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.

PHILIPPA BREDENKAMP

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.

The third factor to understand is the amount of energy needed to chew the food. A lot of people believe celery is a negative-kilojoule food because you burn more kilojoules chewing celery than the amount of kilojoules celery contains. The amount of kilojoules burned during chewing is minimal.

It's estimated that around 46kJ are burned if you chew for 1 hour. This would mean in order to burn off the kilojoules in one stick of celery (i.e. to make it a negative-kilojoule food), you would need to chew that stick for a full 6 minutes!

ASHLEIGH CARADAS

Registered dietitian

All food has kilojoules, even if it’s something very low in kilojoules, like spinach. The reason why certain vegetables and highly nutritious foods might be considered negative-kilojoule foods is because they contain nutrients (like magnesium, chromium or vitamin B12) that could help us burn carbohydrates and fats. But no food has negative kilojoules; everything has caloric value! 

PHILIPPA BRAMWELL-JONES

Registered dietician and certified health coach, Robyn Rees and Associates

The old equation that “kilojoules in = kilojoules out” has, thankfully, been refuted by medical science. We have lived under the threat of the word “kilojoule” for so long that it has become conditioned in many of us. It has become how we hierarchy foods. I believe a shift in focus is needed – to what nourishment food provides.

So, negative-kilojoule foods? To me, it makes no sense. It marginalises the benefits of food and maintains that authoritarian view that the “kilojoules” food contains dictates whether or not it sits on your plate. For me, the food that sits on my plate is the one that nourishes me.