Significant evidence suggests they’re lithovoric/xylophagic autotrophs that supplement their diets with scavenging.

Powerful stomach acids and cellulytic enzymes allow them to break down cellulose (wood fiber) into sugars which are then broken down into glycogen stored in the liver. This allows rapid and energetic bursts of speed, paired with oxygen-storing proteins in the muscle that allow aerobic respiration to continue long after a human runner tires out. This also makes it more difficult to choke them out, as they can go for 5-10 minutes without air.

Creatine allows rapid rephosphorylation of spent ADP, though the release of metabolic byproducts and therefore oxidative stress necessitates the consumption of antioxidants in order to prevent cancers and accelerated aging.

The consumption of raw minerals such as zinc and iron allow for a truly gargantuan volume of semen to be produced very rapidly. They produce a dilute sulfuric acid in their semen, which helps break down the rock they consume and tunnel through.

Interestingly enough, the fat, distended stomachs needed to metabolize all that cellulose made them ill suited to crawling between walls. Instead, rape dwarves have evolved to rely on external digestion. I’ve observed groups ejaculating onto wooden building materials and absorbing the products as food, which at that point is predigested and less voluminous than raw wood itself.

Scavenging is purely opportunistic. They will eat leftovers if given the chance.