Review of the TMAO Heart Disease Study | Understand Nutrition
The basis for Koeth et al.’s hypothesis is that vegetarians experience less heart disease than omnivores. Some studies (here and here) have indeed found this association. However, two studies (here and here) that compared vegetarians to other health-conscious people who ate meat and the association between meat consumption and heart disease disappeared.
These studies suggest that meat-eating itself is not the problem; rather an unhealthy lifestyle is to blame.