His diet probably wasn't the cause for his illness, but as far as I understand, he refused ordinary medical procedures (surgery) that had a good track record against his particular type of cancer, in favour of observing a special diet. The diet didn't help and the cancer progressed to the point where surgery was no longer an efficient treatment.

From the article stratoscope (one comment tree down) links to: "My pancreas levels were completely out of whack, which was really terrifying … considering everything.” (Jobs died after a long battle with pancreatic cancer in October 2011) Food for thought..