While genes do predispose people to obesity, changes in dietary and lifestyle factors are the primary reasons for the massive increase in obesity rates in the last five decades, new research suggests.
The study, “Quantifying the impact of genes on body mass index during the obesity epidemic: longitudinal findings from the HUNT Study,” was published in the journal The BMJ.
Since 1975, the percentage of adults who are obese worldwide has nearly tripled. This increase is primarily considered a result of people eating more calorie-dense foods and getting less physical activity, but genetic makeup also affects how a person puts on weight.
The aim of the new study was to assess how temporal lifestyle changes have affected people with different genetic predispositions.
Researchers in Norway looked at data from 67,305 people from a Norwegian tuberculosis screening program. Their body mass index (BMI, a measurement that reflects a person’s weight relative to their height) was measured at various points between 1963 and 2008.
Based on variants known to influence weight gain, the researchers divided the people into five groups, from lowest to highest genetic risk of obesity. The researchers then looked for patterns and changes over time.
Unsurprisingly, at all time points, people with a predisposition to obesity did, in fact, have a higher BMI than people without such predispositions. The data also illustrated that people with the highest genetic risk experienced the greatest increases in BMI over the years. But interestingly, increases in BMI were also seen in people in the lowest genetic risk group.
In a press release, Maria Brandkvist, a PhD candidate at the Norwegian University of Science and Technology and co-author of the study, gave the example of a 35-year-old male who is genetically predisposed toward being heavier. On average, this individual would weigh 8.5 pounds more than a male of similar age without the genetic predisposition in the 1960s.
Today, the difference would be 30.5 pounds: 14.9 due to the genetic predisposition, and an extra 15.6 “simply as a result of living in our ‘obesogenic’ environment,” Brandkvist said, adding that this excess of weight “is caused mostly by today’s unhealthy lifestyle,” but is also influenced by the interaction of genes and the environment.
“This study provides evidence that genetically predisposed people are at greater risk for higher BMI and that genetic predisposition interacts with the obesogenic environment resulting in higher BMI,” researchers concluded. “Regardless, BMI has increased for both genetically predisposed and non-predisposed people, implying that the environment remains the main contributor.”
It should be emphasized that this study, by its design, cannot determine a cause-and-effect relationship in any regard; it only shows trends and associations.