Wood is a form of plant matter that contains stored energy captured through photosynthesis. This raises the question: if wood is composed of complex organic molecules, does it contain calories, and why can the human body not use it as fuel? The answer lies in the fundamental chemistry of the wood structure and the specific capabilities of the human digestive system. While we can measure the energy held within wood, our bodies lack the biological tools necessary to unlock that power, making it inaccessible for nutrition.
The Chemical Energy Stored in Wood
A calorie is a unit of energy, often measured by scientists as calorific value—the heat released when a material is completely burned. This measurement confirms that wood contains substantial stored chemical energy, comparable to many digestible foods.
The energy in wood is held within the carbon-hydrogen bonds of its molecular structure, established when the tree converts sunlight into sugars. Dry wood typically holds an average calorific value of 4.4 to 4.5 kilocalories per gram, nearly equivalent to the four kilocalories per gram found in pure sugar or starch. This shows that wood is energy-dense; the issue is the human body’s inability to access it. When wood is burned, combustion breaks these chemical bonds, releasing the stored energy as heat and light.
Why Human Digestion Fails Against Cellulose and Lignin
The stored energy in wood is locked away primarily in two complex polymers: cellulose and lignin. Cellulose, which makes up 40 to 55% of wood, is a long-chain carbohydrate composed of thousands of glucose units. Lignin, comprising 18 to 35% of wood, is a complex, three-dimensional, non-carbohydrate polymer that provides structural support.
The glucose units in cellulose are connected by beta-1,4-glycosidic bonds, which are the fundamental barrier to human digestion. To break these bonds and release glucose for energy, the body requires the specific enzyme called cellulase. Humans do not produce cellulase in their digestive tract, meaning the cellulose molecule remains intact as it passes through the system.
This differs from starch, the primary carbohydrate found in foods like potatoes and grains. Starch is also a glucose polymer, but its structure uses alpha-glycosidic bonds, which are easily broken down by human enzymes like amylase. The difference in the chemical geometry of the bond—alpha versus beta—determines whether a plant material is a source of energy or indigestible bulk.
Lignin presents a formidable chemical challenge because its complex, irregular structure acts as an impenetrable matrix that encases the cellulose fibers. No animal, including humans, produces the necessary enzymes (ligninases) to break down this aromatic compound. Herbivores like cows and termites can gain energy from wood only because they host symbiotic microorganisms in their gut that produce cellulase and other necessary enzymes.
The Physical Outcomes of Consuming Wood
Since chemical barriers prevent nutritional uptake, consuming wood results in zero usable calories and can lead to serious physical consequences. The material is not dissolved by stomach acids or digestive enzymes, remaining as rough, hard, and abrasive fragments. These fragments can cause mechanical damage throughout the gastrointestinal tract, leading to irritation and internal injury.
The physical consistency of wood, even if finely ground, means it does not pass through the intestines easily. The non-dissolving material can accumulate, leading to gastrointestinal impaction, or a blockage. This often requires emergency medical intervention, as the digestive tract is not designed to handle unyielding, coarse material.
Undigested wood acts similarly to insoluble dietary fiber, but at a dangerous extreme. While fiber aids in waste movement, wood offers no nutritional return and carries significant risk of injury. Furthermore, wood often contains foreign contaminants, such as chemical treatments or harmful fungi, which can introduce toxins into the body upon ingestion.