The McVitamins website tells us:

The prostrate is a small chestnut sized gland located just beneath the urinary bladder

No, not quite! This is a very common error. It is now so common that the prostate, the gland surrounding the neck of the bladder in male mammals (which comes from Greek prostates, one that stands before) is commonly misspelled as prostrate meaning lying stretched out on the ground with one’s face downwards (which comes from Latin prosternere, to throw down). It is also common to find people prostating themselves.

If you Google prostrate you will find several sites for prostrate [sic] cancer. These are either mistakes or deliberate attempts to capture people searching using the wrong term. Also on Google there any many links to individuals who would prostate [sic] themselves before all manner of things but not a bladder.

So be careful! This is an area for embarrassment. Your spellchecker will not pick these up but an editor or proof-reader will.