Why do overweight people get skin darkening in skin folds?
It may be acanthosis nigricans - this is where the folds of skin, particularly the neck and tummy become significantly darker than other skin. It should improve with weight loss and can be improved by using creams containing retinoids (Vitamin A) and skin lighteners.
The most common cause is due to a high production of insulin and this then stimulates skin cells to be produced at a much faster rate. It can also be a result of hormonal changes, sometimes seen in pregnancy, abnormal thyroid function and polycystic ovaries. Certain medicines can also cause it.