Howdy, and welcome to pets.ca! I don't know the answer to your vit D query, but I do know that if you want your kitties to keep a nice figure, dry food is far from ideal. I would especially stay away from any "indoor" formulas. They tend to be far too high in carbohydrates for an obligate carnivore and are actually more likely to make your cats fatter. Really, all these
indoor/senior/breed-specific/diet/sensitive-stomach/dental/etc. foods are just marketing gimmicks by the petfood companies. A low-carb wet food would be a much better choice, for reasons that you can read about here:
http://www.catinfo.org/ and here:
http://www.catnutrition.org/index.php and here:
http://www.littlebigcat.com/index.ph...=show&item=016
You're right about it being easier to prevent weight gain than it is to deal with it after the fact, so good for you for being proactive. Ditching the dry would be the best way to get a handle on it.