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.
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.
The study you point to here is a guideline on conducting studies. It was unfortunately not available online so I can't evaluate their recommended methodology. Looking for actual studies that tried to do randomized feeding trials, I found "A randomized controlled-feeding trial based on the Dietary Guidelines for Americans on cardiometabolic health indexes"[1] as a top hit, which fortunately had the full text [2] available.
Randomized controlled-feeding sounds good, let's check it out. After trudging through this for a bit I came to the meat of the methodology:
> Participants were provided a daily meal checklist (Supplemental Figure 1) that included each menu item with space for documenting the amount consumed; the time each item was consumed; a checkbox to confirm having only eaten study foods; a checkbox to confirm not taking any medications, supplements, or other remedies; space for documenting any adverse events related to eating the meals; and space for documenting any nonstudy foods, drinks, medications, supplements, or other remedies. They were also instructed to return all unwashed packaging; visual inspection was documented by the metabolic kitchen. In addition to the checklists and returned packaging, participants were educated on food safety as well as provided tips on managing challenging social situations while participating in a feeding study. Repeated reinforcement of the value of honesty over perfection was provided. Study coordinators reviewed the returned checklists with the participants to verify completeness.
I'm not sure what the intention of your comment is. Yes, I linked to the guidelines on feeding studies because that is entirely the point of my comment.
You linked to a study where food was provided to the subjects (the food obviously nutritionally selected and provided per the study groups), and the subjects obviously are assumed to stick to the provided food and to accurate report what they ate among that reported food (with the study counting packaging, remainders, etc). This is a *UNIVERSE* better than the classic "tell us how many eggs you ate over the past two months" type nutrition studies, which are by far the most common (e.g. the Nurses' Health Survey).
Are you expecting the people to be inprisoned? I mean, there are in-patient studies but they are obviously massively more difficult to carry out.
> Are you expecting the people to be inprisoned? I mean, there are in-patient studies but they are obviously massively more difficult to carry out.
I expect rigorous methodologies to be employed before conclusions are drawn or held to be widely applicable. Self-reporting is intrinsically flawed. It does not seem like feeding studies as defined here addresses this or has been validated to produce superior results -- detected non-compliance was significant (though they did not report the difference between self-reported non-compliance and methodologically detected non-compliance) and undetected non-compliance was of course not measured.
Would I expect to be only satisfied by imprisonment or inpatient studies? I don't even think I'd be satisfied by that! The differences in activity would make all such results difficult to interpret. But if inpatient is the best we can do, but it's difficult, then we have to live with the fact that our understanding of nutritional interventions is extremely dubious. You can't just accept bad science because it's the best you can do.
Prisons give you the control needed, but prisons generally are not realistic to how people could live their lives. When you are locked in a cell most of the day that limits movement (in ways different from an office where people get up to go to meetings and the like). Prisons will get you your 20 minutes a day of exercise, but it isn't representative of how most people will exercise (even counting only those who go to the gym). As such you can get a lot of data but it is unknown which data applies to normal people who live lives in ways that are likely different in ways that matter.
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.