Ingesting large amounts of riboflavin, commonly known as vitamin B2, causes urine to take on a bright yellow color, according to Gizmodo. Riboflavin is found in high amounts in many multivitamin supplements.
Studies about riboflavin absorption and excretion show that about 27 milligrams of riboflavin is the maximum amount the body absorbs in a single dose, and the body excretes half of all excess riboflavin in the form of urine, as Gizmodo explains. High doses of up to 100 milligrams of riboflavin are found in some multivitamin formulations. In such a 100 milligram dose of riboflavin, approximately 73 milligrams go unabsorbed, and the body excretes half of that in its urine. The excess riboflavin in the urine is what causes the urine to become a bright yellow color.
Individuals are far less likely to get enough riboflavin from food to cause their urine to become bright yellow. An excellent source of riboflavin is spinach, which contains 0.42 milligrams of riboflavin per cup of spinach, according to the World's Healthiest Foods. To consume 27 milligrams of riboflavin, the maximum amount that the human body can absorb in a single dose, an individual would need to eat about 64 cups of spinach. Other good sources of riboflavin in food include beet greens, soybeans, tempeh, eggs and almonds.