Hormonal

Exercise combined with calorie restriction does not induce browning (brite cell formation) in human subcutaneous white adipose tissue; instead, it is associated with decreased UCP1 expression, suggesting a 'whitening' or adaptive response to energy deficit rather than increased thermogenesis.

If you are losing weight through exercise and diet, do not expect your fat cells to turn into 'brown fat' that burns extra calories. Your body actually adapts by reducing the expression of thermogenic proteins (like UCP1) in your white fat, likely to conserve energy during a deficit. This is a normal, adaptive response to weight loss, not a failure of your exercise program. The weight loss is driven by the calorie deficit, not by creating new heat-generating fat cells.