Why are humans unable to use cellulose as a source of glucose?

One of the remarkable features of cellulose is its structure, which is primarily comprised of glucose molecules linked by beta-1,4-glycosidic bonds. These bonds form when a hydroxyl group on the first carbon (C1) of one glucose molecule connects with the fourth carbon (C4) of an adjacent glucose molecule, creating a linear chain.

The robust nature of these bonds gives cellulose its strength and high tensile properties, which is why it is an essential component of plant cell walls. However, this resilience also means that breaking these bonds requires specific enzymes that can attack this unique linkage. Unlike other carbohydrates such as starch or glycogen, which have alpha linkage, the beta-linkage in cellulose cannot be easily broken down by human digestive enzymes.

Our digestive system secretes various enzymes that are specialized to break down different types of molecules. For instance, salivary and pancreatic amylases are adept at cleaving the alpha-glycosidic bonds found in starch, thus converting it into simpler sugars like maltose and glucose, which can be absorbed by the body.

However, humans do not have the enzyme cellulase that can degrade cellulose due to the beta-1,4-glycosidic bonds. While some microorganisms and animals, such as termites and ruminants, produce cellulase or host symbiotic bacteria that can break down cellulose, humans lack this capability. As a result, our body treats cellulose as insoluble fiber, which aids in digestion by providing bulk but is not a source of glucose for energy.

Cellulase deficiency in humans is not a condition in the same sense as other enzymatic deficiencies because, in our evolutionary history, there was no requirement for humans to evolve the ability to produce cellulase. This is largely due to our diet and the ready availability of other carbohydrate sources that we can more readily digest.

Insoluble fibers like cellulose pass through our digestive system untouched, providing benefits such as promoting a healthy digestive tract and preventing constipation. Despite cellulose's role as a non-digestible fiber, our inability to break it down into glucose does not categorize as a 'deficiency' per se. It's simply a reflection of our physiology and the specific set of enzymes that our bodies naturally produce.