An essential nutrient often lacking in vegetarian diets is vitamin B12, which is primarily found in animal products. This vitamin is crucial for maintaining healthy nerve cells and producing red blood cells. Vegans and many vegetarians may need to supplement, as inadequate intake can lead to deficiencies, resulting in fatigue and neurological issues. Fortified foods such as plant-based milk or cereal may provide alternative sources of B12, but monitoring and possibly supplementing is essential for those avoiding animal products.