Too much glucosamine can kill beta cells, according to an article posted online before publication in the Journal of Endocrinology.
This is not good news for us. We need all the beta cells we can get
But it hasn't been clear if taking glucosamine orally would have the same effect as altering cell metabolism so that the glucose the cells took up was shunted into the hexosamine pathway. This pathway is an alternative way of metabolizing glucose.
The new study suggests that yes, oral glucosamine can cause harm to beta cells. In fact, it can kill them. The amounts used in the study, 1500 mg a day, were higher than those usually used for joint pain, about 5 to 10 times the recommended dose. And the study was in rodents. But as the authors note, some people take more and more glucosamine if the lesser amounts don't stop the pain.
However, they say they don't think that glucosamine taken in smaller doses would be harmful.
I think what this shows is that we really need to be careful with both supplements and prescription drugs, because in most cases we don't yet know exactly what they're doing. You have pain. You take something for the pain. But whatever you're taking may cause problems in the future.
Metabolism of glucose is complex. No one understands it all. Sometimes in vitro (in cell cultures) effects are not replicated in vivo (in the whole organism). But if glucosamine can kill beta cells in vitro, which happened in this study, I would definitely not want to be taking it for joint pain.
I did try it myself years ago, and after about 6 weeks my fasting blood glucose levels started going up. Because of the time delay, it took me a while to make the connection. But when I stopped taking the glucosamine, the fastings returned to where they had been.
Bottom line: only you can decide if whatever pain relief you get from glucosamine is worth the risk of increasing insulin resistance or harming beta cells. Just be aware of possible problems and be cautious.