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Questions about calorie deficit needed for weight loss have been discussed many times here and there plenty of sources pointing out that body fat contains about 7700 Kcal/kg. The standard answer is therefore that a deficit of 7700 Kcal is needed to lose 1 kg of body weight in the form of body fat. But this 7700 Kcal only takes into account fat tissue, which is 86% fat and 14% water, while the rest of the body contains 60% water.

So, the 7700 Kcal equals 9000 Kcal/kg *0.86, but it seems to me that since the body will keep its water percentage fixed at 60%, one should calculate the calorie deficit needed for weight loss per kg as 9000 kcal/kg * 0.4 = 3600 Kcal/kg, much lower than the usual figure.

While some sources do point out that weight loss may be more than expected due water loss, this is usually mentioned as a transient effect when people start to lose weight, not as continuous process that accompanies the loss of body fat in a steady way.

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    The 3500 calories = 1 lb of fat loss is outmoded, based on limited data and doesn't hold up under scrutiny. About the only truth is burn more than you eat and you will lose weight (Eventually).
    – JohnP
    Sep 14, 2018 at 19:52
  • @JohnP - do you have a source for that? Apart from the self evident statement that different fats can have different calorific values, it would seem that the key part of the question is a good one - whether to factor in that losing 1kg of fat is prima facie also linked to losing an additional 1.5kg of water, if the percentage of water overall is relatively constant, (and it's not clear if quoted calorific values routinely factor in the water or not, or if water is lost with fat loss: a 2.5x difference would be far beyond "outmoded data" I rhink)
    – Stilez
    Sep 15, 2018 at 23:19
  • @Stilez I will find some of the refutations. It is like 220-age, it was a bad interpretation that gets repeated because it is easy. I will have to investigate the water correlation, that is the first I've heard of "permanent" water loss. The body is remarkably good at homostasis, so I would doubt that as well.
    – JohnP
    Sep 16, 2018 at 15:23
  • @Stilez here is one review of 5 weight loss studies: nature.com/articles/ijo201351
    – JohnP
    Sep 16, 2018 at 15:25
  • @JohnP Just a clarification of the “burn more than you eat” sentence. The statement is evidently true. Keep in mind the key goal is not to burn more, but to eat less. In the past few years, the role of exercise in weight loss has lost a lot of significance. Diets are also only effective by a small margin. Permanent weight loss demands a permanent cut of calories consumed.
    – Narusan
    Sep 16, 2018 at 16:33

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Weight loss induced by calorie restriction is rather complex phenomenon which depends on plethora of factors like metabolic pathways, energy conservation, nutrient composition, BMI, age etc. (Reducing Calorie Intake May Not Help You Lose Body Weight, D. Benton et al, Perspectives on Psychological Science, June 28, 2017 ). However as a general guideline it can be taken that:

Reduced energy intake (by about 500 kcal/d) will promote gradual weight loss over time at the expected rate of 1–2 kg per month.

"2006 Canadian clinical practice guidelines on the management and prevention of obesity in adults and children", CMAJ, April 10, 2007, Vol. 176, No. 8

After simple arithmetic transformation, ~10 000 kcal calorie restriction leads to the loss of 1 kg body mass. If we take an average of 25% of mass of human body is attributed to the fat tissue so roughly 40 000 kcal restriction is required to loose 1 kg of body fat.

For additional information you can consult:

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