A group of 50 adult male humans were in a clinical study to determine how much niacin is toxic. During the first 2 weeks of the study, members of the group consumed only a formula diet that was nutritionally complete, except it contained 24 mg niacin, which is 1.5 times the RDA for niacin (males). At that point in the study, all members reported feeling fine, and they could function normally. During the next week, they were given a formula that contained 48 mg niacin. Within a few days of consuming this formula, 90% of the participants reported feeling ill, and they complained of having upset stomachs and flushing (redness) on their faces. At that point, researchers gave the subjects a formula that contained 16 mg niacin. After a week of consuming the new formula, all participants reported feeling well again. Based on this information, the researchers concluded that the ________.
a) Toxicity level of niacin is 48 mg
b) Toxicity level of niacin is 24 mg
c) Toxicity level of niacin is 16 mg
d) Niacin has no toxic effects