When you eat very few calories but still get all your important nutrients, your body quickly switches to burning fat for fuel, which drops your blood sugar and insulin and raises ketones—putting you into a state called ketosis in just two or three days.

When people with excess body fat eat very few carbs and more fats, their blood ketone levels rise quickly within two weeks, showing their body has switched to burning fat for fuel.

Elite race walkers who follow a keto diet for 3 weeks start burning fat for fuel during intense exercise, and their bodies show signs of ketosis — like having 1 mmol/L of ketones in the blood — even when working hard.

When your body makes less insulin (like during fasting or on a low-carb diet), it starts producing more ketones - these are alternative fuel molecules your body can use instead of sugar.