I had very bad heat intolerance when I had my biggest relapse ever anumber of years ago. Since then, I always felt better when it was 20C or less.
January 2010 I started up with Niacin (vit B3) and in the summer of 2010 I went to south east asia (for a reunion, I would not have made such a long trip to such a hot place otherwise).
Obviously I was dreading the heat but actually I was fine. I did some googling on Niacin and heat intolerace and I found some studies on Niacin and heat stress in live stock (hope I do not offend anyone)
As it happens Niacin drops the core temperature of cattle making them better heat resistant. Google Niacin and heat stress and you'll get loads of info.
http://en.engormix.com/MA-dairy-cattle/ ... 579/p0.htm
Although I still dont feel great when it's real hot, it is definately better. Might be something to try for people who struggle with heat.