The study says the participants were 62% female, overwhelmingly ate more vegetables and supplements, exercised more than the control group, were mostly white, as well as self reported glucosamine supplementation - without dosage information. Overall it might be more indicative of wealth, education, and fitness than the effect of glucosamine. The paper itself says it’s not able to differentiate the effect from lifestyle changes:
> 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.
The last time I did googling on efficiency of glucosamine (several years ago, to combat runner's knee) I only found clinical trials sponsored by companies which produce glucosamine. Who's sponsoring this study?
That is possible but the large sample size may help. The study involved roughly a half million people with around one hundred thousand supplementing with glucosamine. With that many people and these big numbers the results are rather strong.
Sampling addresses variance, not confounding factors.
If the sample is not representative of the broader population, then in no way will any amount of sampling correct that error.
Observational studies often involve quite large samples, but they remain unreliable because it is extremely difficult to address all possible confounding factors. Experimental intervention is the gold standard for a reason, and absent the ethical or practical impossibility of experimentation, should be used here.
