The best time to take vitamin C can be determined by the two main physiological functions of vitamin C absorption and excretion, so that the blood concentration of vitamin C in the body can be maintained for a longer period of time. It can be taken half an hour to one hour after a meal, when the food in the gastrointestinal tract is fully absorbed and vitamin C can be fully digested in the intestine, thus keeping the blood concentration of vitamin C for a longer period of time. It can also be taken at bedtime, as vitamin C is mainly excreted during sleep, when the blood flow in the kidneys is reduced, which has a greater effect on maintaining the blood concentration of vitamin C.