Since the title of the study already gives it away, I suggest that we address the phrase “non-randomized”, which describes an essential characteristic of a recent 8-week study that was conducted by scientists from the Freie Universität Berlin (Germany), right away. Yes, the study was designed as a non-randomized controlled clinical trial with two arms, involving healthy volunteers only. And yes, it is indeed very likely that the subjects who chose to be in the fasting group had a positive bias towards ‘not eating’ once a week…
…but hey, let’s be honest: Isn’t that the “real world”? I mean people who will subscribe to any form of intermittent fasting protocol usually do so voluntarily. The fact that the study was nonrandomized is thus in line with the authors’ intention which was to “evaluate whether intermittent fasting is an effective preventive measure and whether it is feasible for healthy volunteers under everyday-conditions.” (my emphasis in a quote from Kessler 2017).
The study itself consisted of an 8-week intervention period, followed by a four-month follow-up (the total study duration was thus 6 months). Outcomes were assessed at baseline, after eight weeks, and after six months. As previously hinted at, fasting was not obligatory, volunteers who were not interested in fasting served as a control group.
The intervention itself is easy to describe: Participants in the fasting group were asked to continue their regular nutritional habits on the non-fasting days, while the control group maintained their habitual nutrition throughout the whole period. More specifically, …
- fasting was done on a self-chosen weekday (e.g. Monday)
- patient adherence was defined by conducting at least six of the eight eight overall days of fasting during the eight week intervention period
- fasting days were defined as days during which the subjects abstained from solid food for at least the period between 00:00 and 23:59, and accumulated a maximum total energy intake of 300 kcal/d resulting from defined fasting beverages
- allowable “fasting beverages” were standard fruit and/or vegetable juices 2 x 200 ml/d, clear vegetable broth (maximum 300 ml/d), ≥ 750 ml/d hot, unsweetened herbal teas, ≥ 1500 ml/d of non-sparkling, resp. tap water at room temperature
- for better compliance cardamom, vanilla, and cinnamon were allowed in small quantities in order to enhance the taste of grain based liquids
- unsweetened black tea, green tea, and black coffee were also allowed in small quantities (maximum 2 servings)
Study outcomes included changes of metabolic parameters (insulin, glucose, insulin-resistance, IGF-1, BDNF, lipids, liver enzymes, HbA1c) and coagulation-markers; bioelectrical impedance analysis; body mass index; abdominal girth; blood pressure; general quality of life (WHO-5 questionnaire), as well as mood and anxiety (HADS, POMS, Flourishing-Scale, VAS, Likert-scales). The intervention consisted of a fasting day which was repeated every week for eight weeks, with abstinence from solid food between 00:00 and 23:59 at a minimum and a maximum caloric intake of 300 kcal on each fasting day. A per protocol analysis was performed, p-values <0.05 were considered as significant.
A sample fasting day: 200ml fruit or vegetable juice in the morning, 300ml vegetable stock for lunch, 200ml vegetable or fruit juice in the evening plus 2250 ml of tea and water distributed over the whole day. Laxative measures before, during and after the fasting days were not part of the protocol (e.g. standard Polyethylenglycol solution, Glauber or Epsom salt).
The analysis of the subjects’ (N=22 in the fasting, N=14 in the control group) baseline data showed no significant between-group differences. Overall, the scientists observed that the “aseline values for participants of both groups reflect a very healthy, middle-aged and predominantly female population” (Kesseler 2017) – a group with a high variance in BMI (despite normal baseline BMI, hip measurement and abdominal girth values) and a similar distribution pattern for IGF-1 baseline values.
Adherence to fasting was OK – accordingly, it doesn’t mess with the results
Of the twenty subjects in the intervention group, nine subjects completed six fasting cycles, four subjects completed seven fasting cycles, and seven subjects completed eight fasting cycles.
What do we know about intermittent fasting (Patterson 2017): Mostly based on rodent studies, we know that intermittent fasting and restricting the availability of food to the normal nighttime feeding cycle improve metabolic profiles and reduce the risks of obesity and obesity-related conditions, such as nonalcoholic fatty liver disease, and chronic diseases, such as diabetes and cancer. For humans, this is not satisfactorily studied, though. What appears to be relatively certain is that intermittent fasting regimens are not harmful physically or mentally (i.e., in terms of mood) in healthy, normal weight, overweight, or obese adults.
The mechanism that’s behind the beneficial effects of fasting is complex and involves (a) changes in the gut microbiome, (b) direct and indirect effects of the circadian clocks, and (c) lifestyle changes related to diet (e.g. no late-night chips binges, involuntary reduction in total energy intake, etc.), activity and sleep (Patterson 2017).
Overall (humans + rodents), studies suggest that almost any intermittent fasting regimen can result in some weight loss. Among the 16 intervention trials included in Pattersons 2017 review, 11 reported statistically significant weight loss. With regard to the often-cited beneficial effects on insulin concentrations, improvements in lipids, or reductions in inflammatory factors, the contemporarily available evidence is mixed, with alternate-day fasting, in particular, resulting in both, weight loss, as well as reductions in glucose and insulin concentrations, in the three pertinent studies in Patterson et al.’s 2017 review. According to the authors of the review, alternate day fasting may yet not be practical because it leads to intense hunger on fasting days. The hunger pangs can be avoided or ameliorated by modifying the ADF regimens (e.g. protein-modified fast on every other day).
In that, it is important to remember, though that “research has not demonstrated that alternate-day fasting regimens produce superior weight loss in comparison to a standard, continuous calorie restriction weight-loss plans” (Peterson 2017). Furthermore, potential interactions of intermittent fasting and other health behaviors, such as diet, sleep, and physical activity have not sufficiently been studies. The same goes for links between intermittent fasting regimens and clinical outcomes, such as diabetes, cardiovascular disease, cancer, or other chronic diseases, such as Alzheimer’s disease.
Figure 1: From baseline, to wk 8 there was a sign. reduction in body fat % (relative 6%, absolute -1.79% | Kressler 2017)
In view of the fact that the scientists’ sensitivity analysis that included only patients with eight cycles revealed no significant differences for the evaluated outcomes, it doesn’t seem as if the failure to adhere to all 8 fasting days in 50% would have ruined the data, though.
The number of adverse events, i.e. 69 vs. 17 in the fasting vs. control group confirms that fasting, especially when your body isn’t accustomed to it, can have side effects.
The most common side effects that could be related to fasting were headache, migraine, nausea, ravenousness, circulatory disturbance, hunger, general feeling of weakness, tiredness, stomach ache, meteorism, heartburn and cold sensations in the body.
The beneficial effects of fasting on (a) total body fat and (b) mood and anxiety scores were small, but measurable and statistically significant… albeit probably without practical relevance.
For me, personally, it is not surprising that an intervention that consisted of only one fasting day per week did not have statistically significant effects on the subjects’ body weight, BMI, glucose management or blood pressure. In fact, I am rather surprised that Kessler et al. found significant treatment effects on total body fat and the results of the HADS mood and anxiety test.
Figure 2: Changes in HADS scores and blood pressure from baseline to wk 8 (V1) and 6-mo follow-up (V2 | Kessler 2017)
Even if you’re a fasting advocate, though, you will probably have to agree with the scientists and admit that “none of the between-group differences were clinically relevant” (Kessler 2017).
The assumption that the mood changes, the authors detected after the follow-up were a result of fasting seems to be very questionable, … to say the least.
This is particularly true in view of the fact that the pronounced improvement in HADS scores occurred after the intervention phase. At the end of the 8-week intervention, the changes in HADS total scores were identical in both groups (-0.82±2.67 for fasting, -1.93±3.34 for control, p = 0.39). The subsequent increase which was supposedly related to the previous fasting could thus have been mere coincidence – or, even worse, a result of the subjects being allowed to finally eat whenever you wanted, again 😉
Similar caution should be exerted with respect to the – on an absolute basis – quantitatively small changes in body fat.
While the reduction in body fat occurred, when you would expect to see it, the absolute changes were small (from 28% to 26%) and despite their statistical significance, potentially an artifact of the measuring method. Especially in view of the fact that similarly relevant changes in waist circumference were not observed, it is furthermore questionable, whether that’s metabolically relevant visceral and belly fat or rather relatively benign appendicular fat the subjects lost.
Safe and feasible, but not really useful?! “Fast once a week and get all the benefits of calorie restriction/fasting?” While you may have previously suspected that this claim must be a scam, you do now have Kessler’s latest study to prove it: there are no clinically relevant benefits of once-a-week fasting in healthy volunteers – at least not a very uncontrolled, but realistic real-world scenario (which does allow for (over-)compensation on non-fasting days) as it was used in the study at hand.
This lack of practically relevant differences in the short run is in line with a recent, more tightly controlled study, I discussed only recently. Said study compared the effects of intermittent (two days per week) and continuous caloric restriction only to find virtually identical weight and fat loss in both groups (learn more).
The overall take-home-message is thus “fasting is feasible and safe”, not “fasting is an easy way to get jacked, healthy and happy”… although I bet you will see this study being misinterpreted in this way on pro-fasting media outlets very soon.
Against that background and in view of the previously outlined “?” we have to put behind the alleged improvements in mood and body fat (see last paragraphs of this article), I would not interpret the study at hand as evidence in favor of weekly one-day juice-/broth-fasting. I mean, if (intermittent) fasting – in which way or form you may use it – promotes weight/fat loss, there’s a 99.9% chance it does so because it modulates your energy balance; and in most of the cases that’s not due to innate effects like the previously described increase in RMR with breakfast skipping & co, but rather by simply reducing your food intake – effects on the circadian rhythm and gut hormones, on the other hand, contribute to the various health benefits of (intermittent) fasting | Comment!
Conley, Marguerite, et al. “Is two days of intermittent energy restriction per week a feasible weight loss approach in obese males? A randomised pilot study.” Nutrition & Dietetics (2017).
Kessler, Christian S., et al. “A Non-Randomized Controlled Clinical Pilot Trial on 8 Weeks of Intermittent Fasting (24h per week).” Nutrition (2017).
Patterson, Ruth E., and Dorothy D. Sears. “Metabolic Effects of Intermittent Fasting.” Annual Review of Nutrition 37.1 (2017).