Can Smarts Help Shield Folks from Obesity? Maybe Not

A teenager’s brain power appears to have little bearing on whether they will become overweight or obese as adults.

British researchers found that, on average, sharper teens weighed only slightly less in adulthood than siblings who scored lower on tests of thinking skills, according to a new study published April 13 in the journal PLOS Medicine.

The difference amounted to just under a half-pound for a 6-foot-tall adult, said lead author Liam Wright, a senior research fellow in population health at University College London.

“We found a very small association that in practice means that, on average, siblings with higher cognitive ability are unlikely to weigh much less than siblings with lower cognitive ability,” he said.

The research refutes prior studies that have linked low cognitive scores in teens to higher risk of obesity in later life.

That’s because those earlier studies looked at general populations, and didn’t take into account other powerful factors besides smarts that could influence a person’s weight, Wright said.

“The problem with comparing people from the general population according to their cognitive ability and BMI is that unobserved factors may explain the association,” he said. (BMI, or body mass index, is an estimate of body fat based on height and weight.)

To account for those unknown factors, Wright and his colleagues analyzed data on 12,250 siblings from more than 5,600 U.S. households. These sibs were followed from adolescence to age 62 as part of four separate studies.

Comparing siblings could help account for some hidden factors that might influence weight, since brothers and sisters share a common background, Wright said. For example, they have similar genetics and are raised in the same household.

“Many studies by necessity use simple observational designs where correlations can be explained by factors that were not measured in the study,” he said. “Sibling designs are an improvement, in the sense that they can account for factors that are shared between siblings without needing to measure them.”

The researchers first looked at everyone included in the combined data set and found that low-scoring teens did indeed appear to weigh more. Cognitive ability was measured through math and reading tests.

But when researchers specifically compared siblings, the difference in weight based on cognitive ability all but vanished.

Mayo Clinic neurologist Dr. David Knopman noted that low-scoring teens in this study should not be considered developmentally disabled.

“This article was definitely not talking about cognitive impairment. It was merely talking about high versus low cognitive test scores in presumably cognitively intact adolescents or children,” Knopman said. “I would definitely not use the word impaired to refer to the low-performing folks.”

The findings run counter to the general assumption that obesity is a condition guided strictly by self-control and decision-making, said Andrew Brown, biostatistics core director for the Center for Childhood Obesity Prevention at the Arkansas Children’s Research Institute.

Under this line of thinking, sharper people are better able to use nutritional and other health-related information to avoid excess weight.

“Implicitly, a lot of people indicate that obesity is caused by choices, and choices relate to cognition,” Brown said. “It’s the ability to think about and understand choices, if you use cognitive ability as a proxy for the ability to ‘make good choices.'”

Mental ability also is associated to higher pay and better education, which, theoretically, would lead people to live in safer neighborhoods with access to healthier food.

But factors other than smarts likely have a greater impact on obesity risk, Wright said.

“The heritability of BMI is high — greater than 50% — so genetics are important within a population,” he said. “But obesity rates have also increased massively over the past four decades, far faster than any genetic changes could have occurred, so it’s clear that environmental factors have large effects on obesity, too.”

One such factor could be the increased “availability of cheap, energy-dense foods” like fast food and processed foods, Wright said.

More information

Johns Hopkins has more about obesity prevention.

SOURCES: Liam Wright, PhD, senior research fellow, population health, University College London, U.K.; Andrew Brown, PhD, biostatistics core director, Center for Childhood Obesity Prevention, Arkansas Children’s Research Institute, Little Rock; David Knopman, MD, neurologist, Mayo Clinic, Rochester, Minn.; PLOS Medicine, April 13, 2023