Yes, constipation is one of the more common side effects when you first start intermittent fasting — but it is almost always temporary and fixable. The main causes are eating less food volume overall (less bulk moving through the gut), changes in gut bacteria, dietary shifts, and in many cases simply not drinking enough water.
When you shorten your eating window, you typically eat less total volume than before. Less food means less bulk in the digestive tract — and less bulk means fewer triggers for a bowel movement. This single factor explains most fasting-related constipation.
Beyond volume, several other changes can slow things down during the adjustment period:
Reduced gut motility. The movement of food through the intestines (peristalsis) is partly triggered by eating. When you eat less frequently, peristalsis slows. This is a normal physiological response, not a sign that something is wrong — but it can feel uncomfortable, especially if you were previously eating five or six times a day.
Dietary changes. Many people change what they eat at the same time they start fasting — cutting carbs, removing processed foods, reducing or eliminating grains. The gut bacteria that thrived on your old diet need time to adjust to the new one. During that transition, digestion can slow and constipation is common.
Dehydration. Stool consistency is largely determined by how much water it contains. When insulin drops during fasting, the kidneys excrete more sodium — and water follows. If you're not actively replacing this through drinking, your colon will extract more water from waste as compensation, making stools harder and more difficult to pass.
Electrolyte shifts. Lower insulin means your body excretes more sodium, potassium, and magnesium. These minerals play a role in smooth muscle function throughout the gut. If electrolytes fall significantly, gut motility can slow further.
1. Drink significantly more water. This is the most immediate fix. Aim for at least 2–2.5 litres of water per day, ideally spread throughout your fasting window. Adding a small pinch of sea salt to water supports electrolyte balance and helps retain hydration in tissues.
2. Add fermented vegetables to your meals. Kimchi and sauerkraut are particularly effective. They introduce beneficial bacteria that improve gut motility and support regular bowel function. Start with two to three tablespoons per day alongside your main meal.
3. Eat more leafy greens and non-starchy vegetables. Spinach, rocket, courgette, broccoli, and celery provide fibre that bulks up stool without spiking blood sugar. These are fully compatible with intermittent fasting and actively support regular digestion.
4. Make sure you're eating enough. Some people accidentally undereat when they shorten their eating window. If your body is significantly under-fuelled, it will slow gut motility as part of its conservation response. Eat substantial meals when your window is open.
5. Walk after meals. Light walking — even 15–20 minutes after eating — stimulates bowel motility reliably. It's one of the simplest and most effective tools for resolving fasting-related constipation.
6. Give it time. For most people, constipation during the first weeks of fasting resolves on its own within two to three weeks as the gut adapts to the new eating rhythm. If it persists beyond four weeks despite the adjustments above, consider consulting a healthcare provider.
Want to learn more? Read our full article: Does Intermittent Fasting Cause Constipation?
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This is for informational purposes only and is not medical advice.