I love Dwarves!!! I usually explain this in my own setting by having them keep pens of moles/ground hogs. They also eat insects in large quantity, by farming and or gathering them. Further, fungus is a viable food source, mushrooms can be grown in caverns. Mosses and lichen are some of the most nutritious food stuffs on the planet. Add underground streams and lakes, and you start to really get a plethora of vegetation, and the added bonus of fish/snakes/ and other water dwelling creatures.
Love Dwarves. Love your entries, I know i'm late, but I couldn't help but post to this one.