We can't digest hair because our bodies lack the enzymes to break down keratin, the tough protein that makes up hair.
Here's a more detailed explanation:
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Keratin's Robust Structure: Hair is primarily composed of keratin, a fibrous structural protein. Keratin is incredibly strong and durable due to its tightly packed structure and cross-linking. This structure makes it resistant to breakdown by most biological processes.
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Lack of Necessary Enzymes: Human digestive systems contain a variety of enzymes designed to break down carbohydrates, proteins, and fats. However, we don't produce keratinase, the enzyme specifically needed to break down keratin.
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Harsh Conditions Required: Breaking down keratin requires extremely harsh conditions. As referenced, the process requires very acidic or alkaline environments and temperatures exceeding 100 degrees Celsius (212 degrees Fahrenheit) for a significant period. These conditions are impossible to achieve and maintain within the human digestive system.
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Analogy: Think of trying to digest wood. Wood is primarily cellulose, and like keratin, our bodies don't produce the necessary enzymes (cellulase) to break it down.
In summary, our bodies simply aren't equipped with the right tools (enzymes) and conditions to break down the robust protein structure of hair. This is why it passes through our digestive system undigested.
