The nutrient that is essential for the formation of red blood cells is vitamin B12, also known as cobalamin. This vitamin plays a crucial role in the production of red blood cells in the bone marrow. A deficiency in vitamin B12 can lead to anemia, characterized by a reduced number of red blood cells, which can cause fatigue, weakness, and other health issues. Dietary sources of vitamin B12 include animal products such as meat, dairy, and eggs, making it particularly important for individuals following vegetarian or vegan diets to find alternative sources or supplements.