Still, the authors accept correlation as a potential objection, despite doing their best to control for it. I suppose they need a randomised controlled study or

Still, the authors accept correlation as a potential objection, despite doing their best to control for it. I suppose they need a randomised controlled study or something to be more certain. "Third, regular glucosamine use may be a marker for a healthy lifestyle, but it is hard to distinguish the confounding effects of a healthy lifestyle from the impact of regular supplementations in an observational study. Although we had carefully adjusted for potential confounding lifestyle-related factors in our analyses, we could not exclude the possibility that the results were confounded by unmeasured lifestyle-related factors. In general, with the current observational study design the possibility of residual confounding due to imprecise measurements or unknown factors cannot be excluded for all findings in our study, despite our careful adjustment of all measured confounders."