happy science: Negative Calorie Food: Science Myths and Legends

The theory goes, negative calorie foods contain fewer calories than they take to digest. Celery is a reported 'negative calorie food', so if one lump (this is a hypothetical example) contains 6 calories, but it takes 10 calories to digest the celery then you use 4 calories and do not gain any.

I had some faith in the principal of this idea, it seemed to make sense in my head. Maybe I had just turned my skeptical inquisitive brain off, or maybe, a part of me just wanted to accept that it might be a possibility..

A?quick google search revealed a host of information, news articles and blog posts on this subject. Dr Stu posted a blog?post summarising some of the literature about negative calorie foods?very recently.

Sadly, but predictably, I discovered, there are no scientific?studies to support this 'fact', but there is an awful lot of opinion flying around... and here's why it is incredibly difficult to prove or disprove if the statement is true or not:

• How do you measure how many calories it takes to just digest something? The body is always burning calories, so ?how do you measure the number of calories used only in digesting one item of food?

Difficult, no 'real' answer to this question.

General consensus is that 10% of your daily calorie output is used in digestion..?This is?interpreted to mean that out of a recommended 2000 calories per day for a human lady,??200 calories per day that are used for digestion.

But how much of that is used to digest one item, like celery? Do some items of food require more 'digestive energy' as they contain 'tougher' substances (like fibre in celery)?
Interesting side observation here, the Westerterp review,? found that?when?food was consumed?with?alcohol, this?increased DIT.?So, alcohol seems to increase the amount of energy it takes to digest something. The relationship?between alcohol intake and weight is not clear, a positive correlation between moderate?alcohol intake and activity levels has been found?(US population) however, as you can probably guess it has been found that, alcoholics exercise less, but also have a lower body weight.?A more recent and detailed study?of 37,000 people, investigated the amount and frequency of drinking alcohol (they did correct for?levels of activity, but some information was missing) determined that the people that drank alcohol the least (but frequently) had lower BMI values than those that drink more, less frequently.

I always (maybe presumed in my head) as?alcohol is such an incredibly high energy source, (second only to fat in terms of energy density)?that there was a link between alcohol and obesity.. but?as with most?presumptions, ?it doesn't seem that clear.

Back to the original question!

Celery contains mostly water and fibre, and Dr Stu's blog explains, although fibre alone hasn't been tested,?adding fibre to a meal reduces the DIT.

This means that actually, less energy is used to burn off the calories in the meal that contains fibre. Which goes against the view that low calorie foods take more energy to burn than they contain. This doesn't absolutely answer the question of 'how many calories it takes to digest different food stuffs', but at the moment, it seems to be the closest answer we have

This leads us swiftly on to the next question...

• How do you measure exactly how many calories the body takes from something? The celery might contain 6 calories, but do you gain all the calories?

Difficult again... View from Susan Jebb from the Medical Research Council's Unit of Human Nutrition Research in Cambridge?in response to?this question on?The Naked Scientists website -

'Not all of the calories that are actually in a food will be absorbed, digested and available for the body to use.? What happens is that the calories which are in food, which will be released if we were just to burn it, as we might do in the bomb calorimeter in the laboratory, those calories cannot all be absorbed by the body.? Some will be lost in the faeces and the remainder will be digested.? About, perhaps, 10% of the total calories we consume might actually appear at the other end of the gut.? Once calories have been absorbed, again, they're not all fully available.? Some will be lost in urine for example.? The final loss of calories happens because some of the energy is fermented by the bacteria in the gut and so, it?s not available to humans.? It?s actually burned off by the bacteria that are living inside us.? And so, the consequence of all of that is that not all the calories that are actually in the food will be available for the body to use.? But in fact, the losses are proportionately quite small.? The calories that you see written on the back of a food pack have already had all of these adjustments made for the amount that will be digested and absorbed.? And so, the calories you see on the packet is actually not the total calories in that food.? It?s the so-called metabolisable energy, the amount of energy which is going to be available to the body.'

So, although celery might lower the DIT, you might not get all of the calories from the celery anyway.

• How long does it take to chew something, and how on earth do you work out how many calories it uses?

A comment published in the New England Journal of Medicine, suggests that, on average 11 calories per hour are used to chew chewing gum (unspecified number of participants).?This?value is an average and extrapolated from?measurements of 12 minutes of chewing.?Unless you chew your food like a cow, for hours on end, the calories you burn chewing celery are going to be pretty insignificant (1-2)?

So, due to lack of evidence, I would summarise that we do not have a conclusion as to if negative foods do exist or not. The DIT studies suggests food high in fibre does not take increased energy to digest and can lower the energy used to digest other foodstuffs, however, as the calorific content of celery is pretty low anyway, if eaten alone, fewer calories could be taken by the body than are used in eating it.

One thing is for sure unfortunately, if you are dieting, you are going to get fewer calories from a stick of celery than a slice of cake.

... as always, please share any extra information below!

I created this blog post after?spotting a brief exchange on Twitter, between @tkingdoll and @noodlemaz, about negative calorie foods, like celery that reportedly contain fewer calories than they take to digest.

References:

Westerterp KR: Diet induced thermogenesis. Nutr Metab (Lond) 2004, 1:5.

Mikkelsen PB, Toubro S, and Astrup A. Effect of fat-reduced diets on 24-h energy expenditure: comparisons between animal protein, vegetable protein, and carbohydrate. Am J Clin Nutr 2000 72: 5 1135-1141
Levine J, Baukol P, Pavlidis I. ?The energy expended in chewing gum.? New Engl J Med. 1999 Dec 30; 341(27): 2100.

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