At the core of Sustain lie the two most popular choices of protein powder, whey (in isolated and concentrated fractions) and casein (micellar). These two protein-types display potent effect on inhibiting protein breakdown and promoting protein synthesis, but not if used alone. Whey protein, the "fast-acting" protein, rapidly enters the bloodstream and exerts a considerable effect on increasing muscle protein synthesis (growth) rates. While this is ideal in some circumstances, the rapid entrance and clearance to and from the bloodstream does little to reduce protein breakdown. Casein protein on the other hand, the "slow-acting" protein, coagulates in the stomach forming a more timed-release of amino acids into the bloodstream. This effect raises muscle protein synthesis rates to a lesser extent than whey, but exerts a much more significant effect on protein breakdown inhibition over time and ensures muscle protein synthesis rates remain elevated for upwards of eight hours!