By Martina M. McGrath, MD
July 7, 2017
Alternate day fasting (ADF) is a popular weight loss method, promoted via weight loss books and media. In the most widely promoted form, participants are advised to eat ~25% of their daily caloric needs on fasting days, alternating with unrestricted intake on nonfasting days. It has been suggested that it produces more significant weight loss and greater compliance than standard caloric restriction, as daily compliance is not necessary.
Several short-term studies have shown up to 7% weight loss at three months using this approach, with improvement in parameters such as insulin sensitivity, blood pressure and lipid profile.1,2 However, despite this positive data, no prior randomized controlled trials had directly compared the two weight loss methods for efficacy and patient adherence.
A group from the University of Illinois has reported results of a one-year controlled trial of 100 metabolically healthy obese adults, randomized to alternate day fasting versus daily caloric restriction versus control.3 Their primary outcome was change in body weight at 6 and 12 months; with dietary adherence and changes in risk factors for cardiovascular disease as secondary outcomes. Both intervention groups had all meals provided during the first three months of the trial and received weekly nutrition counseling for the next three months. The control group attended for assessment of weight and metabolic parameters but got no specific intervention. In the first six months, participants in the alternate day fasting group were advised to take 25% of recommended daily calories on fasting days and 125% on nonfasting days. The calorie restriction group were prescribed 75% of recommended calories daily.
From 6 to 12 months, all participants were advised to maintain their weight, with the alternate day fasting group switched to 50% and 150% intake on fasting and nonfasting days, whereas the caloric restriction group was switched to 100% of daily requirements each day. At baseline and six months, total energy expenditure was measured using doubly labeled water to provide accurate assessments of caloric requirements. Participants also completed seven-day food diaries and had their level of physical activity recorded with activity trackers during the study.
There was significant patient dropout across the study. In the alternate day fasting group, 38% of those enrolled dropped out, and around one-third of those were due to dissatisfaction with the diet. In the restricted calorie and control groups, 29% and 26% of participants dropped out respectively. As might be anticipated in a trial with such intensive intervention, large proportions of dropouts in each group were due to scheduling conflicts. On review of prescribed versus actual calorie intake, participants in the alternate day fasting groups ate more than prescribed on fasting days and less than prescribed on nonfasting days. In contrast, those prescribed caloric restriction achieved closer adherence to their prescribed calories.
Weight loss at six months was similar between the two intervention groups; -6% and -5.3% of body weight in the alternate day fasting and caloric restriction groups respectively.3 Similarly, weight regain showed no difference between the groups by 12 months. In terms of secondary outcomes, no consistent differences were seen between the groups for blood pressure, heart rate, plasma lipids, glucose, or markers of insulin resistance.3
How do we interpret the findings? Firstly, it would appear that alternate day fasting is more difficult to adhere to than previously described. There was considerable patient dropout in the fasting group. Indeed, the very detailed approach taken by the authors to estimate actual calorie intake shows that despite being provided with meals and intensive nutritional support, most participants fell into a pattern closer to traditional dieting. Secondly, both weight loss approaches were moderately successful but neither was superior in this analysis. Finally, in terms of assessing the change in risk factors, this was a healthy obese population with baseline blood pressure of ~120s/80s and LDL cholesterol ~110mg/dL. Therefore, the study was likely underpowered to detect significant differences in these parameters. However, given the lack of difference in weight loss, it would appear unlikely that large effects on metabolic parameters would be seen. This appears to be in contrast to other studies of alternate day fasting but there are considerable conflicting data, with varied study populations and designs, making accurate comparisons challenging.4
- Johnson JB, Summer W, Cutler RG, et al. Alternate day calorie restriction improves clinical findings and reduces markers of oxidative stress and inflammation in overweight adults with moderate asthma. Free Radical Biology & Medicine 2007;42:665-74.
- Hoddy KK, Kroeger CM, Trepanowski JF, Barnosky A, Bhutani S, Varady KA. Meal timing during alternate day fasting: Impact on body weight and cardiovascular disease risk in obese adults. Obesity 2014;22:2524-31.
- Trepanowski JF, Kroeger CM, Barnosky A, et al. Effect of Alternate-Day Fasting on Weight Loss, Weight Maintenance, and Cardioprotection Among Metabolically Healthy Obese Adults: A Randomized Clinical Trial. JAMA Internal Medicine 2017.
- Harvie M, Howell A. Potential Benefits and Harms of Intermittent Energy Restriction and Intermittent Fasting Amongst Obese, Overweight and Normal Weight Subjects-A Narrative Review of Human and Animal Evidence. Behavioral Sciences 2017;7.
Dr. Martina McGrath is an Instructor in Medicine at Harvard Medical School, and a member of the Renal Division, Department of Medicine, at Brigham and Women’s Hospital, both in Boston. Dr. McGrath is the Medical Editor for the Trends in Medicine blog.