Published OnlineFirst October 9, 2013; DOI: 10.1158/1055-9965.EPI-13-0511 Cancer Epidemiology, Research Article Biomarkers & Prevention An Exploratory Study of Respiratory Quotient Calibration and Association with Postmenopausal Breast Cancer Ross L. Prentice1, Marian L. Neuhouser1, Lesley F. Tinker1, Mary Pettinger1, Cynthia A. Thomson2, Yasmin Mossavar-Rahmani3, Fridtjof Thomas4, Lihong Qi5, and Ying Huang1 Abstract Background: The respiratory quotient (RQ), defined as the ratio of carbon dioxide exhaled to oxygen uptake, reflects substrate utilization when energy is expended. Fat and alcohol have RQ values of approximately 0.7, compared with 1.0 for carbohydrate, and approximately 0.8 for protein. Here, the association between RQ and postmenopausal breast cancer risk is studied. Methods: Paired RQ measurements were obtained, separated by approximately 6 months, for women in the reliability subset of a Women’s Health Initiative (WHI) Nutrition and Physical Activity Assessment Study. Linear regression of the average of the paired log RQ assessments on a corresponding log food quotient (FQ) average and other study subject characteristics, including age, body mass index, race, and education, yielded calibration equations for predicting RQ. Results: Calibration equations, using any of food frequency, food record, or dietary recall data, explained an appreciable fraction of measured log RQ variation, and these were used to compute calibrated RQ estimates throughout WHI cohorts. Calibrated RQ estimates using 4-day food record (4DFR) data related inversely (P ¼ 0.004) to (invasive) breast cancer risk in the WHI Dietary Modification trial comparison group, and corre- sponding RQ estimates using food-frequency data related inversely (P ¼ 0.002) to breast cancer incidence in this cohort combined with the larger WHI observational study. Conclusion: Although preliminary, these analyses suggest a substantially higher postmenopausal breast cancer risk among women having relatively low RQ. Impact: RQ elevation could provide a novel target for breast cancer risk reduction. Cancer Epidemiol Biomarkers Prev; 22(12); 2374–83. Ó2013 AACR. Introduction provides an attractive strategy to advance epidemiologic There is little reliable information on energy expendi- research in this area. ture patterns in relation to chronic disease risk. Studies Biomarker substudies within Women’s Health Initia- relating self-reported macronutrient consumption to risk tive (WHI) cohorts include objective measures of short- reveal few established or probable associations (1, 2), term total energy expenditure using a doubly-labeled perhaps because of dietary assessment measurement water (DLW) technique, and protein consumption from issues. The incorporation of objective measures of energy 24-hour urinary nitrogen (3, 4). Linear regression of log- consumption and expenditure in epidemiologic studies biomarker values on corresponding log self-report dietary assessments in combination with such study subject char- acteristics as body mass index (BMI), age, and race led to Authors' Affiliations: 1Division of Public Health Sciences, Fred Hutch- "calibrated" total energy and protein consumption assess- inson Cancer Research Center, Seattle, Washington; 2Division of Health ments that make an adjustment for measurement error in Promotion Sciences, Zuckerman College of Public Health, University of self-reported dietary assessment. Associations of calibrat- Arizona, Tucson, Arizona; 3Department of Epidemiology and Population Health, Albert Einstein College of Medicine, Bronx, New York; 4Department ed total energy and protein estimates with risk were found of Preventive Medicine, University of Tennessee Health Sciences Center, for several diseases (5–9), even though mostly not evident Memphis, Tennessee; and 5Department of Public Health Sciences, Uni- versity of California, Davis, California when using uncalibrated dietary data. The Nutrition and Physical Activity Assessment Note: Supplementary data for this article are available at Cancer Epide- miology, Biomarkers & Prevention Online (http://cebp.aacrjournals.org/). Study (NPAAS) in WHI also included indirect calorim- etry (IC), which assesses oxygen uptake and carbon Corresponding Author: Ross L. Prentice, Division of Public Health Sciences, Fred Hutchinson Cancer Research Center, 1100 Fairview Ave- dioxide exhalation. Under controlled conditions, IC nue North, P.O. Box 19024, Seattle, WA 98109-1024. Phone: 206-667- provides a noninvasive assessment of resting energy 4264; Fax: 206-667-4142; E-mail: [email protected] expenditure (REE) during the assessment period (10). doi: 10.1158/1055-9965.EPI-13-0511 Recently, the REE data were used, in conjunction with Ó2013 American Association for Cancer Research. DLW and dietary self-report data, to develop calibrated 2374 Cancer Epidemiol Biomarkers Prev; 22(12) December 2013 Downloaded from cebp.aacrjournals.org on October 1, 2021. © 2013 American Association for Cancer Research. Published OnlineFirst October 9, 2013; DOI: 10.1158/1055-9965.EPI-13-0511 Respiratory Quotient and Postmenopausal Breast Cancer estimates of activity-related energy expenditure (11) in study enrollees at nine WHI clinical centers. Black and WHI cohorts. Included in the IC instrument’s data out- Hispanic women were oversampled as were women in put is a respiratory quotient (RQ) assessment, which the extremes of body mass index (BMI) and relatively reflects substrate oxidation during the IC testing period. younger postmenopausal women. The RQ is defined as the ratio of carbon dioxide exhaled Women were excluded from the NPAAS for having any to oxygen inhaled as the body expends its energy reserves. medical condition precluding participation, weight insta- For carbohydrate oxidation the RQ is 1.0, whereas for fat bility in preceding months, or travel plans during the and alcohol it is approximately 0.7, and for protein it is study period. A 20% reliability subsample repeated the close to 0.8, depending on the specific amino acids under- entire biomarker study protocol at approximately 6 going oxidation during the test (12). Therefore, RQ esti- months after the original protocol application. All women mation provides the opportunity to contrast fat and car- provided written informed consent for their WHI partic- bohydrate (stored as glycogen) utilization in relation to ipation, and for their participation in the NPAAS. chronic disease risk. Here, we develop RQ calibration equations based on NPAAS protocol and procedures corresponding food quotients (FQ), defined (13) as the The NPAAS study protocol involved two clinical center sum of 0.7, 0.8, and 1.0 times the estimated percentage of visits separated by a 2-week period, along with at-home energy from fat plus alcohol, protein, and carbohydrate activities. The first visit included eligibility confirmation, respectively, with dietary data from each of three assess- informed consent, DLW dosing for short-term energy ment procedures, in combination with pertinent study expenditure assessment, completion of FFQ and other subject characteristics. RQ estimates using each dietary questionnaires, collection of a blood specimen and spot assessment procedure are then developed for women urines after DLW dosing, and training in 4DFR comple- throughout WHI cohorts, and these estimates are associ- tion. Between the two clinic visits, participants completed ated with subsequent invasive breast cancer incidence a 4DFR and collected 24-hour urine on the day prior to the during WHI cohort follow-up. second clinic visit. At the second clinic visit, participants provided addi- tional spot urine specimens and a fasting blood sample, Materials and Methods 4DFRs were reviewed, and participants completed an IC WHI cohorts protocol. The first of three 24HRs was obtained in the 1 to 3 The design of the WHI clinical trial (CT) and observa- weeks after visit 2, and then monthly thereafter for the tional study (OS) and enrollee characteristics have been other two. presented previously (14–16). Briefly, all women were IC was carried out using a standard protocol (19), with postmenopausal and in the age range of 50 to 79 when either a DeltaTrac II Respiratory Gas Analyzer (Datex- enrolled at 40 U.S. clinical centers between 1993 and 1998. Ohmeda Inc.) or a SensorMedics VMAX metabolic cart The CT enrolled 68,132 women to either or both of the (SensorMedics Inc). All metabolic carts were calibrated Dietary Modification (DM) trial (48,835 women) or to each day according to the manufacturer’s instructions, overlapping postmenopausal hormone-therapy trials and gases were monitored during each test. Participants (27,347 women). The DM trial randomly assigned 40% of arrived on day 15 after a 12-hour fast and rested in a enrollees (19,541 women) to a low-fat eating-pattern inter- semireclined position in a thermally neutral room for 30 vention, and 60% (29,294 women) to a usual diet compar- minutes followed by a 30-minute test under a canopy ison group (DM-C). The companion WHI OS is a prospec- (eight clinics) or using a non–rebreathing-fitted mouth- tive cohort study that enrolled 93,676 postmenopausal piece (one clinic). Data points were obtained every min- women in the age range of 50 to 79 years from 1994 to ute, but the first 10 minutes of collected data were not 1998. The OS and DM cohorts were drawn from essentially included in subsequent data analysis as 10 minutes are the same catchment populations, with substantial overlap needed to achieve steady-state conditions. Steady state in baseline data