We live in the Middle East so well used to heat when we go out but of course have ac at home. We went to Italy mid June and it was about 32 -36 degrees and humid. On our 3 rd night dd threw up all her dinner 2 hours after eating and 15 mins after she was in bed, prob due to heat cos after that it was just pure torture, she would eat breakfast - weetabix but I really had to coax her and dinner 10 hours later, again weetabix and again silly games books etc to make her eat, she never drinks anything during the day but she was drinking milk and water like it was going out of fashion, she always had her cup in her mouth. And yes she lost weight. On our 2nd day back home she started to eat normally again. I guess there is nothing you can do really, I worried for the first 4 days but then saw dd was fine so just let her get on with it.