Written by: Kelley R. Hill, MSN, RN-BC
In a Nutshell
- Intermittent fasting focuses on when you eat
- Popularly used for weight loss, it may also have other health benefits
- Many fasting options are available and should be individualized
Intermittent fasting is the practice of restricting eating to within a specific timeframe and has surged recently in popularity. While it appears to promote weight loss and may improve metabolic health, science has yet to definitively conclude that intermittent fasting is both sustainable and beneficial.
Intermittent Fasting is based in the science of our circadian rhythm, which helps regulate metabolism – the rate at which the body burns calories. This is why when we eat may make a difference. Planned periods of time where we intentionally do not eat also prevents grazing and snacking which can sabotage many weight loss plans.
Weight loss associated with intermittent fasting has been attributed to a number of factors – increased calorie burn, decreased appetite, decreased consumption, smarter food choices, behavior changes, and commitment to improved overall lifestyle (such as including regular exercise). However, none of the theories have been proven.
Scientific & Expert Support
Intermittent fasting, in which eating is restricted to a specific window of time, is popular but not yet proven as an effective weight loss strategy.
JAMA found in its largest trial to date, no evidence that intermittent fasting works as a weight loss strategy. Many experts still assert that, with regard to weight loss, what matters is the quality and quantity of food you are eating, not when you eat it. The most common weight loss approaches emphasize moderate daily caloric restriction as a reliable and safe first option. In other words, simply consume slightly less calories every day.
Experts do agree that restricting meals to earlier in the day at minimum improves gut health and sleep.
The long-standing, widely recognized and supported position that a minimum of 1,200 calories per day is required to maintain basic metabolic function and general health. Reducing your daily caloric intake by 500 calories – either through eating less or exercising more – generally results in safe weight loss of 1 lb. per week (0.45 kg). You should not expect to lose weight more rapidly than this.
Intermittent Fasting may be beneficial in lowering blood pressure, improving insulin sensitivity, reducing cholesterol, improving mental state, reducing belly fat, reducing inflammation, and improving cellular repair. The evidence for many of these claims, however, has not reached a definitive level.
Common Approaches to Intermittent Fasting
If you’re considering time-based fasting, there are a number of approaches available from beginner to advanced. Each form encourages no-calorie beverages during the fasting period such as water, black coffee, and other zero-calorie drinks for adequate hydration and to help control hunger. No plans promote the consumption of excessive fast foods and junk foods during the eating window.
Most studies focused on potential benefits of fasting have inconsistencies and have been limited to a small number of participants over a short period of time. Larger studies over longer periods of time are needed to achieve relatively certainty in the results.
A good option for those who cannot fast is a transition to cleaner eating. Strenuous exercise should be avoided during fasting periods.
STRENGTH OF EVIDENCE: B
The available evidence on the efficacy of intermittent fasting is based on mostly on small trials and industry-funded research, and popular supporters. The TREAT randomized clinical trial was the largest study to date and the results did not show that intermittent fasting was more effective that eating the same amount of calories throughout the day.
While intermittent fasting has many famous fans and influencers who swear by it, the evidence just doesn’t support its use.
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- The TREAT Randomized Clinical Trial
- The Effectiveness of Intermittent Fasting to Reduce Body Mass Index and Glucose Metabolism: A Systematic Review and Meta-Analysis - PubMed (nih.gov)
- Effects of Intermittent Fasting on Health, Aging, and Disease | NEJM
- Harvard University – Harvard Medical School. (2020). Special health report – The diet review. Cambridge, MA: Harvard Health Publishing.
- Intermittent fasting: What questions should we be asking? - ScienceDirect
- Metabolic Effects of Intermittent Fasting | Annual Review of Nutrition (annualreviews.org)
- Effects of intermittent and continuous calorie restriction on body weight and metabolism over 50 wk: a randomized controlled trial | Request PDF (researchgate.net)
- Impacts of Intermittent Fasting on Energy Balance and Associated Health Outcomes - Full Text View - ClinicalTrials.gov
- The role of intermittent fasting and meal timing in weight management and metabolic health - PubMed (nih.gov)
- Ten-Hour Time-Restricted Eating Reduces Weight, Blood Pressure, and Atherogenic Lipids in Patients with Metabolic Syndrome - PubMed (nih.gov)