if dragons were able to blow fire with a breath, what did they eat?

Most fictional dragons are carnivorous. However, there are some omnivorous species as well. Dragons produce flame by way of a gland deep in thed throat. (The gland is also responsible for their smoky scent) When a dragon flames, a flammable compound produced by the gland coats the beast's mouth. The compound ignites when in contact with air, producing the flames. As the dragon exhales, thed flames follow its breath.

Well, I'm no dragon expert, but according to a book I read, dragons only need moonlight to eat. But again, I'm not an expert, so I may be wrong.

They are carnivorous, but in two series with dragons that I've read (Inheritance and Dragon Riders of Pern), the dragons also eat some sort of plant that helps with fire breathing.

they ate meat and a special kind of rock which ignited the carbondioxide which made fire

They catch animals,other dragons, and humans then slowly burn them!!

They eat meat, but the gases they make for the dijestion are that they breath fire