Losing weight may be a lot harder these days than it was in the 1970s.

That means when it comes to their health, many millennials could be starting off on the wrong foot.

The increase in obesity among adults in the United States is well known. It has doubled over the past four decades.

But the results of a new study suggest that this increase may not be as simple too much food and too little exercise.

Read More: Learn More About Obesity »

BMI and Physical Activity Increase Alongside Obesity

In a study published online this month in Obesity Research & Clinical Practice, researchers looked at the change in body mass index (BMI) between the 1970s and 2000s. This measure of body weight in relation to height is often used an indicator of obesity.

“This analysis would be akin to: ‘When I was your age this is how BMI related with diet and physical activity,'” study author Jennifer Kuk, PhD, an associate professor in the School of Kinesiology and Health Science at York University, Toronto, said in an email to Healthline.

BMI increased by 10 percent in men and 11 percent in women between 1971 and 2008.

Several other factors related to body weight also increased during that time, including total calories consumed and carbohydrates in the diet. However, fat and protein intake declined.

Leisure time physical activity also increased, although information was only available between 1988 and 2006. Men increased their activity by 47 percent during that time and women by 120 percent.

The data for the study comes from an annual health survey of Americans, with dietary information available from 36,377 adults and physical activity information from 14,419 adults.

Read More: Obese People Have Slim Chance of Obtaining Normal Body Weight »

Losing Weight More Difficult Today

For a long time, weight gain has been seen as simply a matter of calories in versus calories burned. If you eat more but don’t increase how much you exercise, you will gain weight.

For the most part, this holds true.

“For any individual, if you want to lose weight — given all else is the same — you will need to eat less and/or increase your physical activity to lose weight,” said Kuk.

Diet and exercise will always remain the foundation of maintaining a healthy weight. But the new study suggests that over time, other factors have made it more difficult for people to shed the pounds.

“To lose a given amount of weight today,” said Kuk, “you may need to eat even less and/or increase your physical activity even more now than previously.”

These results were the same even when the researchers took into account age, education, ethnicity and smoking status.

Read More: Obesity Is Biologically 'Stamped In,' Diet and Exercise Won't Cure It »

Many Factors Influence Body Weight

Doing research on weight gain or loss is notoriously difficult. It’s nearly impossible to control every little factor that can affect body weight. And studies that do tend to be small.

The current study, though, provides a snapshot of how obesity and BMI have changed over time. But it has its own limitations.

Participants in the national survey reported their own diet and physical activity levels. Some people may have underestimated how much food they ate or exaggerated how often they got off the couch each week. Measurements of physical activity using wearable monitors have been used only recently.

However, because the trends seen in the new study are consistent over time, the researchers are confident in the connections they found between BMI, food intake and physical activity.

The researchers also point to studies that hint at the role of other factors that may encourage weight gain. Possible suspects include chemicals in the environment, too little or too much sleep, artificial lights at night and the bacteria in the gut.

Read More: Artificial Light Associated with Obesity, Study Says »

Millennials Starting Off on Wrong Foot

The new study didn’t look at the rise in obesity for different age groups, so it’s difficult to know if millennials are struggling more with obesity than older people are today.

But millennials are coming of age when much of our environment pushes us toward gaining weight by encouraging us to move less and eat more junk food.

This early start with obesity could also be difficult to leave behind after adolescence. Children who are obese are likely to remain obese into adulthood.

But that doesn’t mean weight loss is out of reach for millennials — or prior generations.

Doctors and other health professional can provide guidance on the best approaches to weight loss, such as well-designed weight loss programs that include both dietary changes and increased physical activity.

Starting earlier in life could also give millennials a jumpstart on their health.

“Younger clients in my practice see more significant weight loss with the same behavior changes,” registered dietitian Caroline Fornshell, RD, founder of LWell in Yorktown, Virginia, said in an email to Healthline. “This is because older adults are more likely than younger adults to have health or inflammatory concerns, as well as much less strength and capability for strenuous exercise.”