Impact of health on intelligence

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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. [1] 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.

Contents

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. [2] [3]

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. [4]

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. [5]

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. [6]

Breastfeeding

Studies often find higher IQ in children and adults who were breastfed. [3] [7] 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. [8]

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. [9] [10] Other studies have failed to replicate any correlation between the FADS2 gene, [11] breastfeeding and IQ, while others show a negative effect on IQ when combining bottled feeding, and the "G" version of FADS2. [12]

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. [13] [14]

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. [15]

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. [16]

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. [17]

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. [18] Developed nations fortify several foods with various micronutrients. [19]

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. [20]

More speculatively, other nutrients may prove important in the future. Vitamin B12 and folate may be important for cognitive function in old age. [21] Fish oil supplement to pregnant and lactating mothers has been linked to increased cognitive ability in one study. [22]

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. [23] [24] [25] [26] [27] [28]

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. [29] 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. [30]

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). [31] The prevalence was even higher previously since the worldwide prevalence of stunting is declining by about half of a percentage point each year. [32] 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. [33] [34]

Toxins

Industrial chemicals

Certain toxins, such as lead, mercury, toluene, and PCB are well-known causes of neuro-developmental disorders. Recognition of these risks has led to evidence-based programmes of prevention, such as elimination of lead additives in petrol. Although these prevention campaigns are highly successful, most were initiated only after substantial delays. [35]

Policies to manage lead differ between nations, particularly between the developed and developing world. Use of leaded gasoline has been reduced or eliminated in most developed nations, and lead levels in US children have been substantially reduced by policies relating to lead reduction. [36] Even slightly elevated lead levels around the age of 24 months are associated with intellectual and academic performance deficits at age 10 years. [37]

Certain, at least previously, widely used organochlorides, such as dioxins, DDT, and PCB, have been associated with cognitive deficits. [38]

A Lancet review identified 201 chemicals with the ability to cause clinical neurotoxic effects in human adults, as described in the peer-reviewed scientific literature. Most of them are commonly used. Many additional chemicals have been shown to be neurotoxic in laboratory models. The article notes that children are more vulnerable and argues that new, precautionary approaches that recognise the unique vulnerability of the developing brain are needed for testing and control of chemicals in order to avoid the previous substantial before starting restrictions on usage. [39] An appendix listed further industrial chemicals considered to be neurotoxic. [40]

Alcohol and drugs

Fetal alcohol exposure, causing Fetal alcohol syndrome, is one of the leading known causes of intellectual disability in the Western world. [41]

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. [42] Effects on fetal development are minimal when compared with the well-documented adverse effects of tobacco or alcohol use. [43]

Maternal tobacco smoking during pregnancy is associated with increased activity, decreased attention, and diminished intellectual abilities. [44] However, a recent study finds that maternal tobacco smoking has no direct causal effect on the child's IQ. Adjusting for maternal cognitive ability as measured by IQ and education eliminated the association between lower IQ and tobacco smoking. [45] But another study instead looking at the relationship between environmental tobacco smoke exposure, measured with a blood biomarker, and cognitive abilities among U.S. children and adolescents 6–16 years of age, found an inverse association between exposure and cognitive ability among children even at extremely low levels of exposure. The study controlled for sex, race, region, poverty, parent education and marital status, ferritin, and blood lead concentration. [46]

Healthcare during pregnancy and childbirth

Healthcare during pregnancy and childbirth, access to which is often governed by policy, also influences cognitive development. Preventable causes of low intelligence in children include infectious diseases such as meningitis, parasites, and cerebral malaria, prenatal drug and alcohol exposure, newborn asphyxia, low birth weight, head injuries, and endocrine disorders. A direct policy focus on determinants of childhood cognitive ability has been urged. [2]

Stress

A recent theory suggests that early childhood stress may affect the developing brain and cause negative effects. [47] Exposure to violence in childhood has been associated with lower school grades [48] and lower IQ in children of all races. [49] A group of largely African American urban first-grade children and their caregivers were evaluated using self-report, interview, and standardized tests, including IQ tests. The study reported that exposure to violence and trauma-related distress in young children were associated with substantial decrements in IQ and reading achievement. Exposure to violence or trauma was correlated with a 7.5-point (SD, 0.5) decrement in IQ and a 9.8-point (SD, 0.66) decrement in reading achievement. [48]

Violence may have a negative impact on IQ, or IQ may be protective against violence. [49] The causal mechanism and direction of causation is unknown. [48] Neighborhood risk has been related to lower school grades for African-American adolescents in another study from 2006. [50]

Infectious diseases

A 2010 study by Eppig, Fincher and Thornhill found a close correlation between the infectious disease burden in a country and the average IQ of its population. The researchers found that when disease was controlled for, IQ showed no correlation with other variables such as educational and nutritional levels. Since brain development requires a very high proportion of all the body's energy in newborns and children, the researchers argue that fighting infection reduces children's IQ potential. The Eppig research may help to explain the Flynn effect, the rise in intelligence noted in rich countries. [51] They also tested other hypotheses as well, including genetic explanations, concluding that infectious disease was "the best predictor". [52] Christopher Hassall and Thomas Sherratt repeated the analysis, and concluded "that infectious disease may be the only really important predictor of average national IQ". [52]

In order to mitigate the effects of education on IQ, Eppig Fincher & Thornhill repeated their analysis across the United States where standardized and compulsory education exists. [52] [53] The correlation between infectious disease and average IQ was confirmed, and they concluded that the "evidence suggests that infectious disease is a primary cause of the global variation in human intelligence". [52]

Tropical infectious diseases

Malaria affects 300–500 million persons each year, mostly children under age five in Africa, causing widespread anemia during a period of rapid brain development and also direct brain damage from cerebral malaria to which children are more vulnerable. [54] A 2006 systematic review found that Plasmodium falciparum infection causes cognitive deficits in both the short- and long-term. [55] Policies aimed at malaria reduction may have cognitive benefits. It has been suggested that the future economic and educational development of Africa critically depends on the eradication of malaria.

Roundworms infect hundreds of millions of people. There is evidence that high intensities of worms in the intestines can affect mental performance, [56] but a systematic review in 2000 and a 2009 update found that there was insufficient evidence to show that deworming treatments improve cognitive performance or school performance in children. [57] [58]

HIV infection in children in sub-Saharan Africa affects their motor development, but there is insufficient evidence to show a slowing of language development. [59]

Effects of other diseases

There are numerous diseases affecting the central nervous system which can cause cognitive impairment. Many of these are associated with aging. Some common examples include Alzheimer's disease and Multi-infarct dementia. Many diseases may be neurological or psychiatric and may primarily affect the brain. Others may affect many other organs, like HIV, Hashimoto's thyroiditis causing hypothyroidism, or cancer. According to a 2015 report in The American Scholar, an assortment of neglected tropical diseases as well as some recently identified pathogens such as Pseudo-nitzschia have also been found to erode human intelligence. [60]

Major depression, affecting about 16% of the population on at least one occasion in their lives and the leading cause of disability in North America, may give symptoms similar to dementia. Patients treated for depression score higher on IQ tests than before treatment. [61] [62]

Myopia and hyperopia

A 2008 literature review writes that studies in several nations have found a relationship between myopia and higher IQ and between myopia and school achievement. Several, but not all, studies have found hyperopia to be associated with lower IQ and school achievements. A common explanation for myopia is near-work. Regarding the relationship to IQ, several explanations have been proposed. One is that the myopic child is better adapted at reading, and reads and studies more, which increases intelligence. The reverse explanation is that the intelligent and studious child reads more which causes myopia. Another is that the myopic child has an advantage at IQ testing which is near work because of less eye strain. Still another explanation is that pleiotropic gene(s) affect the size of both brain and eyes simultaneously. [63] A study of Chinese schoolchildren found that after controlling for age, gender, school, parental myopia, father's education, and books read per week, myopia was still associated with high nonverbal IQ. Nonverbal IQ was a more important explanation than books read per week. [64]

Other associations

Long working hours (55 vs. 40) was associated with decreased scores on cognitive tests in a 5-year study on midlife British civil servants. [65]

See also

Related Research Articles

The Flynn effect is the substantial and long-sustained increase in both fluid and crystallized intelligence test scores that were measured in many parts of the world over the 20th century. When intelligence quotient (IQ) tests are initially standardized using a sample of test-takers, by convention the average of the test results is set to 100 and their standard deviation is set to 15 or 16 IQ points. When IQ tests are revised, they are again standardized using a new sample of test-takers, usually born more recently than the first; the average result is set to 100. When the new test subjects take the older tests, in almost every case their average scores are significantly above 100.

Discussions of race and intelligence – specifically, claims of differences in intelligence along racial lines – have appeared in both popular science and academic research since the modern concept of race was first introduced. With the inception of IQ testing in the early 20th century, differences in average test performance between racial groups were observed, though these differences have fluctuated and in many cases steadily decreased over time. Further complicating the issue, modern science has shown race to be a social construct rather than a biological reality, and intelligence has no undisputed definition. The validity of IQ testing as a metric for human intelligence is itself disputed. Today, the scientific consensus is that genetics does not explain differences in IQ test performance between groups, and that observed differences are environmental in origin.

<span class="mw-page-title-main">Malnutrition</span> Medical condition

Malnutrition occurs when an organism gets too few or too many nutrients, resulting in health problems. Specifically, it is "a deficiency, excess, or imbalance of energy, protein and other nutrients" which adversely affects the body's tissues and form. Malnutrition is not receiving the correct amount of nutrition. Malnutrition is increasing in children under the age of five due to providers who cannot afford or do not have access to adequate nutrition.

Iodine deficiency is a lack of the trace element iodine, an essential nutrient in the diet. It may result in metabolic problems such as goiter, sometimes as an endemic goiter as well as congenital iodine deficiency syndrome due to untreated congenital hypothyroidism, which results in developmental delays and other health problems. Iodine deficiency is an important global health issue, especially for fertile and pregnant women. It is also a preventable cause of intellectual disability.

Prenatal development includes the development of the embryo and of the fetus during a viviparous animal's gestation. Prenatal development starts with fertilization, in the germinal stage of embryonic development, and continues in fetal development until birth.

<span class="mw-page-title-main">Stunted growth</span> Reduced growth rate in human development

Stunted growth, also known as stunting or linear growth failure, is defined as impaired growth and development manifested by low height-for-age. It is a primary manifestation of malnutrition and recurrent infections, such as diarrhea and helminthiasis, in early childhood and even before birth, due to malnutrition during fetal development brought on by a malnourished mother. The definition of stunting according to the World Health Organization (WHO) is for the "height-for-age" value to be less than two standard deviations of the median of WHO Child Growth Standards. Stunted growth is usually associated with poverty, unsanitary environmental conditions, maternal undernutrition, frequent illness, and/or inappropriate feeding practice and care during early years of life.

<span class="mw-page-title-main">Protein–energy malnutrition</span> Medical condition

Protein–energy malnutrition (PEM), sometimes called protein-energy undernutrition (PEU), is a form of malnutrition that is defined as a range of conditions arising from coincident lack of dietary protein and/or energy (calories) in varying proportions. The condition has mild, moderate, and severe degrees.

The study of height and intelligence examines correlations between human height and human intelligence. Some epidemiological research on the subject has shown that there is a small but statistically significant positive correlation between height and intelligence after controlling for socioeconomic class and parental education. The cited study, however, does not draw any conclusions about height and intelligence, but rather suggests "a continuing effect of post-natal growth on childhood cognition beyond the age of 9 years." This correlation arises in both the developed and developing world and persists across age groups. An individual's taller stature has been attributed to higher economic status, which often translates to a higher quality of nutrition. This correlation, however, can be inverted to characterize one's socioeconomic status as a consequence of stature, where shorter stature can attract discrimination that affects many factors, among them employment, and treatment by educators. One such theory argues that since height strongly correlates with white and gray matter volume, it may act as a biomarker for cerebral development which itself mediates intelligence. Competing explanations include that certain genetic factors may influence both height and intelligence, or that both height and intelligence may be affected in similar ways by adverse environmental exposures during development. Measurements of the total surface area and mean thickness of the cortical grey matter using a magnetic resonance imaging (MRI) revealed that the height of individuals had a positive correlation with the total cortical surface area. This supports the idea that genes that influence height also influence total surface area of the brain, which in turn influences intelligence, resulting in the correlation. Other explanations further qualify the positive correlation between height and intelligence, suggesting that because the correlation becomes weaker with higher socioeconomic class and education level, environmental factors could partially override any genetic factors affecting both characteristics.

<span class="mw-page-title-main">Child development</span> Developmental change in children

Child development involves the biological, psychological and emotional changes that occur in human beings between birth and the conclusion of adolescence.

Environment and intelligence research investigates the impact of environment on intelligence. This is one of the most important factors in understanding human group differences in IQ test scores and other measures of cognitive ability. It is estimated that genes contribute about 20–40% of the variance in intelligence in childhood and about 80% in adulthood. Thus the environment and its interaction with genes account for a high proportion of the variation in intelligence seen in groups of young children, and for a small proportion of the variation observed in groups of mature adults. Historically, there has been great interest in the field of intelligence research to determine environmental influences on the development of cognitive functioning, in particular, fluid intelligence, as defined by its stabilization at 16 years of age. Despite the fact that intelligence stabilizes in early adulthood it is thought that genetic factors come to play more of a role in our intelligence during middle and old age and that the importance of the environment dissipates.

Zinc deficiency is defined either as insufficient zinc to meet the needs of the body, or as a serum zinc level below the normal range. However, since a decrease in the serum concentration is only detectable after long-term or severe depletion, serum zinc is not a reliable biomarker for zinc status. Common symptoms include increased rates of diarrhea. Zinc deficiency affects the skin and gastrointestinal tract; brain and central nervous system, immune, skeletal, and reproductive systems.

<span class="mw-page-title-main">Micronutrient deficiency</span> Medical condition

Micronutrient deficiency is defined as the sustained insufficient supply of vitamins and minerals needed for growth and development, as well as to maintain optimal health. Since some of these compounds are considered essentials, micronutrient deficiencies are often the result of an inadequate intake. However, it can also be associated to poor intestinal absorption, presence of certain chronic illnesses and elevated requirements.

Iodine deficiency is a widespread problem in western, southern and eastern parts of China, as their iodized salt intake level is much lower than the average national level. Iodine deficiency is a range of disorders that affect many different populations. It is estimated that IDDs affect between 800 million and 2 billion people worldwide; countries have spent millions of dollars in implementing iodized salt as a means to counteract the iodine deficiencies prevalent today. With China accounting for "40% of the total population", it bears a large portion of those who are iodine deficient.

<span class="mw-page-title-main">Nutritional neuroscience</span> Scientific discipline

Nutritional neuroscience is the scientific discipline that studies the effects various components of the diet such as minerals, vitamins, protein, carbohydrates, fats, dietary supplements, synthetic hormones, and food additives have on neurochemistry, neurobiology, behavior, and cognition.

Prenatal cocaine exposure (PCE), theorized in the 1970s, occurs when a pregnant woman uses cocaine and thereby exposes her fetus to the drug. Babies whose mothers used cocaine while pregnant supposedly have increased risk of several different health issues during growth and development.

Prenatal memory, also called fetal memory, is important for the development of memory in humans. Many factors can impair fetal memory and its functions, primarily maternal actions. There are multiple techniques available not only to demonstrate the existence of fetal memory but to measure it. Fetal memory is vulnerable to certain diseases so much so that exposure can permanently damage the development of the fetus and even terminate the pregnancy by aborting the fetus. Maternal nutrition and the avoidance of drugs, alcohol and other substances during all nine months of pregnancy is important to the development of the fetus and its memory systems. The use of certain substances can entail long-term permanent effects on the fetus that can carry on throughout their lifespan.

<span class="mw-page-title-main">Environmental enteropathy</span> Disorder of chronic intestinal inflammation

Environmental enteropathy is an acquired small intestinal disorder characterized by gut inflammation, reduced absorptive surface area in small intestine, and disruption of intestinal barrier function. EE is most common amongst children living in low-resource settings. Acute symptoms are typically minimal or absent. EE can lead to malnutrition, anemia, Stunted growth, impaired brain development, and impaired response to oral vaccinations.

<span class="mw-page-title-main">Undernutrition in children</span> Medical condition affecting children

Undernutrition in children, occurs when children do not consume enough calories, protein, or micronutrients to maintain good health. It is common globally and may result in both short and long term irreversible adverse health outcomes. Undernutrition is sometimes used synonymously with malnutrition, however, malnutrition could mean both undernutrition or overnutrition. The World Health Organization (WHO) estimates that malnutrition accounts for 54 percent of child mortality worldwide, which is about 1 million children. Another estimate, also by WHO, states that childhood underweight is the cause for about 35% of all deaths of children under the age of five worldwide.

The first 1000 days describes the period from conception to 24 months of age in child development. This is considered a "critical period" in which sufficient nutrition and environmental factors have life-long effects on a child's overall health. While adequate nutrition can be exceptionally beneficial during this critical period, inadequate nutrition may also be detrimental to the child. This is because children establish many of their lifetime epigenetic characteristics in their first 1000 days. Medical and public health interventions early on in child development during the first 1000 days may have higher rates of success compared to those achieved outside of this period.

The effects of early-life exposures to anesthesia on the brain in humans are controversial. Evidence from nonhuman primate research suggests significant developmental neurotoxicity and long-term social impairment, with a dose–response relationship where repeated exposures cause a more severe impact than single ones. Research in humans has not found conclusive clinical evidence of cognitive impairment; however, systematic reviews imply the possibility of greater behavioural impairments in children exposed to anesthesia before the age of three than control subjects.

References

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