In some people obesity is an outcome of their genetic makeup, but in most others it is the result of a pro-obesity environment.
Researchers at the Norwegian University of Science and Technology say that the rise in obesity witnessed in recent times can be best explained from an interaction between genes and other factors, such as diet, lifestyle, and physical activity, which has shifted more towards a pro-obesity, or obesogenic, environment.
Researchers came to this conclusion after an extensive longitudinal study spanning over four decades of data from more than 100,000 people.
An important message from the study is that it appears that environments that increasingly favor obesity contribute more to the obesity epidemic than genetic factors.
Although previous research had suggested that genetic vulnerability had larger consequences after the onset of the obesity epidemic than before, the new Norwegian dataset provides convincing results to the contrary, with a large sample size and range of years of assessments and ages.
For instance, the study extrapolated that a 35-year-old man of average height living in the 10960s with pro-obesity genes would, on average, weigh around 3.9 kilograms (kg) more than his counterparts without pro-obesity genes. If the same man remained 35 years old but lived in Norway today, his vulnerable genes would make him more than 6.8 kg heavier.
In addition, both the obesity-predisposed man and his non-predisposed peers would have gained an extra 7.1 kg simply as a result of living in our obesogenic environment.
In other words, the obese man’s 13.9 kg excess weight is caused mostly by today’s unhealthy lifestyle, but also by how his genes interplay with the environment.
In their study paper, the researchers note that even though worldwide obesity has nearly tripled over the last 4 decades, scientists are still unclear about the causes of the epidemic.
While many similar studies have also concluded that the causes likely result from interactions between genes and environment, they have relied mainly on short age spans and follow-ups and self-reported body weight.

