Association between intelligence quotient and obesity in England

1 Faculty ofMedicine, University of Versailles Saint-Quentin-en-Yvelines, Montigny-le-Bretonneux, France 2 Research andDevelopment Unit, Parc Sanitari Sant Joan de Déu, Universitat de Barcelona, Barcelona, Spain 3 Instituto de Salud Carlos III, Centro de Investigación Biomédica en Red de Salud Mental, Madrid, Spain 4 Cambridge Center for Exercise Science, Anglia Ruskin University, Cambridge, UK 5 ICREA, Barcelona, Spain

In the past years, there has been a controversy regarding the potential association between intelligence quotient (IQ) and obesity. [6][7][8][9][10] For example, one study conducted in the United Kingdom showed that childhood general intelligence was negatively associated with adult obesity at age 51 years after adjusting for education, earnings, mother's body mass index (BMI), father's BMI, childhood social class, sex, and BMI at 16. 10 In contrast, it was found in another UK prospective study of more than 17 400 individuals followed from birth that adjusting for education attenuated the inverse association between childhood IQ and adult obesity at age 42 years to the null. 7 This finding was later corroborated in a meta-analysis of 26 studies, which found no significant relationship between intelligence and obesity after adjusting for education. 8 Although these studies have advanced the field, there are two important limitations that should be mentioned. First, to the best of our knowledge, all of them failed to conduct simultaneous adjustment for a variety of potential confounders. Specifically, none of the studies have taken into account smoking status, 11,12 alcohol dependence, 13,14 drug use, 15,16 and common mental disorders, 17,18 despite the fact that these factors have been reported to be associated with both obesity and IQ. Second, the previous studies were not nationally representative, [6][7][8][9][10] and it is thus difficult to extrapolate their findings.
Therefore, the present study aimed to analyze the association between IQ and obesity in England using nationally representative community-based data while adjusting for key confounders including sex, age, ethnicity, marital status, qualification, employment, income, chronic physical conditions, loneliness, social support, stressful life events, smoking status, alcohol dependence, drug use, and common mental disorders. Verbal IQ is referred to as IQ in this manuscript for the sake of brevity.

Obesity (dependent variable)
BMI was calculated as weight in kilograms divided by height in meters squared based on self-reported weight and height. Using the standard World Health Organization (WHO) definition, obesity was defined as ≥30 kg/m 2 .

Control variables
The control variables were selected based on past literature. 6

Chronic physical conditions
The following disorders were included: cancer, diabetes, epilepsy/fits, migraine or frequent headaches, cataracts/eyesight problems, ear/hearing problems, stroke, heart attack/angina, high blood pressure, bronchitis/emphysema, asthma, allergies, stomach ulcer or other digestive problems, liver problems, bowel/colon problems, bladder problems/incontinence, arthritis, bone/back joint/muscle problems, infectious disease, and skin problem. 21 These conditions had to be diagnosed by a doctor or other health professional and be present in the previous 12 months. The total number of chronic physical conditions was further calculated for each individual.

Loneliness
This was assessed with an item from the Social Functioning Questionnaire (SFQ). Respondents were asked to assess to what extent they had felt "lonely and isolated from other people" in the past 2 weeks with response options, "very much," "sometimes," "not often," and "not at all." In the analyses that follow, these response options were dichotomized with those who responded "sometimes" and "very much" being categorized as lonely. 21

Social support
This was assessed with a 7-item measure. Using answer options "not true" (score = 0), "partly true" (score = 1), and "certainly true" (score = 2), participants responded to statements that inquired if family and friends did things to make them happy, made them feel loved, could be relied on no matter what, would see that they were taken care of no matter what, accepted them just the way they are, made them feel an important part of their lives, and gave them support and encouragement. Responses were added to create a scale score that could range from 0 to 14. The internal consistency of the scale was good: Cronbach's α = .89.

Stressful life events
Eighteen items were used to assess different stressful life events (eg, serious illness, death of an immediate family member, and major financial crises). 21 The number of stressful life events was further calculated for each participant and ranged from 0 to 18.

Smoking status
Participants were asked about their smoking status and were classified as never smokers (never) and past or current smokers (quit/current). 21

Alcohol dependence
Excessive alcohol consumption was screened using the Alcohol Use Disorders Identification Test (AUDIT). Alcohol dependence was assessed with the Severity of Alcohol Dependence Questionnaire (SADQ-C) in participants with an AUDIT score of 10 or above.
Scores of four or above indicated alcohol dependence in the past 6 months. 21

Drug use
Each individual was asked if he/she had used in the past year one of the following drugs: cannabis, amphetamines, cocaine, crack, ecstasy, heroin, acid or LSD, magic mushrooms, methadone or physeptone, tranquilizers, amyl nitrate, anabolic steroids, and glues. Those who claimed to have used at least one of these drugs were considered to be drug users.

RESULTS
There were 6798 individuals aged ≥16 years included in the present study ( Note. IQ was assessed using the National Adult Reading Test (NART). In this analysis, the continuous IQ variable was used, and the estimates represent the change in odds associated with a 10-point decrease in IQ. Obesity was defined as a body mass index (BMI) of ≥30 kg/m 2 . *Adjusted for sex, age, ethnicity, marital status, qualification, employment, income, chronic physical conditions, loneliness, social support, stressful life events, smoking status, alcohol dependence, drug use, and common mental disorders.

F I G U R E 1 Prevalence of obesity by intelligence quotient (IQ) score
Note. IQ was assessed using the National Adult Reading Test (NART). Obesity was defined as a body mass index (BMI) of ≥30 kg/m 2 .
Bar denotes upper end of 95% confidence interval.

Main findings
To the best of our knowledge, this is the first nationally representative study investigating the association between IQ and obesity, while it is also the first study that adjusted for a variety of potential confounders.
In this sample of almost 6800 individuals, we found that the prevalence of obesity decreased from approximately 21% in the IQ 70-79 group to 15% in the IQ 120-129 group. In addition, after adjusting for potential confounders (ie, sociodemographic, physical, psychological, behavioral, and psychiatric), we found that IQ was negatively associated with obesity with the OR (95% CI) for IQ 70-79 (vs 120-129) being 1.72 (1.21-2.41). All potential confounders had little influence in the association between IQ and obesity in our study.

Interpretation of the findings
Recently, there has been some controversy on the association between IQ and obesity, and several studies have suggested that education may play a major confounding role in this association. 7,8 Specifically, one large prospective study from the United Kingdom found that lower childhood IQ scores increased the risk of adulthood obesity but that this association was no longer significant after adjustment for education. 7 Based on these findings, the authors emphasized the fact that education is likely to be a strong confounding factor in the association between childhood IQ and adulthood obesity. Although these results are of particular interest, other authors have reported a significant relationship among IQ, BMI changes, 9 and obesity 10  intelligence on adult obesity, regardless of education or earnings. 10 Our study results concur with the latter studies that found that education is unlikely to be a confounder in the association between IQ and obesity.
There are several hypotheses to explain the IQ-obesity relationship.
The fact that the potential confounders assessed in this study had little influence in the association between IQ and obesity points to the possibility that other factors that were not investigated in this current study may be important. For example, people with high IQ may be more likely to engage in healthy behaviors than those with low IQ. One study found that adulthood consumption of fruits and vegetables was more frequent and adulthood consumption of chips and cakes/biscuits was less frequent in those with high childhood mental ability. 23 That same study further highlighted the fact that there was a positive association between high mental ability and exercise habit. These results were corroborated in a recent study that found that high IQ in youth increased the odds of moderate cardiovascular activity and strength training in middle age, whereas it decreased the odds of having a sugary drink in the previous week and heavy alcohol consumption. 24 It is also possible that obesity may be a risk factor for low IQ. It has been observed that BMI is negatively associated with both cognitive function and word-list learning, 25 while there is some evidence that high BMI may alter gray matter volume. 26

Clinical implications and directions for future research
Although the present findings provide valuable information on the link between low IQ and obesity, it is important to understand that IQ is a nonmodifiable risk factor that is rarely assessed in the general population. Therefore, the development of obesity prevention programs focusing on intelligence is difficult to implement. Nevertheless, IQ may be regularly assessed in specific situations such as the follow-up of children with developmental difficulties or the follow-up of adults with psychiatric disorders. Our findings suggest that low IQ is an independent risk factor for obesity even after adjusting for several potential confounding factors. Thus, we believe that individuals with low cognitive abilities should be screened for obesity on a regular basis.
Furthermore, the management of individuals with low IQ should be transdisciplinary, and should involve several health professionals (eg, dietitian, physiotherapist, and general practitioner) in order to evaluate their health behaviors (eg, diet and physical activity) that may lead to obesity.

Strengths and limitations
The strengths of this study include the large sample size and the use of nationally representative data. However, our findings should be considered in the light of several limitations. First, IQ was estimated with the NART, a test requiring good understanding of English, and this may have biased the present results. Second, BMI was based on selfreported weight and height, and it is thus possible that the prevalence of obesity was underestimated in this sample as people tend to underreport their weight. 27 Third, this was a cross-sectional study and thus no conclusions about causality or temporality of the association between IQ and obesity can be drawn.

CONCLUSIONS
There was a negative association between IQ and obesity in the UK population even after adjustment for a variety of potential confounders. Further research is needed to gain a better understanding of the mechanisms involved in this relationship.

ACKNOWLEDGMENTS
We would like to thank the National Center for Social Research and the University of Leicester who were the Principal Investigators of this survey. In addition, we would also like to thank the UK Data Archive, the National Center for Social Research, and other relevant bodies for making these data publically available. They bear no responsibility for this analysis or interpretation of this publically available dataset.

ETHICAL APPROVAL
Ethical permission for the study was obtained from the Royal Free Hospital and Medical School Research Ethics Committee. All participants provided informed consent before their inclusion.