David Foster -- Whitney Houston Had Bad DNA

Mega-music producer David Foster says Whitney Houston's deteriorating voice was not the result of drug abuse ... she was just saddled with bad DNA.

Foster knew Whitney's highs and lows better than most. He produced some of her tracks from "The Bodyguard," including "I Will Always Love You."

Foster is almost shockingly candid in the video ... in so many words saying she didn't have much of a voice for most of her career.