It’s not just hard to lose weight. It’s harder than it has ever been.

According to a recent study published in the journal Obesity Research & Clinical Practice, past generations could eat more, exercise less, and still not weigh as much as we do.

For the study, researchers at York University and the University of Alberta collected data on the eating habits of 36,400 American adults between 1971 and 2008, as well as data on the physical activity levels of 14,419 adults between 1988 and 2006. After examining their body mass indexes, physical activity routines, caloric intake, and amounts of carbohydrates, protein, and fat consumed, they found that given the same eating and exercise habits, people weigh more now than they used to.

“For a given food intake, people are about 10 percent heavier in 2008 than they were in 1971 and about 5 percent heavier for a given exercise level in 2008 than they were in 1988,” says study co-author Jennifer L. Kuk, P.h.D., associate professor of kinesiology and health science at York University in Toronto. “This study suggests that there is more than diet and exercise that is contributing to the rise in obesity.” According to the U.S. Centers for Disease Control and Prevention, between 1988 and 1994, 58.8 percent of American adults were overweight or obese. Between 2011 and 2012, 74.9 percent were.

Why It’s So Hard to Lose Weight Nowadays

So if it’s not just poor eating habits and days spent sitting in an office chair that are making it so hard to lose weight, what is? Experts share six things that have been making it extra hard to lose weight lately.

1. Pollution

The air is getting murkier – and air pollution influences the body’s metabolism, making it easier to gain weight and store fat, says Kuk, noting that this may explain why many wild animals and lab rats are also gaining weight every decade. What’s more, a 2015 study published in Occupational and Environmental Medicine found that people who were regularly exposed to noise from trains, aircraft or vehicles had larger waistlines than those who weren’t subjected to constant traffic noise.

2. Food Additives and Chemicals

While the exact effects of pesticides, antibiotics, preservatives and other chemicals in our food and water supply is still up for major debate, research suggests they all may be “detrimental to our waistlines,” says Holly F. Lofton, an internal medicine physician with the Weight Management Program at NYU Langone Medical Center. After all, their use has been steadily increasing along with our weight. Between just 1996 and 2011, U.S. pesticide use increased by an estimated 404 million pounds, or 7 percent, according to the Centre for Sustaining Agriculture and Natural Resource. And, according to one animal study in PLOS ONE, those pesticides result in insulin resistance and increased fat storage.

3. Later Nights

You can call yourself a night owl, but humans aren’t nocturnal. In fact, Obesity research suggests that eating at night – even if you don’t eat any extra calories – contributes to weight gain by altering the body’s circadian rhythms. And people who eat most of their food at night have higher body mass indexes than people who eat earlier in the day do, according to one findings in the International Journal of Obesity. Meanwhile, nighttime light exposure via computers, phones and tablets is linked to disrupted sleep, which affects your body’s levels of stress hormones, your metabolism and how much fat you burn and how much you store around your stomach, Kuk says.

RELATED: What’s More Important for Weight Loss: Exercise, Healthy Eating, or Sleep?

4. Increased Medication Use

“Many medications decrease your metabolism or increase your tendency to store fat,” Kuk says, including: antidepressants, antihistamines, antipsychotics, beta-blockers, corticosteroids and even some medications for the treatment of Type 2 diabetes. What’s more, children born to mothers on these drugs may be genetically prone to weigh more as they grow up, Lofton adds.

5. Sugar Highs

All calories are not created equal. “One hundred calories from sugar affects your body a lot differently than 100 calories from protein,” Lofton says. Translation: Sugar spikes your blood sugar to increase the amount of calories you store in your fat cells, but protein helps build muscle to keep your metabolism revved. And according to the Obesity Research & Clinical Practice study, between 1971 and 2008, carbohydrate intake increased 10 to 14 percent while protein intake decreased 5 to 9 percent. Meanwhile, in 1999, the average American consumed a record-high average of 155 pounds of added sugar per year, according to the United States Department of Agriculture.

RELATED: Surprising Signs That You’re Eating Too Much Sugar