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If you are attempting to lose weight via dieting and/or exercise, you will see this number EVERYWHERE on the internet.
If you create a deficit of 3500 calories you will lose one pound, as simple as that.
But is that really true?
Surely there are many factors which will affect how true this is?. That different energy sources have different energy densities, such as muscle and fat, for one.
And where does this number even come from, how is it derived?
The original 3500 calorie theory comes from a correspondence published in 1959, by a Dr. Max Wishnofsky, where he equates it to pounds lost in observed obese patients.
It's further perpetuated by badly applied mathematics. White adipose tissue has the responsibility for energy storage in the body. This tissue is composed of anywhere from 60% to 85% lipid (fat). If you take the commonly accepted 9 calories of energy per gram of fat (Which is also not quite accurate), you get the following formula:
9 (calories/gram) * 454 grams (grams in a pound) * .85 = 3465 calories.
So, the original estimation was that there were 3500 calories in a pound of human fat, so to lose a pound you had to burn 3500 calories. Since this "makes sense", it was widely repeated and used, and became entrenched. It is not accurate, and implying that to lose a pound of fat you have to burn exactly 3500 calories is erroneous.
Edit: While researching something else, I came across this article, titled "Farewell to the 3500 calorie Rule", on Today's Dietitian website. One paragraph from the article:
It's been estimated that the 3,500-calorie rule is cited in more than 35,000 educational weight-loss sites.1 In September, the Journal of the American Medical Association published a patient handout titled Healthy Weight Loss, in which the first sentence states, "A total of 3,500 calories equals 1 pound of body fat. This means if you decrease (or increase) your intake by 500 calories daily, you will lose (or gain) 1 pound per week."2 Undoubtedly, the 3,500-calorie dogma still is being taught even though it's been shown that it simply doesn't work this way. So where did the 3,500-calorie weight-loss wisdom come from? It originated from researcher Max Wishnofsky, MD, in 1958, who calculated that 1 lb of fat stores approximately 3,500 kcal of energy.3 It was appealingly simple, and it stuck.
It is, at least, overly simplified.
The average amount of water in adipose tissue is 13%. In addition to that, it also contains approx. 3% protein. The remainder is fat.
Lard, which should have a similar calorific value as human fat, provides ~8980 calories per kg.
To simplify this calculation we assume that 1 kg of protein provides 4000 calories and 1 kg of fat 9000 calories (in other words ratios of 1:4 and 1:9).
The average kilo of adipose tissue (fat tissue) contains 840g fat and 30g protein. Which amounts to a calorific value of 7680 calories per kilo.
Translated into pounds that is 3484 calories per pound, which is pretty close to the number in question.
The problem is that this is based on average values that have wide ranges attached to them.
The water content of fat tissue can vary between ~4 and ~40% and the protein content between ~2 and ~3.5%.
This means the calorific value can vary between 5540 and 8540 calories per kilo (or 2510 and 3870 per pound) of fat tissue.
Sadly, it also seems that the water content is lower for those above standard weight. So whoever needs to lose a couple of pounds because of health reasons should rather apply 3900 per pound.
The numbers are based on this study. It is a bit old but as the numbers are based on actual tissue samples, I doubt they are too far away from the truth. What might have changed is the average. At least in some countries, as obesity rates are much higher than 50 years ago.
Regarding the water content of body fat, try a watermelon test. Replace an intake of 1000 calories with watermelon worth 500 calories. Drink as much water as you usually do. The extra water intake will most likely lead to an increase body weight on the next day, even though your calorific intake was lower.
The above-mentioned study refers to: "THE CHEMICAL COMPOSITION OF ADIPOSE TISSUE OF MAN AND MICE." LORETTE W. THOMAS, Department of Physiology, Edinburgh University. (1962)
Every pound of pure body fat that is metabolized yields approximately 3500 kcals, thus a daily caloric deficit of 500 kcals theoretically results in fat loss of approximately one pound per week if the weight loss comes entirely from body fat [7].
However, a static mathematical model does not represent the dynamic physiological adaptations that occur in response to an imposed energy deficit [8]. Metabolic adaptation to dieting has been studied in overweight populations and when observed, reductions in energy expenditure amount to as little as 79 kcal/d [9], to as much as 504 kcal/d beyond what is predicted from weight loss [10].
https://www.ncbi.nlm.nih.gov/labs/pmc/articles/PMC4033492
Exploring reference [7], What is the Required Energy Deficit per unit Weight Loss?:
[...] it is now generally acknowledged that this rule of thumb is an oversimplification (1). But under what conditions is this rule of thumb appropriate? In other words, what are the factors that determine the cumulative energy deficit required per unit weight loss?
Note, 3000 kcal = 32.2 MJ per kg:
Figure 1 The predicted energy density of weight loss expressed as a function of A)initial body fat content or B) initial body weight of women.Data points depict the calculated weight loss energy densities from several published studies in both obese and lean subjects.
There is somewhat of a cluster around 3500 calories, slightly below even. (A rule-of-thumb of less calories is better for losing weight since less calories need to be burned.)
The fact that weight loss typically slows over time for a prescribed constant diet (9, 38) suggests that either the energy expenditure decreases with time, or the dietary intervention is relaxed over time, or both.
