A recent study has shown there's more to the old 'it takes a 3500 calorie deficit to lose a pound' formula.

‘In 1950s, medical researcher Max Wishnofsky found that a pound of fat tissue represents 3,500 kilo-calories. But, researchers have now found that it over predicts how much people will lose by a sizable margin.’

But, according to Hall, Wishnofsky made a couple spurious assumptions. First, he assumed that when you lose weight you only lose fat tissue. The much bigger mistake Wishnofsky made was misunderstanding how our bodies react to weight loss. As soon as we start cutting calories from our diet, the number of calories our body expends begins to fall. The reason Wishnofsky, and so many others since, have botched this biological fact is that it's fairly counterintuitive.





The tendency is to assume that as you lose weight, the same calorie cut back should prove even more effective once you are lighter, and, presumably, in need of less food. At the very least, it should continue to produce the same results as it was when you were heavier. Hall has, in many ways, spearheaded the movement to shed the nutrition world of the 3,500 calorie rule.
In 2011, he created a model, called the Body Weight Planner, which directly challenged the adage. Drawing from a vast pool of data, the tool approximated metabolic changes in people trying to lose weight, and showed how greatly the 3,500 calorie rule overestimates weight loss.
Hall said, "When we made the comparison between the 3,500 calorie rule and the revised prediction, we found that it over predicts how much people will lose by a sizable margin." Hall prefers to say that losing weight is difficult. Most people who try to lose weight, he says, end up back to where they started in less than a year. But he blames popular but misleading rules for the long-term failure of so many diets.
Source-ANI