Proper childhood nutrition appears critical for cognitive development; malnutrition can lower IQ. For example, iodine deficiency causes a fall, in average, of 12 IQ points.[68]It is expected that average IQ in third world countries will increase if malnutrition of various kinds is eradicated.

One recent study found that a group of children with the "C" version of the FADS2 gene who were breastfed acquired on average 7 IQ points higher than to those with the "G"allele of the gene.[69][70] Other studies have failed to replicate any correlation between the FADS2 gene,[71] breastfeeding and IQ, while others show a negative effect on IQ when combining bottledfeeding, and the "G" version of FADS2 .[72]

People with a higher IQ have generally lower adult morbidity and mortality. Post-Traumatic Stress Disorder[73] and schizophrenia[74][75] are less prevalent in higher IQ bands. People in the midsts of a major depressive episode have been shown to have a lower IQ than when without symptoms and lower cognitive ability than people without depression of equivalent verbal intelligence.[76][77]

A study of 11,282 individuals in Scotland who took intelligence tests at ages 7, 9 and 11 in the 1950s and 1960s, found an "inverse linear association" between childhood IQ scores and hospital admissions for injuries in adulthood. The association between childhood IQ and the risk of later injury remained even after accounting for factors such as the child's socioeconomic background.[78] Research in Scotland has also shown that a 15-point lower IQ meant people had a fifth less chance of living to 76, while those with a 30-point disadvantage were 37% less likely than those with a higher IQ to live that long.[79]

A decrease in IQ has also been shown as an early predictor of late-onset Alzheimer's Disease and other forms of dementia. In a 2004 study, Cervilla and colleagues showed that tests of cognitive ability provide useful predictive information up to a decade before the onset of dementia.[80] However, when diagnosing individuals with a higher level of cognitive ability, in this study those with IQs of 120 or more,[81] patients should not be diagnosed from the standard norm but from an adjusted high-IQ norm that measured changes against the individual's higher ability level. In 2000, Whalley and colleagues published a paper in the journal Neurology, which examined links between childhood mental ability and late-onset dementia. The study showed that mental ability scores were significantly lower in children who eventually developed late-onset dementia when compared with other children tested.[82]

IQ is also negatively correlated with certain diseases.

Several factors can lead to significant cognitive impairment, particularly if they occur during pregnancy and childhood when the brain is growing and the blood-brain barrieris less effective. Such impairment may sometimes be permanent, or may sometimes be partially or wholly compensated for by later growth. Several harmful factors may also combine, possibly causing greater impairment.

Developed nations have implemented several health policies regarding nutrients and toxins known to influence cognitive function. These include laws requiring fortification of certain food products and laws establishing safe levels of pollutants (e.g. lead, mercury, and organochlorides). Comprehensive policy recommendations targeting reduction of cognitive impairment in children have been proposed.[83]

In terms of the effect of one's intelligence on health, in one British study, high childhood IQ was shown to correlate with one's chance of becoming a vegetarian in adulthood.[84] In another British study, high childhood IQ was shown to inversely correlate with the chances of smoking.[85]

There is also a relationship between longevity and intelligence.