3500 kcal equals 1 pound of body weight: fact or myth?

March 18, 2012 § Leave a comment

In the article ‘What’s a few billion neurons between friends?‘ I expressed surprise that the figure of 100 billion neurons in the human brain was often repeated despite no one seeming to know where the figure came from. It reminded me of a claim I heard regarding the usual answer to a common dieting question: ‘how many calories do I need to burn to lose 1lb of weight’. There are different variations to this question but the answer is almost invariably the same – 3,500 calories.

By chance I caught the end of a radio interview with a lady who was questioning the provenance of this figure, which like the number of neurons in the brain, had gained general acceptance. I didn’t hear the name of the interviewee and was unable to follow up the claim that the 3,500 calories figure is a myth, but I now believe the person involved was Zoë Harcombe.

In her article ‘1 lb does not equal 3,500 calories‘ Zoe says that she asked several organisations (the National Health Service (NHS); the National Institute for Clinical Excellence (NICE); the Department of Health; the National Obesity Forum; the Association for the Study of Obesity; the British Dietetic Association and Dieticians in Obesity Management) where the 3,500 figure came from but none of them were able to provide a satisfactory answer. This is an interesting line of investigation which I will comment on later but first I want to focus on her claim that ‘you will struggle to find anyone demonstrate the precise calculation behind this’.

True, I did struggle at first but only because attempts to Google the answer only brought up page after page of links to forums, blogs and articles asking how many calories are in a pound of weight. However, searching more academic sources I found articles referencing the work of Max Wishnofsky. Wishnofsky’s 1958 article ‘Caloric Equivalents of Gained or Lost Weight‘ provides the following explanation:

“… the average fat content of human adipose tissue taken from various parts of the bodies of well-nounished subjects is 87 per cent. One pound (454 g) of human adipose tissue, therefore, contains 395 g of fat. The caloric value of one g of animal fat is 9.5 ; consequently, the caloric equivalent of one pound of human adipose tissue may be considered to be about 3,750 cal.”

My research also produced several references to Dr Gilbert B Forbes. Forbes observed that body weight changes involves both body fat and fat-free body mass and devised a formula to reflect this. As Forbes states in ‘Body fat content influences the body composition response to nutrition and exercise‘:

“In most situations involving a significant change in body weight, both fat-free body mass (FFM) and body fat participate, but the relative contribution of FFM and fat to the total weight change is influenced by the initial body fat content.”

Hall revisited Forbes’s work and devised a new formula. This formula produces results that ‘… compared favorably with data from human under-feeding and over-feeding experiments and accounted for previously unexplained trends in the data’. In another journal article Hall asks the question “under what conditions is it appropriate to use this rule of thumb and what are the factors that determine the cumulative energy deficit required per unit weight loss?” The rule of thumb Hall refers to is the need to create a calorie deficit of 3,500 calories in order to lose one pound of weight. Hall comes to the conclusion:

“The rule of thumb approximately matches the predicted energy density of lost weight in obese subjects with an initial body fat above 30 kg but overestimates the cumulative energy deficit required per unit weight loss for people with lower initial body fat.”

Returning to the point raised by Zoe Harcombe, several organisations she questioned were unable to say where the 3,500 calorie figure came from. It is a figure repeated by “government literature, in just about every diet book, in private health booklets and all over the internet’. Zoe’s investigations show that too often received wisdom is accepted without looking deeper and checking sources. Since my first draft of this article I have looked deeper and after checking Zoe’s sources I realise that not only did she know about Wishnofsky’s work she even referenced his 1958 article in her book The Obesity Epidemic.

Other than the point about the need for trusted organisations to be more careful about the provenance of the information repeated to the public, an interesting point to arise from this investigation is that the truth about how many calories are in a pound of body weight is more complex than is commonly recognized.

My investigations show that 3,500 figure is not wrong per se and is certainly not a myth, but rather it is a rule of thumb that cannot be applied blindly. So do 3,500 kcal equals 1 pound of body weight? The answer is ‘sometimes’.

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