The Malassezia fungus, other fungi, and yeasts can get out of hand if you eat a lot of sugar. Not only that, but too many sweets and sugary drinks in your system quickly increase your blood sugar and insulin levels. As a result, your body secretes hormones that boost oil production. The Malassezia fungus feeds off your body’s natural oils, leaving oleic acid on your scalp. This oleic acid can cause your scalp to itch or shed skin cells (per Head & Shoulders).
A 2019 study in the Journal of Investigative Dermatology looked at how diet might affect people with seborrheic dermatitis. The researchers looked at the diets of more than 4,000 people, where 14.5% of them had seborrheic dermatitis. People who ate more fruits had a lower risk of the skin condition. Those who consumed alcohol, potatoes, and processed and unprocessed meat were more likely to have seborrheic dermatitis. This was particularly the case in women.