Losing muscle after 40 can slow your metabolism and mess with blood sugar, but whether it directly causes insulin resistance is less clear.

Research supports that when people lose muscle, their bodies burn fewer calories at rest, and several studies show that bed rest or inactivity—which causes muscle loss—makes it harder for muscles to remove sugar from the blood, backing up the blood sugar claim. There's also evidence that older men who kept muscle gained less belly fat, suggesting muscle loss can lead to more fat. But the link to insulin resistance is tricky: one study found older people with less muscle tend to have higher insulin resistance, but other research shows that having insulin resistance can actually predict future muscle loss, not the other way around. So it's possible the relationship goes both ways, and the post's cause-and-effect claim might be oversimplified.