Can Human Beings Survive on Grass? A Look at the Digestive Differences

January 11, 2026 •

4 min read

In principle, a human could physically eat grass, but a study published in the Proceedings of the National Academy of Sciences confirms that we cannot effectively digest it. Our digestive systems lack the necessary enzymes and structure to process the cellulose, meaning that while you might fill your stomach, you would ultimately starve. This biological limitation reveals a fundamental difference between humans and grazing animals.

Quick Summary

Humans cannot survive on a grass-only diet because we lack the enzyme to digest cellulose, the plant's primary structural component. Unlike ruminant herbivores, our single-chambered stomach and shorter digestive tract are inefficient at fermenting and breaking down fibrous plant matter. Consuming grass provides virtually no nutritional value and can cause digestive distress and tooth damage.

Key Points

Indigestible Cellulose: Humans cannot digest the cellulose that constitutes most of grass because we lack the necessary enzyme, cellulase.
Ruminant Digestive Contrast: Unlike ruminant herbivores with multi-chambered stomachs and specialized microbes, the human single-chambered stomach is inefficient for fermenting tough plant fibers.
Minimal Nutritional Value: Even though grass contains some nutrients, they are locked within indigestible cell walls, providing virtually no usable calories to the human body.
Risks of Consumption: Eating large amounts of grass can cause digestive distress like bloating and diarrhea, as well as wear down tooth enamel due to its high silica content.
Evolutionary Adaptation: The human digestive system evolved to process nutrient-dense foods like meat, cooked starches, and diverse plants, resulting in a smaller, more energy-efficient gut than that of herbivores.
Cultivated Grass Products: The only way humans consume grasses for nutrition is by processing specific varieties, such as extracting juices from young wheatgrass or harvesting seeds from grains like wheat and rice.

The Core Reason: Indigestible Cellulose

At the heart of the matter lies cellulose, a complex carbohydrate that forms the rigid cell walls of plants like grass. A glucose polymer, cellulose is the most abundant organic compound on Earth, representing a huge potential energy source. However, accessing this energy is a challenge for humans because we do not produce the enzyme cellulase, which is required to break the beta-glycosidic bonds that link the glucose units in cellulose.

While humans can digest starch, another glucose polymer with different bonds, cellulose passes through our digestive system largely intact. It functions as insoluble dietary fiber, adding bulk to our stool and aiding bowel movements, but offers no usable calories or absorbable nutrients. The complex structure of cellulose is further reinforced by other components like hemicellulose and lignin, making it even more resistant to degradation.

The Beta-Glycosidic Bond Barrier

The chemical difference between starch and cellulose is subtle yet profound. In starch, the glucose units are connected by alpha-1,4-glycosidic bonds, which human enzymes like amylase can easily break down. In contrast, cellulose's glucose units are linked by beta-1,4-glycosidic bonds. Our digestive enzymes simply cannot recognize or act on these specific beta linkages, making the vast majority of the calories locked within grass inaccessible to us.

Human vs. Herbivore Digestive Systems

Humans and grass-eating herbivores, such as cows and sheep, have fundamentally different digestive systems adapted to their respective diets.

Ruminant Digestive System

Multi-chambered stomach: Ruminants possess a four-chambered stomach, including the large rumen, where symbiotic bacteria and other microorganisms reside.
Microbial fermentation: These specialized microorganisms produce the enzyme cellulase, fermenting the ingested grass and breaking down the cellulose into volatile fatty acids (VFAs), which the animal then absorbs and uses for energy.
Rumination: The process of "chewing the cud" involves regurgitating partially digested food from the rumen to be re-chewed, further breaking down the fibrous material for maximum nutrient extraction.

Human Digestive System

Single-chambered stomach: Humans have a simple, single-chambered stomach not designed for fermenting tough plant fibers.
Limited fermentation: While we have gut bacteria in our large intestine that can break down a small amount of dietary fiber, the process is not extensive enough to provide significant energy or nutrients from cellulose.
Nutrient-dense reliance: Our digestive system evolved to process a diverse, omnivorous diet that includes nutrient-dense foods like meat, cooked vegetables, fruits, and starches, which are much easier and faster to digest.

Comparison Table: Human vs. Ruminant Digestion


Feature	Human Digestive System	Ruminant Digestive System (e.g., Cow)
Stomach	Single-chambered (monogastric)	Four-chambered (rumen, reticulum, omasum, abomasum)
Cellulose Digestion	Cannot directly digest; lacks cellulase enzyme	Breaks down via microbial fermentation in the rumen
Nutrient Absorption	Efficiently absorbs nutrients from starches, proteins, fats	Absorbs volatile fatty acids (VFAs) produced by microbes
Processing	Linear digestion from mouth to anus	Regurgitation and re-chewing (rumination) for efficiency
Adapted For	Varied, omnivorous diet including animal and processed foods	Herbivorous diet based on tough, fibrous plant matter

Nutritional Deficiencies and Risks of Eating Grass

Attempting to subsist on grass would lead to severe malnutrition and starvation. Grass is very low in the essential nutrients, calories, and fats humans need to survive. Even though it contains some vitamins and minerals, they are locked within the indigestible cellulose and cannot be absorbed by the human body. A high-fiber diet of only grass would likely cause significant digestive distress, including bloating, gas, and severe diarrhea, which could lead to dangerous dehydration.

Furthermore, grass contains a high concentration of silica, an abrasive compound also found in sand and rock. Constant chewing would rapidly wear down human tooth enamel, leading to irreversible dental damage over time. Grazing animals have teeth adapted for this wear and tear, but humans do not.

Can any human consume grasses?

While your lawn is not an option, some parts of certain grasses have been cultivated over millennia to be digestible by humans. Cereal grains like wheat, rice, and corn are the seeds of grasses, and their starches are processed into digestible food. Young wheatgrass and barley grass are also juiced, a process that mechanically breaks down the cell walls to release vitamins and minerals in a liquid form that can be absorbed by the body. This is a far cry from eating a handful of lawn clippings.

The Evolutionary Context of Human Diet

The inability to survive on grass is a result of millions of years of evolutionary history. As early human ancestors moved into more open, savannah-like environments, their diet diversified beyond the fruits and leaves that characterize other primates. The inclusion of more energy-dense foods, like meat, cooked starches, and processed plants, provided the high-quality nutrition necessary to fuel a larger, more complex brain. This shift led to a smaller, more efficient gut compared to herbivores, reducing the time and energy spent on digestion.

The adoption of fire for cooking, a milestone in human evolution, further enhanced this trend. Cooking breaks down tough food fibers and makes nutrients more bioavailable, allowing humans to extract more energy from a wider variety of sources. This combination of technological and biological adaptations cemented our omnivorous nature and moved us far away from being simple grazers.

Conclusion: Can human beings survive on grass?

The answer is a definitive no. Our digestive system, the absence of the cellulase enzyme, and our evolutionary dietary adaptations all prevent us from extracting any meaningful nutrition from grass. While it's not toxic, consuming it as a sole food source would quickly lead to starvation, digestive issues, and dental damage. The best use of grass for humans remains transforming it into highly digestible grains or juicing specific varieties, techniques that our ancestors developed to access nutrients locked within plants. In a survival situation, seeking out digestible food sources like tubers, insects, or seeds would be far more effective than trying to subsist on grass.

Live Science: Why Can't Humans Eat Grass?

Frequently Asked Questions

Humans do not produce cellulase because we evolved as omnivores, not specialized herbivores. Our digestive system is adapted for a varied diet of meat, fruits, and processed plant foods, and our ancestors did not rely on large amounts of raw, fibrous grasses for survival.

Yes, eating a significant amount of grass can cause digestive upset, including bloating, gas, and diarrhea, because our body cannot break it down effectively. This can also be caused by ingesting bacteria, pesticides, or other contaminants from untreated lawn grass.

Ruminants like cows and sheep have a multi-chambered stomach with a large rumen that houses symbiotic microbes. These microbes produce cellulase, which breaks down the cellulose through fermentation. Ruminants also regurgitate and re-chew their food (cud) for more efficient digestion.

Yes, humans can and do get nutrients from other parts of grass plants. Cereal grains like wheat, rice, and corn are the seeds of grasses and provide starch and other nutrients. Juices from young wheatgrass and barley grass also contain vitamins and minerals released by mechanical breakdown.

Grass contains silica, an abrasive mineral that can wear down human tooth enamel over time with constant chewing. Grazing animals have teeth that are adapted to withstand this abrasion, but human teeth are not.

No, eating grass is not a viable survival strategy. While it might temporarily fill your stomach, it provides virtually no calories and could lead to debilitating digestive problems, including dehydration from diarrhea. You would die of starvation and malnutrition.

The main difference is the cellulose content. Edible leafy vegetables like lettuce have much lower levels of cellulose and higher concentrations of digestible nutrients compared to tough, fibrous grass. Humans can easily access the nutrients in salad leaves, whereas the nutrients in grass are locked away.