7700 Kcal deficit per per kg weight loss, or just 3600 Kcal deficit per kg weight loss?
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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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:
Managing Overweight and Obesity in Adults: Systematic Evidence Review
from the Obesity Expert Panel;
Diet in the management of weight loss, I. Strycher, CMAJ. 2006 Jan 3; 174(1): 56–63
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