Great, then I can also get mandatory personalized ads based on my current exact nutritional needs!

Yes. All studies have limitations. You report them and interpret the results accordingly.

Sure! Start broad, trying to determine what may be interesting and then design stricter and stricter experiments.

Unfortunately yes, that's the problem with a lot of the studies people are making life decisions on, psychology, dietary supplements... Between studies funded by companies and/or authors with conflict of interest, unreliable methods (in vitro antioxidants) or cohorts selection issues (grad students that need the money so they lie on selection interviews for experiments or trials)... In those cohort studies, what people eat is highly dependent on their wealth, education, localisation, jobs... And they rarely control for everything (mostly because it is impossible and if they asked people to remember instead of giving them devices they don't may not have the money to spend on a carefully designed experiment).

> We really need a safe, unobtrusive device that can record body's micronutrient intake on a daily basis.

Impossible.

This is how most nutrition "science" is conducted.

Every time you see a food study like this look at the details. You'll be surprised how often the data are based on these questionnaires.