If you eat healthy and exercise regularly and still can’t lose weight, your thyroid might not be working like it should. This happens in about 5% of people, and it's most common in women and people over 60. In addition to weight gain, it can also cause fatigue, joint or muscle pain, and depression. Medications can help, so get it checked if you think it might be an issue.
If you're genetically predisposed to gain weight easily, that may be another strike against you. Even if you don't actually gain weight, you may still gain inches around the waist. This weight gain can be so frustrating, it's easy to become obsessed with losing it, starving yourself or exercising too much or maybe even looking into the latest plastic surgery procedure.
People naturally lose muscle after 40, especially women after menopause. Because muscle burns more calories than fat, this can slow down your metabolism and make it harder to shake those stubborn pounds. Strength-training exercises -- lifting weights or doing body-weight exercises, like push-ups and squats -- at least twice a week can help you keep those muscles.