Being an ER nurse means a lot of 12-hour shifts.
"You just eat mindlessly and you just slowly start to increase the weight," said nurse Claudia Guevara.
At one point, 5-foot-5 Guevara tipped the scales at nearly 200 pounds. In a year, she's lost 65 pounds.
She may blame her weight gain on stress eating and inactivity, but experts say there are some lesser-known factors contribute to weight gain too.
"Obesity, just like anything else in life, is multi-factorial," said Dr. Soha Dolatabadi, Good Samaritan Hospital.
Internist Dr. Soha Dolatabadi says many people might be surprised to know how air pollution can affect key hormones that control body weight.
And while air-conditioning is great, experts say being in temperature-controlled spaces reduces how many calories we burn.
Then there's the vicious cycle of not sleeping.
"So when you lay down, all the soft tissues around your airways kind of constrict the airways, so you can't really get good air, so you can't really sleep well," said Dolatabadi.
Which leads to the disruption of sleep and appetite hormones.
Some things you can't control. For example, how old your mom was when she gave birth to you. Studies show the older a woman is when she gives birth, the higher her child's risk for obesity.
And then there's ancestors' lifestyle and environment. Research suggests some of those influences could last two generations.
"Whatever your grandma made and ate, probably became the stock, the routine food that your parents ate," said Dolatabadi.
Some scientists believe what our ancestors experienced can alter the way our brains are wired, so what they ate leaves a sort of "genetic mark" on our DNA.
"All we can do is to become aware of it," said Dolatabadi.
Claudia is more aware. She still snacks all day, but only on fruits, veggies, lean meats and whole grains. She also does a Zumba class daily and stays active.