[woobar]

All these factors are accounted for. I.e. in a controlled environment individual metabolic rate, weight, gender, etc will be measured.

[HelloMcFly]

What wouldn't likely be measured with any sort of accuracy is individual variance is gut microbiota, which I'd assume could lead to notable differences. Even so, I think we'd get more signal than noise doing our best to measure and factor out what we are currently able to.

[biomcgary]

Sequencing to identify gut microbiota is now common in research. The general makeup of gut bacteria is relatively stable, so weekly or monthly sampling should be adequate.

[DrScump]

Microbiome, one of the most important factors, is not accounted for. Neither is genetics.