Just one point of data: I kept a spreadsheet when I lost 59 pounds in 96 days. I had values for my personal base burn as a function of current weight and per task (usually a rower and hiking), and a daily deficit of 2000 calories correlated fairly well with a daily loss of .55 pounds (in round numbers; I don't want to sound like the proverbial economist with a sense of humor. I also went over some and under some, used nutritional labels and activity estimates that rounded to the nearest 10, &c.)
I was not scientifically rigorous so grain of salt, but over three months or so, I anecdotally found that 3500 calories of deficit correlated very well with a pound of loss. After that I stayed pretty constant while I was paying attention, 1-2 years. Bit North of there now, but I don't do much counting anymore.
This is a good point. While "calories in != calories out" within a broad range of caloric inputs, because humans have some built-in ability to absorb fluctuations in food intake centered around each person's metabolic setpoints, you can definitely get some play at the extrema of the caloric intake/expenditure axis.
In the opposite direction from your example, if someone has a hard time gaining weight, they may find that eating 3000 cal/day has no effect but eating a carefully measured 6000 cal/day definitely moves the needle upward.
The problem in ...
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