Exercise and Prolonged Fasting to Improve Brown Adipose Tissue Function in Postmenopausal Women

Postmenopause is associated with a substantial increase in cardiometabolic risk, partly driven by the decline in estrogen levels and alterations in cellular energy metabolism. Mitochondrial dysfunction has been documented in skeletal muscle of postmenopausal women and is associated with metabolic inflexibility, reduced fat oxidation, ectopic fat accumulation, and insulin resistance. However, whether mitochondrial dysfunction is also present in adipose tissue of postmenopausal women, and whether it can be reversed through lifestyle interventions, remains largely unknown.

Brown adipose tissue (BAT) is a mitochondria-rich and metabolically active tissue involved in thermogenesis and substrate utilization. Although human BAT has traditionally been studied in the supraclavicular region, emerging evidence from our research group suggests the presence of a metabolically active adipose tissue depot in the dorsocervical region, referred to as interscapular brown adipose tissue (iBAT). Preliminary observations indicate that this depot may have distinct metabolic characteristics. However, iBAT has not yet been characterized in postmenopausal women.

Aerobic exercise is a potent non-pharmacological stimulus for mitochondrial remodeling and improves mitochondrial biogenesis and oxidative capacity. Exercise-derived lactate may also promote adipose tissue browning and mitochondrial remodeling. Prolonged fasting induces profound metabolic adaptations characterized by increased fat mobilization and changes in substrate utilization. Preliminary data from our group suggest that five days of prolonged fasting substantially increase fat oxidation during exercise, providing a rationale for investigating whether fasting and exercise may have complementary effects on mitochondrial function and metabolic flexibility.

Based on this rationale, the main hypothesis of EMPOWER-BAT is that iBAT in postmenopausal women presents mitochondrial dysfunction and that aerobic exercise, five-day prolonged fasting, and their combination can improve mitochondrial function through lactate-related signaling pathways, leading to enhanced whole-body metabolic flexibility. The main objective is to characterize mitochondrial function in iBAT and determine the effects of these interventions on iBAT mitochondrial function and metabolic flexibility.

EMPOWER-BAT is a randomized controlled trial involving 60 postmenopausal women aged 45-65 years with a body mass index ≥25 kg/m². Participants will be randomized (1:1:1:1) into four groups: a control group maintaining its usual lifestyle; an aerobic exercise group performing supervised exercise for 60-90 minutes at 65% of heart rate reserve; a prolonged fasting group following a medically supervised fasting protocol providing approximately 600 kcal/day; or a combined group completing both exercise and fasting. All interventions will last five consecutive days.

Before and after the intervention, iBAT biopsies will be collected from the dorsocervical region to assess mitochondrial function and adipocyte characteristics. Metabolic and physiological assessments will include indirect calorimetry at rest and during exercise, body composition, cardiorespiratory fitness, and blood and urine biomarkers. Continuous glucose and ketone monitoring will also be performed during the intervention. Four weeks later, metabolic and physiological assessments will be repeated to determine whether changes in metabolic flexibility are maintained after participants return to their usual lifestyle.

Inclusion criteria

• Women aged 45 to 65 years.
• Postmenopausal status, defined as ≥ 12 consecutive months without menstruation.
• Women with amenorrhea shorter than 12 months will be included if their follicle-stimulating hormone levels are >30IU/liter.
• BMI between ≥25 kg/m2.
• Sedentary or low-active (not meeting physical activity guidelines).
• Willing to be randomized to any of the four groups.
• Able and willing to adhere to all study procedures and show availabity in their schedule.
• Must be willing to have biological samples stored for future research.

Exclusion criteria

• Diabetes mellitus (determined based on fasting glucose levels defined by ADA criteria).
• Any other active endocrine disease (thyroid disease, any signs of Cushing's syndrome, adrenal disease and lipid-associated disorders such as familial hypercholesterolemia).
• Any cardiac disease (i.e., ischemic cardiac disease, arrhythmias, severe heart failure).
• Use of medication known to influence glucose and/or lipid metabolism or brown fat activity (e.g., beta-blockers, antidepressants, corticosteroids).
• Current or recent (past 6 months) use of hormone replacement therapy, including systemic estrogen therapy (oral or transdermal), combined estrogen-progestin therapy, tibolone, or selective estrogen receptor modulators.
• Clinically relevant abnormalities in clinical chemistry or electrocardiogram at screening (to be judged by the study physician).
• A first-degree family member with sudden cardiac death.
• Any chronic renal or hepatic disease.
• Abuse of alcohol or other substances.