Fatness and Thinness in the Middle Ages

Maybe not fatty (I couldn't say either way), but definitely carby. It's possible that Vietman's single most recognizable cultural export is rice noodle soup.

Paper and wood pulp are carbohydrates, but you cannot get any energy from eating those.

Petrol is 12kcal/g, more calorie dense than fat at 9kcal/g, but you can't get any energy from eating it.

"Ah, I mean the digestible things" I hear you say; well then - we've already established that "a calorie is not a calorie" if you need to add exceptions. But, with this exception:

Different kinds of foods have a different calorie extraction/absorption profile. paper/petrol have 0; pure sugar has 100%, most things defined as "food" are high on that scale, but usually not 100%; It also takes nontrivial energy to digest some foods. This is often lumped up into "atwater factors", the values of which seem to vary by 20-80% among different individuals and the same individual at different times.

The statement that "a calorie is just a calorie" is incredibly useless in the context of human metabolism.

Not true. There's a difference between complex carbohydrates provided by fruits, vegetables meat and other unprocessed food. Simple carbohydrates like are shorter and break down faster providing less energy per food unit consumed. Complex carbohydrates are longer and take longer for our bodies to breakdown giving more energy over a longer period of time.

That doesn't mean each calorie in what we call 'food' is as accessible to the body's metabolic processes as a calorie in other food. If i wrap a pellet of food inside a non-digestible shell, it will still burn in a bomb calorimeter or will still have about the same caloric measurement in constituent macro-analysis.

This doesn't mean calories are a useless model of energy uptake from foods, just that they are not perfect.

We also have to take into account that the food industry has not been exactly neutral with regards to this metric. If calorie counting is a thing, then they will game the imperfectness of the metric in any way possible to get the desired 'low' score on the label while still maximizing your addictive consumption cravings regardless of how fat it will or will not make you. So we can/could expect the 'calorie' as a unit of measure to become less and less acurate as a result of this gamign of the metric.

But, again, this still doesn't mean that there is no relation between food intake, energy expenditure and weight gain as some would like to portray.

If your consumption pattern is making you fat, reducing consumption while changing nothing about the diet composition or timing (yes, when and how you consume also has an influence on your metabolism) will still make you gain less weight or even make you lose weight depending on the ratio change.

The reason that previous obvious statement is still vilified is that there is an enormous industry around consumption and managing consumption. It is not by coincidence that the advice from last century about consumption moderation 'eat less' has largely been replaced by an 'eat different' message.

Finally, there is an enormous hypocrisy in creating a 'market' system that evolves through rewards the most ferocious and amoral predators that can push whatever 'food' optimized for maximal consumption addiction, and then turning around to the 'consumer' and proclaiming 'personal responsibility' and 'moderation'.

This antiquated trope is like HSV; it just won't go away.

Edit:

To clarify; consider the effects of {hypo,hyper}thyroidism, which include weight-gain and weight-loss, respectively. Regardless of diet.

Now consider the fact that what you eat affects how the endocrine system functions, similar to how thyroid disease wreaks havoc with it.

That's just one example, and I'm no expert, but have read some books on the subject. Another I vaguely recall is the pituitary gland getting damaged or otherwise malfunctioning causing uncontrolled weight-gain regardless of diet.

It's these edge cases of malfunctioning human bodies that helped illuminate how its energy storage is moderated with hormones. It didn't take much to confirm different foods trigger different hormone responses once scientists knew what hormones to look for.

My sister had serious fertility problems. She read many articles on the topic over the years and we talked about me possibly serving as a surrogate for her should she be simply unable to have a child of her own. She forwarded me some portion of those articles, so I am somewhat well read on the topic of fertility issues for American women.

It takes about 25,000 stored calories to support a pregnancy. Being underweight suppresses ovulation. Anorexic women can outright lose their periods.

I suspect this fact probably helped women pass themselves off as male soldiers and male pirates historically. Such professions are pretty physically demanding and I suspect it may have caused a lot of women to have fewer periods or no periods while passing themselves off as male, thereby helping them to hide their gender.

My oldest son has pointed out that we are probably underestimating the number of women who did that because, by definition, those that fully succeeded were never identified and died as male. A recent-ish story supports that suggestion.