Final answer:
Skin flushing can occur when the niacin intake exceeds the upper limit of 35 mg/day, leading to the dilation of blood vessels. Most niacin is excreted through urine, and exercise can increase this excretion.
Step-by-step explanation:
Flushing of the skin may occur when niacin exceeds the UL of 35 mg/day. The UL, or Tolerable Upper Intake Level, represents the maximum daily intake unlikely to cause adverse health effects. When niacin intake surpasses this level, it can lead to skin flushing due to the dilation of blood vessels. In addition, large doses of niacin are often excreted through urine, since 40 to 80% is usually eliminated in this way for adults, with a range of 20 to 70 µg daily. For children, this range is approximately 20 to 35 µg. Factors like exercise may increase urinary iodine, thus affecting overall excretion.