I think weight gain affects asthma negatively.
Weight gain generally causes loss of breath. This happens particularly if one's airway condition isn't good. Also steroid medications often increase weight.
Fred Pescatore's 'The Allergy and Asthma Cure' might help. This book devotes one chapter to weight loss diet program. I've not read the chapter but I'm sure the book is helpful for those who's interested in nutritional approach to asthma.
My diet is based on the book. Hope this would help.