A 20-year old man in Taiwan went to a dermatology clinic for a strange rash that had developed across his shoulders and chest. The raised, red, and itchy condition had been bothering him for a full month. By this point, he had also developed patches of pigmented skin interlaced with the red rash.

According to a case report in the New England Journal of Medicine, a skin biopsy showed swelling between his skin cells and inflammation around blood vessels, but testing came up negative for other common signs of skin conditions, leaving doctors with few leads. The doctors ultimately came to a diagnosis not by analyzing his skin further but by hearing about his diet.

The man's chest and shoulders, showing his rash and hyperpigmentation. Credit: New England Journal of Medicine, 2026

The man told doctors that two months prior to his clinic appointment—a month before his rash developed—he had switched to a ketogenic diet, which is a high-fat but very low-carbohydrate eating pattern. This diet forces the body to shift from using glucose (sugar derived from carbohydrates) as an energy source to fat instead.

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© New England Journal of Medicine, 2026