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I have often heard people say that celery will help weight loss because digesting it consumes more calories than is contained in celery itself.

Is this true and, if so, is the weight loss significant? E.g. if someone ate 1kg of celery, would it noticeably affect their weight?

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FWIW, I personally heard that it was because it makes you lose appetite. –  Borror0 Mar 13 '11 at 16:20
    
I've heard the same claim about coffee. –  Sklivvz Mar 13 '11 at 22:35
    
I haven't accepted an answer yet because I have not yet seen an answer to: is the weight loss significant? –  Wikis Mar 16 '11 at 11:02
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I’ve heard the same claim about beer – which is of course a spoof of the often-parroted claim that one beer contains less calories than it takes the body to heat the cooled beer up to body temperature (which is only off by a factor of … oh, a thousand). –  Konrad Rudolph Mar 16 '11 at 12:20
    
"would it noticeably affect their weight?" Yes, they would weigh 1kg more. –  Colin D Mar 15 '13 at 16:52
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4 Answers

The reason for this claim is most likely the high content of dietary fiber in celery. Dietary fiber cannot be used as energy by humans, but it still causes some feeling of satiety.

You don't get many calories from celery due to the high amount of fiber (the energy content of celery is only 14 kcal/100g), but digestions itself takes some amount of energy.

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-1. How many calories does digesting it take? "Some" isn't a number! –  billynomates Apr 20 '11 at 13:06
    
from wikipedia : "A stalk of celery provides 6 calories to the body, but the body expends only half of a single calorie digesting it" en.wikipedia.org/wiki/Negative_calorie_food#Foods –  Duralumin Mar 18 '13 at 9:57
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I don't mean to step on the obvious here, but if 1kg of celery per day was all you ate (meaning that you allow no other caloric intake) you would certainly lose weight and probably quite rapidly. However, over an extended peiod of time, you would eventually become malnourished and probably die.
Conversely, adding 1kg of celery to a diet (assuming a healthy diet at 1800-2000 kcal/day) certainly has some benefits, mostly due to its being high in fiber and water. These same properties also may cause some loss of water weight through increased diuresis initially, however I think even this would be barely noticeable and replaced very quickly. It would probably be detectable only if you were to weigh yourself carefully before and after voiding.
Even taking into consideration indiviual variations in metabolism and calories burned while chewing, the idea is that digesting food is only 5-15% of the average person's daily energy expenditure (according to some sources, alcohol and protein take the most energy to digest link found at http://www.nutritionandmetabolism.com/content/1/1/5) it seems like any impact it would have on weight and body mass in general would be negligible, because the energy requirements to burn one pound of body fat is somewhere around 3500kcal.

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10% of energy expenditure would indeed equal 10 Watt = 864 KJ/day = 200 kcal/day. Adding a kilo of celery might take another 100 kcal, but it contains 140 kcal. So no. –  MSalters Mar 22 '11 at 12:09
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The Urban Legend web-site, Snopes argues that this legend is true.

It is the bodily energy devoted to the digestion of the green stalks that exhausts calories.

They also emphasize that it works as a displacement too:

those who are eating it aren't eating something else

Their references seem to be largely based on newspaper articles, which isn't reassuring. (They also cite Food Reference but they aren't terribly wedded to the claim, prefixing it with "supposedly".)

The Straight Dope tried an ad hoc experiment, showing that even ignoring the cost of digestion, over a period of an hour eating celery, the total calories consumed were outweighed by the base rate of calorie-burning by a person eating (or even resting)

After an hour I'd eaten eight stalks. (I was interrupted a couple times, thankfully.) All things considered, I think I packed away as much of the stringy stuff as could reasonably be expected. Total consumption: 514 grams. Total calories ingested: 72.

Bear in mind that you burn roughly 60 calories per hour while asleep, 85 while eating [...] According to one calorie calculator I found, I need 78 calories per hour just to support my body weight.

So both sites, with limited rigour, suggest that yes, eating celery burns more calories than it provides.

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Thanks, but is the weight loss significant? That is still not answered. –  Wikis Nov 18 '11 at 20:35
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No. If you are expecting significant weight loss by effectively reducing your calorie intake by one meal on one day, I have bad news for you... –  Oddthinking Nov 18 '11 at 21:33
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To "support my body weight" presumably requires more energy than to digest celery. Wikipedia's negative calorie food article asserts that the amount of energy required to digest celery is much lower than the energy contained in celery (although, like the Snopes and Straight Dope articles, the references seem largely unreliable). –  Douglas S. Stones Mar 15 '13 at 15:44
    
"support my body weight" simply refers to (awake?) resting levels. The effort to chew and digest would be on top of that. The references aren't great, it is true, but it is one of those esoteric questions that is unlikely to have a large, reproduced, peer-reviewed experiment to directly test it, so I think they are as good as we can hope to find. –  Oddthinking Mar 15 '13 at 22:11
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According to the Straight Dope, celery helps you lose weight by having an extremely low amount of calories, it is not negative.

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