People are bad at reporting what they eat. That's a problem for dietary research

A lot of people seem to have a purely emotional relationship with resources which logic doesn't seem to be able to penetrate. Food and finances seem similar here. For years I tried to get my wife to stick to a grocery budget. That is, we have $n per week for all groceries. She'd blow badly over the limit every time. "But we needed [food]" or "These were toiletries, so they don't _count_ as groceries." Ultimately we never had an real success sticking to a grocery budget, and ultimately the solution was me working towards better paying jobs.

This feels a LOT like weight loss. Calories in --> calories out is conceptually very simple, but in practice more people struggle with it than not. It certainly cannot be the case that they struggle with the concept; they struggle with actually putting it into action. Lying to themselves, twisting themselves into philosophical knots, and probably most often, conceding to their cravings. Food acquisition is one of out more basic drives, so it should probably be no surprise that people struggle to intellectualize it.

(as and aside, there are also people who wrongly believe that calories in --> calories out is a flawed concept because not all people have the same metabolism, or not all calories are equal. Both of these are true, but none of them actually negate the premise. For whatever your metabolism, and whatever your category of calories, fewer calories will still produce weight loss. It may feel unfair that someone doesn't have to work as hard as you to produce the same result, but this is actually true in all areas of life. Now that said, improving the quality of your calories is very important, and should not be ignored -- but it also does not negate the premise.)

"> Is coffee good for you? What about wine or chocolate? Scientists trying to answer these questions"

These are dumb questions to ask in the first place, because the "you" and "good" here are too personal for any general answer to be useful to most people. Unfortunately, this is not just lazy writing that took complex questions and simplified them to the point of uselessness - we really are asking these kinds of questions :(

Most of this doesn't generalize to populations the size of the world in the way something like "physics" does, because, for starters, we aren't very deterministic or very homogeneous at large scale.

Instead, you end up with millions to tens of millions of people in a subgroup particularly affected or unaffected by something because of genetic variation, etc.

Any reasonable scientist knows this. Instead, the main reason to try to answer these questions framed like this seems to be either to get funding, or to make headlines.

Sometimes we can answer extreme versions of this question (IE it seems data suggests alcohol is fairly universally bad for almost any person, definition of bad, and amount), but that's pretty rare. This then gets used as a "success" to do more poorly designed and thought out studies.

Just because we want to know things doesn't mean we should use mechanisms that we know don't work and produce mostly useless results. This is true even when we don't have lots of mechanisms that do work or produce useful results.

It's much slower and much more expensive, but what we learn is at least more useful.

It's really hard, slow, and expensive to answer questions about particle physics - this doesn't mean we revert to asking atoms to self-report their energy levels and publishing headlines about how "larger atoms that move around more live longer" or whatever based on the results. Instead, we accept that it will hard, slow, and expensive, and therefore, we better get started if we want to ever get somewhere.

Eventually you learn recipes and their values. I memorised a lot of basics. But mostly I cut out non-vegetable carbohydrates and ate a ton of salads with nonfat Greek yogurt and hot sauce as a dressing, and whey protein.

For weighing things, I have a kitchen scale that lets me tare it with something on it. I find it easier to tare a container of an ingredient, then dose some of that ingredient out, then reweigh it to get the delta I put in. For things which have a dash of an ingredient I'll just guess. A few grams here and there won't really matter much.

For partitioning a meal: Sometimes I weigh my portion. Over time I've trained myself to estimate the weight of what I take such that my visual estimates are reasonable. Eventually my visual estimates have gotten better.

A lot of your other challenges are just not that important: If you're off by a few calories in either direction, it's not a big deal. It'll average out in the long run. If you're systematically off, you'll eventually recalibrate your goals anyway based on how you feel and/or your weight patterns vs what the calorie counts tell you.

Such studies do exist - randomized feeding trials. In these studies the participants are provided all meals and snacks, and sometimes are under constant surveillance for weeks and sometimes months on end.

https://pubmed.ncbi.nlm.nih.gov/39134209/

Obviously such studies are far more invasive and expensive to run than the classic "fill out of a survey" observational study [1], so they tend to be the outliers. But they exist and have incredibly useful results.

[1] There is a widely cited nutritional survey vehicle called the Nurses' Health Study, and it is the foundation of countless largely disposable nutrition clickbait results. This survey-based observation has been used to prove that meat is bad for you, and good for you. That artificial sweeteners make you thinner, and fatter. And on and on. That single "every now and then try to remember the kinds of things you ate over the past period of time" survey is the root of an incredible amount of noise in nutrition science.