More research is showing that it's harder for women to lose weight compared to men.
Laura Sullivan loves everything about her husband Randy - except how he loses weight fast and she barely moves the scale when they diet.
"We eat similar. It flies off of him and doesn't for me," said Sullivan. "I have to work really hard, especially since I hit 40, it's gotten worse and worse."
Randy understands his wife's frustration. The couple decided to get healthier a few years ago. He quickly lost 50 pounds.
"It does seem kind of unfair. She works harder than I do," said Randy. "But for me, it just comes easy. It comes off."
"We need to recognize that there are real differences. It's not in her head. She's not doing anything wrong. We just can't have the same expectations," said Dr. Brett Cohen with Memorial Healthcare System.
A recent study at Ohio State University showed that a chemical reaction involving a particular enzyme may cause the body to store fat, and that enzyme is only active in women. The problem appears to be estrogen, which women lose as they age, allowing the enzyme to run wild.
"It's like a light went off at 40, like happy birthday to me, I don't get to lose weight anymore, you know," said Laura Sullivan.
Other studies show when diet and exercise are combined, men and women have different physical reactions.
"Men respond to that by having more comfort and not feeling as hungry. Women don't get that same response. Physiologically, women are more hungry to eat," said Cohen.
Because men have more testosterone, which builds lean muscle, men can lose weight more quickly as muscle burns energy faster, say doctors.
Doctors hope new research can lead to more sex-specific weight-loss techniques.