Can Human Beings Digest Grass? The Biological Reasons We Can't

November 12, 2025 •

4 min read

The human body does not produce the enzyme cellulase, which is required to break down cellulose, the primary component of grass. Consequently, while it is not poisonous, eating grass would provide minimal to no nutritional benefit for human beings.

Quick Summary

Humans cannot effectively digest grass due to the lack of the specific enzyme needed to break down cellulose. The human digestive system is anatomically different from herbivores like cows, which rely on specialized stomach compartments and symbiotic bacteria to extract nutrients.

Key Points

Missing Enzyme: Humans lack cellulase, the enzyme required to break down the complex carbohydrate cellulose found in grass.
Cellulose vs. Starch: The molecular bonds in cellulose are different from those in starch, and human enzymes cannot break them down.
Herbivore Adaptations: Animals like cows have specialized multi-chambered stomachs and symbiotic bacteria that produce cellulase, allowing them to digest grass efficiently.
Acts as Fiber: When humans eat grass, the indigestible cellulose passes through the system as roughage, providing little to no nutritional value.
Gut Microbiome Contribution: Some human gut bacteria can ferment insoluble fiber, but their contribution to cellulose digestion and energy extraction is minimal compared to herbivores.
Nutritional Inadequacy: Even if digestible, grass is not nutrient-dense enough to sustain humans, who have evolved to thrive on a varied omnivorous diet.

The Inability to Digest Cellulose

At the heart of the matter is a complex carbohydrate called cellulose. Cellulose is a glucose polymer that forms the primary structural component of plant cell walls, giving grass its stiffness and rigidity. For the body to extract energy from any carbohydrate, it must first break it down into simple sugar units. In the case of starch, the chemical bonds are easily cleaved by the human enzyme amylase, which is present in saliva and the pancreas. However, the glucose units in cellulose are joined by a different type of bond, known as a beta-acetyl linkage.

Humans, like all mammals, simply do not possess the enzyme, known as cellulase, that is necessary to break these specific beta-acetyl bonds. Without cellulase, the large cellulose molecule remains intact and passes through our digestive tract mostly untouched. This is in stark contrast to herbivores, which have evolved unique ways to process this tough plant fiber.

The Complex Composition of Grass

Beyond just cellulose, grass is a composite material that presents further challenges to human digestion. A plant cell wall also contains other reinforcing polymers, such as hemicellulose and the incredibly durable lignin. Lignin is a complex aromatic polymer that binds to cellulose and hemicellulose, further increasing the cell wall's strength and resistance to degradation. This makes it difficult for any digestive process, even with cellulase, to access the energy stored within.

Furthermore, many grasses contain high levels of silica, a hard, abrasive compound that can cause significant wear and tear on teeth over time. Herbivores that graze on grass, such as cows, have specialized, continuously growing teeth to compensate for this constant abrasion, an adaptation that humans lack.

A Tale of Two Digestive Systems: Human vs. Ruminant

The most striking difference between humans and grass-eating animals lies in the structure and function of their digestive systems. Humans are monogastric (single-chambered stomach) omnivores, adapted for a varied diet of fruits, vegetables, grains, and meat. Ruminants, such as cows, are herbivores with a complex, four-chambered stomach specifically designed for processing fibrous plant material.


Feature	Human Digestive System	Ruminant Digestive System
Stomach	Single-chambered	Four-chambered: Rumen, Reticulum, Omasum, Abomasum
Enzyme Source	Produced by the host (amylase, protease)	Symbiotic bacteria produce cellulase in the rumen
Digestion Process	Swift enzymatic and acidic breakdown in stomach and small intestine	Extensive fermentation by microbes in the rumen, followed by rumination
Cellulose Breakdown	None; passes as indigestible fiber	Efficiently broken down by microbes in the rumen
Nutrient Absorption	Primarily in the small intestine	Nutrients (like volatile fatty acids) absorbed in the rumen; microbial protein also digested

In a cow's rumen, billions of symbiotic bacteria and other microorganisms produce cellulase and other enzymes, breaking down tough plant fibers through a fermentation process. The animal then regurgitates and re-chews this material, known as cud, to further break it down. This lengthy and specialized process allows the cow to absorb the volatile fatty acids produced by the microbes, which serve as its main source of energy.

The Role of Our Gut Microbiome

While humans can't produce cellulase, our own gut microbiome does contain some bacteria capable of fermenting insoluble fibers, including trace amounts of cellulose. These bacteria reside primarily in our large intestine, and their fermentation produces short-chain fatty acids (SCFAs), which can provide a small amount of energy to the host. However, this process is far less efficient than the system found in ruminants, and the amount of energy gained is minimal.

Furthermore, a 2024 study suggests that some cellulose-degrading bacterial strains, likely acquired from ruminant contact during the domestication of animals, are now becoming rare in industrialized societies. This suggests our modern, low-fiber diets have led to a loss of microbial diversity that could impact our gut health. The study authors speculate this might be why evidence for cellulose fermentation in the human gut is so scarce today. For more on this, see the study in Science.

What Happens When Humans Eat Grass?

If a human were to eat grass, it would pass through the digestive system mostly intact, acting as roughage or dietary fiber. This undigested fiber is beneficial for gut health in moderate amounts, aiding in regular bowel movements and contributing to satiety. However, consuming large quantities could cause digestive distress, bloating, and nutrient malabsorption. In times of extreme famine, people have reportedly consumed grass out of desperation, but it cannot sustain life due to its lack of bioavailable nutrients. Trying to subsist on a grass-only diet would lead to severe malnutrition and potentially life-threatening bowel perforation.

Conclusion: Our Omnivorous Design

The biological inability to digest grass is a testament to the specialized nature of our digestive system. Unlike herbivores designed to extract nutrients from fibrous plants, humans have evolved as omnivores, with a shorter intestinal tract and the enzymatic machinery to process a diverse diet. The key difference lies in the missing cellulase enzyme and the vastly different digestive anatomy. While we may share some symbiotic gut bacteria with herbivores, our capacity to break down cellulose remains insignificant. Our primary nutritional intake comes from digestible carbohydrates, proteins, and fats, not the tough cell walls of the plants around us. Though grass can act as a source of fiber, attempting to use it as a primary food source is a futile and dangerous endeavor.

Frequently Asked Questions

Humans cannot digest cellulose because our bodies do not produce the necessary enzyme called cellulase. This enzyme is required to break down the specific chemical bonds in the cellulose molecule.

If a person eats grass, it will pass through their digestive system largely undigested. It may act as a source of dietary fiber, but it provides no significant nutritional value and could cause digestive discomfort in larger quantities.

No, grass is not inherently poisonous for humans, but it is also not nutritious and is not meant for consumption. Consuming large amounts can lead to digestive issues.

Cows and other ruminants have a multi-chambered stomach, with the largest chamber (the rumen) housing billions of symbiotic microorganisms that produce cellulase and ferment grass into energy.

Yes, some species of bacteria in the human large intestine can ferment some insoluble fibers, including trace amounts of cellulose. However, this process is not efficient enough to provide significant energy from grass.

Grains are the seeds of cultivated grasses and are highly digestible for humans, unlike the fibrous leaves and stems of grass. During the plant's life cycle, nutrients move from the leaves to the seeds, making the seeds more concentrated and nutritious.

Indigestible fiber, like the cellulose found in grass, is beneficial in the human diet as roughage. It adds bulk to stool, aids in the movement of food through the gut, and supports overall bowel health.

Evolving to digest grass would require significant anatomical and genetic changes over a very long period. It's an unlikely and impractical proposition given our current diverse diet and the lack of evolutionary pressure to do so.