Because intelligence is a thing which the brain does, and the brain requires nutrients to do anything, it is obvious that extreme malnutrition can cause a significant IQ deficit. However, some people doubt that the level of malnutrition needed to inhibit intelligence is common in modern human populations. This view is contradicted by the fact that, as shown in several reviews, cognitive abilities can probably be enhanced by taking multi-vitamins.
Schoenthaler and Bier (1999) reviewed 13 randomized control trials in which gains on non-verbal IQ tests were compared between groups of people given nutritional supplements and groups given placebos.
As can be seen, the mean effect of nutritional supplements was to increase a group’s average non-verbal IQ by 3.2 points and while not all studies produced an effect size which was statistically significant, they did all produce effects in the expected direction.
Closer analysis of these studies has suggested that most participants in both groups experience the same change in IQ (as a result of “learning” the test) and that the higher gains among groups given nutritional supplements is due to a minority of participants, around 20%, experiencing IQ gains of nearly 15 points.
Eilander et al. (2010) restricted their sample to studies with treatments that lasted at least 4 weeks, on children, and on healthy samples. Across 12 randomized controlled trails, the standardized mean difference between those given a multi vitamin supplement and those not was 0.14 SD for fluid intelligence (p=.083) and =-.03 SD (p=.74) for crystalized intelligence. Across 4 trails, a SMD of .30 SD (p=.044) was found on academic performance while no significant effect was found for short term memory, sustained attention, or cognitive processing speed.
Surprisingly, trials conducted in high socio-economic status nations found larger effects than did those conducted in developing countries. This may be because a lack of macro nutrients prevented children in developing nations from using micro nutrients properly.
Lastly, Grima et al. (2012) meta-analyzed randomized control trials looking at how multi vitamin supplements impact cognitive ability in adults. In total, they looked at 10 studies covering 3,200 participants. However, these studies looked at different cognitive abilities. Two studies looked at general cognitive ability and both found no significant effect. One study looked at reasoning ability and found no significant effect. Across three studies (n=1,233) supplementation was found to improve verbal fluency but this was not statistically significant (SMD=.06, P=.26). Two studies looked at numerical ability and both found a positive effect. Visual perception was investigated by three studies, two of which found no effect and one of which found a positive effect. Reaction times was looked at in two studies. One found a positive result (choice reaction time) and one did not (simple reaction time). Four studies looked at immediate recall and together they produced a positive effect (SMD=.32, P=<.01). Three studies looked at delayed recall and together they produced an insignificant negative effect (SMD=-.14 P=.33).
In total, these reviews suggest that multivitamins can improve intelligence, but this effect is likely limited to just a few cognitive abilities and a minority of people.
Race and Food Consumption in America
Before looking at how different essential nutrients predict IQ, and how Blacks and Whites differ in said essential nutrients, let’s take a look at racial differences in food consumption. Self-report data indicates that African Americans eat more fruit, vegetables, meat, and sugar, than White Americans do. They also have higher rates of obesity.
In light of these facts, it is odd to think, as many egalitarians do, that Blacks score poorly on IQ tests due to nutritional deficiencies. (Unless, of course, you think that Blacks are more likely than Whites to lie about eating healthy food.) Nonetheless, let’s take a look at how each essential nutrient relates to cognitive ability and the degree of Black and White Americans differ in their levels of those nutrients.
Essential Nutrients, Intelligence, and Race
Research on essential nutrients typically falls into three categories: cross-sectional, longitudnal, and randomized control trials. Cross-sectional studies simply look at whether, at a given point in time, levels of different nutrients in the body predict a person’s cognitive ability. These studies typically control not only for social variables such as wealth or education, but also biological variables, such as the levels of other nutrients in that person’s body. Longitudinal studies look at how nutrient levels predict future changes in cognitive ability. These studies are typically done with children or elderly participants. RCTs are experiments in which one group is given a nutrient supplement and another is given a placebo and the cognitive abilities of these two groups are compared before and after treatment.
RCTs can obviously establish that a nutrient has a casual impact on intelligence. However, a lack of positive effects in RCT does not show that a given nutrient doesn’t impact intelligence. A lack of effect from supplements can be caused by the body not absorbing the nutrients in the supplement in the same way that it does when the nutrients are found in regular food.
Because of this, if longitudinal and cross-sectional evidence supports a causal hypothesis while RCT data either does not or simply doesn’t exist, then we can justifiably say that there is reasonable evidence for a real effect of a nutrient on intelligence. With that being said, let’s look at each nutrient, one by one.
Vitamin B and Folate
Feng et al. (2006) reviewed six longitudinal studies which found a significant relationship between vitamin B-12 and either dementia or cognitive decline and 3 longitudinal studies which found no relationship. Feng et al. also conducted a study of their own which found no relationship between vitamin B levels and cognitive ability in a sample of 451 elderly Chinese people.
Balk et al. (2007) reviewed 14 RCTs which gave elderly participants vitamin D and Folate supplements and then measured the impact this had on their cognitive ability. These studies consistently found no effect.
Given the inconsistent results of longitudinal studies and consistently negative results of RCTs, there is only very weak evidence linking vitamin B to intelligence. Vitamin B deficiency is very rarevery rare in both races but slightly more prevalent in Whites (2.2%) than Blacks (1.2%). It probably plays no role in the Black-White IQ gap.
Harrison (2012) reviewed 7 studies which found a significant cross-sectional relationship between vitamin C intake and cognitive ability and 3 which did not. Harrison also reviewed 2 longitudinal studies which found that vitamin C intake predicted cognitive decline and one RCT which found that vitamin C supplementation had no effect on cognitive ability. Given the inconsistent findings of cross-sectional research, the limited number of longitudinal studies, and lack of effect in the RCT, there is only weak evidence that vitamin C impacts intelligence.
Vitamin C deficiency is more common among Whites (7.1%) than among Blacks (4.3%), but it is somewhat rare within both races. It is unlikely that it has a significant impact on the Black-White IQ gap.
Anastasiou et al. (2014) reviewed several previous meta-analyses which showed that there is a cross-sectional relationship between vitamin D levels and cognitive ability. Two longitudinal studies have looked at whether vitamin D levels predict later cognitive ability in non-elderly populations. One study found no effect while another, with a sample of 6,494 participants, found that vitamin D have a positive relationship with verbal fluency, verbal memory, and mental processing speed, and a curvilinear relationship with word recall. Among elderly subjects, four studies have produced positive results and two have produced insignificant results when looking at how vitamin D levels predict later cognitive decline. Five RCTs have been carried out on vitamin D and all five have shown no effect. Given the inconsistent longitudinal findings and the consistent negative findings of RCTs, there is only weak evidence that vitamin D impacts intelligence.
Vitamin D deficiency is found in about 1 in 33 Whites and 1 in 3 Blacks. This is because Black’s darker skin blocks vitamin D absorption from the sun. The evidence linking Vitamin D to intelligence is not very strong, but if it does impact IQ then it certainly plays a role in the Black-White IQ gap. The racial gap in vitamin D deficiency is caused by genetics however, and so this would support the hereditarian view on racial intelligence differences.
Morris et al. (2002) reviewed six previous cross-sectional studies five of which found a significant association between vitamin E levels and cognitive ability. They then conducted a longitudinal study with a sample of 2,889 elderly people and found that greater vitamin E consumption predicted with lesser cognitive decline even after controlling for age, race, sex, education, smoking, alcoholic consumption, total caloric intake, and intake of vitamin C, carotene, and Vitamin A.
Hang et al (2006) reviewed three previous RCTs which found no effect of vitamin E supplementation on cognitive ability before conducting their own, with a sample of over 6,000 women, which also found no effect.
Cross-sectional studies point to a real effect, but RCTs do not and there has only been one longitudinal study that I am aware. Given this, I conclude that there is moderate evidence that vitamin E impacts intelligence.
Vitamin E deficiency is more common among Blacks (1.2%) than among Whites (0.6%) but is very rare in both races. Given this, it is unlikely that it plays a significant role in the Black-White IQ gap.
Gogia and Sachdev (2012) meta-analyzed the results of 8 RCTs and found that zinc supplementation had no effect on infant cognitive ability. Bhatnager and Taneja (2001) and Black (2003) reviewed five RCTs not included in Gogia and Sachdev’s meta-analysis three of which found no effect and two of which found a positive effect. Tupe and Chiplonkar (2009) and Penland (2005) also found that zinc supplementation had a positive effect on cognitive ability while Rico (2006) did not.
Given the lack of longitudinal evidence and wild inconsistency of RCTs, the evidence does not favor zinc having an effect on intelligence.
McGregor and Ani (2001) reviewed cross-sectional evidence going back to 1919 showing a relationship between iron deficiency and cognition performance as well as seven longitudinal studies all of which showed that iron deficiency predicted lower future cognitive ability.
Sachdev et al. (2005) meta-anlayzed 17 RCTs and found a standardized mean difference between treatment and placebo groups of 0.30 (p=<.001). Four of these studies used full scale IQ tests and they produced a combained standardized mean difference of .41 (P<.001).
The evidence that iron levels impact intelligence is very strong. It used to be the case that African Americans were more likely than White Americans to have an iron deficiency. However, this is no longer the case.
This chart displays data on 1-3 year olds. Given that many African American adults alive today were born when there were racial differences in iron deficiency, it may play some role in the current Black-White IQ gap.
Back in the 1970’s, roughly 15% of Blacks and 5% of Whites had an iron deficiency meaning that there was a 10 point gap. If we assume that iron deficiency lowers a person’s IQ by 0.4 SD, or 6 points, as implied by Sachdev’s meta-analysis of RCT studies, then iron deficiency would have caused a Black-White IQ gap of .04 SD, or 0.6 points. This is a little less than 4% of the Black-White IQ gap which existed in the 1970s.
Happily, this environment disadvantage has already been completely eradicated and any impact it had on racial intelligence disparities will disappear over the next few decades.
Conclusion for America
Nutritional deficiencies likely play little to no role in the American Black-White IQ gap. Nutritional deficiencies are generally rare in both races and sometimes favor Blacks. Moreover, there is no strong evidence linking most nutrients to IQ. The exceptions to these two major trends are iron and vitamin D but, as we have seen, the racial gap in vitamin D is caused by genes and the racial gap in iron has been eliminated.
However, malnutrition does differ significantly by race internationally. Africa has significantly higher rates of malnurishment than Europe or America does, and so this might play a role explaining why Africa has a very low mean IQ.
The International Picture
To test the effect that malnutrition has on international IQ differences, in another post I created a measurement of national malnutrition rates based on the prevalence of the health effects of malnutrition such as stunting, wasting, and being underweight.
I found that malnutrition did have a strong relationship with national IQ (r=-.498, p>.0001). However, the racial composition of a national had an even stronger relationship with national IQ and continued to predict national IQ after malnutrition was controlled for. Moreover, the gap between Black and White nations was virtually unaffected by controlling for malnutrition.
These are only the results of one analysis, but they provide some reason to be skeptical of the idea that malnutrition explains Africa’s low IQ.