Fasting can cause fatigue in the short term but often leads to increased energy levels over time as the body adapts.
When you first start fasting, your body relies on glucose from food for quick energy. Without eating, glucose stores drop after about 10 to 12 hours, which can make you feel tired, lightheaded, or moody. This happens because insulin levels fall sharply, and your body has not yet switched to other fuels. Experts note these side effects like headaches usually fade as you get used to it. For more details, see https://hcafarwest.com/blog/entry/intermittent-fasting-what-really-happens-to-your-body[3] and https://www.massgeneralbrigham.org/en/about/newsroom/articles/pros-and-cons-of-intermittent-fasting[4].
After this initial phase, fasting triggers a metabolic switch. Your body starts burning stored fat for energy through fatty acids and ketones. This shift, often around day three in longer fasts, creates a hypometabolic state that conserves energy and boosts fat use. Studies show this can improve metabolic flexibility, reduce inflammation, and even enhance comfort for daily activities. Check https://pmc.ncbi.nlm.nih.gov/articles/PMC12736288/[1] and https://www.massgeneralbrigham.org/en/about/newsroom/articles/pros-and-cons-of-intermittent-fasting[4].
In trials with fasting-mimicking diets, people lost weight, saw lower blood sugar, and had better markers like reduced IGF-1, which links to aging. Ketosis from fasting burns fat efficiently, leading to reports of higher energy once adapted. Animal studies back this with better brain function and less fatigue. See findings from https://www.science.org/content/article/five-day-fasting-diet-could-fight-disease-slow-aging[2] and https://primaryimmune.org/about?id=k_intermittent-fasting-for-weight-loss[7].
Longer fasts under medical watch show drops in blood pressure and inflammation, aiding overall vitality. Intermittent fasting also improves insulin sensitivity and glucose control, which supports steady energy. Stay hydrated to avoid dehydration-related tiredness. Learn more at https://www.frontiersin.org/journals/nutrition/articles/10.3389/fnut.2025.1664811/full[5].
Sources
https://pmc.ncbi.nlm.nih.gov/articles/PMC12736288/
https://www.science.org/content/article/five-day-fasting-diet-could-fight-disease-slow-aging
https://hcafarwest.com/blog/entry/intermittent-fasting-what-really-happens-to-your-body
https://www.massgeneralbrigham.org/en/about/newsroom/articles/pros-and-cons-of-intermittent-fasting
https://www.frontiersin.org/journals/nutrition/articles/10.3389/fnut.2025.1664811/full
https://www.news-medical.net/news/20251217/Intermittent-fasting-edges-daily-calorie-cuts-for-blood-pressure-and-long-term-cardiovascular-risk-estimates.aspx
https://primaryimmune.org/about?id=k_intermittent-fasting-for-weight-loss
