People and dogs have red kidneys. Cats have yellow ones.

Cat kidneys get their yellow color from tiny fat droplets packed inside some of their cells.

Scientists have known about these droplets for more than 100 years. But for a long time, they didn't know why cats had so many or what the droplets did.

In 2026, researchers in the UK and Japan took a fresh look.

Keep reading for a simple step you can take at home today.

The UK Team Explores a Connection to Kidney Disease

Chronic kidney disease (CKD) is common in older cats. According to Cornell University's Feline Health Center, it affects up to 4 in 10 cats over age 10. For cats over 15, that number rises to as many as 8 in 10.

Researchers at the University of Nottingham closely analyzed the fats in cat kidneys. They also examined kidney samples from dogs and Scottish wildcats for comparison.

Domestic cats had distinctive fat compounds that set their kidneys apart from those of dogs and Scottish wildcats. Some of these compounds were more prominent in older cats.

The team suggested that these features could be related to CKD in cats. In people and other animals, too much fat inside kidney cells can damage those cells.

However, these distinctive compounds also appeared in healthy cats. Having fat droplets in the kidneys does not, by itself, signal kidney disease.

The Japanese Team Uncovers the Secret Behind Each Cat's Scent

In August of the same year, a team led by Iwate University in Japan published an intriguing finding. Their work involved a familiar feline expression.

You know the look: a cat sniffs another cat's scent, then pauses with its mouth slightly open, looking a little dazed. That's called the flehmen response.

In the experiments, cats lost interest when they repeatedly smelled the same urine sample. But a sample from a different cat caught their attention again. They could tell the scents apart.

Analysis revealed 13 distinctive fat compounds in the urine. Each cat had its own mix of these compounds, and that mix stayed fairly consistent over time. The cats could tell one another apart based on these differences alone.

These same compounds were also found in the fat droplets inside the kidneys.

The researchers think these droplets store the scent compounds, helping each cat's scent stay consistent over time.

How these droplets affect CKD still needs more study.

You can't see these droplets at home, but you can notice changes in your cat's weight, drinking, and urination.

Three Things Cat Caregivers Should Know

1. These findings don't mean you need to cut dietary fat

Fat droplets in kidney cells do not automatically mean a cat is eating too much fat. These studies don't mean your cat needs a low-fat diet.

2. Weigh your cat under consistent conditions

When you see your cat every day, gradual weight loss can be hard to spot. A separate study reviewed records from 569 cats with CKD. The records showed a trend toward weight loss as early as three years before diagnosis.

Start while your cat is healthy: weigh them once a month, using the same scale at roughly the same time of day. Watch for a gradual drop in your cat's weight. The trend matters more than any single reading.

One option is to weigh yourself holding your cat, then subtract your own weight. To track smaller changes, use a pet scale that measures in grams. If your cat is comfortable in a basket, you can weigh them in it, then subtract the empty basket's weight.

3. Watch for changes in drinking and urination, too

Notice whether your cat is drinking more or leaving larger urine clumps in the litter box.

If your cat keeps losing weight or is drinking and urinating noticeably more, schedule a veterinary checkup to find out why. Don't wait for next month's weigh-in.

Weigh Your Cat Today

The mystery of cats' yellow kidneys isn't fully solved yet. But there's something you can do at home today.

Today's weight gives you a starting point for noticing changes next month and beyond. Take that first step in kidney care: weigh your cat today.

Start kidney care while your cat is healthy. Greycoat Research is here to help.

Explore kidney care at Greycoat Research →

About the Author and Research Team

Alex Shin, Pharmacist | Greycoat Research

20+ years of experience advising pet caregivers on medication use

Consultations for caregivers of cats with kidney disease · Nutritional formulation advisor

Greycoat Research works with veterinarians, pharmacists, and researchers in South Korea and Japan to study feline kidney health and nutrition.

References

1. Brociek RA, Alborough R, Kotowska AM, et al. "Lipid droplets in felid kidneys: prevalence and composition by lipidomics." Frontiers in Veterinary Science. 2026;13:1711591.

2. Ichizawa S, et al. "Signatures of branched-chain fatty acids derived from a kidney reservoir confer stable chemical individuality on domestic cats." Current Biology. 2026.

3. Freeman LM, Lachaud M-P, Matthews S, et al. "Evaluation of weight loss over time in cats with chronic kidney disease." Journal of Veterinary Internal Medicine. 2016;30(5):1661–1666.

4. Cornell Feline Health Center. "Chronic Kidney Disease."

Educational Disclaimer

This article is for educational purposes only and is not a substitute for professional veterinary advice, diagnosis, or treatment. It is not intended to diagnose, treat, cure, or prevent any disease.

Please consult your veterinarian before making changes to your cat’s diet, supplements, or care routine.