How Much Fasting Is Necessary to Lose Weight?

The main findings of this study were a 3% weight loss in 8 weeks in both groups with a 550-calorie restriction (unintentional) and good adherence to both diets. Interview with:
Sofia Cienfuegos
PhD Candidate, Human Nutrition.
Department of Kinesiology and Nutrition
University of Illinois at Chicago  What is the background for this study?

Response: Intermittent fasting has been gaining a lot of popularity lately mainly due to a large group of the general population that don’t like to keep tabs on their caloric intake. Intermittent fasting is a safe and effective alternative to weight loss that does not require people consciously restricting calories. Time restricted feeding (TRF) is one specific type of IF that has been gaining a lot of popularity and public interest lately. Previous studies in TRF with the 16/8 method made people want to try and follow this diet strategy achieving promising results. Some people were wondering if shortening the feeding  window even further would induce better results in terms of weight loss and cardiometabolic health.

Based on this question, we decided to test two different short time restricted feeding methods (18/6 and 20/4) to see if they could induce even better results in weight and health outcomes. We were also wondering if people were able to stick to these interventions considering the short eating window. What are the main findings?

Response: The main findings of this study were a 3% weight loss in 8 weeks in both groups with a 550-calorie restriction (unintentional) and good adherence to both diets. Also, we found significant reductions in insulin resistance and oxidative stress in both intervention groups. What should readers take away from your report?

Response: One of the first takeaways is that participants were actually able to stick to both diets. We had no dropouts from our intervention groups, meaning they liked both TRF methods. Second, that 18/6 induces the same results as 20/4 in terms of metabolic health and weight loss. This means that shortening the window any further than 6 hours would not necessarily result in incremental benefits, at least according to our study.

Previous studies with the 16/8 TRF method have shown a 3% weight loss in 12 weeks whereas this one showed 3% in 8 weeks. This means that weight loss appears to be more efficient with 18/6 and 20/4 rather than 16/8. For people with insulin resistance this method would be a good dietary alternative, our study showed results in the reduction of fasting insulin and insulin resistance.

Also, both diets showed an unintentional calorie restriction of 550 calories. This could be particularly useful for health professionals when working with patients that have a hard time adhering to a low-calorie diet. What recommendations do you have for future research as a result of this work?

Response: Future research should focus on the effect of short window TRF (18/6 and 20/4) and metabolic syndrome, PCOS or type 2 diabetes. Also, future research should test the weight loss effects of TRF and low carb, low fat or low glycemic load diets and see which diet pattern could induce better results when combined with time restricted feeding. Is there anything else you would like to add?

Response: It is always important to consult with your primary care physician before starting any fasting regimen. Also consult with your pharmacist how to work your medication around your fasting regimen. People under 18 years of age should not fast as well as women trying to conceive, pregnant or breastfeeding. People with low BMI or history of eating disorders should avoid doing IF. 


Sofia Cienfuegos, Kelsey Gabel, Faiza Kalam, Mark Ezpeleta, Eric Wiseman, Vasiliki Pavlou, Shuhao Lin, Manoela Lima Oliveira, Krista A. Varady. Effects of 4- and 6-h Time-Restricted Feeding on Weight and Cardiometabolic Health: A Randomized Controlled Trial in Adults with Obesity. Cell Metabolism, 2020; DOI: 10.1016/j.cmet.2020.06.018

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