What is intermittent fasting?
Intermittent fasting focuses on when you eat rather than what. Common approaches include 16:8 (eating within an 8-hour window), 5:2 (about 500–600 calories on two days a week) and alternate-day fasting. The theory is that longer fasts trigger a "metabolic switch" from glucose to fat-derived ketones (de Cabo & Mattson, 2019).
Does intermittent fasting work?
Most trials find weight loss similar to ordinary calorie restriction. In the 12-week TREAT trial of 16:8 eating, weight loss didn't differ meaningfully from the control group, and participants lost some lean mass (Lowe et al., 2020). It works when it helps you eat less overall. Some research suggests eating earlier in the day, in step with your circadian rhythm, may help more.
Side effects and who should be careful
Hunger, headaches and irritability are common early on, and overeating during the eating window cancels out the benefit. Eat enough protein and strength-train to protect muscle. People with diabetes on insulin or sulfonylureas risk low blood sugar, and pregnant women, teens and anyone with an eating-disorder history should avoid it unless a clinician advises otherwise.
This article is general health information and is not a substitute for diagnosis or treatment. If symptoms persist, see a doctor.
FAQ
- Can I drink coffee while fasting?
- Black coffee, water and unsweetened tea are generally fine and won't break a fast in a meaningful way.
- Will fasting slow my metabolism?
- Any significant weight loss can lower calorie burn slightly. Protein and resistance training help preserve metabolic rate.
Sources reviewed: 2026-10-25
References
- de Cabo, R., & Mattson, M. P. (2019). Effects of intermittent fasting on health, aging, and disease. New England Journal of Medicine, 381(26), 2541–2551.
- Lowe, D. A., Wu, N., Rohdin-Bibby, L., et al. (2020). Effects of time-restricted eating on weight loss and other metabolic parameters: The TREAT randomized clinical trial. JAMA Internal Medicine, 180(11), 1491–1499.