You might have been there before: you clean up your diet and hit the gym five times a week, but at some point, the scale stops budging no matter what you do. A new study sheds light on this frustrating phenomenon: when it comes to weight loss, environmental and lifestyle factors matter more than we may think, and it's affecting millennials' waistlines.

The study, published in Obesity Research & Clinical Practice, looked at the diets and physical activity habits of over 35,000 people who participated in a survey between 1971 and 2008. The researchers found that while calorie intake increased over the years, physical activity did as well. Unfortunately, researchers also noted that even though millennials are exercising more than previous generations, they still are 10 percent heavier than those who ate similar calorie amounts in the '70s and five percent heavier than those who exercised just as much in the '80s. In other words, if you're a 25-year-old today, you have to eat less and exercise more than your 25-year-old counterpart from previous generations in order to not gain weight.

So what gives? Study author Jennifer L. Kuk says this is because weight loss isn't just about a simple calories-in-calories-out formula. Instead, our weight is the result of many different lifestyle factors, like medication use, pollutants, genetics, timing of food intake, stress, gut bacteria, and even nighttime light exposure. "[This study] indicates there may be other specific changes contributing to the rise in obesity beyond just diet and exercise," she says. "Ultimately, maintaining a healthy body weight is now more challenging than ever." Great.

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The good news is this study confirms that what you do when you're not sitting down for a meal or hopping on the treadmill can set yourself up for even more weight-loss success. Check out these lifestyle tips that will help the scale budge: