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.

If a woman who is overweight but otherwise healthy eats very few carbs for three months, her body starts making more of a fuel called beta-hydroxybutyrate — this means her body is switching to burning fat for energy instead of carbs.

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.

Among middle-aged men who lift weights and are eating fewer calories, a ketogenic diet that keeps ketones high does not lead to better preservation of muscle strength or body composition than a non-ketogenic low-carb, high-fat diet over eight weeks.

Eating mostly fats and very few carbs for 3 months helps overweight people lose fat and weight, on average about 5% body fat and 3.5 kg.