Confusing title, here’s what I mean.
Let’s say I’m on a weight loss diet for a week with a 500 kcal daily deficit and lost 2 kg (I don’t know if the math checks out, it’s just an example). If I had a 500 kcal surplus daily for a week, would I gain those 2 kg? Or is there a difference because of absorption/metabolism/whatever and I’d either gain less or more than that?
Let’s assume your math is correct (on mobile and don’t remember the values off the top): Yes. It’s that simple, calories in, calories out. If you eat 500kcal less than you use one week, and 500kcal more the next week, you’ll be at 0.
Now, important note: kcal values, as well as TDEE,. are approximations. Estimated a averages. They’re accurate enough in most cases, but they aren’t 100%. If you do a practical test don’t freak out if your results are slightly off one way or the other.
Additionally about eating more or less than you use, not total. If your TDEE is 2000 kcal (for example), and you ear 1500 one week, and 2500 the next… but you ran a marathon in one of those weeks, or were just much more active in general, your results will vary.
When you say “eating with a 500 kcal daily deficit”, that by definition means “absorbing 500 kcal less than my metabolism is burning.”
But because there can be absorption differences and, more importantly, differences in resting metabolic rate, then yes, the same quantity of food can have a different effect on weight gain.
There is a difference, based on the metabolic “waste” to reduce certain foods to their base nutrients and also based on your absorption that can be raised or lowered based on your microbiome and fiber intake.
Also 1kg/2lb of muscle is approx 4000kcal and 1kg of fat is approx 9000kcal (in or out)
So to lose 1kg of fat you need to have a 9000kcal deficit but this is not so directly to be measured as you can lose weight from fat, muscle, and even water.
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BUT THAT DIFFERENCE DOES HAVE ANY DIFFERENCE? ABSOLUTELY NOT
It’s a difference impossible to be measured without proper equipment and even so, it’s too little to deserve your attention. If you are trying to gain or lose weight, basically stay on a kcal surplus/deficit and observe the difference mostly in the mirror as it goes. If you’re on a kcal deficit or surplus you will for sure lose/gain weight just pay attention to the speed it happens bc it can mean you are not doing it in the best way. If I gain 1kg in a week, I know for sure that this is mostly water/fat