Energy Restriction and Hormones in Premenopausal Women

Overview

A growing body of epidemiological and biological evidence strongly suggests that physical activity may reduce the risk of breast cancer. Although the mechanism remains unclear, possible links between reduced risk and exercise include favorable alterations in body composition and positive changes in the hormonal milieu. One hormonal biomarker of breast cancer, circulating estrogen, is postulated to be reduced by chronic physical activity, presumably due to disruptive effects of exercise upon menstrual cyclicity, and the potential for loss of body fat with subsequent reductions in the peripheral biosynthesis of circulating estrogens. Although studies have shown that chronic exercise can reduce circulating estrogen, we know little about the magnitude and duration of exposure to an energy deficit required for these changes. Additionally, no studies have addressed the degree to which peripheral production of estrone, versus the ovarian production of estradiol, is altered with exercise that promotes weight/fat loss. A second biomarker of breast cancer, insulin-like growth factor-1 (IGF-1), is presumably increased with exercise but reduced with exercise if energy balance is negative. No prospective studies have addressed whether a moderate aerobic exercise program that results in weight loss will lead to significant changes in IGF-I levels, particularly in individuals of differing initial energy stores. Metabolic energy availability is an important contributing factor in the development of reproductive cancers. However, current methods for assessing energy availability, which include anthropometric measures, calculations of energy balance, evaluation of various serum and urinary biomarkers are prone to measurement error, not sensitive to alterations in energy availability, and are sometimes affected by disease states. The current project includes the introduction of a novel approach to estimating energy status by measuring metabolic hormones in plasma: insulin, IGF-I, insulin-like growth factor binding protein-3 (IGFBP-3) and leptin. This study tested whether a program of moderate aerobic exercise that is combined with a moderate level of dietary restriction would result in significant decreases in two biomarkers of breast cancer, circulating estrogens and IGF-I.

This study used a randomized prospective design (block; 3:1 allocation) to test the effects of a four month (four menstrual cycles) intervention of moderate aerobic exercise (4 times per week, 60 minutes/session) combined with caloric restriction designed to produce a weekly energy deficit of -20%. The aims of the study were: 1) To test the hypothesis that a moderate level of low energy availability created through a combination of exercise and caloric restriction will lower circulating estrogens and IGF-I; 2) To test the hypothesis that exercise-induced decreases in body fat will contribute substantially to the lowering of circulating estrogens changes with training; 3) To validate a novel method of assessing energy status that represents an improvement of existing. Eumenorrheic, untrained women between 25-40 years (n=47) were randomly assigned to one of two treatments groups: exercise with caloric-restriction (EX+CR; n=36) or light conditioning reference (LC; n=11) groups. To determine treatment effects on circulating estrogens, reproductive function and IGF-I, both serum and urinary levels of hormones were monitored for a control period of 2 months (2 menstrual cycles), i.e., Screening and Baseline, followed the 4 month (4 menstrual cycles: intervention 1-4) experimental period. The exercise/diet intervention began on the first day of the third month (menstrual cycle) and continue for four menstrual cycles thereafter (intervention 1, 2, 3, and 4). A post-study measurement was taken during cycles days 1-7 of the seventh menstrual cycle. Baseline energy needs were assessed during the baseline cycle. Resting metabolic rate and non-exercise physical activity were added to determine a caloric need for the day. Caloric intake was supervised throughout the entire study, and meals were comprised of 55% carbohydrates, 30% fat, and 15% protein. Exercise training was supervised, and maximal aerobic capacity (VO2max) was calculated. Menstrual status was assessed through analysis of daily urinary metabolites of estrone-1-glucuronide (E1G), pregnanediol glucuronide (PdG), and midcycle luteinizing hormone (LH). Underwater weighing and a digital scale were used to assess body composition, and fasting blood samples were collected to assess metabolic hormones.