A recent (and IMHO, fascinating) study published in the journal Neurology suggests that heavier people score lower on cognitive tests and have a higher risk of dementia later in life than their thinner counterparts.
I go into this post with the full knowledge that perhaps:
a) people will attack me just for drawing attention to this research
b) people will attack the quality of the research
c) people will get pissed at the results
d) people will get pissed at me
e) and various other permutations of people getting their panties in a wad.
That’s their right, of course.
HOWEVER, this is my blog and I’ll post what I please–and I happen to find this line of research both provocative and important. So, the content of this post is under the fold, proceed at your own risk.
The goal of these researchers’ was simple: to determine whether body mass index (which is a ratio of your height to weight and, yes, does have its flaws) is associated with cognitive function and cognitive decline in healthy men and women. The reason they wanted to investigate this question is that previous links have been found between dementia and obesity, however they are not clear enough to provide a causal link between being overweight and cognitive problems. It is important to note that they used healthy participants whose only difference was whether they were of normal BMI or obese BMI (greater than or equal to 30). All subjects were free of dementia at the start of the study.
About 3300 French people participated in the study, 1660 men and 1576 women, who were either age 62, 52,42, or 32 at the study’s start. During routine medical examinations (required by French employment law for all workers), participants were given cognitive tests. In addition, participants aged 62 were screened for cognitive decline. Data was collected in 1996, and then five years later in 2001.
Data Collection:Data was collected in the form of medical history (self-reported), education level, lifestyle/family information, occupation, exercise level, stress levels, blood pressure, medications, as well as a calculation of BMI. Four cognitive tests were used: a word-recall test (short term memory), the Wechsler Adult Intelligence Survey, a selective attention test, and a delayed free recall test of material learned earlier (long term memory). Baseline (1996) levels were compared to 5-year levels (2001), and subjected to chi-square and t tests to prove significance.
Results: (this is the part that some folks aren’t gonna like)
At the baseline, cognitive performance was higher when BMI was lower, and this correlation persisted after adjusting for age, sex, education, physical activity, and region of residence. The delayed-recall task (long term memory) was the most highly associated with BMI, however the intelligence survey and word recall tests were all significantly correlated. These results persisted after adjusting the results for the other factors listed above. Higher BMI changes were also associated with poorer performance on the selective attention test at the follow-up timepoint.
“Whatever the test, we found a negative association between BMI at the baseline and the improvement of cognitive performance…..In this analysis, the association between BMI and decline in [the intelligence test] remained significant after multivariate adjustment.”
The authors formulate several conclusions from their data:
1) Higher BMIs are associated with poorer cognitive performance in a linear way
2) Higher BMIs at baseline were associated with a higher level of cognitive decline
My thoughts are this: while correlations are not causations, this study is quite compelling because it is the first that has followed a large number of participants longitudinally on several cognitive tests, and that adjusts for a host of physical/social/economic factors which could have been considered confounds. Furthermore, this study included participants from the south of France which are overall much healthier than the general Westernized-country population. I would love to see this study replicated in the USA, although I would wager than a much larger impact would be seen—so in that case, these results might be considered too mild.
Why would a higher BMI mediate cognitive decline? There are a few hypotheses. One postulates a direct, negative action of excess fat tissue on neuronal tissue through adipocyte-produced chemicals (see Bray 2004). Leptin, for example, is thought not only to regulate food intake but also plays a role in the learning and memory process. This is supported by leptin knockout mice models, which have changes in the morphology of brain structures (like the hypothalamus) and cognitive impairment. Another hypothesis is that the atherosclerosis and vascular disease which often accompanies obesity has a negative impact on brain function. It is well established that cardiovascular disease is associated with obesity and dementia (see Decarli 2004). And also, diabetes (and insulin resistance) is associated with cognitive decline. Recently, a study demonstrated a pathology of neuronal plaques (a symptom of Alzheimer’s) in insulin-resistant mice (see Ho 2004). In any event, the preponderance of evidence is becoming undeniable (although the human capability of denial does astound me): obesity is not good for your brain.
Source: Cournot et al. 2006. Relation between body mass index and cognitive function in healthy middle-aged men and women. Neurology. Oct (1), 67; 1208-1214.