Weight gain at the time of menopause

At the time of menopause when the ovaries stop producing significant amounts of hormones, central weight gain, increasing insulin resistance and metabolic syndrome (a combination of diabetes, high blood pressure and obesity) are commonly seen. This phenomenon has been discussed in the media in articles such as: "Why women pile on the pounds as they age - and it may not be their fault".

Oestrogen receptors in the brain control food intake, energy expenditure and body fat distribution. Oestrogen deficiency can increase hunger, slow fat metabolism and encourage central fat gain. As oestrogen levels fall, this turns on an enzyme called ALDH1A1 which causes us to store fat, particularly visceral fat around the internal organs. This is the worst kind because it sends out metabolic signals to the rest of the body! Decreases in testosterone might also account for muscle loss and a 4-5% decrease in metabolic rate.

Hormone replacement therapy shouldn't cause weight gain unless the doses of hormones are far too high for the individual  (too much oestrogen can cause bloating, fluid retention and weight gain). In fact some clinical studies have indicated that hormone replacement therapy prevents or reduces weight gain and body fat gain,[1] particularly abdominal fat gain[2].

[1] Relationship between hormone replacement therapy use with body fat distribution and insulin sensitivity in obese postmenopausal women. 

[2] Effects of hormone replacement therapy on weight, abdominal fat distribution and lipid levels in Japanese postmenopausal women