A dark patch on your neck could be your body's warning about kidney health, study suggests

Dark, thickened patches on the neck that many people dismiss as a cosmetic issue may actually be a visible warning sign of kidney disease in people with type 2 diabetes, according to a new study by Indian researchers.

The condition, called acanthosis nigricans, is characterised by dark, velvety, thickened skin, usually appearing on the neck, underarms, and sometimes the face.

The new study, published in the Indian Journal of Nephrology, suggests that acanthosis nigricans may be important beyond its role in predicting diabetes. It may also indicate a higher risk of kidney damage among people who already have diabetes.

Researchers found that diabetic patients with moderate-to-severe acanthosis nigricans (AN), which is linked to obesity and insulin resistance, were more than four times as likely to have diabetic nephropathy, one of the leading causes of chronic kidney disease and kidney failure.

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“It is a dark, thick, velvety type of skin. It stands out from the rest of the skin. It is also present below the armpits and sometimes on the face also,” said Dr Anoop Misra, chairman, Fortis C-DOC Centre of Excellence for Diabetes, Metabolic Diseases and Endocrinology, and senior author of the study.

“It is a marker most often for insulin resistance. Once this is seen, we alert the person that they may have diabetes and advise them to get themselves checked.”

The research

The researchers suggest that a simple visual examination during a routine clinic visit could help doctors identify patients who need closer monitoring of kidney function—particularly in resource-limited settings where laboratory screening is not readily available.

Researchers studied 300 adults with type 2 diabetes—150 with moderate-to-severe acanthosis nigricans and 150 without the skin condition. They assessed kidney damage, nerve damage, eye complications, and cardiovascular disease.

The findings were striking: 50% of people with acanthosis nigricans had diabetic nephropathy compared with 17.3% of those without it. After adjusting for age, sex, obesity, diabetes duration, and lifestyle factors, acanthosis nigricans remained independently associated with more than a fourfold higher risk of kidney disease.

Dr Misra said the link had been suspected for years because acanthosis nigricans reflects insulin resistance, but it had never been proven as a predictor of organ complications.

“We were thinking for several years that it might also predict this. But this was never proven. Nobody had shown it before,” he said.

More relevance in South Asia, including India

The reason this finding is particularly relevant for India is that acanthosis nigricans is much more common among South Asians, who are already at high risk of diabetes and its complications.

“Across the world, it is not very common in white people. In South Asians especially, it is quite common,” Dr Misra said.

Previous studies have shown that acanthosis nigricans is frequently seen among obese Indian children and adults because of insulin resistance. But the unanswered question was whether the skin marker indicated damage to other organs.

“The question was whether it could produce problems in other organs of the body,” Dr Misra said.

The study found that among people with similar weight and diabetes status, those with acanthosis nigricans had significantly more kidney problems. Increased urinary albumin—a marker of early kidney injury—was almost three times more common among those with the skin condition.

Dr Misra believes the message is not only for specialists but also for general doctors and families.

“This can be seen by relatives of the patient, by nurses, by MBBS doctors. It is very easy to see. Just by looking at the neck, it can be noted in one minute.”

He stressed that acanthosis nigricans should prompt closer monitoring of kidney function in diabetic patients.

“Everybody with diabetes should be evaluated for kidney disease, but for anybody with diabetes and acanthosis nigricans, we have to be very particular not to miss evaluation.”

The study authors suggest that this simple physical sign could become a low-cost screening tool, especially in settings where access to regular kidney testing is limited. “In settings where tests are not available, a simple observation can help diagnose the kidney disease. The patients can be advised to go for further evaluation,” says the study's author, Dr Koel Dutta, Department of Diabetes, Fortis C-DOC Centre for Excellence, New Delhi.

However, Dr Misra cautioned that acanthosis nigricans itself is not the cause of kidney disease. Rather, it is a visible marker of the underlying biological processes—especially insulin resistance—that increase metabolic risk.

The good news is that addressing the root cause may reduce risk.

“Anything which reduces weight will lower chances of having kidney problems,” Dr Misra said. He added that newer medicines, including GLP-1 receptor drugs that help reduce weight and blood sugar, may also have kidney benefits, while other medications specifically protect kidney function.

The researchers say more studies are needed before acanthosis nigricans is included in formal diabetes screening guidelines. But for millions living with diabetes, a simple look in the mirror may offer an early warning.

“This is not a message only for super specialists. This is a message for general practitioners and the public,” says Dr Misra.