Food with negative calories: eat more to weigh less!

On 20 February 2015, the Daily Mail, a newspaper with a readership of millions, published an article on foods which allegedly burn more calories than they contain. They were said to include cucumber, asparagus, cauliflower, celery, lean meat, tomatoes, papaya, chilli, and apples.

The internet is also awash with similar lists of foods that supposedly contain ‘negative calories’. Those lists usually include fruits like apples, berries, oranges, and melons, but they may also contain vegetables, including asparagus, broccoli, courgettes, and cauliflower. Quite apart from the fact that none of these, nor any other food, contains fewer calories than they provide. It is doubly ironic that many of them, especially the listed fruits, are actually high in calories.

A ‘calorie-free’ apple comes in at around 80 kcal, and an orange has more than 100 kcal. And what about the ‘calorie-free’ melon? A small honeydew melon can easily contain more than 300 kcal. So, someone trying to lose weight by restricting their intake to 1200 kcal a day can easily eat their full allowance just by grazing on a few pieces of fruit through the day — or they can completely ruin their calorie deficit. Although vegetables are usually a little lower in calories, it’s worth checking them on a nutrition chart. Carrots have between 20 and 40 kcal each, and if you’re like me and often nibble away at a few carrots as a snack, you can easily take in 200 kcal that way.

The body does use a certain amount of energy in processing food, but only around 1 to 30 per cent of the energy contained in the food itself. For example, about half a calorie is required to digest 6 calories’ worth of celery (Upton, 2008). That’s only a fraction of what the body absorbs. Proteins normally require more energy to digest than carbohydrates or fat since the body finds it more difficult to break protein down and transform it into energy. So, a protein-rich diet isn’t bad from that perspective, but it’s still not a miracle cure that will allow you to eat unlimited amounts of food, or even lose weight by eating.

In the EU it’s actually legally permissible to label foods ‘calorie-free’ if they have fewer than 5 kcal per 100 g. But even Coke Zero and Diet Coke have a tiny amount of caloric energy (around 1 kcal per 100 ml).

The only truly calorie-free — or rather, calorie-negative — foodstuff is water. It contains no calories whatsoever. But our system does need to use some energy to heat it up to body temperature and to process it. A couple of glasses of water require only a minimal amount of energy to process, but it is highly advisable to drink a lot of water while dieting anyway. As with everything, though, it’s important to remember here that ‘the dose makes the poison’. Suddenly downing several litres of water can create a mineral imbalance in the body, as it flushes out important salts. And that can cause serious health problems.