The finding of another possible cause stemmed from the observation that children of women who smoked during pregnancy are more likely to be diagnosed with ADHD. Given that nicotine is known to cause hypoxia (too little oxygen) in the uterus, and that hypoxia causes brain damage, smoking during pregnancy could be an important contributing factor leading to ADHD. It may even help explain in part the increase in ADHD diagnoses, as the number of women smokers has increased. However, there are not nearly enough women smoking during pregnancy to account for all the ADHD diagnoses, and the mothers of many of those diagnosed with ADHD did not smoke during or before pregnancy. It is also possible that cause and effect could be confounded in this study, since many mothers who smoke during pregnancy may be ADHD suffers themselves; therefore the cause may simply be the shared genetic material of mother and child, rather than the mother's smoking.
Brain development in the uterus and during the first year of life, possibly related to drug use during pregnancy or environmental toxins may also be possible causes, but again, little proof seems to exist as yet, and it can again be pointed out that the parents of many if not most individuals with ADHD did not consume drugs during regency.
It has also been suggested that ADHD may be the result of a poor diet and other external factors, rather than from any physiological source. Studies of changes in diets of children provide some anecdotal and scientific evidence for this, but current majority opinion seems to be that the available evidence is insufficient to either prove or disprove this. However, it has been noticed that some children with ADHD seem to be addicted to milk. It has been proposed by Norwegian and British scientists that this is due to the casomorphins, peptides formed by incomplete digestion of the whey protein.
It has, however, been established conclusively that a small percentage of children are sensitive to dyes and other food additives, sugar, caffeine, etc.
Children with ADHD have lower levels of key fatty acids. In fact, one study found that the lower the levels, the worse the symptoms. The possibility that fatty acid deficiency is a trigger for ADHD is especially plausible as nutrition scientists have recently demonstrated that the American diet is extremely deficient in omega-3 fatty acids. At the same time, ADHD diagnoses are rapidly increasing. More support for this idea comes from findings that breast-fed children have much lower levels of ADHD, and that until quite recently, infant formula contained NO omega-3 fatty acids.
A recent randomized double-blind experiment compared a fatty acid supplement with placebo in children with developmental coordination disorder (which exhibits a high degree of overlap with ADHD diagnoses). Fatty acid supplements improved spelling, reading, and behavior after three months. While not directly showing a causal link between ADHD and fatty acids, increased levels of fatty acids has a beneficial effect on related behavior.
However, creating a deficiency of omega-3 fatty acids in pregnant rats produces pups that are hyperactive and that have altered brain levels of dopamine in the same brain regions as seen in humans and other rat models of hyperactivity.