When people don't eat for 3 days, their body produces more ketones (a type of fuel made from fat), which can be measured as rising levels of beta-hydroxybutyrate in the blood.

Eating earlier in the day for 4 days slightly increased ketone levels in the morning for overweight people compared to eating throughout the day.

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.