I would think that fasting can create a cranky person at times. I know it does with some people just dieting.
The person didn't ask for advise, so I guess he was somewhat insulted that you thought he needed help. Apparently he was just wanting people to support whatever he was doing and felt like he knows what he is doing. You did say that it wouldn't work well for you...so the comment was more of an overall warning for others reading his post. But like I said, I think he was looking for support and he didn't take it as support. The troll comment was a little over the top though.
Don't stop trying to help. Others may benefit from your comment whether he did or not.