Health can affect intelligence in various ways. Conversely, intelligence can affect health. Health effects on intelligence have been described as being among the most important factors in the origins of human group differences in IQ test scores and other measures of cognitive ability. 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 barrier of the child is less effective. Such impairment may sometimes be permanent, sometimes be partially or wholly compensated for by later growth.
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.
Improvements in nutrition (often involving specific micronutrients) due to public policy changes have been implicated in IQ increases in many nations (as part of the overall Flynn effect), such as efforts fighting iodine deficiency in the U.S.
Nutrition Malnutrition may occur during several periods of growth, such as pregnancy, during breastfeeding, infancy, or childhood. It may also happen due to deficiencies of different nutrients, such as micronutrients, protein or energy. This may cause different effects. Timing Some observers have argued that malnutrition during the first six months of life harms cognitive development much more than malnutrition later in life. However, a study from the Philippines argues that malnutrition in the second year of life may have a larger negative impact than malnutrition in the first year of life. While it is debatable whether or not as an infant or after two years is the worst time for malnourishment, the bottom line in these studies is that not having enough nutrients at a young age negatively effects learning.
Intrauterine growth retardation
Undernutrition during pregnancy, and other factors, may cause intrauterine growth retardation (IUGR), which is one cause of low birth weight. However, it has been suggested that in IUGR the brain may be selectively spared. Brain growth is usually less affected than whole body weight or length. Several studies from developed nations have found that with the exception of extreme intrauterine growth retardation also affecting brain growth, and hypoxic injury, IUGR seems to have little or no measurable effect on mental performance and behavior in adolescence or adulthood. For example, acute undernutrition for a few months during the Dutch famine of 1944 caused a decrease in mean birthweight in certain areas. This was later associated with a change in performance on IQ tests for 18–19 years old Dutch males draftees from these areas compared to control areas. The subjects were exposed to famine prenatally but not after birth. During the famine, births decreased more among those with lower socioeconomic status (SES), whereas after the famine, there was a compensatory increase in births among those with lower SES. Since SES correlates with IQ, this may have hidden an effect caused by the undernutrition.
Breastfeeding Studies often find higher IQ in children and adults who were breastfed. It has been proposed that omega-3 fatty acids in breast milk, known to be essential constituents of brain tissues, could at least partially account for an increase in IQ.
Recently, however, the longstanding belief that breastfeeding causes an increase in the IQ of offspring was challenged in a 2006 paper published in the British Medical Journal. The results indicated that mother's IQ, not breastfeeding, explained the differences in the IQ scores of offspring measured between ages 5 and 14. The results of this meta-analysis argued that prior studies had not controlled for the mother's IQ. Since mother's IQ was predictive of whether a child was breastfed, the study concluded that "breast feeding [itself] has little or no effect on intelligence in children." Instead, it was the mother's IQ that had a significant correlation with the IQ of her offspring, whether the offspring was breastfed or was not breastfed.
One study found that breastfeeding was linked to raised IQ (as much as 7 points when not controlling for maternal IQ) if the infants had an SNP coding for a "C" rather than G base within the FADS2 gene. Those with the "G" version showed no IQ advantage, suggesting a biochemical interaction of child's genes on the effect of breastfeeding. Other studies have failed to replicate any correlation between the FADS2 gene, breastfeeding and IQ, while others show a negative effect on IQ when combining bottled feeding, and the "G" version of FADS2.
Infancy Two studies in Chile on 18-year-old high-school graduates found that nutritional status during the first year of life affected IQ, scholastic achievement, and brain volume.
Micronutrients and vitamin deficiencies Micronutrient deficiencies (e.g. in iodine and iron) influence the development of intelligence and remain a problem in the developing world. For example, iodine deficiency causes a fall, on average, of 12 IQ points. These deficiencies could technically show up in medical scans at various ages.
Policy recommendations to increase availability of micronutrient supplements have been made and justified in part by the potential to counteract intelligence-related developmental problems. For example, the Copenhagen consensus, states that lack of both iodine and iron has been implicated in impaired brain development, and this can affect enormous numbers of people: it is estimated that 2 billion people (one-third of the total global population) are affected by iodine deficiency, including 285 million 6- to 12-year-old children. In developing countries, it is estimated that 40% of children aged four and under have anaemia because of insufficient iron in their diets.Behrman, J.R., Alderman, H., and Hoddinott, J., "Hunger and Malnutrition ," Copenhagen Consensus 2004.
A joint statement on vitamin and mineral deficiencies says that the severity of such deficiencies "means the impairment of hundreds of millions of growing minds and the lowering of national IQs." Since the brain is not fully developed until age 25, this can be effecting people through their late teens.UNICEF and The Micronutrient Initiative, "Vitamin & Mineral Deficiency: A Global Progress Report ," March 2004.
Overall, studies investigating whether cognitive function in already iron-deficient children can be improved with iron supplements have produced mixed results, possibly because deficiency in critical growth periods may cause irreversible damage. However, several studies with better design have shown substantial benefits. One way to prevent iron deficiency is to give specific supplementation to children, for example as tablets. However, this is costly, distribution mechanisms are often ineffective, and compliance is low. Fortification of staple foods (cereals, flour, sugar, salt) to deliver micronutrients to children on a large scale is probably the most sustainable and affordable option, even though commitment from governments and the food industry is needed. Developed nations fortify several foods with various micronutrients.FOOD FORTIFICATION TECHNOLOGY Food Fortification: Technology and Quality Control. (FAO Food And Nutrition Paper - 60)
Additional vitamin-mineral supplementation may have an effect also in the developed world. A study giving such supplementation to "working class", primarily Hispanic, 6–12-year-old children in the United States for 3 months found an average increase of 2 to 3 IQ points. Most of this can be explained by the very large increase of a subgroup of the children, presumably because these were not adequately nourished unlike the majority. The study suggests that parents of schoolchildren whose academic performance is substandard would be well advised to seek a nutritionally oriented physician for assessment of their children's nutritional status as a possible etiology.
More speculatively, other nutrients may prove important in the future. Vitamin B12 and folate may be important for cognitive function in old age. Erratum in: Am. J. Clin. Nutr. 77 (2): 523. Fish oil supplement to pregnant and lactating mothers has been linked to increased cognitive ability in one study.
Another study found that pregnant women who consumed 340 grams of low-mercury containing fish with fatty acids per week have benefits that outweigh the risks for mercury poisoning. They were less likely to have children with low verbal IQ, motor coordination and behavioral problems. However, foods containing high amounts of mercury, such as shark, swordfish, king mackerel and tilefish, might cause mental retardation.Pregnant Women: Eat More Fish or Not? - To Your Health - MSNBC.comFish Diet in Pregnancy May Hone Kids' IQMedical News: Eating Fish During Pregnancy Provides 'Brain Food' for Child - in OB/GYN, Pregnancy from MedPage TodayPregnant? Omega-3 Essential for Baby's Brain
Protein and energy malnutrition
One study from a developing country, Guatemala, found that poor growth during infancy, rather than low birth weight, was negatively related to adolescent performance on cognitive and achievement tests. A later related very long-term study looked at the effect of giving 6–24-month-old children in Guatemala a high protein-energy drink as a dietary supplement. A significantly positive and fairly substantial effects was found on increasing the probability of attending school and of passing the first grade, increasing the grade attained by age 13, increasing completed schooling attainment, and for adults aged 25–40 increasing IQ test scores.
Stunting 31% of children under the age of 5 in the developing world are moderately (height-for-age is below minus 2 standard deviations) or severely stunted (below minus 3 standard deviations). The prevalence was even higher previously since the worldwide prevalence of stunting is declining by about half of a percentage point each year. A study on stunted children aged 9–24 months in Jamaica found that when aged 17–18 years they had significantly poorer scores than a non-stunted group on cognitive and educational tests and psychosocial functioning. Giving a nutritional supplementation (1 kg milk based formula each week) to these already stunted children had no significant effect on later scores, but psychosocial stimulation (weekly play sessions with mother and child) had a positive effect.
Toxins
Drug use Alcohol in pregnant women causes fetal alcohol syndrome, one of the leading known causes of intellectual disability in the Western world.
Current cannabis use was found to be significantly correlated in a dose-dependent manner with a decline in IQ scores, during the effect of the use. However, no such decline was seen in subjects who had formerly been heavy cannabis users and had stopped taking the drug. The authors concluded that cannabis does not have a long-term effect on intelligence. However this is contradicted by the long-term longitudinal study, carried out by Otago and Duke universities, which found that regular use of marijuana in teenage years affects IQ in adulthood even when the use stops. The drop in IQ was 8 points. Adults smoking marijuana had no lasting effect on IQ.Nature Journal. August 2012 and 30mt Radio discussion on Radio NZ with authors Effects on fetal development are minimal when compared with the well-documented adverse effects of tobacco or alcohol use.