Though the rule has been on the books since the 1950s, it is churning new controversy at a time when butter consumption is on the rise in America as it's increasingly thought to be healthier than margarine. Butter made from grass-fed cows, such as Kerrygold, is a staple in some diets and for the "bulletproof coffee" movement, where such butter is mixed with coffee and MCT oil for purported — but debated — weight-loss benefits.