Does intermittent fasting really work?

Does intermittent fasting really work?
Sarah Pupkes Clark, PharmD, Fairview Pharmacy Services

Background: Calorie restriction is a known method of weight loss but is often difficult to sustain and associated with rebound weight gain. Intermittent fasting (IF) is an alternate mechanism for weight loss in which eating is restricted to defined periods of time. In addition to weight loss, prior studies have shown that IF can lead to improved glucose metabolism, reduced insulin resistance, and even a lengthened lifespan. However, due to the inconsistent findings of previous studies and varied methods of IF, high quality evidence supporting IF is lacking.

Objective: To explore the effects of IF on weight and glucose metabolism in the general population. 

Study Design: This study is a systematic review and meta-analysis in which two investigators selected articles by searching Cochrane, Pubmed, and Embase. Studies were included if they lasted a minimum of 4 weeks. The included population was not pregnant and did not have diabetes mellitus, chronic liver or renal disease. These studies were compared to a control group of studies examining calorie restriction or regular diet. Based on this inclusion criteria, 12 studies (n=545 subjects total) were included in the analysis. The majority of studies included were 8-12 weeks in length, with 3 that were 24 weeks duration. Study participants (61.5% female, 38.5% male) had no differences in BMI or fasting glucose levels at baseline. Statistical analysis was used to determine the weighted mean difference (WMD) between the baseline and post intervention measurements of total body weight, fasting glucose, BMI, fat mass, lean mass, insulin resistance via homeostatic model assessment (HOMA- IR), leptin, and adiponectin. 

Results: Eight of the 12 articles reported BMI results; the BMI was lower in the IF group by 0.75 kg/m2  [95% CI -1.44 to -0.06; P=0.033]. Total body weight was measured in 10 of the 12 studies and was found to be lower in the IF group but was not statistically significant -1.94 kg [95% CI -5.20 to 1.3; P=0.241]. Eight articles tested fasting blood glucose levels, and the IF group had fasting glucose values reduced by a WMD of 4.16 mg/dL [95% CI -6.92 to -1.40; P=0.003] post intervention. An overall reduction in HOMA-IR by 0.54 [95% CI -1.05 to -0.03; P=0.038] in the IF group was reported in six studies. Lean mass and fat mass did not show significant differences after the IF diet intervention. However, adiponectin levels were increased WMD 1008.87 ng/mL [95% CI 140.45 - 1877.29; P=0.023] while leptin levels were decreased WMN -0.51 ng/mL [95% CI -0.77 to -0.24; P<0.001] in the IF group compared with the control group post intervention.

Conclusions: Overall, this meta-analysis did not show significant reduction in weight with an IF diet. However, a decline in BMI and fasting blood glucose was observed along with an improvement in insulin resistance. Prior studies have shown that an IF diet has a greater impact on those with prediabetes than those without; indicating that an IF diet could show more benefits in those with impaired glucose metabolism. It is important to realize the studies included in this meta-analysis were of a short duration, making it difficult to determine the lasting effects of an IF diet. Consider consulting a dietician for their expertise on this topic. 

Key Point: Intermittent fasting does not lead to significant weight loss when compared to both calorie restriction and a regular diet, but it has shown to reduce BMI, fasting blood glucose levels, and insulin resistance.

References: 

  1. Cho Y, Hong N, Kim, K, et al. The effectiveness of intermittent fasting to reduce body mass index and glucose metabolism: a systematic review and meta-analysis. J. Clin. Med. 2019;8(10):E1645.