Humans cannot effectively digest grass, a common plant material. While grass is technically non-toxic, the human digestive system is not equipped to break down its primary structural component. This fundamental difference means consuming grass provides virtually no nutritional benefit and can lead to various uncomfortable physical effects. The reasons behind this inability are rooted in the specific enzymes and digestive structures present in the human body compared to those found in specialized herbivores.
The Role of Cellulose in Digestion
Humans cannot digest grass primarily due to cellulose, a complex carbohydrate forming plant cell walls. Cellulose is a polysaccharide with strong beta-1,4-glycosidic bonds linking its glucose units. Human digestive enzymes break down alpha-glycosidic bonds, like those in starch. However, the human body does not produce cellulase, the enzyme necessary to break these beta-1,4-glycosidic linkages.
Without cellulase, cellulose molecules remain largely intact as they pass through the digestive tract. Instead of being broken down into absorbable sugars, cellulose acts as dietary fiber, adding bulk to waste material and aiding in bowel movements. While this fiber is beneficial for gut health by promoting smooth intestinal function, it means that the energy and nutrients locked within the plant cell walls of grass are inaccessible to the human body. Consequently, grass offers virtually no caloric or nutritional value to humans.
Specialized Digestive Systems of Herbivores
Animals that digest grass possess specialized adaptations humans lack. Herbivores, such as cows, sheep, horses, and rabbits, rely on symbiotic microorganisms within their digestive tracts. These microorganisms produce the necessary cellulase enzyme, enabling them to break down cellulose into usable nutrients.
Herbivores are broadly categorized into two groups based on where this microbial fermentation primarily occurs. Ruminants, like cows, have multi-chambered stomachs, with the rumen serving as a large fermentation vat where microorganisms break down cellulose before the food reaches the true stomach and small intestine. This process often involves regurgitation and re-chewing of partially digested food, known as “chewing the cud,” to further reduce particle size and maximize nutrient extraction. Conversely, hindgut fermenters, such as horses and rabbits, perform microbial fermentation in enlarged sections of their large intestine, specifically the cecum and colon. These specialized digestive organs provide an environment for the cellulose-digesting microbes to thrive, allowing the animal to absorb the byproducts of fermentation, such as volatile fatty acids, for energy.
Consequences of Humans Eating Grass
Consuming grass would not provide sustenance and could lead to several unpleasant physical effects. Since the human digestive system cannot break down cellulose, grass passes largely undigested through the gastrointestinal tract. This indigestibility can lead to significant gastrointestinal discomfort, including bloating, gas, stomach cramps, and diarrhea. The fibrous nature of grass, coupled with its abrasive silica content, can also cause mild irritation to the digestive lining and contribute to the wear and tear on teeth over time.
Beyond discomfort, grass offers negligible nutritional benefits because its nutrients remain encased within indigestible cellulose. While small amounts might be ingested without severe harm, relying on grass as a food source would quickly lead to malnutrition and starvation, as the body cannot extract the necessary calories, proteins, or other essential nutrients. Furthermore, grass found in natural environments may contain pesticides, herbicides, parasites, or bacteria from animal waste, posing additional health risks if ingested.