How much fluid is actually in a lumbar disc?

70-90% The normal young adult disk is about 70-90% water, but it is held in the cells and other parts of the disk. Some of the molecules act like sponges to suck in and hold onto the water molecules. As we stand and sit and work throughout the day, some gets squeezed out; when we lie down and rest, those molecules suck it back in. As the disk ages, the percentage will decrease.
90% water at birth. Which decreases over time as part of the aging process to 70% by the 6th decade.