Seborrheic Dermatitis Diet Treatment: 8 Foods to Avoid. Seborrheic dermatitis is an inflammation of the skin. It is the red, itchy rash that appears on the scalp with flaky scales. This is a very common skin disease and is very similar to eczema, psoriasis, or any other allergic reaction. It could also affect the scalp as well as other parts of the body such as your face, nose, forehead, chest, head and shoulders. Seborrheic dermatitis can either be mild or severe.