The enzyme (produced by many herbivores, but not by humans) that converts cellulose (plant fiber) to glucose. Engineered animal feeds often contain supplementary cellulase; I've seen it as an ingredient in some freaky human dietary supplements, but I'm not too sure of the wisdom of introducing foreign enzymes into the human digestive cycle. Further, most Americans don't get enough fiber, but get quite sufficient glucose from bread, cereal, pasta, fruit, and starchy vegetables.