Strictly speaking, any amount of calories will break a fast because consuming calories provides energy to the body, ending the fasting state
. There is no universally agreed-upon calorie threshold that allows you to remain fully fasted, as individual metabolism and fasting goals vary. However, some experts and fasting protocols allow for a modified fast where a small number of calories are consumed without fully negating some fasting benefits. Common guidelines include:
- Staying under about 50 calories during fasting periods to maintain some fasting benefits, though this is not scientifically definitive
- Some modified fasting diets permit up to 30% of daily calorie needs on fasting days (e.g., about 500-600 calories for a 2,000-calorie diet), but this is more of a calorie restriction than a true fast
- The type of calories matters: carbohydrates can quickly break ketosis and fasting benefits, while fats and certain low-glycemic foods may be less disruptive
In practice, for strict intermittent fasting aiming for maximal benefits like autophagy and ketosis, it is recommended to consume zero calories during fasting windows, sticking to water, black coffee, or unsweetened herbal teas
Summary
- Any calories technically break a fast.
- Under 50 calories may be tolerated in some modified fasting approaches.
- Zero calories is best for full fasting benefits.
- Carbohydrates break fasting effects more than fats.
- Individual responses vary, so personal goals and metabolism influence what "breaks" a fast.
This means even a small snack or a few calories in a drink can end your fast, but some people use minimal calories strategically depending on their fasting style and goals
