Is obesity associated with a decline in intelligence quotient during the first half of the life course? What are the causal effects of breastfeeding on IQ, obesity and blood pressure? Evidence from comparing high-income with middle-income cohorts
Cross-sectional studies have found that obesity is associated with low intellectual ability and neuroimaging abnormalities in adolescence and adulthood . Some have interpreted these associations to suggest that obesity causes intellectual decline in the first half of the life course . We analyzed data from a prospect i ve longitudinal study to test whether becoming obese was associated with intellectual decline from childhood to midlife . We used data from the ongoing Dunedin Multidisciplinary Health and Development Study , a population -representative birth cohort study of 1,037 children in New Zeal and who were followed prospect ively from birth ( 1972 - 1973 ) through their fourth decade of life with a 95 % retention rate .  Intelligence quotient ( IQ ) was measured in childhood and adulthood . Anthropometric measurements  were taken at birth and at 12 subsequent in-person assessment s. As expected , cohort members who became obese had lower adulthood IQ scores . However , obese cohort members exhibited no excess decline in IQ . Instead , these cohort members had lower IQ scores since childhood . This pattern remained consistent when we accounted for children 's birth weights and growth during the first years of life , as well as for childhood-onset obesity . Lower IQ scores among children who later developed obesity were present as early as 3 years of age . We observed no evidence that obesity contributed to a decline in IQ , even among obese individuals who displayed evidence of the metabolic syndrome and /or elevated systemic inflammation Background A novel approach is explored for improving causal inference in observational studies by comparing cohorts from high-income with low- or middle-income countries ( LMIC ) , where confounding structures differ . This is applied to assessing causal effects of  breastfeeding  on child blood pressure ( BP ) , body mass index ( BMI ) and intelligence quotient ( IQ ) . Methods St and ardized approaches for assessing the confounding structure of breastfeeding by socio-economic position were applied to the British Avon Longitudinal Study of Parents and Children ( ALSPAC ) ( N ≃ 5000 ) and Brazilian Pelotas 1993 cohorts ( N ≃ 1000 ) . This was used to improve causal inference regarding associations of breastfeeding with child BP , BMI and IQ . Analyses were extended to include results from a meta- analysis of five LMICs ( N ≃ 10 000 ) and compared with a r and omized trial of breastfeeding promotion . Findings Although higher socio-economic position was strongly associated with breastfeeding in ALSPAC , there was little such patterning in Pelotas . In ALSPAC , breastfeeding was associated with lower BP , lower BMI and higher IQ , adjusted for confounders , but in the directions expected if due to socioeconomic patterning . In contrast , in Pelotas , breastfeeding was not strongly associated with BP or BMI but was associated with higher IQ . Differences in associations observed between ALSPAC and the LMIC meta- analysis were in line with those observed between ALSPAC and Pelotas , but with robust evidence of heterogeneity detected between ALSPAC and the LMIC meta- analysis associations . Trial data supported the conclusions inferred by the cross-cohort comparisons , which provided evidence for causal effects on IQ but not for BP or BMI . Conclusion While reported associations of breastfeeding with child BP and BMI are likely to reflect residual confounding , breastfeeding may have causal effects on IQ . Comparing associations between population s with differing confounding structures can be used to improve causal inference in observational studies
The authors even suggested in this case increasing educational levels as a means of reducing obesity .
Indeed , we have recently reported results from two experimental studies showing that mental work up-regulates appetite-stimulating hormones and promotes overconsumption of food ( 3,4 ) .
Furthermore , a careful examination of our data revealed that when the subjects were categorized on the basis of the mental workload intensity , those for whom the intensity of the mental effort was greater significantly increased their spontaneous food intake as compared with those characterized by a less strenuous cognitive effort .
In other words , the harder we think the more we eat .
This hyperphagic effect of intense thinking was also accompanied by more plasma glucose instabilities ( 5 ) .
The fluctuations in plasma glucose levels that were observed are in agreement with the literature showing that a period of intense cognitive processing leads to a measurable decrease in levels of blood glucose ( 6 ) , a fall that has also been associated with feeling less energetic ( 7 ) .
Accordingly , food intake might be perceived as a strategy for allowing the individual to recover optimal body functioning .
These observations are also relevant for students having hard times to learn at school and for whom study ing is arduous and mentally challenging .
On the basis of our observations , we find it realistic to postulate that those susceptible to commit more cognitive effort to take in charge st and ard mental tasks are more likely to over-eat and gain weight compared with those for whom the accomplishment of mental tasks at the same level of performance is elementary or more natural .
Along the same lines , it would not be surprising to find a higher dropout rate for those ‘ mentally stressed ’ students .
In a system where academic performance continues to rank as high priority in public schools , it seems obvious that some students are genetically more gifted and likely to perform at a higher level ( 8) .
As previously shown , better cognitive abilities are predictive of more important life achievements such as earning a doctorate , securing a patent , or publishing a novel or major literary work ( 9 ) .
It may be somewhat paradoxical that the amount of knowledge-based work is more important in the higher socioeconomic groups whereas population data show a higher prevalence of obesity for the socioeconomically disadvantaged .
Thus , individuals highly involved in mental work are not all likely to over-eat and other healthy behaviours such as regular physical activity may counterbalance the net impact on energy balance .
As mentioned in the paper by Yu and co-workers ( 1 ) , educational attainment seems to be an important mediator of this relationship .
In an early view issue of this journal , Yu and co-workers ( 1 ) published a systematic review and meta- analysis about the mutual relationship between obesity and intelligence quotient ( IQ ) .
The authors observed that a lower IQ score in childhood was associated with obesity in later adulthood .
Although it is not possible to definitively rule out reverse causation in the IQ – obesity relationship , two main mechanisms have been put forward by the authors that could explain why a low IQ has been found to be a predictor of obesity .
First , higher intelligence may reduce the risk of obesity by promoting more healthful behaviours .
For example , data from a cohort study showed that children with higher mental ability scores reported significantly more frequent consumption of fruit and vegetables and more high-intensity physical activity participation in adulthood ( 2 ) .
Second , higher childhood IQ may predict favourable socioeconomic positions later in life .
With the advent of computers and new technologies , the pendulum has swung far in the direction of excessive mental activities , as opposed to excessive physical activities .
A careful examination of computer-related activities reveals that they represent a particular type of sedentary activities ; they are stressful and biologically dem and ing for the body and deserve to be counterbalanced by an adequate physical activity regimen ( 10 ) .