Fasting is a relatively well-studied metabolic state in sports and physical exercise due to studies of the “Ramadan” period observed by Muslim athletes [12, 14]. However, only a single study has reported its effect during a resistance training program aimed at achieving skeletal muscle growth [30]. Our data demonstrate that during a RT program, TRF was capable of maintaining muscle mass, reducing body fat, and reducing inflammation markers. However, it also reduced anabolic hormones such testosterone and IGF-1.

A key point of the TRF approach utilized in the present study is that total daily calorie intake remained the same while the frequency of meals (i.e. time between meals) was altered. This is dissimilar to many other IF regimens. There are a number of different IF protocols, most of which have the goal of reducing total energy intake. Additionally, unlike ADF and some other forms of IF, the regimen utilized in the present study employed the same schedule each day, consisting of 16 h fasting and 8 h feeding.

Although IF has received a great amount of attention in recent years, the majority of studies have investigated the effects of IF in overweight, obese or dyslipidemic subjects [19–21, 47–50]. However, little is known about the effects of such nutritional regimens in athletes, and more specifically, in body builders or resistance-trained individuals. The present study provides the first in-depth investigation of IF in this population of athletes. With the exception of reduced triglycerides, our results do not confirm previous research suggesting a positive effect of IF on blood lipid profiles [17–19, 47, 49, 51, 52], however, it has to be taken into account that our subjects were normolipemic athletes. The magnitude of reduction in triglycerides was also smaller than is typically seen in individuals who have elevated concentrations prior to IF.

As reported, a decrease of fat mass in individuals performing IF was observed. Considering that the total amount of kilocalories and the nutrient distribution were not significantly different between the two groups (Table 2), the mechanism of greater fat loss in IF group cannot simply be explained by changes in the quantity or quality of diet, but rather by the different temporal meal distribution. Many biological mechanisms have been advocated to explain these effects. One is the increase of adiponectin that interacts with adenosine 5′-monophosphate-activated protein kinase (AMPK) and stimulates Peroxisome proliferator-activated receptor gamma coactivator 1-alpha (PGC-1α) protein expression and mitochondrial biogenesis. Moreover, adiponectin acts in the brain to increase energy expenditure and cause weight loss [53]. It is notable that in the present study, the differences in adiponectin between groups remained even when normalized relative to body fat mass, whereas the significant decrease of leptin (that might be considered a unfavorable factor for fat loss) was no longer significant when normalized for fat mass. Other hypothesis is an enhanced thermogenic response to epinephrine [54] or an increase in REE [55] after brief periods of fasting, but our preliminary data didn’t support this point.

Interestingly, although reductions in the anabolic hormones testosterone and IGF-1 were observed, this did not correspond to any deleterious body composition changes or compromises of muscular strength over the duration of the study. It has been previously reported that men performing caloric restriction have lower testosterone than those consuming non-restricted Western diets [56], however, the present experiment did not restrict calories in the IF group. In animal models, IF influences the hypothalamo-hypophysial-gonadal axis and testosterone concentration probably through a decrease in leptin-mediated effects [57], but it must be considered that mice on a an every-other-day feeding regimen consume about 30–40 % less calories over time compared to free feeding animals and that in our study, no differences in leptin concentration were seen when normalized for fat mass. Also, the reduction of IGF-1 in the TRF group deserves some discussion. A previous study by Bohulel et al. [11] reported no changes in the GH/IGF-1 during Ramadan intermittent fasting. Even though it is plausible that IF mimics caloric restriction through common pathways (e.g. AMPK/ACC) (adenosine 5′-monophosphate-activated protein kinase/acetyl-CoA-carboxylase) [58], recent data on humans showed no influences of caloric restriction on IGF-1 [59, 60]. It is possible that the increase of adiponectin and the decrease of leptin could influence the IGF-1 concentration, even though it is unclear to what extent changes in adipokines impact circulating IGF-1 levels following weight loss [59].

Previous studies have reported mixed results concerning the ability to maintain lean body mass during IF, but the vast majority of these studies imposed calorie restriction and did not utilize exercise interventions [22]. In our study, the nutrient timing related to training session was different between the two groups, and this could affect the anabolic response of the subjects [61] even though these effects are still unclear [62]. However, we did not find any significant differences between groups in fat-free mass, indicating that the influence of nutrient timing may be negligible when the overall content of the diet is similar.

There is an increasing amount of data suggesting that IF could potentially be a feasible nutritional scheme to combat certain diseases. In the present study, both blood glucose and insulin concentrations decreased in the IF group. The potential of IF to modulate blood glucose and insulin concentrations has previously been discussed, but primarily in the context of overweight and obese individuals [3]. The concurrent increase in adiponectin and decrease in insulin may be related to modulation of insulin sensitivity, as adiponectin concentrations have been positively correlated with insulin sensitivity [21, 50, 63, 64]. Moreover, related to the well-known anti-inflammatory effect of adiponectin, it is possible that the reduction of inflammatory markers is related to the improvement of insulin sensitivity. Inflammation plays an pivotal role in insulin resistance development through different cytokines that influence numerous molecular pathways. For example, insulin resistance could be triggered by TNF-α via JNK and IKKβ/NF-κB (jun amino-terminal kinase/inhibitor of NF-κβ kinase) pathways, which may increase serine/threonine phosphorylation of insulin receptor substrate 1. Moreover IL-6 could decrease insulin sensitivity in skeletal muscle by inducing toll-like receptor-4 (TLR-4) gene expression through STAT3 (activator of transcription 3) activation. This relationship is potentially bidirectional as the activation of IKKβ/NF-κB signalling could, in turn, stimulate the production of TNF-α [65]. Modulation of some of these inflammatory markers by IF was seen in the present study: TNF-α and IL-1β were lower in the TRF group than ND at the conclusion of the study, while IL-6 appeared to decrease in the TRF group, but was not significantly different from ND. Previous information on the impact of IF on inflammatory markers is limited, but a previous investigation by Halberg et al. [66] reported no changes in TNF-α or IL-6 after two weeks of modified IF in a small sample of healthy young men.

Although a reduction in T3 was observed in the IF group, no changes in TSH or resting energy expenditure were observed. The observed reduction in RR in the TRF group indicates a very small shift towards reliance on fatty acids for fuel at rest, although a significant statistical interaction for RR was not present. Fasting RR has been previously reported to be a predictor of substantial future weight gain in non-obese men, with individuals who have higher fasting RR being more likely to gain weight [67]. Interestingly, it was reported by Seidell et al. [67] that although RR was related to future weight gain, RMR was not. It should be noted that individuals with the highest risk of future weight gain had fasting RR > 0.85 (as compared to individuals who had RR < 0.76). In the present study, the RR at the end of the study in both the TRF group and ND group do not directly fall into either of these categories (RR = 0.81 and 0.83, respectively).

Based on the present study, a modified IF protocol (i.e. TRF) could be feasible for strength athletes without negatively affecting strength and muscle mass. Interestingly, even though androgen concentrations were lowered by TRF, there was no difference in muscle mass changes between groups (+0.64 kg in TRF vs +0.48 kg in ND). Caloric restriction in rodents has been reported to decrease testosterone and IGF-1 even though human data on long-term severe caloric restriction does not demonstrate a decrease in IGF-1 levels, but instead an increased serum insulin-like growth factor binding protein 1 (IGFBP-1) concentration [60, 68]. However, no data are available for most forms of IF. Decrease the activity of the IGF-1 axis could be a desirable target for reducing cancer risk [69], but it is also well known that the activation of the IGF-1/AKT/mTOR (insulin-like growth factor-1/protein kinase B/mammalian target of rapamycin) pathway is one of the keys for muscular growth. In addition to altering IGF-1, fasting can promote autophagy [28], which is important for optimal muscle health [70]. Additionally, there is a possibility that the different eating patterns of the groups in the present study impacted the relative contributions of different hypertrophic pathways in each group.

Some limitations of the present study should be taken into account. One is the different timing of meals in relationship to the training sessions that could have affected the subjects’ responses. On this point, there is not a consensus among researchers. The beneficial effects of pre-exercise essential amino acid-carbohydrate supplement have been suggested [61], but the same group found that ingesting 20 g of whey protein either before or 1 h after 10 sets of leg extension resulted in similar rates of AA uptake [62]. Additionally, other studies have reported no benefit with pre-exercise AA feeding [71, 72]. Another limitation of the present study is that the energy and macronutrient composition of the diet was based on interview, and this approach has known weaknesses. Because of the limitations of this method, it is possible that differences in energy or nutrient intake between groups could have existed and played a role in the observed outcomes.