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Medical Literacy

Insulin Resistance Skin Signs: 7 to Check and the 2 Tests That Confirm It

Dark, velvety patches in the skin folds are the classic skin sign of insulin resistance. Most people with them have become resistant to insulin, and two ordinary blood tests settle the question.

Ageless Coach Editorial Team · May 28, 2026

In 30 Seconds
The patch has a name. The American Academy of Dermatology says acanthosis nigricans darkens and thickens one or more areas of skin, often feels like velvet, and vigorous scrubbing will not get rid of it. AAD
It usually means insulin. Mayo Clinic says most people who have acanthosis nigricans have also become resistant to insulin, and that insulin resistance is what leads to type 2 diabetes. Mayo Clinic
Two tests confirm it. NIDDK says prediabetes is diagnosed with an A1C of 5.7 to 6.4 percent or a fasting glucose of 100 to 125 mg/dL. Many people can prevent or reverse it. NIDDK

This is for anyone who has found a dark, velvety patch on the back of the neck, in an armpit, or in the groin. You may have wondered if it is dirt, sun, or something else. This page answers three questions. What is the patch? Does it mean insulin resistance? What should you ask for at your next visit?

The short answer: the patch is usually acanthosis nigricans. Mayo Clinic says most people who have it have also become resistant to insulin. NIDDK says insulin resistance and prediabetes usually cause no symptoms at all. That is why the skin matters. It is often the first thing you can see.

The conclusion is hopeful. Cleveland Clinic says treating the cause treats the skin, and NIDDK says many people can prevent or reverse prediabetes. Below: the seven signs, a quick sorter for which situation is yours, the two tests to ask for, and the plan.

1
The Signs · 7

Seven skin signs that point at insulin, not dirt

The American Academy of Dermatology says acanthosis nigricans causes one or more areas of skin to darken and thicken, and that the skin often feels like velvet. People sometimes think a dye or something they touched stained the skin and try to scrub it away. It does not wash off.

Seven things to check, in the order most people notice them.

  1. A dark, velvety band on the back of the neck. Mayo Clinic says the main sign is dark, thick, velvety skin in body folds and creases, and that it often appears on the back of the neck.
  2. The same darkening in the armpits or groin. Mayo Clinic names the armpits and groin as the other two usual places. Cleveland Clinic adds that the patches most often show up on the skin folds of the neck, armpits and groin.
  3. Patches under the breasts. Cleveland Clinic lists the skin under the breasts among the most common sites, and says the markings can appear anywhere on the body.
  4. A crop of new skin tags. Mayo Clinic says the affected skin might develop skin tags. Cleveland Clinic lists skin tags among the symptoms.
  5. Itching in the patch. Mayo Clinic says the affected skin might be itchy. Cleveland Clinic lists itchiness too.
  6. A mild odor from the fold. Mayo Clinic says the skin might have an odor. Cleveland Clinic describes foul-smelling skin patches.
  7. It fails the scrub test. The AAD says vigorous scrubbing will not get rid of it, and Cleveland Clinic says washing will not remove it. A stain comes off. This does not.
97.6 million
NIDDK says that in 2021 about 97.6 million adults in the United States had prediabetes, and that people with insulin resistance and prediabetes usually have no symptoms.
Source: NIDDK

Color is not the whole story. Cleveland Clinic says the patches run from light brown to black, and on darker skin the shade change can be easy to miss. The velvet feel is the more reliable clue. Run a finger across the area and compare it with the skin beside it.

2
Which Applies To You · 4

Four situations, and what each one means for your next step

Cleveland Clinic says acanthosis nigricans often happens to otherwise healthy people, but in some cases it is a sign of an underlying condition. The cause decides the next step, so sort yourself first.

Find the line that fits you.

  1. It came on slowly, in the folds, and you carry extra weight or diabetes runs in your family. This is the common pattern. Mayo Clinic says the risk is higher with obesity and with a family history, especially where type 2 diabetes is common. Ask for the two blood tests in the next section.
  2. It came on slowly and you are slim and otherwise well. Cleveland Clinic says anyone can develop it, including otherwise healthy people. It also says the patches do not always mean diabetes. Still ask for the tests. NIDDK says prediabetes usually has no symptoms, and weight is only one risk factor on its list.
  3. You started a new medicine before it appeared. Mayo Clinic names high-dose niacin, birth control pills, prednisone and other corticosteroids as possible causes. The AAD says changing medicines might be an option. Do not stop anything on your own. Bring the bottle to the visit.
  4. It appeared suddenly or is spreading fast. Cleveland Clinic says symptoms tend to develop slowly, over months or even years, and that if they appear suddenly you should see a dermatologist because it may be a sign of cancer, though this is rare. Mayo Clinic says the same: see a provider, especially if the changes are sudden.
A dermatologist examines a woman's facial skin with a handheld dermatoscope in a clinic
Cleveland Clinic says the velvety markings most often show up on the skin folds of the neck, armpits and groin and under the breasts.

Two more things the sources want you to know. Cleveland Clinic says the condition is not contagious and not harmful in itself. And the AAD says that sometimes the dark patches have no connection to any medical condition, which is exactly why a dermatologist, not a guess, should settle it.

Most people who have acanthosis nigricans have also become resistant to insulin.

Mayo Clinic

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One sheet: the seven signs, the four-way sorter, the two tests with their numbers, and what to say at the visit.

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INSULIN RESISTANCE: SEVEN SKIN SIGNS, TWO TESTS
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3
The Tests · 2

The two blood tests that confirm it, with the numbers

NIDDK says health care professionals usually do not test for insulin resistance itself, because that test is mainly used in research. They test for prediabetes, the stage insulin resistance leads to. Two blood tests do the job.

The A1C test. NIDDK says it shows your average blood glucose over the past three months, and that a result between 5.7 and 6.4 percent means prediabetes.

The fasting plasma glucose test. NIDDK says it shows your blood glucose at the time of the test, and that a result between 100 and 125 mg/dL means prediabetes.

A gloved hand holding two blood sample tubes against a plain background
NIDDK says prediabetes is diagnosed with an A1C of 5.7 to 6.4 percent or a fasting glucose of 100 to 125 mg/dL.
5.7 to 6.4
NIDDK says an A1C between 5.7 and 6.4 percent means prediabetes. A fasting glucose of 100 to 125 mg/dL means the same thing.
Source: NIDDK

NIDDK adds two useful details. The A1C test can miss prediabetes in people with certain health conditions. In that case a doctor may run an oral glucose tolerance test instead, where 140 to 199 mg/dL means prediabetes. Insulin resistance can also raise blood fats, so your doctor may check cholesterol and triglycerides.

If you are 35 or older, NIDDK says testing is reasonable on age alone. If the tests are normal, it says to retest at least every three years. If they show prediabetes, retest every year. That last number matters. NIDDK says people with prediabetes have a high chance of developing type 2 diabetes within 5 to 10 years. It also says you can take steps to prevent it.

Cleveland Clinic says the skin usually improves when the cause is treated, and that weight loss, exercise and a nutritious diet help reduce insulin levels. The patches may even disappear completely. NIDDK puts a number on the weight: in the Diabetes Prevention Program, losing 5 to 7 percent of starting weight reduced the chance of developing diabetes.

✗
Sorted

Three mistakes people make with these patches

Each of these delays the one thing that helps, which is treating the cause.

Scrubbing harder. The AAD says people mistakenly believe a dye or something they touched discolored the skin, and that vigorous scrubbing will not get rid of it. Cleveland Clinic says washing will not remove it either.

Buying a fade cream first. Cleveland Clinic says not to use skin care products on the dark spots without talking to a provider, because they may not work and may irritate the skin and make it worse. Prescription creams and retinoids exist, but Cleveland Clinic puts treating the underlying condition first.

Waiting to see if it goes away. Cleveland Clinic says it will not go away on its own. Mayo Clinic says people who have it are much more likely to develop type 2 diabetes. The patch is an early warning, and early is the useful part.

📋Your Coach's Recommendations
1
Check the three folds in a mirror this week, by feel as well as by eye.
Mayo Clinic names the armpits, groin and back of the neck. Cleveland Clinic adds under the breasts and says the shade can be light brown to black. Velvet texture that does not scrub off is the tell.
2
Ask for an A1C and a fasting glucose, and say why you are asking.
Say this at the visit: "I have a dark velvety patch in a skin fold and I have read it can point to insulin resistance. Can we check an A1C and a fasting glucose?" NIDDK says 5.7 to 6.4 percent, or 100 to 125 mg/dL, means prediabetes. If you are over 50, ask for cholesterol and triglycerides at the same draw.
3
Treat the cause for 90 days before you treat the patch.
Cleveland Clinic says weight loss, exercise and a nutritious diet reduce insulin levels and improve the skin. NIDDK says losing 5 to 7 percent of body weight cut diabetes risk in the Diabetes Prevention Program. Walk after meals, add two strength sessions a week, and keep the retest date NIDDK sets: one year if prediabetes, three if normal.

To your health,

Ageless Coach

Age Strong. Live Long.

Trusted Sources Behind This Article
American Academy of DermatologyMayo ClinicCleveland ClinicNIDDK

This article is education, not medical advice. Mayo Clinic says to consult your health care provider if you notice changes in your skin, especially if the changes are sudden. Ageless Coach is an affiliate partner and may earn a commission on purchases made through links on this page.

Frequently Asked Questions

What are the skin signs of insulin resistance?
Mayo Clinic says the main sign of acanthosis nigricans is dark, thick, velvety skin in body folds and creases, most often the armpits, groin and back of the neck. It says the skin might also be itchy, have an odor and develop skin tags. The AAD says vigorous scrubbing will not remove it.
Can I have these patches if I am slim?
Cleveland Clinic says anyone can develop acanthosis nigricans, including otherwise healthy people, though the risk is higher with excess weight or a family history. NIDDK lists age 35 and older, family history, inactivity, smoking and PCOS among risk factors for insulin resistance, none of which require excess weight.
Which test should I ask my doctor for?
NIDDK says the A1C test and the fasting plasma glucose test are the usual ways to diagnose prediabetes. An A1C of 5.7 to 6.4 percent or a fasting glucose of 100 to 125 mg/dL means prediabetes. The test for insulin resistance itself is mostly used in research.
Will the dark patches go away if I lose weight?
Cleveland Clinic says the condition will not go away on its own, but that treating the cause treats the skin. It says weight loss, exercise and a nutritious diet help reduce insulin levels and improve the skin, and that the patches may even disappear completely.
When is a dark patch an emergency?
Cleveland Clinic says the patches normally develop slowly over months or years, and that if they appear suddenly you should see a dermatologist because sudden onset may, rarely, be a sign of cancer. Mayo Clinic says to consult your provider about skin changes, especially if they are sudden.
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