Vitamin D is a stable vitamin not reduced by most household cooking methods, according to nutrition experts at World's Healthiest Foods. Lessening heat and shortening cooking times are the best way to avoid a reduction in the vitamin content of most vitamin D-rich foods.
While the precise temperature cooked foods begin to lose their vitamin D content is unclear, studies suggest that moderate to high temperatures over long periods reduce -but do not completely eliminate- vitamin concentration. Additionally, foods cooked in oil typically lose more vitamin D than those cooked using other methods, according to World's Healthiest Foods. Sources of vitamin D include eggs, milk, mushrooms, salmon and tuna.