Intermittent fasting and health: side effects, precautions and special cases
Intermittent fasting is safe for most healthy adults, and its usual complaints (headache, lightheadedness, constipation, irritability) have known causes and simple fixes. But it isn't for everyone, and not in any form: pregnancy, eating disorders, medicated diabetes or being underweight all call for medical supervision.
This guide covers the side effects and how to head them off, then the situations people ask about most: thyroid, insulin resistance, women and the menstrual cycle, being over 50, sleep and constipation.
The essentials in 5 points
- Headaches and lightheadedness are almost always a lack of salt and water: a pinch of salt or electrolytes fixes it without breaking your fast.
- You can fast with hypothyroidism, but levothyroxine goes on an empty stomach, 30-60 minutes apart from your coffee.
- With insulin resistance or prediabetes, an earlier eating window has improved insulin sensitivity in trials; if you take medication, check with your doctor first.
- Women: 12-14 hour windows often sit better. If your cycle changes or food starts causing anxiety, shorten the fasting window.
- After 50, the priority is protein (0.55-0.7 g per pound, 1.2-1.6 g/kg) and strength training so you don't lose muscle.
Reading path
The articles on this topic in the order that makes sense. Each one answers a specific question.
Headaches, dizziness, irritability the first few days… almost all of it has a fix. What is normal, what is not, and how to feel better.
Read · 6 min →Intermittent fasting can cause constipation indirectly: you eat less volume and less fiber, drink less water, and get fewer digestive cues across the day. It usually resolves by increasing water, fiber (vegetables, fruit, beans, seeds) and physical activity; black coffee and magnesium also help move things along.
Read · 5 min →If you have hypothyroidism or other thyroid issues, fasting takes more care. What to watch for and when to check in with your doctor.
Read · 5 min →Insulin resistance is behind a lot of stubborn weight and prediabetes. How fasting may help improve your sensitivity.
Read · 6 min →Bad sleep sabotages your fast, and late dinners sabotage your sleep. How to line both up so they work together.
Read · 5 min →Fasting is controlled stress. In some people it pushes cortisol up. How to tell if that is happening to you, and what to do about it.
Read · 5 min →Metabolism, muscle and hormones all shift with age. Fasting still works, but it needs a few adjustments.
Read · 6 min →Women can do intermittent fasting: in 8-week trials using a 4-6 hour window, pre- and postmenopausal women lost 3-4% of their body weight with no changes in estrogen, testosterone, or SHBG. Signals of hormonal disruption come mostly from aggressive protocols (alternate-day fasting, steep deficits), so it's best to start with gentle windows (12/12 to 14/10) and keep an eye on your cycle.
Read · 7 min →Fasting lowers insulin and your kidneys flush out more sodium and water (the natriuresis of fasting), which explains much of the headache, dizziness and fatigue of the first hours and days. Sodium, potassium and magnesium without sugar have no calories and do not break your fast; on fasts over 18-24 hours, or with heavy sweating, a pinch of salt in your water is usually enough.
Read · 5 min →What the science says
- Dietary fiber increased stool frequency versus placebo (OR 1.19) in a meta-analysis of 5 trials (Yang 2012). [1]
- 8 weeks on a 4-6 h window: -3% body weight in premenopausal and -4% in postmenopausal women, with no change in testosterone, SHBG or estrogen (Cienfuegos 2023). [2]
- Insulin drives sodium reabsorption in the kidney; its fall explains the natriuresis of fasting and sodium retention on refeeding (DeFronzo 1981). [3]
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Frequently asked questions
Who shouldn't do intermittent fasting?
Does intermittent fasting cause constipation?
Can I do intermittent fasting with type 2 diabetes?
Does fasting affect sleep?
Sources cited on this page (3)
- Effect of dietary fiber on constipation: A meta analysis. World Journal of Gastroenterology, 2012.
- Effect of time‐restricted eating on sex hormone levels in premenopausal and postmenopausal females. Obesity, 2023.
- The effect of insulin on renal sodium metabolism. Diabetologia, 1981.