A deficiency of biotin, also called vitamin B7, can contribute to brittle nails and hair problems.
Biotin is a water-soluble B vitamin involved in metabolizing fats, carbohydrates, and proteins. It also supports enzymes that help cells process nutrients, making it important for normal skin, hair, and nail maintenance. When a genuine deficiency occurs, symptoms can include hair thinning or loss, skin rashes, fatigue, and brittle or splitting nails.
True biotin deficiency is uncommon because the vitamin is found in foods such as eggs, nuts, seeds, legumes, meat, fish, and some vegetables. Certain conditions can increase risk, including prolonged consumption of raw egg whites, severe malnutrition, some inherited metabolic disorders, and long-term use of particular medications. Cooking egg whites reduces a protein that can interfere with biotin absorption.
Brittle nails and hair breakage have many possible causes, including repeated chemical treatments, dehydration, thyroid disease, iron deficiency, and other nutritional problems. Biotin supplements are therefore not a universal fix, and high-dose use can interfere with some laboratory tests, including tests for thyroid hormones and cardiac markers.