumed their meals at 8 am, 1 pm, and 8 pm, while the TRF group consumed their meals at 1 pm, 4 pm, and 8 pm. Subsequently, all participants underwent supervised training conducted thrice weekly, consisting of heavy compound movements to failure. Body composition, resting metabolic rate, basal levels of anabolic and hunger hormones, and muscular performance were examined pre- and post- the 8-week intervention. The TRF group experienced significant decreases in FM, leptin, total testosterone, adiponectin, IGF-1, and increases in the adipokine adiponectin. Although, it must be noted that significant differences for leptin were no longer apparent once corrected for BM. Furthermore, there were no significant differences found in fat free mass (FFM) or muscular performance. Finally, a similar investigation was carried out in resistance trained females by Tinsley and colleagues in 2019 [22
]. Twenty-four participants underwent 8-weeks of supervised training while, much like Moro et al. (2016), following either a 16/8 fasting protocol or eating regularly from breakfast until the end of day. Furthermore, all participants consumed an average of ~1.6g/kg/day of protein. In a per protocol analysis of 24 participants who completed the intervention and complied with study procedures, a significant group by time interaction indicated greater loss of FM in the TRF and TRF plus beta-hydroxy-beta-methylbutyrate (HMB) supplementation groups relative to the normal diet group at the 4-week timepoint, but the FM loss only remained statistically significant for the TRF plus HMB group by the end of the study. However, no significant differences between groups were observed in the intention-to-treat analysis that included all 40 participants who entered the study. Once again, no difference in changes in FFM were noted between groups, with all groups demonstrating an increase. Similarly, all groups displayed increased muscle thickness of the elbow flexors and knee extensors as well as muscular performance improvements without differences between groups. With the disparate findings in FM changes between the three previous studies, and the substantial interest in TRF programs to alter body composition, further investigation is required, employing a more robust method of body composition assessment (e.g., multi-compartment models compared to dual-energy X-ray absorptiometry [24
]). Furthermore, due to the limited literature in resistance training populations, there is a need for further exploration of whether TRF influences adaptations to resistance training when calorie and protein intakes are controlled. Thus, the current investigation sought to expand on the work of Tinsley et al. [22
] and Moro et al. [21
] by further exploring the impact of short-term TRF. More specifically, Tinsley et al. 2019 [22
] noted significant improvements in body composition at four weeks in the TRF group, but these changes no longer remained at eight weeks. Thus, four weeks was chosen for our investigation in order to further elucidate the potential effects of shorter time periods of calorie restrictive dieting. Additionally, 4-week caloric restriction time periods may also be more representative of field implementations of TRF in recreational populations (e.g., improvements in body composition or weight while peaking for strength sports). Furthermore, due to the limited research on resistance training and TRF, this data could not only be telling of the more acute changes in body composition, muscular performance, resting energy expenditure, and blood biomarkers, but be built upon by future research studies that implement a similar design for longer time periods (e.g., 8–24 weeks). Therefore, our objective was to investigate the effects of 16/8 TRF versus a normal meal distribution with equivalent calorie deficits and protein intake in both groups on measures of body composition, muscular performance, resting energy expenditure, and blood biomarkers following 4-weeks of resistance training in recreationally active men.
Recently, fasting regimens such as TRF have gained considerable popularity despite limited empirical support for their advantages when compared to traditional energy-restricted diets. The present study demonstrated short-term TRF did not elicit more favorable alterations in body composition when compared to a normal meal distribution in isocaloric and isonitrogenous conditions. However, our data also suggest that 16/8 TRF does not negatively influence the ability to maintain FFM over four weeks in a hypocaloric state when elevated dietary protein intake and a strenuous resistance training program are present. Additionally, the implementation of daily 16-h fasting periods did not compromise improvements in muscular strength or power over the course of the intervention.
While the current study was shorter than the three previous TRF and resistance training interventions (i.e., four as opposed to 8-weeks [21
]), relatively similar body composition changes were still noted. Tinsley and colleagues [23
] did note apparent differences in LST changes between fasting and non-fasting groups over the course of eight weeks, which, although not statistically significant, may have practical implications for those performing resistance training for body composition enhancement. The researchers postulated that these disparities might be attributed to the differing average daily protein intakes between the groups (ND: 1.4 g/kg/day; TRF: 1.0 g/kg/day). However, despite the lower protein intake, LST was maintained with TRF, most likely due to the resistance training stimulus. In an attempt to eliminate discrepancies in protein intake, Moro and colleagues [21
] and Tinsley at al. (2019) [22
] matched protein between groups at 1.9 g/kg/day and 1.6 g/kg/day, respectively. Following 8-weeks of 16/8 TRF in resistance-trained males, Moro et al. reported FFM was maintained, lending credence to Tinsley et al.’s (2016) reported hypothesis that protein intake was the likely source of the possible discrepancies in lean mass changes in their previous investigation. Additionally, Tinsley et al. (2019) reported increases in FFM regardless of group (i.e., TRF, TRF plus HMB, control), despite differences in meal timing. Subsequently, the present study similarly supports the three previous interventions, with no differences in the maintenance of FFM between groups, despite differences in meal timing and the resultant discrepancies in daily energy and protein distribution. With the implementation of a 25% caloric deficit, significant decreases in BM attributed to loss of both FM and FFM would be expected in the absence of a structured exercise program and adequate protein intake. As both groups similarly decreased BM, FM, and BF% following the 4-week intervention, and FFM did not change, it can be inferred that the exercise and nutrition intervention was effective for promoting loss of body fat without concurrent lean mass loss. Previous research suggests that a protein intake of ≥2 g/kg/day may be required to maintain FFM during an energy-restricted diet in active individuals [38
] as protein is critical for upregulating muscle protein synthesis, the driving force behind FFM adaptive responses to chronic resistance exercise. While our participants were not consuming 2 g/kg/day, they indeed consumed ~1.8 g/kg/d, a quantity surpassing the 1.6 g/kg/d threshold highlighted by Morton and colleagues [41
] for increasing muscle mass when engaging in resistance training as well as in the suggested range of 1.4–2.0 g/kg/day recommended by the International Society of Sports Nutrition [37
]. It is plausible the three day per week, full body resistance exercise program, along with a higher protein diet, contributed to these results.
REE decreased similarly for both groups following the 4-week intervention, however, this is in line with previous findings. A systematic review of 90 investigations by Schwartz and Doucet [42
] demonstrated an average of 15.4 kcals/kg of weight loss lowering of REE. Interestingly, the average decrement was greater in interventions lasting two to six weeks. The slowing of metabolic rate with weight loss is often attributed to reductions in FFM. However, as our participants maintained FFM, this is not likely to be a contributing factor to the reduction in REE observed in the present investigation.
One of the major discrepancies between the current findings and Moro et al. is that of reductions in FM with the implementation of TRF. Unlike Moro et al. [21
], this study demonstrated that when daily caloric intake was equated, the alterations in meal frequency did not play a role in the reductions of FM or BF%. These findings confirmed our hypothesis that overall caloric balance would be the driving factor in changes in body composition, despite alterations in meal timing. However, the differing durations and participant characteristics, particularly training status, between these two studies are noteworthy. Additionally, as both investigations employed self-reported dietary assessments, the reported nutritional intakes should be viewed cautiously.
Participants in the present study increased BP1RM
to a significant degree, although improvements did not differ between diets. While participants did not increase whole body FFM to a significant degree, results from muscle ultrasonography did demonstrate significant increases in VL and BB CSA. It is plausible that the improvement in both lower body and upper body strength measures over a short period of time can be partially attributed to increases in CSA [43
]. While the CSA increases are notable, previous reports highlight that a strong relationship between increases in CSA and muscular strength are not well established until elite levels of training status and proficiency have been achieved [44
]. Given the short training duration in the present investigation, it is more likely that substantial neural adaptations were stimulated in response to the intense resistance training program implemented in recreationally trained participants, and that these adaptations may have primarily contributed to the observed strength increases [47
]. Nevertheless, the increases in muscular strength were similar to outcomes reported by the previous studies in this area [21
]. While muscular strength improved, muscular endurance did not improve for either group in our investigation. This is unlike Tinsley and colleagues [23
], who reported muscular endurance improvements in the lower body. The lack of improvements in muscular endurance in our investigation are likely due to the style of training, rather than the dietary strategies involved.
Another contrary finding from the previous investigation [21
] was in regard to the biomarkers serum testosterone and plasma cortisol. Moro and colleagues [21
] reported decreases in testosterone in the TRF group over the course of 8-weeks. Conversely, we found no significant differences between groups at any point throughout the investigation with regard to serum testosterone. While the short time period of the current study (4-weeks) may have played a role in these findings, previous investigations have found detectible changes in as little as one to two weeks of resistance training [50
]. It is worth noting that while we did detect a main effect for time with regard to decreases in testosterone, the actual physiological relevance of observed decreases is highly questionable as serum testosterone levels have been shown to fluctuate as much as 43% throughout a 24 h period [52
]. Furthermore, shifts in sleep patterns may disrupt serum testosterone levels by as much as 57 ng/dl [53
]. Thus, the minor alterations observed are likely not physiologically relevant, despite statistical significance, and should not be over-interpreted.
Additionally, our findings demonstrated a rise in plasma cortisol only in the ND group over the course of 4-weeks. Rises in cortisol have been shown to occur during hypocaloric periods [55
], with greater increases found when participants were asked to closely monitor their diets [55
]. Additionally, fasting has demonstrated an ability to alter the normal circadian rhythm of cortisol rises and falls [60
]. While speculative, the period of TRF may have altered the normal spike seen at the time of day when cortisol was assessed. While cortisol assessments were taken at the same approximate time pre- and post-intervention (±2 h), time from the waking hour was not quantified, which may have also affected the measurement [61
]. However, the previous investigation of Tinsley et al. indicated no alteration of the cortisol awakening response or changes in average cortisol concentrations with eight weeks of TRF plus resistance training in females [22
]. Future investigations should explore the implications of TRF on cortisol levels and rhythms further. Additionally, the discrepancies between the present study’s findings and that of Moro and colleagues (i.e., no change in cortisol) [21
] might be attributable to the implementation of a weight-maintenance diet, rather than an energy-restricted diet.
One of the novel findings of the present investigation was the lack of impact of TRF on perceived athlete readiness. Despite the length of daily calorie abstinence, perceived recovery between training sessions and perceived readiness did not differ between diets. This aspect has not been previously addressed with regard to TRF in resistance training populations, and may be useful when deciding to implement TRF in actively training individuals. Furthermore, the dietary strategy did not significantly impact feelings of energy, motivation to perform physical tasks, fullness, desire to eat, or hunger. However, it is important to highlight that the timing of the surveys may have impacted these findings as they were completed prior to the participants’ workouts, which fell within their feeding window.
There were a number of limitations to the current study that should not be overlooked when interpreting the findings. First, the use of self-reported dietary intake and basing our dietary analysis on a total of 12 days throughout the 4 weeks are limitations. However, efforts were made by the research team to ensure participants were adhering to all dietary guidelines including interviewing participants during each workout (i.e., three times per week) and reviewing dietary logs at the beginning of each week. We also recognize that the 4-week duration of the study is a limitation and more research needs to be conducted that implements longer timeframes (e.g., 8–24 weeks). Furthermore, our participants’ circadian schedules, sleep schedules, and work/academic schedules varied, which may also influence our findings, especially hormone assessments. Additionally, we only examined recreationally active men. Future investigations should explore diverse populations and various degrees of training status including applications to sport specific performance. Moreover, variants of TRF structure should be explored such as employing TRF on weekdays with returns to normal meal patterns on weekends. Furthermore, future investigations should examine the impact of workout timing (i.e., training performed during or outside the feeding window) on potential performance and body composition outcomes in conjunction with TRF.