Cats’ Hair and Whiskers Suggest a Vegan Diet in Lab Tests — Despite Being Carnivores
Most cat owners know one thing for sure: cats need meat. Unlike humans, they cannot make essential nutrients from plant foods, making them true carnivores. Yet isotopic measurements of domestic cats’ fur suggested something counterintuitive: Chemically, they were more like plant eaters than meat eaters.
In a study published in Frontiers of ecology and evolutionResearchers at the University of Vienna found that the explanation comes from the way cats process and incorporate food proteins into their tissues. Their results indicate that isotopic measurements do not always reflect what an animal actually eats.
“When we tested the cats’ hair for nitrogen isotopes, the results gave the impression that they ate mainly plants,” first co-author Viktoria Zechner said in a press release. “This means that isotopic examination of animal hair alone can sometimes be misleading about their diet.”
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Tracking Cat Diet With Nitrogen Isotopes
To determine what animals eat, researchers often turn to stable isotope analysis. Nitrogen, an element present in all living tissues, exists in two stable forms. As energy moves up the food chain, from plants to herbivores to predators, the heavier form tends to accumulate.
This model is measured using a value called δ15N. In general, higher levels of δ15N indicate a higher position in the food web. But tissues don’t reflect food perfectly. The difference between the isotopic value of an animal’s diet and that of its body tissues is known as the trophic discrimination factor, or TDF. In many species, this enrichment is between 3 and 5 parts per thousand.
To see how domestic cats compared, researchers examined the fur of 35 indoor cats fed commercial diets. They also analyzed the whiskers of 14 of them and measured nitrogen values in supermarket cat food. For comparison, they examined the scalp hair of 653 people who identified as vegan, vegetarian, or omnivore.
Nitrogen isotope patterns in domestic cats
In humans, the results met expectations. Omnivores showed higher δ15N values than vegetarians, and vegans showed the lowest values. The cats were different. Their fur and whiskers produced δ15N values closer to those observed in vegans than in carnivorous humans.
The main difference was the trophic discrimination factor. In human omnivores, there was a significant discrepancy between the isotopic values of food and hair. In cats, this gap was much smaller, averaging around 1.6 parts per thousand.
In practical terms, this means that a cat’s fur reflects the nitrogen signature of its food with very little modification.
Researchers think this may come from cats’ remarkable efficiency in processing proteins. Because their diet is made up of animal proteins that closely match their own amino acid requirements, they can incorporate these building blocks into tissues such as keratin with minimal chemical alteration. Fewer metabolic adjustments mean less isotope enrichment.
Humans and many other animals appear to modify dietary proteins more extensively before incorporating them into body tissues. This additional treatment tends to increase the δ15N values.
Implications for the interpretation of animal diets
“This doesn’t mean that cats eat like vegans,” said first co-author Hannah Riedmüller. “But it overturns long-held assumptions about carnivore isotopic signatures.”
Stable isotope analysis is widely used to study animal diets, but the study shows that physiology can influence how tissues reflect food. In cats, efficient protein utilization reduces the difference between food and fur in ways that standard models do not always predict.
The researchers looked only at hair and whiskers, without knowing whether blood, muscle or bone would show similar patterns. The mechanisms underlying the low trophic discrimination factor of cats also remain under investigation.
For ecologists, these results are a reminder that isotopic values cannot be read in isolation. Understanding how an organism processes nutrients is just as important as knowing what it consumes.
For ecologists, the results emphasize that isotope data should be interpreted alongside biology, not apart from it.
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