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My teenage daughter and her friends were recently talking about "negative calories". They were curious about the story that eating and digesting celery chewed up more energy than was in the celery. Yes indeed, what they’d heard is that crunching through a celery stick will make you lighter. So I had to check out this "negative calories" stuff. Even though I trained as a doctor, I had a grand total of only eight hours of lectures on Nutrition. Nutrition is a huge topic, so I went to the experts, the nutritionists, who spend years studying this enormous body of knowledge. They told me that nobody had ever done the experiment of feeding people nothing but celery. This is partly because the volunteers would soon get sick if they ate no proteins or fats, and partly because celery has so few calories that any measurements would be difficult . One way to look at this "negative calories" story is to consider Basal Metabolic Rate (BMR) – the amount of energy needed to keep you alive when you are just resting. The nutritionists measure this by putting the volunteer in a box, and measuring how much oxygen they burn up, and how much carbon dioxide they generate. These are easy to measure when you eat a big meal of a steak and lots of potatoes, and almost impossible to measure when you have a small meal of a few sticks of celery. Surprisingly, just in digesting an average meal, we humans increase our BMR by 50% for about four hours. In other words, it costs energy to digest that meal. If you’ve ever had a tough steak, you might think that most of that extra energy you burn up comes from the act of chewing. But no, the vast majority of that extra energy is burnt in the gut. About half is used in mechanically breaking down the food into smaller lumps in your stomach, then pushing it along the 10-metre length of your gut, and then making special enzymes to chemically break the food into individual molecules, and finally, pushing those microscopic molecules into the cells lining your gut. The other half of the energy is spent stitching those tiny molecules back together to make bigger molecules (eg, join amino acids to make proteins). So you might ask why break the molecules down, only to build them up again? The answer is that if only really really small chemicals can get across the lining of your gut, bigger things (like bacteria and viruses) will find it really hard to push in. All this activity (breaking chemicals down, and building them up again) increases your BMR from 1 to 1.5. But if a human exercises really hard, such as in an Olympic 100-metre sprint, they can crank up their BMR to 20 times normal – but only for 10 seconds or so. Horses can do much better – in the Melbourne Cup, thoroughbreds can reach a BMR of 45 times normal for a few minutes. But the BMR king is the python, who can maintain its BMR at 45 times normal for weeks at a time, while digesting its meal of a whole calf. So for some animals, there is a huge metabolic cost in digesting a meal. In humans, the energy needed to process a meal varies with the food. This "energy tax" is about 25% for proteins; 2–3% for fats; and about 5–10% for average carbohydrates. So if you ate 100 kilojoules of each, after digestion you’d be left with 75 kilojoules from the protein, 97–98 kilojoules from the fat, and 90–95 kilojoules from carbohydrates.But nibbling a stick of celery is nothing like chewing on a chunk of cow. Celery is about 95% water, 2.2% digestible carbohydrates and 1.8% indigestible carbohydrates. There are also insignificant amounts of protein and fat. The digestible carbs in 100 grams of celery will give you 32 kilojoules, while the indigestible carbs will give you 16 kilojoules. And the energy needed to process all these carbs will be about 4 kilojoules – which leaves you with about 44 kilojoules. That’s a small number, but definitely not a negative number. So you do get a few calories or kilojoules from eating celery.One thing’s for sure, though: if your mouth is full of celery, there’s no room to stuff in fatty, high-kilojoule foods like chips and hamburgers.