Negative Calorie Food: Science Myths and Legends



Celery Cupcakes: A dieters dream?
A popular diet belief is that there are some amazing diet wonderfoods that contain negative calories, like celery. So if you eat lots of them, you will burn extra calories; without even lifting a finger.

The theory goes, negative calorie foods contain fewer calories than they take to digest. Celery is a reported 'negative calorie food'. If one lump (this is a hypothetical example) contains 6 calories, but it takes 10 calories to digest then the body uses 4 calories and does 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, there is 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.

The only way to put any kind of measurement to the energy used during digestion of one meal/food stuff is by measuring 'diet induced thermogenesis' (DIT) which is an increase in energy expenditure above basal fasting level, divided by the energy content of the food ingested (Westerterp et al 2004).

And this is measured, as stated in Dr Stu's blog post, by using a whole body calorimeter (a respiration chamber). More (very interesting) information about how whole body calorimeters work and what they measure here.

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)?

No-one has tested people eating celery or any other reportedly negative calorie foods specifically, however, from other studies it is concluded that; Protein rich foods, like dairy and meat cause an increase in DIT,  fatty foods cause a very small increase in DIT, and carbs lie in the middle between the two (Mikkelsen et al 2000),. So, proteins use more energy, carbs are in the middle and fatty foods use little energy to digest.

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 weightA 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.





Comments

  1. I always kind of held on to the idea that apples were calorie neutral or negative because they are such hard work to eat. I doubt it though as they are quite a high calorie fruit.

    ReplyDelete
  2. It stands to reason that ice water should be a negative calorie refreshment, since water has 0 calories and yet your body must expend energy to replenish the heat that is transferred while it rises to body temperature. I can't think of much else that would take more energy to passively eat and digest than you get back in return. However, if you include in the equation the energy you expend gathering and preparing food... then you have a lot more options to work with. One extreme being running after a wounded animal for several hours until it collapses from exhaustion. In an urban setting you might choose to walk to the market and carry the heavy grocery bags back home. Or you could decouple the activity phase from the eating phase entirely, by, I don't know, maybe exercising more and eating less.

    ReplyDelete
    Replies
    1. It should be, but ice water isn't really a 'food'. But, maybe fat free frozen yoghurt is!

      Delete
  3. Icewater is mostly drunk in warm environments; it should be suspected to reduce the costs of cooling of. So the direction of these thermal energetic effects depends almost entirely on ambient temperature. Same goes for frozen food stuff..

    The contraintuitive link between BMI and alcohol consumption might be due to loosing muscle weight while gaining fat. I think regular drinkers often get fat -especially around the waist- but if they loose muscle mass at the same time, they may well loose weight in the process of getting fat.

    ReplyDelete
  4. Negative Caloric Foods are an interesting subject that I believe should be broken into distinct categories: both the Raw Food and the Preparation techniques effects on the energy expended digesting it. Ice Water for example would be an example of preparation vs. raw nature given one can disregard energy required in assimilation of nutrients into the body; room temperature water would then be a effectively zero caloric intake. Preparation of Meat, however, plays a huge role in digestive energy expenditures: Raw, unmutilated meat takes much more effort to digest than the cooked, transformed counterpart.

    So if people are looking to increase their baseline metabolic rate then ordering their steak rare could be an option. I have read some articles stating less nutrients are actually extracted from undercooked meats though...

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