Stanley Hazen of the Cleveland Clinic in Ohio and his colleagues treated human platelets, which form blood clots, with a compound called TMAO. This is made in the body from a waste product of gut microbes, and has been linked to heart disease. The team found that TMAO made the platelets form artery-blocking clots faster. The researchers increased blood TMAO levels in mice by feeding them a diet that was rich in choline, a TMAO precursor, and found that the animals formed clots faster than did those with lower TMAO levels.