"The UK Biobank used a baseline touch screen questionnaire to assess several potential confounders: sociodemographic characteristics (age, sex, ethnicity, Townsend Deprivation Index, education and average total annual household income), lifestyle behaviours (smoking status, alcohol consumption, physical activity, body mass index (BMI) and vegetable and fruit consumption), health conditions (CVD (myocardial infarction, angina or stroke), respiratory disease (chronic obstructive pulmonary disease or emphysema), cancer, digestive disease (liver failure, cirrhosis or alcoholic liver disease), dementia, depression, longstanding illness, hypertension, diabetes and high cholesterol), drug use (chondroitin, aspirin and other non-steroidal anti-inflammatory drug (NSAID) use), vitamin supplementation (vitamin A, vitamin B, vitamin C, vitamin D, vitamin E, multivitamin and folic acid) and mineral and other dietary supplementation (calcium, iron, zinc, selenium and fish oil)). "
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."
