Intermittent fasting (IF) has become a lifestyle of eating during specific period of time and fasting for the rest of the day. IF has shown many of the health benefits. It is not only reducing free-radical production and weight loss, but also the adaptive cellular responses between and within organs that restores glucose metabolism, increases stress resistance and suppresses inflammation (1). Previous studies showed the positive effect of IF in animal models on chronic disorders such as obesity, diabetes, cardiovascular disease, cancer and neurological disorders (2, 3). Ramadan intermittent fasting (RIF) is one type of IF that performed by about 1.5 billions of Muslims worldwide annually from dawn to sunset for one month. The time periods of RIF vary from 9 to 22 h per day due to geographical differences and seasonal variation (4). Food, water, smoking and sexual activities are forbidden during the fasting daytime hours of Ramadan month. Our previous studies demonstrated that not only RIF can improve anthropometric parameters, but also can improve mental health, fatigue and quality of life (QoL) (5). However, the physiological mechanism behind these beneficial effects need to be elucidated.

Depression is related to irregularities in hypothalamic–pituitary–adrenal (HPA) axis, altered insulin resistance, plasma glucose, pro-inflammatory cytokines, leptin resistance, and disturbance of neurotransmitters, neuropeptides and neurotrophic factors (6, 7). Interestingly, several studies demonstrated that IF has positive effects on the neuroendocrine system and on depression. IF might induce an antidepressant effect on depression, providing potential new treatment options. It included neurotrophic effects and orexin signaling activation that can release endorphin and ketone via the increased of cAMP response element-binding (CREB) phosphorylation (8). Hence, it is of interest to study the effect of IF on biological mediators related to depression, particularly on neurotrophic factors.

Nerve growth factor (NGF) was the first member described among the neurotrophic factors (9). NGF has been known to involve in the regulation of neurotransmitters and neuropeptides synthesis of nerve cells. It is important for growth, maintenance, proliferation and survival of nerve cells (10). Various clinical studies in human and animals showed an association of low NGF levels with depression compared with healthy controls (11). Brain-derived neurotrophic factor (BDNF), another member of neurotrophic factors, was described as being involved in synaptic plasticity (12), depression (8), and increasing survival of dopaminergic neurons of the developing substantia nigra (13). BDNF has been also studied in different neurological-related diseases, such as major depressive disorders, fibromyalgia syndromes and chronic low back pain (14). Glial cell-derived neurotrophic factor (GDNF) is a small protein and member of neurotrophic factors that are responsible for the development, maintenance and survival of different neurons (15). In mice model, induced GDNF-expression showed protective effects to obesity. In addition, several studies showed that GDNF is known to the mechanism underlying depressive disorders (16).

Other biological mediators related to depression and body composition parameters such as insulin growth factor-1 (IGF-1), tumor necrosis factor alpha (TNF-α), interleukin-8 (IL-8), and matrix metalloproteinase-9 (MMP-9) were also determined (17–19). One of the biological mechanisms for depression is related to inflammation. People with major depressive disorder (MDD) and obesity have been shown to have higher inflammatory markers as compared to healthy control (20–22). IGF-1 has been shown to have anti-inflammatory action by inhibiting the expression of proinflammatory cytokines including IFN-γ, IL-1β and TNF-α and enhancing the production of anti-inflammatory cytokines including IL-4 and IL-10 (23). The abnormalities of IGF-1 in MDD patients have been suggested as a marker and predictive role of the neurotrophin for depression and treatment effectiveness (24). Meanwhile, individuals who had increased level of both pro-inflammatory cytokine tumor necrosis factor-alpha (TNF-α) and interleukin 8 (IL-8) have been reported to develop psychiatric disorders such as major depression, bipolar disorder, schizophrenia and sleep disorder (25, 26). Other study showed that depression condition in young patient, levels of serum MMP-9 were increased which might be used as biomarker for bipolar disorder (27). Another study also showed that patients in acute phase of ischemic stroke condition, serum MMP-9 levels were elevated (28). In addition, animal were treated with MMP-9 inhibitor showed improve specific neurobehavioral deficits associated with Alzheimer’s disease (29).

As mentioned before, neurotrophic factors play a role not only related to depressive disorders, but also to body composition parameters. Therefore, this study was an explorative study that aimed at elucidating the changes in neurotrophic factors upon the observation of RIF. Additionally, we aimed to measure the inflammatory cytokine TNF-α, the chemokine IL-8 and MMP-9. Furthermore, in our previous study showed that RIF improved body composition and mental health parameters, fatigue and QoL (5). For the body composition parameters, we measured body weight (BW), body mass index (BMI), skeletal muscle mass (SMM), body fat mass (BFM), fat free mass (FFM), body fat percentage (BFP), body water mass (BWM) and wait and hip ratio (WHR). For mental health parameters, we determined the effects on fatigue [Fatigue Severity Scale (FSS) and visual analogue scale (VAS)], depression (The Hospital Anxiety and Depression Score (HADS) and Beck’s Depression Inventory (BDI)-II) and QoL [Epworth Sleepiness Scale (ESS)] (5). In this study, we aimed to analyze the correlation between measured biological mediators with our previously published clinical parameters including body composition and mental health parameters at all time points.

Local Ethic Committee of Hannover Medical School approved this study (Ethics No. 6899) in accordance with the ethical standards laid down in the 1964 Declaration of Helsinki. This study registered at German Registry of Clinical Trial (DRKS-ID: DRKS00008181). Written informed consent was obtained from all subjects and/or their legal guardian(s).

The most understanding by common people, the effect of IF is related to the alteration of anthropometric parameters, which include BMI, body weight, and fat loss (5, 30–32). Interestingly, our previous studies, RIF has a positive influence to mental health, fatigue and QoL (5, 30). This effect might be mediated by different biological mediators, particularly cortisol and BDNF (33). Another study also observed that RIF improved body composition and did not exacerbate depression in MDD patients (34). In this current study, we wanted to elucidate the effects of RIF at different time points on neurotrophic factors such as ß-NGF, GDNF, BDNF and IGF-1. Additionally, we also measured other biological mediators such as TNF-α, IL-8 and MMP-9.

In our current study, ß-NGF showed significant increase from the middle of Ramadan (T2) to the last days of Ramadan (T3) and one week after the RIF finished (T4). A comparable study measuring NGF during RIF in woman only participants also showed significant increase of NGF during mid and end of RIF (35). It is known that NGF has an important role in the regulation of neurotransmitters and neuropeptides synthesis of nerve cells (10), but there is no previous evidence on the effect of fasting on NGF levels and it effects to improve brain health through the induction of the NGF levels. Interestingly, Beck’s depression inventory (BDI)-II showed negative correlation with ß-NGF levels. A meta-analysis and systematic review observed that NGF levels were significantly lower in MDD than in healthy control (36).

Furthermore, we observed significantly decreased levels of GDNF from one week before the beginning of Ramadan/baseline (T1) to T4, and significantly decreased levels from T2 to T4. Interestingly, GDNF levels has a positive correlation with HADSD at T3 and BFM and WHR at T4. GDNF levels were shown to be significantly decreased in depression (37) involved in the dopamine system and has been linked to a potential treatment in drug abuse (38). Concerning special psychiatric conditions and its correlation to metabolic levels, a study by Skibinska et al. (39) showed that in psychiatric disorders, such as depression, GDNF and metabolic features such as cholesterol, showed significant correlation between GDNF and cholesterol GDNF. Transgenic mice showed higher brown fat mass, energy expenditure and even increased expression in skeletal muscle. Nevertheless, GDNF does not appear as a predictor for treatment remission in other psychiatric disorders, such as general anxiety disorder (40). However, the findings towards GDNF and its (patho) physiological role remain controversial and further studies need to elucidate the role of GDNF. Best to our knowledge, this study is the first study to analyze the effect of fasting on GDNF.

RIF associated with the decrease of BDNF levels in schizophrenia patients (41). Furthermore, an increased BDNF level was linked to type II diabetes mellitus (T2DM) and obesity (42). A study using rats suggested that IF could potentially protect from T2DM by increasing the levels of BDNF and neurotrophin (NT)3 (43). Interestingly, contradictory results were obtained regarding BDNF levels during RIF in healthy participants. Some studies showed a significant increase of BDNF levels during RIF (35). Whereas another study showed a decrease of BDNF levels during RIF (44). These conflicting reports regarding how IF influence BDNF levels might be because of the different time periods of RIF due to geographical differences and seasonal variations. Different time point of collecting the samples also might resulting the differences. Meanwhile, our previous study showed significantly decreased (33), it could be that the latter study include both male and female participants.

Concerning to insignificant changes of IGF-1-levels during RIF, our results were in line with another study conducted on well trained men during RIF (45), although we cannot compare directly with subjects of this study. In contrast to these findings another study showed significant reduction of IGF-1 levels during RIF by adaptation mechanisms. As shown in mice model, it was hypothesized that short time starvation leads to survival mode and reduction of the growth hormone IGF-1 (46). Reduced IGF-1 levels were linked to extended life span in mice (47).

The raw data supporting the conclusions of this article will be made available by the authors, without undue reservation.

The studies involving humans were approved by Ethic Committee of Hannover Medical School (ethics no. 6899; registration code of trial: DRKS00008181). The studies were conducted in accordance with the local legislation and institutional requirements. The participants provided their written informed consent to participate in this study. Written informed consent was obtained from the individual(s) for the publication of any potentially identifiable images or data included in this article.

SG: Data curation, Investigation, Project administration, Writing – original draft, Writing – review & editing. AS: Data curation, Investigation, Methodology, Software, Validation, Writing – original draft, Writing – review & editing. MB: Data curation, Investigation, Methodology, Software, Writing – review & editing. IH: Data curation, Investigation, Project administration, Writing – review & editing. GG: Funding acquisition, Resources, Writing – review & editing. CG: Funding acquisition, Resources, Writing – review & editing. BN: Conceptualization, Data curation, Formal analysis, Investigation, Methodology, Software, Supervision, Validation, Visualization, Writing – original draft, Writing – review & editing.