You age and the calories out decreases; therefore, to maintain a stable body mass, your calories in must decrease as well. If you do not decrease calories in, then the result is gain in mass due to storage of excess calories.
Calories in, calories out.
The calories you consume affect your weight differently depending on your level of metabolic activity. So the key mediator here is how your body processes the calories you consume.
But talking about calories is wrong in any case. All calories are not the same. For example increasing glycemic index by consuming sugars and carbohydrates will lead to greater and more prolonged weight gain than consuming the same calorific load in protein - https://pubmed.ncbi.nlm.nih.gov/12081852/
It's simple misinformation to focus on calories rather than carbohydrate levels, physical activity, healthy gut bacteria etc.
Show me the study that says otherwise. The "complex interaction" theory is just an excuse used by people for failing to cut calories enough to lose weight. Notice that people taking GLP1 drugs don't have to adjust for all these complexities at all.
How does that number on the package get measured?
When I eat nuts, cashews, etc., did 100% of the food get digested and absorbed? A day later or so, there seems to be counterevidence of that plainly visible in the bathroom.
We are talking about the theory that if you eat fructose, or if you eat 'highly processed foods' there is some complex hormone response that causes those calories to be preferentially stored as fat or to slow the metabolism down so they aren't burned.
For sure you are correct when you say that the calories on the label of a food is just an estimate, and not a very good one.
When people talk about "restricting your caloric intake", they are very obviously talking about what's written on the package and not after making calculations based on whether it's fat, protein or carbohydrates. So I think both issues are very tightly related.
What's the diet you would eat every day to get into a calorie surplus even though you are in >500 calorie deficit according to the labels?
However, you can make the calculations based on the numbers I gave in my other comment - which are taken from Why Calories Don't Count from Dr Giles Yeo.
But this is assuming you calculate your daily calorie burn with one diet that is pure protein and then convert to one that has 100% calorie availability, you still can't overcome 500 calories of deficit on paper!
Realistically you would change diets, then find your baseline based on the new diet, and then calculate a calorie deficit from there. In that case you would be very close to a 500 calorie deficit, and you would unavoidably lose about 1lb of fat per week. A 500 calorie deficit sucks but you will adapt to it in a few months if you stick to it. I've managed to lose over 50lbs in the last year, probably close to 75lbs of fat mass. (I did use a GLP1, dieting is hard)
The cause of a failure to lose weight with moderate calorie restriction is always a failure to maintain the calorie restriction aka cheating on the diet (because it is incredibly hard to do when you aren't used to it).
I lived abroad for a year and, when coming back home, started eating healthily (i.e. more greens and less reds, avoiding added salt). In about 3 months, I managed to reduce my weight by 10% - about 6kg. Although I have to admit, if anything, I wanted to keep those kilograms because it was helping me for weight-training lol. My objective was never to lose weight.
oh well, you lost money then. It is not even difficult to understand, some types of food/digestion/absorption will hold more or less water weight to the body so it’s possible to lose total weight even eating above the daily needs, for example a nocarb diet
Whether you gain or lose weight through water weight has little to do with calories at all. If you take creatine, for example, it’s pretty normal to gain a few pounds of water weight until your creatine levels equalize in your body. That doesn’t mean that your body’s “metabolism” changes or something based on the calories you ingest. You’ve just got another pathway to gain/lose apparent weight, albeit temporarily.