Unusual fats in cat kidneys may hold clues to kidney disease

Fat droplets found inside cat kidney cells have carried the same classification since the 1930s.

They have been regarded as harmless incidental findings: pathologists record them and move on. Until this year, that classification had never faced a serious challenge.

Researchers extracted the droplets and used advanced chemical techniques to examine them, revealing an unexpected result.

They found a kind of fat that mammalian kidneys almost never make. Its implications for cat health are still being investigated.

A long-overlooked feature of cat kidneys

Vets have known for many years that cats develop kidney damage more quickly than other companion animals.

According to one veterinary review, kidney disease affects up to 40 per cent of cats aged over ten and as many as 80 per cent of those over 15.

Professor David S. Gardner, of the University of Nottingham, has spent years investigating this puzzle.

Working with Dr Rebecca Brociek and other colleagues, he focused on a feature of cat kidneys that pathologists had recorded in the 1930s but never treated as important.

Minute fat droplets lie within the cells lining the kidney’s filtering tubes. Found in cats of every age and health status, they have been regarded as a feline biological quirk with little apparent significance.

A clear comparison

Gardner’s team collected kidney tissue from pet cats, dogs and Scottish wildcats, a rare native species that eats wild prey rather than commercial food.

They also included captive zoo cats from several locations, using samples from snow leopards and a tiger.

The results showed a marked imbalance. At every age tested, the kidneys of both domestic and wild cats contained considerably more fat than those of dogs. The difference was evident even in kittens under two.

One particular anomaly appeared in 23 of the 24 pet cats: a faint band of unidentified fat.

It did not fit any standard category, and there was no trace of it in dogs or most captive wildcats.

Inside the fat band

After extracting the band and carrying out a more detailed chemical analysis, the researchers found that its molecules belonged to a type of fat rarely seen in mammals.

Known as mono-alkyl-diacylglycerols, the molecules resemble ordinary storage fats, but contain a chemical bond that cells almost never form.

The fats in the cat kidneys also contained unusual fatty acids, including shorter chains and branched structures that mammals do not typically store.

Comparable lipids are found in squid and oily fish, although they are rare in mammals. Before this study, nobody had mapped the actual contents of the droplets.

Their composition is what makes the discovery distinctive. Cat kidney cells appear to produce fat types not made by any other companion animal.

How the pattern changes with age

Kittens had the usual fat band in their proximal tubule cells, located in the part of the kidney that returns nutrients to the blood. Alongside it, the MADAG-like band was only faintly visible.

In older cats, however, the pattern reversed: the normal fat band diminished as the unusual one became dominant. The strongest signal came from the two oldest cats in the sample, both around 15 years old.

Previous research found lipid deposits in the kidneys of nearly 80 per cent of senior cats.

Some accumulation was already present in one in four cats aged one to five. The latest data adds a distinctive chemical fingerprint to those findings.

A possible link to disease

Fat building up outside normal fatty tissue is rarely a good sign. Lipid droplets in the liver or kidney can cause inflammation, scarring and, eventually, cell death.

Cats are particularly susceptible to the gradual formation of scar tissue between the kidney’s filtering tubes.

That scarring is the defining characteristic of feline kidney disease. The researchers suggest that the accumulation of unusual fat could be among the earliest events behind it.

A causal connection has not been established. The lipid pattern occurs in both healthy and ill cats, so the team cannot say that the fats cause disease.

The timing is nevertheless suggestive, as the lipids build up years before symptoms appear.

What sets cats apart

Why this happens in cats but not dogs remains another unanswered question. Cat kidneys have unusually high energy demands, and cats are carnivores adapted to diets rich in protein and fat.

Their genes show evidence of adaptation to fatty foods. It is not yet clear whether those adaptations might become a disadvantage on modern commercial diets.

Earlier research has connected lipid stores in cat kidneys with environmental pollutants that become lodged in the droplets.

The unusual fats could act as a reservoir for those compounds. Diet, biology and exposure may all play a part.

Understanding where the fats come from

For the first time, researchers in the field have a specific chemical target to investigate inside cat kidneys.

Rather than offering only a vague clue, the unusual lipids give scientists a defined, traceable target within kidney cells.

“We are hopeful that soon we will understand why these unusual fats accumulate in domestic cats,” said Gardner.

He envisages a supplement or altered diet that could prevent the buildup and improve pets’ long-term health.

Any intervention is still years away, but researchers now have a clearer path to follow.

The next step is to establish why the lipids form, prevent them from accumulating and test whether doing so reduces kidney disease in later life.