Effects of Estrogen Deficiency on Energy Expenditure

The purpose of this pilot study is to investigate the role of the female sex hormone estrogen, on metabolism, thermoregulation and energy expenditure. Weight and fat gain increase after the menopause, but reasons for this are not clear. Loss of estrogen may cause changes in how women regulate metabolism and thermoregulation, possibly leading to weight gain. Specifically, this study will determine how loss of estrogen affects facultative thermogenesis. Loosely defined, facultative thermogenesis represents heat production that is turned on when needed. For example, when body core temperature falls below a certain threshold, a shivering response is invoked in skeletal muscle to increase heat production and, thus, energy expenditure. However, exposure over several hours to mild cold temperatures that do not trigger shivering (16-20⁰ C) also induces an increase in energy expenditure (cold-induced non-shivering thermogenesis). Although several different tissues may contribute to this response, the recent identification of functional brown adipose tissue (BAT) in humans has promoted an interest in how BAT is activated in humans and its potential role in regulating energy balance and body weight. The investigators will measure BAT activity using PET/CT scans pre and post three months of estrogen suppression.

Inclusion criteria

• Body mass index < 30 kg/m2
• Normal menstrual cycles
• Premenopausal

Exclusion criteria

• irregular menstrual cycles defined as 2 or more missed cycles in the previous year
• on hormonal contraceptive or menopausal therapy
• positive pregnancy test
• intention to become pregnant or start hormonal contraceptive therapy during the period of study
• lactation
• severe osteopenia or osteoporosis (i.e., proximal femur or lumbar spine t scores < -2.0)
• abnormal vaginal bleeding
• thyroid dysfunction
• uncontrolled hypertension
• exercising at least 30 minutes per day at a moderate to vigorous intensity >1 d/wk) over the past 6 months