June 22, 2023

Fueling Facts: Intermittent Fasting – revisited

by Rachel Klomstad, MS, RD

The popularity of intermittent fasting (IF) continues to grow since we last reviewed the topic in our 2020 blog, Intermittent Fasting: What you Need to Know. Increasing numbers of people are incorporating intermittent fasting into their nutrition routine with hopes of gleaning benefits including weight loss, increased metabolism, and increased energy. Animal trials comprised much of the fasting research that supported these outcomes in previous years but now several human studies conducted over the last three years clarifying intermittent fasting’s effects on the body have been published. The primary focus of all IF research historically has been its effect on weight loss; however, many scientists are coming out with data on IF’s influence on a range of health metrics beyond weight. This month’s Fueling Facts highlights emerging IF research and reviews new findings on the effects of IF on body composition, sleep, hormonal regulation, and blood sugar management.


Many human trials have been conducted within the past few years studying intermittent fasting and weight loss. For example, a 2022 randomized controlled trial of 139 obese subjects found no significant difference in weight loss, body fat reduction, or metabolic risk reduction when comparing time-restricted feeding (TRF, a type of IF that involves eating within a specific time window) to daily caloric restriction.1 Multiple human trials conducted in the past three years reinforce these outcomes.2,3 Dr. Krista Varady, renowned intermittent fasting researcher from the University of Illinois, summarizes the bottom line well – “Intermittent fasting produces pretty much the same weight loss and health benefits as daily calorie restriction. Intermittent fasting is another option for people who prefer to watch the clock instead of tracking data in a food record.”4  In other words, if IF generates a calorie deficit and is adopted in the context of a healthy lifestyle (i.e., balanced meals, regular exercise, stress management, adequate sleep, etc.), it can be a successful weight loss tool. This conclusion bolsters the conclusion of our previous blog post, which is that IF can lead to weight loss if the fasting regimen creates a calorie deficit.

Despite the fact that IF may not have a unique impact on body weight, researchers continue to investigate how IF may improve other health outcomes.  


IF may be a helpful approach for improving body composition in a healthy, active population. Trials ranging in duration from eight to fifty-two weeks show notable decreases in body fat with no significant change in fat-free mass in healthy, active males who adopted a hypocaloric IF regimen, participated in regular resistance training, and consumed adequate dietary protein.5,6 It is important to note that in each trial, exercise was performed within the subject’s IF feeding window, allowing the subjects to incorporate snacks or meals as prefuel and refuel. Similarly, another randomized controlled trial found that females participating in resistance training experienced significant body fat loss and muscle hypertrophy after adopting an IF regimen paired with a whey protein supplement for eight weeks.7 These results indicate that healthy males and females can experience fat loss and muscle gain while adopting an IF regimen if resistance training and proper exercise fueling are maintained.


Another secondary outcome that researchers are exploring is sleep and its relationship with time-restricted eating. Some speculate that intermittent fasting may improve sleep via two potential mechanisms: 1) optimized circadian rhythmicity (through restriction of nighttime feedings), and 2) enhanced sleep quality secondary to weight loss.8 However, a 2020 review found the relationship between sleep and IF to be statistically insignificant with respect to sleep quality, duration, efficacy, and latency. Ongoing research examining the interplay between food timing, sleep, and metabolic risk is promising, but lacks the depth of research analysis needed to draw conclusions at this time.8,9,10


Another focus of intermittent fasting research is its potential role in reproductive health. A 2022 review of human trials revealed a decrease in reproductive hormones in females who adopted TRF, which has the potential to improve menstruation and fertility in some individuals.11 In premenopausal females with obesity, multiple studies suggested TRF as a potential therapy for hyperandrogenism in females with polycystic ovarian syndrome (PCOS) since it led to decreased androgen markers and increased sex hormone-binding globulin (SHBG) observed in TRF subjects.11,12 This same 2022 review also cited numerous studies that observed decreased androgens (total testosterone and free testosterone) in active, lean males who adopted an TRF regimen, which is, conversely, an unwanted outcome. This decrease in male sex hormones may have undesirable consequences on metabolic health and libido.11 In each of the four trials reviewed, decreased testosterone, however, resulted in no deleterious body composition change.5,6,13,14


Many question IF’s role in the management of blood sugar, especially in those with prediabetes and diabetes mellitus. A host of research supports IF as effective in reducing body weight, fasting glucose, adiposity, and subsequent insulin resistance as a result of caloric restriction.15 Research investigating benefits of IF for blood sugar control independent of weight loss is limited, however. One study of eight men with prediabetes showed improved insulin levels, insulin sensitivity, beta cell responsiveness, blood pressure, and oxidative stress when subjects adopted an early TRF regimen for five weeks.16 Another 4-day study of eleven overweight adults who adopted an early TRF regimen showed improvements in 24-hour glucose and insulin signaling.17 While further research is required before changes in standard recommendations can be made, existing research supports IF as a safe option for those with prediabetes and diabetes mellitus, unless otherwise noted by your physician.

While there is certainly a correlation between IF and weight loss (as long as the fasting is generating a calorie deficit), scientists continue to explore secondary outcomes. The research will continue to provide us with more insights but, at the present time, our recommendations on intermittent fasting for the general public are unchanged. While intermittent fasting remains safe for most people (excluding those with eating disorders and individuals who are pregnant/breastfeeding), the importance of a balanced lifestyle is necessary to yield the supported benefits. No matter what eating pattern or approach you take, we always recommend keeping the SHIFT Nutrition Pillars in mind. By eating high-quality foods in the right proportions and hydrating at the right times, we aim to create energizing and beneficial effects in the short-term and positive health outcomes in the long-term.

If you are interested in adopting an intermittent fasting regimen, reach out to your SHIFT registered dietitian for more individualized recommendations.

In Real Health,

Rachel & Lauren

SHIFT Registered Dietitians


  1. Liu D, Huang Y, Huang C, et al. Calorie Restriction with or without Time-Restricted Eating in Weight Loss. New England Journal of Medicine. 2022;386(16):1495-1504. doi: https://doi.org/10.1056/nejmoa2114833 
  2. Lowe DA, Wu N, Rohdin-Bibby L, et al. Effects of Time-Restricted Eating on Weight Loss and Other Metabolic Parameters in Women and Men With Overweight and Obesity: The TREAT Randomized Clinical Trial. JAMA Internal Medicine. 2020;180(11). doi: https://doi.org/10.1001/jamainternmed.2020.4153
  3. Varady KA, Cienfuegos S, Ezpeleta M, Gabel K. Clinical application of intermittent fasting for weight loss: progress and future directions. Nature Reviews Endocrinology. 2022;18. doi: https://doi.org/10.1038/s41574-022-00638-x
  4. Dr. Krista Varady—Does Human Research on Intermittent Fasting Support Longevity? blog.insidetracker.com. Accessed June 13, 2023. https://blog.insidetracker.com/longevity-by-design-krista-varady
  5. Moro T, Tinsley G, Bianco A, et al. Effects of eight weeks of time-restricted feeding (16/8) on basal metabolism, maximal strength, body composition, inflammation, and cardiovascular risk factors in resistance-trained males. J Transl Med. 2016;14(1):290. Published 2016 Oct 13. doi:10.1186/s12967-016-1044-0
  6. Moro T, Tinsley G, Longo G, et al. Time-restricted eating effects on performance, immune function, and body composition in elite cyclists: a randomized controlled trial. J Int Soc Sports Nutr. 2020;17(1):65. Published 2020 Dec 11. doi:10.1186/s12970-020-00396-z
  7. Tinsley GM, Moore ML, Graybeal AJ, et al. Time-restricted feeding plus resistance training in active females: a randomized trial. Am J Clin Nutr. 2019;110(3):628-640. doi:10.1093/ajcn/nqz126
  8. McStay M, Gabel K, Cienfuegos S, Ezpeleta M, Lin S, Varady KA. Intermittent Fasting and Sleep: A Review of Human Trials. Nutrients. 2021;13(10):3489. Published 2021 Oct 1. doi:10.3390/nu13103489
  9. Kesztyüs D, Fuchs M, Cermak P, Kesztyüs T. Associations of time-restricted eating with health-related quality of life and sleep in adults: a secondary analysis of two pre-post pilot studies. BMC Nutr. 2020;6(1):76. Published 2020 Dec 17. doi:10.1186/s40795-020-00402-2
  10. Gupta CC, Vincent GE, Coates AM, et al. A Time to Rest, a Time to Dine: Sleep, Time-Restricted Eating, and Cardiometabolic Health. Nutrients. 2022;14(3):420. Published 2022 Jan 18. doi:10.3390/nu14030420
  11. Cienfuegos S, Corapi S, Gabel K, et al. Effect of Intermittent Fasting on Reproductive Hormone Levels in Females and Males: A Review of Human Trials. Nutrients. 2022;14(11):2343. Published 2022 Jun 3. doi:10.3390/nu14112343
  12. Feyzioglu BS, Güven CM, Avul Z. Eight-Hour Time-Restricted Feeding: A Strong Candidate Diet Protocol for First-Line Therapy in Polycystic Ovary Syndrome. Nutrients. 2023;15(10):2260. Published 2023 May 10. doi:10.3390/nu15102260
  13. Stratton MT, Tinsley GM, Alesi MG, et al. Four Weeks of Time-Restricted Feeding Combined with Resistance Training Does Not Differentially Influence Measures of Body Composition, Muscle Performance, Resting Energy Expenditure, and Blood Biomarkers. Nutrients. 2020;12(4):1126. Published 2020 Apr 17. doi:10.3390/nu12041126
  14. Moro T, Tinsley G, Longo G, et al. Time-restricted eating effects on performance, immune function, and body composition in elite cyclists: a randomized controlled trial. J Int Soc Sports Nutr. 2020;17(1):65. Published 2020 Dec 11. doi:10.1186/s12970-020-00396-z
  15. Albosta M, Bakke J. Intermittent fasting: is there a role in the treatment of diabetes? A review of the literature and guide for primary care physicians. Clin Diabetes Endocrinol. 2021;7(1):3. Published 2021 Feb 3. doi:10.1186/s40842-020-00116-1
  16. Sutton EF, Beyl R, Early KS, Cefalu WT, Ravussin E, Peterson CM. Early Time-Restricted Feeding Improves Insulin Sensitivity, Blood Pressure, and Oxidative Stress Even without Weight Loss in Men with Prediabetes. Cell Metab. 2018;27(6):1212-1221.e3. doi:10.1016/j.cmet.2018.04.010
  17. Jamshed H, Beyl RA, Della Manna DL, Yang ES, Ravussin E, Peterson CM. Early Time-Restricted Feeding Improves 24-Hour Glucose Levels and Affects Markers of the Circadian Clock, Aging, and Autophagy in Humans. Nutrients. 2019;11(6):1234. Published 2019 May 30. doi:10.3390/nu11061234


body composition, eating window, fasting, health, healthy eating, intermittent fasting, nutrition, time restricted feeding, weight loss

You may also like

{"email":"Email address invalid","url":"Website address invalid","required":"Required field missing"}