What is Phosphatidyl Serine? Phosphatidyl Serine plays a key role in cellular function, particularly in the brain. This is why it is often recommended for those experiencing cognitive decline. Interestingly, for an athletic population, phosphatidyl serine has also been shown to reduce the stress hormone cortisol. Cortisol stimulates the breakdown of muscle, so reducing cortisol levels is a positive for anyone looking to build muscle. BULK POWDERS™ Phosphatidyl Serine Tablets deliver a