You’ve probably heard of the magic of negative-kilojoule foods - those foods that supposedly contain fewer kilojoules than your body burns to digest them. On top of the list are supposed to be fibre-rich fruits and vegetables, such as apples, celery, cucumber, lettuce and broccoli.
If the science behind this idea is sound, you should be able to lose weight while eating as much of these as your heart – or stomach -desires.
But do negative kilojoule foods exist? We asked three nutrition experts.
Registered dietician, Mindful Eating Dietician Consultancy
The first thing to understand is that there is no naturally occurring food that has zero kilojoules in it; even most man-made, sugar-free foods are sugar-free but not kilojoule-free.
The foods that are often considered “kilojoule-free”, such as vegetables (celery, broccoli, cauliflower, kale, cucumber etc) are the foods that come the closest to being negative-kilojoule foods, however, they do still contain some kilojoules, even if only a tiny amount.
The second thing to understand is something called the “thermic effect” of food. This is the amount of kilojoules used by the body to digest and absorb the food you eat.
In order for a food to be a negative-kilojoule food, the amount of kilojoules burned to digest the food must be more than the total kilojoules the food contains and this is never the case. Only a small percentage, anything from 2 to 20%, of the total kilojoules in the food is used to digest the food.