In this intermittent fasting dietitian summary, we share the basics of intermittent fasting and current evidence, so you can support your clients with navigating the evidence individually.
Are you a registered dietitian (RD) wanting to learn more about Intermittent Fasting and how to support your clients with their nutrition goals?
This content is based on DSC’s Intermittent Fasting dietitian evidence summary. Learn more about Intermittent Fasting, build your expertise, and get access to ready-to-use resources including our Intermittent Fasting dietitian SOAP note summary & Intermittent Fasting Pros & Cons handout.
Created with support from Olivia Farrow, RD, MHSc.
Medically reviewed by Krista Kolodziejzyk, RD, MPH, MBA.
What is Intermittent Fasting (IF)?
Intermittent fasting is a diet approach involving periods of caloric restriction (fasting) followed by periods of eating. Often there is no restriction on food or calories during eating periods (1).
Why are People Trying Intermittent Fasting?
IF is a popular diet approach with publications, books, blogs, news articles, and health gurus discussing this topic (2). IF is promoted widely on social media, mostly based on anecdotal evidence (3).
Most people trying IF are doing it for the purported weight loss effects. As discussed in the literature, this “diet” may seem easier for individuals who struggle (or have found failure in the past) with continued caloric restriction because IF does not tell people what to eat but when to eat, which may be more approachable for some (4).
Methods of Intermittent Fasting
There are several approaches to IF; common methods include:
- Alternate-Day Fasting & Alternate-Day Modified Fasting
Alternate Day Fasting involves alternating eating days and fasting days. Alternate Day Modified Fasting also alternates eating days but allows some food on fasting days (usually, 25% of daily calories are allowed).
- Time-Restricted Feeding
Time-Restricted Feeding involves a consistent schedule where there are eating periods and fasting periods each day.
- Whole-Day Fasting
Whole-Day Fasting involves 1-2 days of fasting with unrestricted eating days for the rest of the week. Fasting days might be consecutive or not, depending on the program.
Intermittent Fasting: Dietitian Evidence Summary
The most robust area of research on IF is in the area of weight loss. Studies examining IF typically compare various IF approaches with “continued energy restriction” (CER) (ie. the typical weight loss diet where there is some form of calorie restriction).
Current evidence suggests:
- IF has similar weight loss effects as CER in adults with “normal”, “overweight”, and “obese” BMI in the short to medium term (less than 12 months studied).
- IF may promote weight loss better in the short-term than no dietary treatment.
- The best evidence on weight improvement with IF is in the Alternate Day Modified Fasting and Whole Day Fasting (2 fasting days) regimes.
Cardiovascular Risk Factors
- Some studies have shown improvements in cardiovascular risk factors with IF including blood lipids, triglycerides and blood pressure (1,6,10,12).
- There have not been any significant differences in cardiovascular risk factor improvement in IF compared with CER.
- It is also unclear whether these improvements are just related to weight loss and whether there is any long-term improvement (7).
- There haven’t been any clinically meaningful improvements in blood glucose or HbA1c seen with IF in the literature.
- Any moderate improvements are not significantly different from those seen with CER.
IF may reduce BMI, waist circumference, and fat mass (1,7,10,11,12) though it may also result in a short-term greater loss of fat-free mass (7,11).
Other Health Parameters
IF is often touted to improve brain health, longevity, inflammation and the gut microbiome. There are also claims that it aids in cancer prevention and enhances cancer treatment.
This has yet to be shown in randomized controlled trials of humans and most of the studies that exist in these areas are in animals (14).
All of these areas represent an exciting opportunity for future research.
Potential Drawbacks to Intermittent Fasting
Common complaints that tend to be higher with an IF regime than with traditional dieting (CER), include:
- Hunger during fasting periods
- Loss of concentration
- Decreased exercise level during fasting periods
- Sleep disturbances
- Feeling cold
The majority of IF regimes do not give recommendations on what to eat, only when to eat; thus there can be a lot of variation in the nutritional quality of the foods individuals choose during eating periods (1).
Intermittent Fasting Dietitian Key Takeaways
- There are various forms of IF including fasting on alternate days, fasting for 1-2 days a week, and fasting during certain times of the day.
- IF typically does not restrict the foods eaten during the eating periods.
- IF can result in small amounts of weight loss in the short term, this weight loss is comparable to a calorie-restricted diet.
- There may be improvements in glucose levels, blood lipids, and blood pressure, these improvements are often not clinically significant and are comparable to a calorie-restricted diet.
- Several non-severe side effects and contraindications associated with IF should be discussed with clients before trialing intermittent fasting.
- Opportunity exists for dietitians to support clients in improving their diet quality alongside IF if they choose to try IF.
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1. Grant M. Tinsley, Paul M. La Bounty, Effects of intermittent fasting on body composition and clinical health markers in humans, Nutrition Reviews, Volume 73, Issue 10, October 2015, Pages 661–674
2. Patterson RE, Sears DD. Metabolic Effects of Intermittent Fasting. Annu Rev Nutr. 2017 Aug 21;37:371-393. doi: 10.1146/annurev-nutr-071816-064634. Epub 2017 Jul 17. PMID: 28715993.
3. Halpern B, Mendes TB. Intermittent fasting for obesity and related disorders: unveiling myths, facts, and presumptions. Arch Endocrinol Metab. 2021 Jan 14:2359-3997000000322. Abstract available from: https://pubmed.ncbi.nlm.nih.gov/33444495/
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7. Cioffi, I., Evangelista, A., Ponzo, V., Ciccone, G., Soldati, L., Santarpia, L., Contaldo, F., Pasanisi, F., Ghigo, E., & Bo, S. (2018). Intermittent versus continuous energy restriction on weight loss and cardiometabolic outcomes: a systematic review and meta-analysis of randomized controlled trials. Journal of Translational Medicine, 16, 371
8. Harris, L., Hamilton, S., Azevedo, L. B., Olajide, J., De Brun, C., Waller, G., Whittaker, V., Sharp, T., Lean, M., Hankey, C., & Ells, L. (2018). Intermittent fasting interventions for treatment of overweight and obesity in adults: a systematic review and meta-analysis. JBI Database System Rev Implement Rep, 16(2), 507-547.
9. Cioffi I, Evangelista A, Ponzo V, Ciccone G, Soldati L, Santarpia L, Contaldo F, Pasanisi F, Ghigo E, Bo S. Intermittent versus continuous energy restriction on weight loss and cardiometabolic outcomes: a systematic review and meta-analysis of randomized controlled trials. J Transl Med. 2018 Dec 24;16(1):371. doi: 10.1186/s12967-018-1748-4. PMID: 30583725; PMCID: PMC6304782.
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11. He S, Wang J, Zhang J, Xu J. Intermittent Versus Continuous Energy Restriction for Weight Loss and Metabolic Improvement: A Meta-Analysis and Systematic Review. Obesity (Silver Spring). 2021 Jan;29(1):108-115. doi: 10.1002/oby.23023. PMID: 34494373.
12. Meng, H., Zhu, L., Kord-Varkaneh, H., Santos, H. O., Tinsley, G.M., & Fu, Peng. (2020).Effects of intermittent fasting and energy-restricted diets on lipid profile: A systematic review and meta-analysis. Nutrition, 77, 110801.
13. Templeman I, Smith HA, Chowdhury E, Chen YC, Carroll H, Johnson-Bonson D, Hengist A, Smith R, Creighton J, Clayton D, Varley I, Karagounis LG, Wilhelmsen A, Tsintzas K, Reeves S, Walhin JP, Gonzalez JT, Thompson D, Betts JA. A randomized controlled trial to isolate the effects of fasting and energy restriction on weight loss and metabolic health in lean adults. Sci Transl Med. 2021 Jun 16;13(598):eabd8034. doi: 10.1126/scitranslmed.abd8034. PMID: 34135111.
14. Stockman, Mary-Catherine et al. “Intermittent Fasting: Is the Wait Worth the Weight?.” Current obesity reports vol. 7,2 (2018): 172-185. doi:10.1007/s13679-018-0308-9