Vegetarians live longer but what does that tell us?
Nothing basically. It has long been known that Seventh Day Adventists, who are mostly vegetarians, live longer. But why? It could be that a strong religious commitment has a destressing effect or it could be that they also deplore smoking. Or it could be that they spend time in church instead of doing dangerous sports. Church is a pretty safe place. So you just can't disentangle cause and effect in the case of the Adventists.
So it is no surprise that a new study just out -- Association of Animal and Plant Protein Intake With All-Cause and Cause-Specific Mortality by Song et al. -- also tells us that vegetarians live longer. Their data was from two high quality U.S. databases but they appear not to have bothered at all with controls.
And you can understand why. Vegetarians will mostly be health conscious people with strong will-power and such people will undoubtedly engage in a range of safer behaviors -- smoking less, avoiding dangerous drugs, exercising more, driving more slowly, climbing fewer mountains etc, etc. And all those things could contribute to a longer lifespan. Vegetarianism may be only the indicator, not the cause.