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Evidence on Intermittent Fasting: Reality Check

Evidence on Intermittent Fasting: Reality Check

by Andrea Chahoud in News

Intermittent fasting (IF) is a dietary approach involving fasting and eating periods. Unlike traditional diets that focus on what you eat, IF focuses on when you eat. The idea behind intermittent fasting is to restrict calorie intake during specific periods of the day or week while allowing for unrestricted eating during other periods. There are several types of intermittent fasting, including periodic fasting, alternate-day fasting, and time-restricted eating.

Types of Intermittent Fasting

Periodic fasting, also known as periodic calorie restriction, is a type of intermittent fasting that involves fasting for one or more days a week. For example, you might fast for two days a week and eat normally for the other five days. Alternate-day fasting is a type of intermittent fasting that involves fasting every other day. On fasting days, you restrict your calorie intake to 500-600 calories; on non-fasting days, you eat normally. This type of fasting is more challenging than periodic fasting because it involves fasting every other day. Time-restricted eating is intermittent fasting that restricts your eating window to a certain number of hours daily. For example, you might fast for 16 hours and eat within an eight-hour window. This approach is easier to implement than periodic fasting or alternate-day fasting because you don’t have to go without food for extended periods.

Effectiveness of Intermittent Fasting

Despite the increasing popularity of IF, the evidence regarding its effectiveness in promoting weight loss is conflicting. I will review the evidence from various meta-analyses. Meta-analysis is an invaluable tool to draw high-level conclusions from multiple studies centered on the same topic or research question. It is a powerful tool for providing a more accurate view of a subject or treatment. It allows researchers to combine data from multiple studies, eliminating the risk of any one study skewing the results. A review of the various meta-analyses (umbrella of meta-analyses) has found at least 28 associations between IF on BMI, body weight, fat mass, LDL-C (bad cholesterol) total cholesterol, triglycerides, fasting plasma glucose, fasting insulin, HOMA-IR (a marker of insulin resistance), and systolic and diastolic blood pressure, mostly in adults with overweight or obesity. However, most of those associations were of very low to low evidence. Only one association was supported by high-quality evidence, the effect of modified alternate-day fasting for 1-2 months on weight loss and fatty liver compared to a regular diet.  IF was most successful for weight loss in the initial 1 to 6 months, followed by a plateau. A recent meta-analysis found that IF diets were not associated with a significantly different body weight at different durations ranging from 2-4 months, 6-8 months, 10-14 months, and 18-30 months compared to calorie-restricted diets. The effectiveness of IF in promoting weight loss and improving overall health is still under debate.

Fasting or Calories Restriction

As IF involves periods of fasting, this naturally leads to a reduction in calorie intake. Whether the effects of intermittent fasting (IF) on weight loss and health benefits are primarily due to the fasting process or the calorie restriction involved is a subject of ongoing debate and research. Although intermittent fasting may have unique metabolic and hormonal effects that go beyond simple calorie reduction based on timing (morning vs evening fasting) and pattern of fasting (alternate day vs. time restricted). However, those changes were not translated into better health outcomes or weight loss. Further research is needed to unravel the underlying mechanisms and determine the specific contributions of each factor. Some individuals may find it easier to adhere to an IF eating pattern and experience positive outcomes, while others may struggle with the restricted eating window given the social implications of eating time and symptoms of fatigue and headache during the fasting periods. Individuals should be mindful of their eating patterns during the normal eating periods. One can still consume excessive calories within the eating window, defeating IF’s purpose.  Finding a balanced and sustainable approach to weight loss that aligns with one’s lifestyle and preferences is crucial for successful weight management and overall health.

References

Patikorn C, Roubal K, Veettil SK, et al. Intermittent Fasting and Obesity-Related Health Outcomes: An Umbrella Review of Meta-analyses of Randomized Clinical Trials. JAMA Netw Open. 2021;4(12):e2139558. doi:10.1001/jamanetworkopen.2021.39558

Harris, Leanne1; Hamilton, Sharon2,3; Azevedo, Liane B.2,3; Olajide, Joan2,3; De Brún, Caroline2,3; Waller, Gillian2,3; Whittaker, Vicki2,3; Sharp, Tracey4; Lean, Mike1; Hankey, Catherine1,∗; Ells, Louisa1,3,∗. Intermittent fasting interventions for treatment of overweight and obesity in adults: a systematic review and meta-analysis. JBI Database of Systematic Reviews and Implementation Reports 16(2):p 507-547, February 2018. | DOI: 10.11124/JBISRIR-2016-003248

Gu L, Fu R, Hong J, Ni H, Yu K, Lou H. Effects of Intermittent Fasting in Human Compared to a Non-intervention Diet and Caloric Restriction: A Meta-Analysis of Randomized Controlled Trials. Front Nutr. 2022 May 2;9:871682. doi: 10.3389/fnut.2022.871682. PMID: 35586738; PMCID: PMC9108547.

Elortegui Pascual, P., Rolands, M.R., Eldridge, A.L., Kassis, A., Mainardi, F., Lê, K.A., Karagounis, L.G., Gut, P. and Varady, K.A., 2023. A meta‐analysis comparing the effectiveness of alternate day fasting, the 5: 2 diet, and time‐restricted eating for weight loss. Obesity, 31, pp.9-21. https://doi.org/10.1002/oby.23568

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