Differences in parts of the brain involved in reward, taste and hunger may be associated with body mass index even in children as young as 4, according to new research. But scientists don't yet know whether those brain differences come before changes in body weight, develop afterward or are influenced by something else.
The observational study, published in Scientific Reports, included 159 children ages 4 to 7. Researchers found that higher BMI was associated with differences in several brain areas, while other measures, including body fat percentage and waist circumference, were not. Because the children were studied at only one point in time, the findings show a connection, not cause and effect.
Researchers at Florida International University used a specialized type of MRI to look closely at brain tissue. They then compared the scans with several measures of body size.
Higher BMI was associated with differences in areas including the insula and nucleus accumbens, which are involved in processes such as taste, reward and motivation. Researchers also saw a possible difference in a small area of the hypothalamus, which helps regulate hunger, although they considered that finding preliminary and said it needs to be confirmed in future research.
Similar patterns have previously been reported in older children and adolescents. Finding associations in children ages 4 to 7 suggests the relationship between body size and brain development may begin earlier than researchers have been able to observe before.
“Our findings push that timeline back by several years, making it one of the earliest investigations of how body weight and brain development may be connected,” lead author Mohammadreza Bayat, a cognitive neuroscience doctoral student at FIU, said in a university news release. “We're not saying these brain differences are causing higher body weight, or that higher body weight is causing brain differences. This study only tells us they're connected.”
One of the more notable findings involved the insula. This part of the brain helps process taste and determine which sensations and experiences get our attention. Previous studies have found that the insula responds to food cues, but researchers have had less information about whether its underlying structure is associated with body size in very young children.
What might explain the differences remains an open question.
The researchers discuss inflammation as one possibility, based partly on previous animal research linking certain diets with inflammation and changes in brain regions involved in eating and reward. But this study didn't measure what the children ate or test them for inflammation, so it cannot show that diet or inflammation explains the brain differences. The scans themselves also cannot reveal exactly what biological process produced the differences researchers detected.
That distinction is especially important for interpreting what the findings mean for families. The study doesn't show that particular foods change children's brains, that the observed brain differences lead children to gain weight or that a child's BMI can be used to predict future obesity.
BMI also produced a somewhat complicated result. Researchers examined four measures related to body size and composition, but BMI was the only one significantly associated with the brain differences. Waist circumference, estimated body fat percentage and whether a child fell into an overweight or obesity category were not.
Researchers caution against interpreting that as evidence that BMI is uniquely good at capturing what's happening in the brain. Accurately measuring body composition in children this young can be difficult, and relatively few children in the study met the threshold researchers used for overweight or obesity. Both factors could have made associations with the other measurements harder to detect.
The study also included an unusually large proportion of children with ADHD: 81 of the 159 participants had ADHD, because the children came from a larger study focused on the condition. Researchers expected the relationship between BMI and brain differences might be stronger among those children, but that wasn't what they found. The overall pattern was similar among children with and without ADHD.
The relatively small study can't rule out more subtle differences involving ADHD, however. Researchers said a larger group of children would be needed to answer that question more confidently.
There are other reasons to be cautious about applying the results broadly. The children came from one metropolitan area in the southeastern United States, 82% were Hispanic or Latino and their parents tended to have relatively high levels of education. The researchers said studies involving children from a wider range of backgrounds will be needed to see whether the same patterns hold elsewhere.
The bigger unanswered question is what happens next.
Following children over time could show whether these brain differences appear before changes in body weight, emerge alongside them or develop afterward. Studies that also measure diet, metabolism and inflammation could help researchers understand what's behind the association.
The research was supported by grants from the National Institutes of Health and in part by the Intramural Research Program of the National Institute of Biomedical Imaging and Bioengineering.