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  • As of 2016, more than 650 million adults globally had obesity, according to the World Health Organization (WHO).
  • Previous studies have linked obesity to a greater risk for cognitive decline.
  • Researchers from University College London provide evidence to cast doubt on a causal link between obesity and cognitive ability.

In 2016, more than 1.9 billion adults worldwide were overweight, of which more than 650 million adults were affected by obesity — a condition where a person’s weight is not within healthy limits, potentially causing other health concerns.

Current projections state by 2025 that number will grow to 167 million adults and children will become overweight or obese.

Past studies have linked obesity to a greater risk for cognitive decline.

Previous research also suggests obesity is a potential risk factor for a number of diseases, including diabetes, cardiovascular disease, hypertension, osteoporosis, rheumatoid arthritis, and cancer.

Now researchers from University College London cast doubt on a causal link between obesity and cognitive ability. They believe the existing findings linking cognitive ability and BMI are biased by shared family factors.

The study was recently published in the journal PLOS Medicine.

According to Dr. Liam Wright, Ph.D., a senior research fellow at the Centre for Longitudinal Studies at University College London in the U.K. and lead author of this study, the research team wanted to study the causal link between cognitive ability and obesity for a few reasons:

“Obesity rates have increased massively over the past four decades, but BMI across the population hasn’t increased uniformly, so the question of why some people are more susceptible to obesity than others is important.

Alongside this, there is a large literature called cognitive epidemiology that finds evidence that cognitive ability is related to almost all measures of health and health behaviors, including obesity.”

“There are some compelling theoretical arguments for why cognitive ability might have a causal effect on health, but unfortunately, most of the cognitive epidemiology literature uses observational research designs that could be biased and not indicate causal effects,” Dr. Wright explained to Medical News Today. “We thought it was important to test for a link between cognitive ability and BMI using a sibling design because it could account for some of the factors that may bias associations found in existing studies.”

For this study, Dr. Wright and his research team compared data from 12,250 siblings from 5,602 households from four separate youth population cohort studies performed in the United States. The data followed each participant from adolescence to age 62.

By comparing the link between cognitive ability and BMI within families, the scientists could account for unobserved factors related to family background.

Sibling designs account for factors that are shared between siblings by design,” Dr. Wright explained. “They don’t require the measurement of these factors — a positive because the measurement is difficult, but also a negative because they don’t show which shared factors matter in actuality.”

“With this caveat, there are four broad factors that we thought could be important: genetics (siblings share 50% of DNA), parental socioeconomic class (wealth, geographic location, etc.), parenting styles (particularly regarding dietary choices), and parental cognitive ability (cognitive ability could operate indirectly!),” he continued. “Again, we didn’t examine these directly.”

Dr. Wright said they thought these factors would increase bias in general population studies, so they would find smaller associations than in previous studies, which he says they did.

“Note, though, sibling designs have their own weaknesses — particularly the potential for siblings to influence each other, e.g. by modeling each other’s behavior,” he added. “This could mean our results are biased, too, though in this case downwards, and thus smaller than the true causal effect.”

When the researchers compared data from unrelated study participants, they found that moving from the 25th to 75th percentile of adolescent cognitive ability was associated with an estimated 0.61 kg/m decrease in BMI, when adjusted for family socioeconomic position.

And when the scientists compared data from siblings, they found that the move from the 25th to 75th percentile of adolescent cognitive ability was associated with only a 0.06 kg/m decrease in BMI.

Dr. Wright says he was not surprised that the association between cognitive ability and BMI was smaller when comparing siblings than it was when comparing the general population, for the broad factors he outlined.

“I was surprised that the association comparing siblings was as small as it was, though,” he continued. “As noted, there are compelling reasons for thinking cognitive ability affects health and health-related decision-making.”

“Two possibilities for this small association are that one, our results were biased towards finding smaller associations (e.g. by siblings influencing each other) and two, reflective decision-making isn’t so important in determining BMI as other factors like satiety, etc.,” Dr. Wright added. “Both of these are speculative.”

MNT also spoke with Dr. Clifford Segil, a neurologist at Providence Saint John’s Health Center in Santa Monica, CA, about this study.

Dr. Segil said he has never seen a link, as both a parent and a neurologist, between people that have obesity or are at a healthy weight and cognitive ability.

“The point of this paper is to say that people with higher cognitive abilities, who have a higher socioeconomic position, have made healthier choices,” Dr. Segil explained. “And when they make healthier choices, maybe their cognitive ability follows their increase for their decreasing BMI.”

“I think obesity being related to cognitive ability is not proven,” he added. “And I think their study design to look at brothers or sisters or siblings in the same family that share the same social economic position mirrors real life.”

After reading this study, Dr. Segil said he would be interested to see if maternal or paternal obesity results in more adolescent cognitive improvements.

“Because after reading this, they said adolescent cognition is associated with lower adult BMI,” he continued. “So I’m curious as to see if the maternal and paternal BMI is associated with higher adolescent cognitive abilities. In other words, having a skinny or obese parent, does that affect their children’s cognitive ability using the same dataset?”

A person has obesity if their current weight is too high for their height.

The most common way of defining whether a person has obesity or not is through the body mass index (BMI).

This tool uses an adult’s height and current weight to determine whether they have obesity or not:

  • Underweight: BMI of less than 18.5
  • Healthy: BMI of 18.5-24.9
  • Overweight: BMI of 25-29.9
  • Obese: BMI of 30 or more

As children and teens are still growing, they require a different BMI calculator to determine obesity, which uses height, age, and gender.

However, the BMI measurement does have its flaws. It cannot determine whether a person’s weight is fat or muscle. Additionally, it does not take into account a person’s bone density, race, or overall body composition.