Vitamin B12 helps to utilize fats and carbohydrates for energy and make new proteins. Vitamin B12 is important in the maintenance of our metabolism, blood cells, and nerve function. Most people get enough vitamin B12 in their diet, but some need supplementation. Certain health conditions such as intestinal or stomach problems, poor nutrition, cancer, HIV, pregnancy, old age, veganism, and alcoholism can cause vitamin B12 deficiency.