The vitamin C blood levels were measured only once. The researchers acknowledged that the lack of further measurement of vitamin C levels might have weakened the findings of their report, because blood levels of the vitamin can vary significantly in one person over time. People who took any supplements had almost the same risk of dying during the four-year period as did people who took no supplements. However, researchers did not focus on the isolated effects of taking vitamin C supplements. When use of all supplements is placed into one category, scientists know that interpreting the data becomes difficult.
Some people take supplements because they are sick.
These and other factors make supplement takers different from people who do not take supplements. The authors of the report acknowledge that their study does not tell us whether adding vitamin C supplements to a vitamin C-rich diet would drive death rates even lower. Men in the group with the highest intake of vitamin C consumed an average of about seven ounces of fruit and almost four ounces of vegetables per day. Women in the highest intake group consumed approximately an ounce more fruit and the same amount of vegetables as the men. People consuming even less vitamin C from their diets than those in the lowest quintile studied might suffer even higher death rates, an idea supported mostly by animal research and reasonable guesswork on the part of scientists.