A 20-year-old man in Taiwan developed a persistent, itchy rash across his shoulders and chest. After a month of unexplained symptoms, a skin biopsy offered no clear answers. Doctors diagnosed the condition only after he mentioned switching to a ketogenic diet two months earlier, a month before the rash appeared.

The case, detailed in the New England Journal of Medicine, describes prurigo pigmentosa, a rare dermatological condition also known as "keto rash." This condition features red, raised bumps and hyperpigmentation, typically on the neck, chest, and back. Its link to metabolic shifts from low-carb diets is well-documented, yet the exact biological trigger remains unknown to medical experts.

While ketogenic diets are popular for weight loss and have clinical uses for epilepsy, they carry documented side effects. Common issues include nausea, fatigue, and bad breath. This mysterious rash adds to the list of poorly understood risks, challenging the diet's reputation as a straightforward metabolic intervention and highlighting gaps in nutritional science.