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Obesity Study Identifies People Who Remain a Healthy Weight Through Life

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A study published in Obesity, the journal of the Obesity Society, has taken a novel approach to understanding why some individuals are more susceptible to weight gain, regardless of their genetic predisposition to obesity. The study was led by Dr. Bram J. Berntzen from the Institute for Molecular Medicine Finland at the University of Helsinki.

Genetics and obesity

Genetics explains roughly 80% of individual variation in body mass index (BMI) in young adulthood, which has made gene studies a key part of obesity research.

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Genome-wide association studies (GWAS) look at tens of thousands of genes that each have a small contributing effect to obesity risk. Using this data, a genetic risk score can be calculated.

Twin studies look at either identical (monozygotic) or non-identical (dizygotic) twin pairs, which can be used to determine the contribution of genetic vs environmental factors to disease. The combination of GWAS and twin studies is a powerful tool for investigating body weight fluctuations.

But previous studies in this area have had some methodological flaws. Most have looked at a pair of twins who had different BMIs. Many of these papers only looked at body weight at a snapshot in time, did not fully consider genetic contributions to obesity or only considered the higher BMI twin as being the outlier. This ignored the possibility that the lower-weight twin might be thinner against the influence of their genetics.

In the new study, researchers utilized data from the Older Finnish Twin Cohort, a long-term health survey of twins in Finland that started in 1975. The authors examined 36-year BMI trajectories of twins born before 1958. They looked at twins whose BMI in young adulthood was below, within or above the genetically predicted range. This approach allowed them to understand resilience against weight gain and susceptibility to it over a considerable period. One drawback of the study approach was that BMI was self-reported.

The authors calculated a risk score based on nearly a million different gene contributions to obesity. They then examined twin pairs where one twin deviated from the body weight anticipated by the risk score.

Weight gain more common

Of the diverging twins, it was more common for a twin to have gained weight above their predicted BMI. Two-thirds of the twin pairs showed this type of variation. In the other third of differing twin pairs, it was the lower-weight twin who had diverged from their genetically predicted weight.

People in the study generally gained weight over time. Individuals below, within and above prediction in 1975 reached, respectively, normal weight, overweight and obesity by 2011, with a mean BMI increase of 4.5. The authors said that individuals who had lower BMIs than predicted “have been protected from weight gain in their environment.”

Berntzen emphasized the role of young adult BMI in determining whether individuals reached a healthy body weight 36 years later. He noted, “It’s vital to study the reasons for weight gain already during childhood before they become young adults.” This indicates that factors influencing weight gain trajectories begin early and continue into adulthood.

Reference: Berntzen BJ, Palviainen T, Silventoinen K, Pietiläinen KH, Kaprio J. Polygenic risk of obesity and BMI trajectories over 36 years: A longitudinal study of adult Finnish twins. Obesity. 2023;31(12):3086-3094. doi:10.1002/oby.23906

This article is a rework of a press release issued by the Obesity Society. Material has been edited for length and content.