Sure, but the more links in the chain, the more control you need in order to prove the causative agent. In the absence of direct experimental evidence (give ani

Sure, but the more links in the chain, the more control you need in order to prove the causative agent. In the absence of direct experimental evidence (give animals aspartame, they get cancer, which we haven't seen), you need to rely on huge observational studies. And these are exactly where you run into all kinds of confounders, like Patient has standard American diet and is overweight but not obese -> that leads to sleep apnea -> chronically tired -> they develop a diet Coke habit -> slight increase in cancer risk. What's causing the cancer here, the diet, the adiposity, the sleep apnea, the lack of quality sleep, the soda itself, the effect of soda on teeth leading to gingival inflammation, or the aspartame in the soda?