Hello Weight gain in men is affected by high stress or cortisol levels in the body. Prolonged heightened levels of cortisol put the body into survival mode and induce the production of fat cells and the decline of metabolism to store food for later use. The cortisol also increases appetite in order to procure energy for ?fight or flight.? Male weight gain naturally follows. High cortisol levels further diminish levels of testosterone. Low testosterone can cause low energy and fatigue, meaning less physical activity and weight gain in men. Thyroid disorders, particularly hypothyroidism, and low levels of the human growth hormone (HGH) often cause weight gain by way of decreasing the body s metabolic rate. You must consult a endocrinologist for the diagnosis nd treatment. Thanks