My health "textbook" says that one pound (454 g) of fat contains 3,500 [kilo-]Calories (14,000 kJ). It provides no references for this statistic.

A calorie is a unit of measure that indicates the amount of energy gained from food or expended through activity. Each time you consume 3,500 calories more than your body needs to maintain weight, you gain a pound of storage fat. Conversely, each time your body expends an extra 3,500 calories, you lose a pound of fat.

However, I couldn't find any results when I searched online to verify this linking back to a credible resource, nor could I find any that denied it linking back to a credible resource. It claims that burning off a pound of fat requires creating a dietary deficit which will consume 3500 Calories. There seems to be an assumption in many online sources as well that 3500 Calories is roughly equivalent to a pound of fat. See:

The last two links make the unreferenced assumption that one pound equals 3500 calories, while the first two seem to both question its veracity and simultaneously justify it.

What's really going on here? Is a pound of fat equivalent to 3500 Calories?

(I put "textbook" in quotes because this book has been verifiably completely incorrect about information in the past, and tends to link to... rather unofficial websites, blog posts, and tabloids on health in its references.)

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    Are you asking about "one pound of adipose tissue" (the tissue type commonly known as "fat"), or "one pound of triglycerides" (the chemical family commonly known as "fat")? – Mark Nov 16 '14 at 7:48
  • @Mark The book doesn't specify, and neither do the sources. If it relevant, that seems pertinent to an answer, though. – user13259 Nov 16 '14 at 7:56
  • It's important to note, for consistency, you are referring to (small c) calories, not (big C) Calories (kilocalories). A calorie is the amount of energy needed to raise the temperature of one gram of water one degree Celsius. – Raystafarian Nov 16 '14 at 11:11
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    @Raystafarian: I interpreted it the opposite way. I believe they are referring to kilocalories, and updated appropriately. I find the continued use of "Calories" as a unit to be bewildering, so please correct me if I have got this wrong. – Oddthinking Nov 16 '14 at 11:51
  • I found this question rather confusing. My first interpretation was "Eating the (equivalent) of 1 lb of (beef) fat would add 3,500 calories to your intake." The first link seemed to be "when it has a dietary energy deficit, the body can extract the equivalent of 3,500 dietary calories from 1 lb of (human adipose tissue) fat". A separate issue is "How much excess dietary calories are required to put on 1 lb of (human adipose tissue) fat?" Can you clarify this? Perhaps a link to the "textbook" and a direct quote would help. – Oddthinking Nov 16 '14 at 11:57

First, it is important to remember that food labeling is not a particularly exact science. Unless completely artificially manufactured, most food is anyway subject to regional or seasonal variations in energy content. Food labeling regulations usually define carbohydrates and protein to have an energy density of 4 kcal/g and fat to have 9 kcal/g. The US regulations can be found here, the EU regulations is here and other countries are likely to operate with the same numbers.

Assuming that "storage fat" in your quote is a colloquial expression for adipose tissue, the next important question is the fat content of adipose tissue. The exact composition of adipose tissue varies significantly between persons, but I've often seen the number 87% be used for the average lipid (fat) content. This number is e.g. used in Wikipedia's article on adipose tissue, it's also used in this book and this article, but without any reference to a source for this number. When examining scientific research, most publications quote much lower numbers, e.g. George L. Baker: Human Adipose Tissue Composition and Age, finding the lipid content to be between 40% and 75%:

The lipid content of adipose tissue accounted for only about 40% of adipose tissue weight in the newborn period and increased with increasing age to 75% in the adult. Concentrations of water, nitrogen, and DNA declined with age.

But if we stay with the energy density of 9 kcal/g and assume adipose tissue to contain 87% fat, one pound (454 g) of adipose tissue would contain 395 g of fat, which again contains 3555 kcal of energy (quite close to the claim). It is also often assumed that 1g of fat contains 37 kJ of energy (another estimate, not quite equivalent to 9 kcal). Using that estimate, 395 g of fat would contain 14615 kJ of enery, which converted to calories is 3491 kcal and even closer to the claim.

I also found this publication: ‘Kevin D. Hall: What is the Required Energy Deficit per unit Weight Loss?’, in which the exact claim (‘an ... energy deficit of 3500 kcal is required per pound of body weight loss’) is compared to a more exact estimation and real world data. At least for the test subjects used in this article, the actual energy deficiency required for one pound of body weight loss varies between 1300 kcal and 4000 kcal.

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    To be clear, the Hall study does not dispute that 1 pound of lost body fat is worth about 3,500 kcals. It points out that actually in a caloric deficit, the body consumes things other than body fat as well (notably lean muscle mass, and excess stored glycogen and water, are all consumed). People who weighed less to begin with, and had a lower % body fat, consumed more of these other, less calorie-dense substances, and so lost weight faster. The study doesn't claim that a pound of fat contains more or less than 3,500 kcals, only that a pound of arbitrary body mass might. – John Doucette Nov 20 '14 at 22:55
  • Yeah, the problem with "1 pound of fat = 3500 calories" is that it's a strictly mathematical equation. The human body isn't a heat machine and actually has incredibly complex mechanisms (like hormones) that affect fat storage and fat loss. These will cause wild variation in the actual "number of calories" to burn a pound of fat. – WakeDemons3 Jul 25 '18 at 17:08

3500 is an average/approximation that attempts to take into account all the considerations mentioned in the comments and more:

  • human fat is mixed with other types of tissue,
  • fat grams actually fall in a range of calories,
  • not everything eaten is absorbed,
  • the fat content of foods is not consistent,
  • etc.

If I could eat somewhere between 3000 and 4000 calories without burning any, I could expect to gain about a pound of stored fat. If I could burn somewhere between 3000 and 4000 calories without eating anything in the meantime, I could expect to lose around a pound of stored fat. However, those statements are approximate and impossible to implement.

This USDA document (dated 1973) describes how the calorie contents and nutrient components of specific foods were determined. Using a "bomb calorimeter", a sample of the food is burned and the heat produced is used to heat a known quantity of water. The temperature of the water is measured before and after and the number of [kilo-]calories of energy produced by the food sample can be computed from that difference.

Those numbers are published in USDA "Standard Reference" documents that manufacturers and researchers (in the US, anyway) use to compute calorie numbers for actual foods based on weight and nutrient content (how many fat grams, protein grams, carb grams are in the food). That's the information that winds up on packaged food nutrition labels.

However, the nutrient and calorie content of food can vary. Here's an FDA document that discusses that issue. So you can't eat the food sample that was measured and we can't measure the food sample that you ate, and there's no guarantee they contain the same number of calories per unit of weight.

Most foods that we eat are not pure fat. Oils and lard come closest, but who eats oil or lard alone? We normally eat a mixture of fat, protein, and carbohydrate. A healthy diet includes a fair amount of fiber. The more fiber consumed, the more calories pass through and are excreted rather than absorbed.

For the average person, trying to remember to record every single calorie consumed is overwhelming. The mint I snagged off my co-worker's desk? The cookies somebody brought to a meeting that I wasn't expecting that I ate three of? (and how could I possibly know how many calories are in a homemade cookie?) The extra soda yesterday afternoon because I was falling asleep?

So in practical terms, on a day to day basis, it's extremely difficult to account for all the variables and accurately track one's caloric intake.

Similarly, it's next to impossible to accurately track one's caloric expenditure. As we exercise and become more fit, the body becomes more efficient. An exercise that burned 400 calories a few months ago may only burn 300 or 250 today.

Another piece of conventional wisdom (called out in the HuffPost article linked in the question) is that a gram of fat contains 9 calories. Again, the 9 cal/gram number is an average. Zoe Harcombe points out in her article linked in the question that she found references for a range from 8.7 to 9.5 for this value. There are 454.5 grams in a pound (based on 2.2 pounds in 1 Kg = 1000 grams). 9 * 454.5 =~ 4090. So according to that, there are approximately a little over 4000 calories in a pound of fat.

Another place we can look for a reference is online food calorie databases like calorieking.com or nutritiondata.com. I went to nutritiondata.com and looked up "olive oil". One of the hits is "Oil, olive, salad or cooking", which is listed as having 884 calories per 100 grams. (884/100) * 454.5 =~ 4018 calories. Other types of fat (canola oil, peanut oil, lard) give similar numbers. Scrolling to the bottom of the page, I see "Source: Nutrient data for this listing was provided by USDA SR-21."

Googling "USDA SR-21", I find this page: http://www.ars.usda.gov/News/docs.htm?docid=18880 which says that "SR" stands for "Standard Reference" and the current version is SR-27, so one of the most respected online sources for nutrition information (nutritiondata.com) is six versions behind the most recent data. This is the "Standard Reference" document I mentioned earlier.

So the original numbers are estimates based on averages from bomb calorimeter measurements made some time in the past. Those estimates are then extrapolated and crunched and rejiggered and reported to more significant digits than is justifiable.

Often calorie numbers are bandied about and used in computations as if they are accurate to 1% or so. Actually, I believe the error is typically on the order of 30% to 50%.

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    This looks solid, but needs more references. – Flimzy Nov 17 '14 at 14:40
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    This makes me wonder about the storage efficiency of biological fat. It's basically a battery, after all; and there's no way that the energy extracted from a pound of fat could be equal to the energy used to generate it. – Ask About Monica Nov 17 '14 at 16:59
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    In practice, it is actually not very hard to track your caloric input and output (people use apps to help, and a simple kitchen scale is sufficient to get very accurate measurements that control for potion size). Changes in weight can also be estimated fairly accurately using moving averages of multiple measurements. People post graphs of this process to places like reddit/r/fitness frequently to demonstrate weight loss. For the overwhelming majority I've seen, a pound lost is 3500 +/-100 kcals missing. There's no way the error is 30-50%. 1-5% seems believable though. – John Doucette Nov 18 '14 at 3:53
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    @JohnDoucette A fairly accurate measurement of both energy intake and energy usage is extremely difficult. Weighing the food is not a problem, but the energy content declared on the food label may be quite different from the actual energy content. The only method to measure the energy usage with reasonable accuracy is to use a breathing mask to measure the oxygen uptake and carbon dioxide output in the air you breathe. Tables and calculations offered by smart phone apps or fitness equipment are very inaccurate. – Tor-Einar Jarnbjo Nov 18 '14 at 14:30
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    My wife spent 24 hours in a specially designed room doing a study on exact calorie intake and energy output at the NIH and it is not a simple thing to calculate. Even with well controlled portions and precise measurement, they can't be completely certain. – mikebabcock Nov 18 '14 at 21:07

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