From Chowhound:
Have you had a friend who visited Europe for a while and raved about how healthy they felt while they were there? They may have even lost weight, even while eating the same amount of food. This is a common experience when people from the U.S. travel abroad, and while there’s no definitive reason for it, many suspect it’s because the EU bans certain ingredients that the U.S. doesn’t.
Many additives and hormones that are present in everyday foods in the U.S. are either banned or heavily restricted in Europe. This is due to the different ways that the Food and Drug Administration (FDA) and the European Food Safety Authority (UFSA) decide which foods should be banned. The EFSA bans foods if there’s a valid chance they could cause harm, while the FDA needs to see proof of harm before banning something. As a result, there are several ingredients that are banned in Europe, but not in the U.S.