30 minutes of intervals? Damn that is alot.
If you are very active, then you will need more calories if you plan on putting on mass. Of course there are different types of training like hypertrophy based and strength based. A mix of both is usually good.
If you are just doing the Intervals for "conditioning" then you can always have something before and right after them if you are not focusing just on fat loss.
The best way for most to gain mass....is to eat more and exercise less....or be like pro athletes who take in like 5000-8000 cal a day.