Protein deficiency is the most direct nutritional cause of hair thinning. Hair is a non-essential use of protein, so when intake is low the body redirects protein away from follicles to vital organs. Diets consistently below 50 grams of protein daily show measurable hair effects over months.
Iron deficiency specifically reduces hair growth rate and increases shedding. Zinc supports follicle structural integrity. Biotin supplements are widely marketed but deficiency is rare and supplementation only helps if you are already deficient.