Whey protein isolate is a byproduct of cheese manufacturing, and it is the liquid that is left over after the milk has been processed. WPI consists of approximately 90% protein, with almost zero carbohydrates and fats. WPI also has almost no lactose (99% lactose-free), this is why those who have sensitivities to lactose often choose WPI as their protein source. WPI has a full amino acid profile, making it one of the best protein sources on the planet.