Support brain health, learning and memory with NOW Magtein® - Magnesium L-Threonate. Magnesium is essential to hundreds of biological processes throughout the body. In the brain, it supports neuronal health and function; but traditional magnesium supplements are not easily accessible to the brain cells that need them for cognitive health. Magtein - Magnesium L-Threonate is a patented form of magnesium that is able to cross the blood-brain barrier to promote optimal magnesium levels in the brain.