[Grishnakh]

Not only that, but people's bodies differ greatly. People have different metabolisms, males use more energy than females, some people are bigger (taller) and heavier than others, etc. Also, people's bodies just work differently, some of which may be attributable to gut bacteria. Some people can get away with eating a lot more calories than others, or different foods, and have a totally different effect.

[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.