Just as antibiotics are associated with faster growth in cattle, a decrease in diversity in the human microbiome is associated with obesity. As the usage of animal antibiotics exploded in the 20th century, so too did usage in humans. The rise coincides with the obesity epidemic. This could be a spurious correlation, of course—lots of things have been on the rise since the ’50s. But dismissing it entirely would require ignoring a growing body of evidence that our metabolic health is inseparable from the health of our gut microbes.