Keto Rash (Prurigo Pigmentosa)

A keto rash, also known as prurigo pigmentosa or Nagashima disease, is an itchy rash with small pink to brown bumps or blisters on your skin. It’s an inflammatory rash that has a symmetrical pattern, usually on your neck, chest and back. The rash has stages, meaning it changes in appearance over time.

Researchers aren’t sure what causes prurigo pigmentosa. But it has a strong link to the keto diet, a very low-carb diet. That’s why it’s called the keto rash. Overall, prurigo pigmentosa is rare. But healthcare providers are seeing more cases of it due to the popularity of this diet.

It can develop days or months after starting a keto diet. But not every person who follows the diet will get a rash. A keto rash can be uncomfortable. But it isn’t harmful to your health.

Symptoms of keto rash

The appearance and symptoms of a keto rash change in stages:

• Early stage: Small, itchy, red to purple bumps appear and may develop into larger, raised patches (plaques).
• Developed stage: Bumps grow larger. They may become fluid-filled (like blisters) and crust over. They often form a net-like pattern.
• Resolving stage: Bumps fade and itching improves. But dark spots (hyperpigmentation) are left behind and can last for several months.

The most common sites of the rash include your neck, chest and back. It rarely affects your face or limbs. Try not to scratch the rash. It can further irritate your skin. If your fingernails break open a bump, it could get infected.

See a dermatologist if you have these symptoms. They can give you a proper diagnosis and rule out other conditions that may be causing a rash.

Keto rash causes

Researchers don’t know the exact cause of prurigo pigmentosa (keto rash). They think ketosis may trigger it. Ketosis happens when your body uses fat as its main fuel source instead of glucose (sugar). Ketosis may happen from:

• Ketogenic (“keto”) diets
• Diabetes
• Fasting
• Post-bariatric surgery
• Anorexia nervosa

Other causes of prurigo pigmentosa may include:

• Metabolic disorders
• Hormonal changes, like pregnancy
• Infections, like H. pylori infection
• Allergic reactions to metals or other chemicals

This rash is most common in Japan. But it’s becoming more common in other countries.

How doctors diagnose this condition

Your provider will review your medical history and do a physical exam. They’ll ask about changes to your eating patterns. Keto rash is rare. So, they’ll need to rule out other types of rashes that look similar.

To confirm a keto rash diagnosis, your provider may recommend the following tests:

• Ketone urine test to check for ketosis 
• Blood tests to check your overall health
• Skin biopsy to rule out other types of rashes
• Patch test to check for skin allergies

How is it treated?

If ketosis is the cause, a keto rash typically goes away once you increase the amount of carbohydrates you eat. Your healthcare provider may recommend eating at least 50 grams of carbs per day. It may take a couple of weeks to get better.

Your provider may also suggest taking an antibiotic, like minocycline, doxycycline or dapsone. These medications have anti-inflammatory effects. So, they help treat the rash even if an infection isn’t the underlying cause.

Most cases of prurigo pigmentosa not caused by ketosis go away on their own.

What can I expect if I have this condition?

The keto rash isn’t dangerous. But without treatment, it can cause lasting spots on your skin that are darker than the surrounding skin (hyperpigmentation). This may affect your appearance and self-confidence.

A keto rash can go away and come back several times. Reach out to your healthcare provider if the rash isn’t improving with treatment. If you have long-lasting hyperpigmentation, ask your provider which treatments can help minimize it.