Can Humans Digest Keratin? The Indigestible Protein Explained

Over 90% of a chicken feather's dry matter is keratin, yet humans cannot derive nutritional value from this protein. The reason is our digestive system is not equipped to break down the resilient, insoluble protein found in hair, nails, and feathers.

Quick Summary

The human body cannot digest keratin, a robust and insoluble protein found in hair and nails, due to the absence of specialized enzymes and the protein's strong structure. Ingested keratin passes through the digestive tract largely unchanged, though large amounts can lead to intestinal blockage.

Key Points

• Indigestible Protein: Humans cannot digest keratin, the protein found in hair, nails, and feathers, because our bodies lack the specific enzymes required.

• Insoluble Structure: Keratin's extreme toughness and insolubility in water, due to extensive disulfide bonds, prevent our standard digestive enzymes from breaking it down.

• Risk of Obstruction: Ingesting large quantities of keratinous material can lead to the formation of a trichobezoar (hairball), causing serious gastrointestinal blockages.

• Specialized Enzymes: The enzymes capable of digesting keratin, known as keratinases, are produced by certain bacteria, fungi, and insects, but not by humans.

• Passes Through Undigested: Small amounts of swallowed hair will simply pass through the human digestive tract and be excreted without being absorbed.

• Different from Soft Keratin: While hard, fibrous keratin is indigestible, the body can synthesize and regulate the softer keratins found in skin and internal linings.

What is Keratin and Why Is It So Strong?

Keratin is a family of fibrous structural proteins that form the primary material of hair, nails, and the outer layer of skin in humans and other animals. This protein is renowned for its durability, a characteristic that arises from its complex molecular structure. The high resilience of keratin is attributed to several key features:

• Extensive Disulfide Bonds: The amino acid cysteine is a major component of hard keratins found in hair and nails. The sulfur atoms in cysteine residues form strong disulfide bonds, creating cross-links that reinforce the protein structure and make it highly resistant to mechanical stress and chemical degradation.
• Insolubility: The tight packing of protein chains and the numerous disulfide bonds make keratin insoluble in water and most organic solvents. This insolubility is a major hurdle for our digestive enzymes, which require soluble or small-particle substrates to function effectively.
• Fibrous Structure: Keratin proteins aggregate into robust intermediate filaments, a cable-like structure that gives hair and nails their physical toughness. This dense, fibrous form is not conducive to enzymatic attack within the human digestive system.

The Human Digestive System's Inability to Process Keratin

Our digestive tract is a marvel of efficiency, equipped to break down most types of proteins, carbohydrates, and fats into usable components. However, this ability does not extend to keratin. The primary reasons for this are rooted in our biological makeup and the sheer recalcitrance of the keratin molecule.

1. Absence of Keratinase Enzymes Enzymes are the body's workhorses for digestion, with specific enzymes (proteases) targeting specific proteins. While humans produce powerful proteases like pepsin and trypsin, these enzymes are not equipped to handle the unique structure of keratin. The class of enzymes that can break down keratin is called keratinase, and it is not found in humans. Keratinase is a specialized enzyme produced by certain bacteria, fungi, and insects.

2. Resistance to Stomach Acid and Proteases Keratin's formidable structure, bolstered by disulfide bonds, makes it highly resistant to the acidic environment of the stomach and the actions of our digestive proteases. Even the strong hydrochloric acid in the stomach cannot effectively unravel the tightly wound protein structure.

3. Inaccessibility of Peptide Bonds Human proteases act by cleaving the peptide bonds within protein chains. In order for this to happen, the enzyme must be able to physically access these bonds. The compact, cross-linked structure of keratin prevents our proteases from reaching and breaking the peptide bonds, leaving the protein intact.

Comparison: Keratin vs. Other Proteins We Digest

To better understand why keratin is indigestible, it's useful to compare its properties with those of a common, easily digested protein, such as casein from milk.

Feature	Keratin (Hair, Nails)	Casein (Milk)
Protein Structure	Fibrous, highly cross-linked with disulfide bonds.	Globular, with a less defined, more open structure.
Solubility	Insoluble in water and digestive fluids.	Soluble and readily disperses in digestive fluids.
Enzyme Requirement	Requires specialized keratinase enzymes, which humans lack.	Easily hydrolyzed by common human proteases like pepsin.
Digestibility in Humans	Indigestible; passes through the digestive tract largely intact.	Highly digestible; broken down into amino acids for absorption.
Formation of Obstruction	Can clump together to form bezoars, causing blockages.	Does not cause blockages; efficiently broken down into nutrients.

The Role of Keratin in Nature and the Risks of Ingestion

While humans are unable to digest keratin, a small number of specialized organisms, such as certain bacteria, fungi, and insects, have evolved the ability to do so. These organisms, including clothes moths, some beetles, and specific bacterial species like Bacillus, produce keratinase enzymes that can break down keratinous materials like wool, feathers, and hair. This is a crucial function in the natural decomposition cycle.

For humans, accidentally swallowing a small strand of hair is usually harmless, as it will simply pass through the digestive system. However, consuming large amounts of keratinous materials, such as hair, over time can be dangerous. The indigestible matter can accumulate in the stomach or intestines and form a dense mass called a trichobezoar, or hairball. This can lead to serious health complications, including nausea, pain, and life-threatening bowel obstruction that requires medical intervention.

The Fate of Indigestible Keratin

What happens to the keratin we ingest? The journey is uneventful from a digestive standpoint.

1. Ingestion: Hair or other keratinous material enters the mouth and is swallowed.
2. Stomach: The material passes into the stomach. The highly acidic environment and the protease pepsin have little to no effect on the strong keratin structure.
3. Small Intestine: The material continues to the small intestine. The more alkaline environment and additional proteases like trypsin and chymotrypsin also fail to break down the insoluble keratin.
4. Colon: As the material moves to the large intestine, most water is absorbed, and the indigestible matter becomes part of the feces.
5. Excretion: The keratin is eventually eliminated from the body as waste, having provided no nutrients in its transit.

Conclusion: We Are Not Built for Keratin

In conclusion, humans cannot digest keratin because we lack the necessary enzymes and the protein's complex, insoluble structure resists our digestive processes. This biological limitation means that hard keratin from hair, nails, and other sources passes through our bodies undigested. While minor ingestion is harmless, significant consumption poses a real risk of intestinal obstruction. While some microorganisms and other animals possess the biological machinery to degrade this tough protein, our digestive system is simply not built for the job.

Microbial production of keratinase from Bacillus velezensis strain MAMA is a great example of organisms that can break down keratin effectively.

Frequently Asked Questions

What happens if you accidentally eat a strand of hair?

Accidentally swallowing a single strand of hair is not harmful. Since the human digestive system cannot break down keratin, the hair will simply pass through your gastrointestinal tract and be excreted from the body.

Why can't humans digest hair and nails?

Humans cannot digest hair and nails because they are primarily composed of hard keratin, a protein with a highly cross-linked structure that is insoluble and extremely resilient. We do not produce the specialized enzymes, called keratinases, that would be necessary to break it down.

Can animals digest keratin?

While most animals cannot digest hard keratin, a few specialized organisms can. Certain insects like clothes moths, some scavenging birds like vultures, and specific bacteria and fungi have evolved to produce keratinase enzymes or house keratin-digesting bacteria to break down this tough protein.

What is a trichobezoar?

A trichobezoar is a medical term for a hairball that forms and becomes lodged in the stomach or intestines. It is a rare condition that occurs when large quantities of hair are ingested over time, and it can cause severe digestive issues or blockage.

Are keratin supplements digestible?

Some keratin supplements are sold as "soluble" or "hydrolyzed" keratin, meaning the protein has been pre-processed to be more digestible. However, the efficacy and necessity of these supplements are debated, as the body produces its own keratin naturally.

What nutrients does the body use to produce keratin?

While we can't digest and absorb keratin directly, our bodies naturally synthesize it using specific nutrients from our diet. Key nutrients for this process include protein, biotin, and vitamin A, found in foods like eggs, salmon, and sweet potatoes.

Is keratin found in foods?

Keratin itself is not found in foods we typically consume, with the exception of certain food additives derived from animal hair or feathers, such as L-cysteine used as a dough conditioner. However, you can eat foods containing the building blocks and nutrients needed for your body to produce its own keratin.