Letter to the Editor from Varady et al.: “Metabolic Impact of Intermittent Fasting in Patients With Type 2 Diabetes Mellitus: A Systematic Review and Meta-analysis of Interventional Studies”
Year: 2021
Type: letter
Abstract: We have read with great interest the systematic review and meta-analysis by Borgundvaag et al. (1) examining the metabolic impact of intermittent fasting in patients with type 2 diabetes. We thank the authors for raising awareness of the possible beneficial effects of these regimens in helping to manage type 2 diabetes. However, there are several critical limitations to this meta-analysis that must be clarified so that the findings are not overinterpreted. First, the meta-analysis mischaracterizes what constitutes as intermittent fasting. The key forms of intermittent fasting have been defined in 2 recent reviews (2, 3). To clarify, intermittent fasting is an umbrella term for 3 distinct diets: (1) alternate day fasting (0-500 kcal fast day alternated with an ad libitum feast day); (2) the 5:2 diet (2 0-500 kcal fast days per week, and 5 ad libitum feast days per week); and (3) time-restricted eating (limiting the eating window from 4 to 12 hours per day). The meta-analysis by Borgundvaag et al. (1) is severely flawed in that it included studies of very low-calorie diets. Very low-calorie diets are different from intermittent fasting in that they involve consuming ~400 to 800 kcal each day, and do not prescribe periods of intentional fasting. It would appear as though the authors included these very low-calorie diet studies because, to date, only 2 intermittent fasting trials have been conducted in patients with type 2 diabetes (4, 5). Including the data from these very low-calorie diet trials to increase the overall sample size of their meta-analysis is highly problematic because it misinterprets the current findings in this field. It is our opinion that the authors should have held off on writing this review until more trials, which actually implement intermittent fasting as an intervention to prevent or treat type 2 diabetes, have been performed. A second major limitation is the comparator group used in the meta-analysis. In the study selection paragraph, it states that studies were included if they “compared any intermittent fasting intervention to a standard diet consisting of either a healthy pattern dietary recommendation with caloric deficit or normal caloric intake (control group).” As such, the comparator group used was an unfortunate blend of subjects undergoing intentional calorie restriction and subjects eating their normal caloric needs each day (controls). It is well known that interventions that prescribe a caloric deficit generally result in weight loss, whereas interventions that allow usual ad libitum intake result in weight maintenance. As such, it is inappropriate to combine a calorie restriction (weight loss) group with a control (weight maintenance) group and use it as a comparator to intermittent fasting. This is particularly unadvisable considering that 1 of the key outcomes measured in this study was weight loss. Including the control subjects in the comparator group dampens the weight loss effects observed, which renders the data reported in this section of the paper flawed and uninterpretable. We suggest that these issues be clarified to readers in general, but particularly, to clinicians wishing to prescribe intermittent fasting to their patients with type 2 diabetes. Although it is true that some pilot evidence exists to show that fasting may help to treat and manage type 2 diabetes, there is simply not enough clinical trial data to perform a meta-analysis on this topic at present. Disclosure: K.V. received author fees from Hachette Book Group for the book, The Every Other Day Diet. The other authors have no conflicts of interest to disclose.
Institution: University of Illinois Chicago
Language: English
Topic: Dietary Effects on Health
Subfield: Physiology (medical)
Field: Medicine
Domain: Health Sciences
Open Access status: bronze
OpenAlex ID: W3158272067