Kai has been warned by her nutritionist that taking large amounts of a certain vitamin can be toxic because it accumulates in the liver. which vitamin?

Iron

A side note about haemochromatosis:

Haemochromatosis is a disease where there is too much iron is in the body.  It is the most common form of iron overload disease. There are two types of haemochromatosis: 

Primary haemochromatosis is a genetic disorder inherited from family members.  People with this condition absorb too much iron and it ends up accumulating in the body, especially in the liver.   Secondary haemochromatosis is caused by other blood-related disorders such as anaemia, or may be due to many blood transfusions, long term alcoholism and/or other health conditions.    If left untreated, iron overload can lead to liver damage. That’s why it’s important to receive treatment as soon as possible after diagnosis to prevent further complications, including liver disease, liver cirrhosis, liver failure, liver cancer, heart disease, arthritis or diabetes. Some organ damage can be reversed if detected early enough and treated appropriately.

cigarettes

If you smoke cigarettes there’s a chance that you are causing damage to your liver – increasing your risk of developing liver cancer and decreasing your liver’s ability to rid your body of dangerous toxins. In turn, this could leave you more susceptible to the damaging effects of some medications on the liver too.