An interesting study by Powell in 2007 illustrates that additives may "hijack" a pathway that exists for calcium phosphate, in which particles passing through the GI tract stick to molecules of the intestinal lumen. See the whole study. He said that removing such additives from the diet of people with Crohn's disease markedly improved symptoms.
In rat studies, it has been shown that the G6PD enzyme increases when they are given food colorings and flavorings. This requirement of more enzyme to metabolize the additives may be a cause of stress for the person deficient in it. Indeed, another study in Nigeria showed that several people with this deficiency had attacks of haemolytic anemia after eating barbecued meat treated with red suya containing a "newly introduced food additive."
Way back in 1975, Worathumrong showed that salicylate inhibits something called the pentose phosphate pathway in cells of people with G6PD deficiency more than other people.