Close-up of butter and coffee in a mug - karins/Shutterstock Nowadays, the term "bulletproof coffee" has outgrown its origin, no longer referring to the specific brand but to the three-part coffee ...

Over ten years ago, Bulletproof popularized mixing coffee with butter as a form of biohacking. Now the company is moving in a new direction, and hopping on a more recent trend: Cold coffee at home.

Drinking coffee on an empty stomach may raise the risk of increased stomach acidity and gastrointestinal discomfort. It may also affect hormone levels.