The primary target to investigate the adherence and feasibility of TRF in an adult working population showed good results in both areas. Participants reached the fasting goal in 72% of recorded days and 78% reported good compatibility with their professional activity. This compatibility applies here to employees of a university, which are, however, composed of many different occupational groups with different tasks. Thus TRF may have the potential to be conducted in a considerable proportion of the adult working population.
With an average loss of -1.3 ± 2.3 kg in weight and − 1.7 ± 3.2 cm in WC, mean differences were only small between baseline and follow-up data. Though there was considerable heterogeneity between participants and weight change ranged from − 8.9 kg to + 3.2 kg while changes in WC were between -13.3 cm and + 4.1 cm. Changes in blood lipids showed more homogeneity but did not deliver the expected improvements, apart from in individual cases. Overall, these results are unsatisfactory with respect to the positive results from trials with rodents (9). In addition to the fact that humans are not kept in cages and the feed supply can be controlled both in terms of time and quantity, one important reason is assumed in the absence of any dietary requirements or instructions. As a consequence some participants reported overeating because of fear of hunger in the fasting phase. For instance, one participant reported to have gobbled up anything he could find by the end of the eating phase. In future studies, this problem has to be explicitly addressed and participants should be given more instructions in terms of nutrition and they need to be looked after more closely.
Independent of changes in body composition, there was a statistically and clinically significant increase in health-related quality of life. Although patient-reported outcomes are constantly gaining more interest and acknowledgment, this is to our knowledge one of the first measurements of HRQoL in intermittently fasting adults (10). This result is especially important because it shows an increase in HRQoL independent of weight loss. Based on the complexity of HRQoL (11), it can be assumed that TRF may have positive physical and psychological effects which to specifically identify exceeds the possibilities of this study.
There are few studies with small numbers of participants examining TRF in humans. Using a mobile app, Gill and Panda observed erratic eating patterns highly variable from day to day with more than half of the 47 participating adults eating for 15 hours, or longer, every day. Furthermore, they report a bias toward eating late and consuming > 35% of calories after 6 p.m. Eight overweight individuals who reduced their daily eating time from > 14 hrs to 10–11 hrs for 16 weeks subsequently reduced their body weight by 3.27 kg (95% CI: 0.908–5.624 kg). In our study, 18 overweight participants reduced their weight by 1.38 kg (95% CI: 0.039–2.717 kg) in 12 weeks, the difference may be due to longer duration and regular individualized feedback in the study by Gill and Panda (12). Additionally, as indicated by small but significant changes in WC and WHtR in our study, TRF may help to loose abdominal fat, an important fact for overall health since abdominal obesity is associated with virtually all kinds of non-communicable diseases and successful interventions are rare (13). Gabel et al. report the effects of a pilot study of 8-hour TRF on body weight and metabolic disease risk factors in 23 obese adults. Participants were allowed ad libitum eating between 10:00 and 18:00 h and water fasting from 18:00 to 10:00 h for 12 weeks. They compared weight loss and other outcomes to a matched historical control group. Except for moderate changes in body weight (-2.6%), energy intake and systolic blood pressure all other variables under consideration (LDL, HDL, triglycerides, fasting glucose, fasting insulin, HOMA-IR, homocysteine) showed no significant differences to controls (14). The 11 obese participants in our study lost − 2.2% body weight confirming the moderate effects of a TRF regimen with ad libitum eating in obese participants.
Gasmi et al. investigated the influence of 12 weeks TRF on muscle performance and immune responses in 20- and 50-year-old men in groups of 10 persons each. They report that their 12 h feeding – 12 h fasting protocol decreases hematocrit, total white blood cells, lymphocytes, and neutrophils but did not affect muscle performance (15). Two other studies independently report results of randomised trials investigating TRF in young males performing resistance training. Moro et al. found a decrease in fat mass in the TRF group while fat-free mass and maximal strength were maintained (16). Tinsley et al. report no changes in total body composition after the eight-week study period in the TRF group despite a reduced energy intake (16). These findings confirm the results from Gasmi et al. that TRF does not restrict the practice of exercise training. This is of course very important since fasting has always been associated with, and criticized for, muscle loss. Though not scientifically approved, TRF has as “Leangains”, a large group of fans in the adherers of the fitness scene, or power athletes and bodybuilders, with the important message that fasting does not compulsorily mean a loss of energy and subsequent muscle performance (17).
Finally, Sutton et al. report a controlled feeding study with early TRF (eTRF) and the improvement of insulin sensitivity, blood pressure and oxidative stress, without weight loss, in pre-diabetic men. Early TRF means that the eating window opens early in the morning, and in this case participants started to eat at 8am and had their last meal before 2 pm. The underlying rationale is to eat in accordance with the circadian rhythms in metabolism. The control group had an identical meal plan, except for the timing, which started at 8am and ended at 8 pm. The authors wanted to know, whether their eTRF schema produces health benefits even without losing weight. After five weeks of controlled feeding, insulin sensitivity and β-cell function increased while postprandial insulin, blood pressure, oxidative stress and appetite in the evening were reduced in the eTRF group (18). Participants in our study had their first meal on average at 10:25am and their last meal at 6:46 pm, and supposedly many of them skipped breakfast. There is evidence from several studies in human and animals that eating at the time of the highest responsiveness of the endocrine system during the active phase of the day in accordance with the circadian rhythm optimizes the body’s food processing capacity (19). Based on evidence mainly from animal studies, Patterson et al. propose a potential mode of action of intermittent fasting (IF), respectively TRF: The association of IF with lifestyle (diet, sleep and activity), the circadian central and peripheral clocks, and the diversity and activity of the intestinal microbiota, may result in a metabolic regulation and subsequent reductions in obesity and other lifestyle-dependent diseases (20).
TRF offers several advantages over other forms of dietary interventions to prevent or treat weight problems and associated disease or disease risks:
TRF offers several advantages over other forms of dietary interventions to prevent or treat weight problems and associated disease or disease risks:
- Low-threshold approach (meaning that the implementation does not necessarily have to be medically supervised)
- No calorie counting
- No dietary restrictions
- Individually adaptable to the daily rhythm
- TRF may improve health even without weight loss
These advantages may be partly counteracted by some pitfalls:
- Continuation of a possibly unhealthy food selection
- Risk of overcompensation due to increased eating during the eating phase
The advantages of TRF make this approach particularly interesting for public health interventions, as the low barriers and ease of implementation can have a positive impact on both entry and adherence. Nonetheless, more research is needed to clearly identify positive and negative impacts in order to weigh the benefits against the risks.
Strengths and Limitations
We had only two drop-outs due to comprehensible reasons, which we consider very few. Overall, the adherence was very good thanks to the motivated participants. In comparison to earlier studies with observational character conducted by the authors, the number of missing data was very small. Unfortunately, the visual analogue scale for the HRQoL was printed on the last page of the questionnaire so that 11 participants simply overlooked it in the baseline assessment. A minor strength of this research is probably a larger sample size than previous studies. With respect to the primary target of the study, one of the strengths is the heterogeneity of the participants with regard to their different professional activities.
The most obvious limitation is the missing control group. This was mainly due to the pilot character of the study and the primary targets, for which a control group was not absolutely necessary. Results need therefore be interpreted with caution. Unfortunately, males are underrepresented, possibly reflecting their less pronounced interest in health. This study was not funded and therefore some examinations which would have been useful could not be carried out for financial reasons.