Thermogenic Responses to Fasting and Overfeeding in Women: a Potential Mechanism Contributing to Postmenopausal Weight Gain

This cross-sectional study will compare fasting and overfeeding effects on 24-hour EE and its components in pre- and postmenopausal women. The investigators will study participants under three conditions in the room calorimeter: 1) eucaloric feeding (energy intake = EE); 2) acute, 24-hour fasting; and 3) acute, 24-hour overfeeding of a low-protein diet (i.e., 200% of energy requirements). Participants will complete the eucaloric study and then complete the overfeeding and fasting studies in random order. For 3 days before each condition, the investigators will provide participants with an outpatient diet designed to meet free-living energy requirements (energy intake = 1.5 x REE) and stabilize macronutrient intake (15% protein, 30% fat, 55% carbohydrate). There will be a 1-4-week washout between conditions. Studies in premenopausal women will be performed during the mid-luteal phase (i.e., days 16-24) when both E2 and progesterone are elevated and, thus, when the potential hormonal effects are maximal.

Pre-study assessments - Once enrolled, participants will complete a pre-study assessment visit in the Clinical and Translational Research Center (CTRC) Outpatient Clinic, and the following assessments will be performed:

1. Resting energy expenditure (REE) - will be measured by indirect calorimetry using a metabolic cart coupled with a canopy system (Parvo Medics TrueOne) and used to estimate energy requirements during the eucaloric condition. REE in premenopausal women will be measured in the luteal phase. Participants will arrive after an overnight fast (>10 hours) between 7 and 9 AM. They will rest while remaining awake for 30 minutes in a temperature-controlled room (~70⁰ F). Oxygen uptake (VO2) and carbon dioxide production (VCO2) will be measured for 20 minutes and averaged over the final 15 minutes.
2. Body composition - Body composition will be measured using dual X-ray absorptiometry (DXA, Hologic Discovery, Waltham, MA).
3. Visceral adipose tissue (VAT) volume - VAT volume and subcutaneous fat area (SFA) will be measured using computed tomography (CT). An instrument dedicated to research activity will be used for all scans (Philips Gemini 64TF). Axial CT images will be obtained through the center of the L2-L3 and L4-L5 inter-vertebral disc spaces.

Study Procedures: Once the pre-screening assessments are completed, participants will be studied under three conditions in the room calorimeter.

Room calorimeter - 24-hour EE and substrate oxidation will be measured using whole-room indirect calorimetry.For three days before each calorimeter study, participants will be instructed to refrain from exercise and provided with all food to stabilize energy and macronutrient intake. The energy content of the diet will be REE x 1.5 (30% fat, 55% carbohydrate, 15% protein). Participants will arrive at 0700h after an overnight fast on the study day. An IV catheter will be inserted in an antecubital vein. Fasting blood samples will be obtained before breakfast and at the end of the 24-hour study and analyzed for leptin, FGF21, and thyroid hormones (TSH, total T3, free T4). The investigators will also measure sex steroids (E2, progesterone) in premenopausal women to confirm that participants are studied in the luteal phase.