MCT Oil

MCT oil breaks a fast. It is pure fat at around 115 calories a tablespoon, so like any caloric food it ends a strict fast the moment you take it.

What makes MCT oil interesting is that it breaks a fast and supports ketosis at the same time, which sounds contradictory until you separate the two questions. Medium-chain triglycerides are absorbed quickly and converted to ketones in the liver, so they raise blood ketone levels rather than lowering them, and because MCT oil contains no protein or carbohydrate, it raises insulin very little on its own. That is the reverse of protein sources like bone broth or collagen: those break a fast and nudge you away from ketosis, while MCT oil breaks a fast yet pushes ketones higher. In both cases it is the calories that break the fast, not the effect on ketosis.

Whether to use it comes down to your goal. If you are fasting for autophagy or a clean fast, the calories break it and the ketones do not buy an exception. If your priority is reaching or deepening ketosis, or you are on a fat-focused dirty fast, some people add MCT oil to coffee deliberately for the ketone boost and the appetite control, accepting that it ends the strict fast. To keep a fast fully intact, the reliable options remain water, black coffee, plain tea, electrolytes, and certain non-caloric supplements.

Source: Ketone and insulin responses to MCT oil in healthy adults (NIH)