You've probably been told at some point that eating celery actually burns more kilojoules than it contains. While celery is low in kilojoules and makes for a refreshing snack, there are no foods out there that count as negative kilojoules. In this video from the SciShow YouTube channel, host Michael Aranda explains what's really going in your body when you eat a so-called "negative kilojoule food". The reality is that all food has kilojoules, and the process of eating it won't instantly burn it off. This is by design. Your body wants to gain as much energy as it can from the food you eat, and it tries to use as little energy as possible to digest it -- even with a super low kilojoule food like raw celery. The high water and fibre composition of celery makes it easy to digest, so your body actually needs to burn less kilojoules to process it.
For example, a full stalk of celery is about 41 kilojoules worth of carbohydrates, sugar and protein. Your body burns about 8 kilojoules digesting it, leaving you with a net gain of around 33 kilojoules. That said, celery and other super low kilojoule foods like carrots, grapefruit, cucumbers and apples make for great snacks when you're trying to optimise your diet. These snacks can make you feel full without feeling guilty, and help keep your goals on track.
Do Negative Calorie Foods Exist? [YouTube]