As a layman non-scientist, I wonder why we see so many relatively low-value observational studies in diet/nutrition, often with results that get contradicted or fail to replicate a few years later.

What would prevent us from doing a study like this: take 1000 diverse subjects, assign each of them a lab assistant, and for 2+ years, diligently track everything they eat, their exercise, sleep, etc. You could pay each participant and staff $100k/each, and the cost would be on the order of $400MM.

May not be epistemologically perfect given that people change their behavior when observed, but still superior to plenty of the stuff getting published now, and likely the knowledge gained for humankind would be worth many billions.