Actually the story is more complicated than that although you are correct in surmising his overweight patients did loose weight but that does not explain why his thin and physically fit heart patients also improved. There were also quite intriguing consequences of the diet such as dramatic improvements with arthritis and asthma.
Check it out:
CBS interview -
http://www.cbsnews.com/video/watch/?id=7420352n
WHEAT BELLY blog -
http://www.wheatbellyblog.com/featured-articles/
Wheat is not alone in its potential unhealthy effects. The year-round consumption of large quantities of carbohydrates is a very recent development in the history of the human body—so recent that it is unlikely that our bodies have evolved to handle it.