Many factors influence your weight beyond your diet and how much exercise you do, and researchers are saying even the decade you were born in can play a crucial role. A new study shows that it's harder for today's adults to maintain the same weight as those who were living 20-30 years ago, even if eating and exercise patterns are kept the same.

The team from York University in Canada analysed data from some 36,377 adults in the US between 1971 to 2008, and found that the average Body Mass Index (BMI) levels were up to 2.3 kg/m2 higher in 2006 compared to 1988, based on the same levels of leisure time physical activity, caloric intake, and macronutrient intake (like proteins and fats).

That means our bodies are an average of 5 percent heavier than they were in the 1980s, after diet and exercise are factored out, and that's certainly puzzling. The academics admit they're yet to figure out just what is causing the increase of BMI over time, but if they can, we might be better able to address a growing obesity problem in the US and beyond.

"Our study results suggest that if you are 40 years old now, you'd have to eat even less and exercise more than if you were a 40-year-old in 1971, to prevent gaining weight," said one of the researchers, Jennifer Kuk. "However, it also indicates there may be other specific changes contributing to the rise in obesity beyond just diet and exercise."

Kuk has offered three potential theories to explain the findings, as The Atlantic reports. First, increased exposure to certain chemicals - like pesticides and flame retardants - could be having an effect. Second, the rise in prescription drugs might be to blame: the number of people in the US relying on them has grown around 10 percent since the late 80s. One in 10 Americans now take five or more prescription drugs regularly.

The final possibility put forward by Kuk is a change in microbiomes. An increase in the amount of meat in our diet, for example, could be changing our gut bacteria to make us more susceptible to putting on weight (remember than many animal products now contain extra hormones and antibiotics to stimulate growth).

Kuk says society should be more respectful of those with weight gain problems because of the numerous potential causes besides a lack of activity and an unhealthy diet. "Ultimately, maintaining a healthy body weight is now more challenging than ever," she says.

The results have been published in Obesity Research & Clinical Practice.