Biomedical subjects

Gary K Grunwald

Publications and source records attributed to Gary K Grunwald.

36 records · Page 2Linked to original sources

PubMed2003

Quantitative CT indexes in idiopathic pulmonary fibrosis: relationship with physiologic impairment.

PURPOSE: To determine whether measurements of skewness, kurtosis, and mean lung attenuation on thin-section computed tomographic (CT) histograms in patients with idiopathic pulmonary fibrosis (IPF) correlate with pulmonary physiologic abnormality in a nonspirometrically controlled multicenter study. MATERIALS AND METHODS: The authors analyzed baseline digital thin-section CT data from 144 patients with IPF who enrolled in a double-blind placebo-controlled clinical effectiveness trial of interferon beta 1a in the treatment of IPF. All patients underwent thin-section CT in the supine position at full inspiration. The lungs were isolated by using a semiautomated thresholding technique, with an upper threshold of -200 HU. An attenuation correction algorithm was used. Pulmonary function tests (PFTs) included forced vital capacity, total lung capacity, forced expiratory volume in 1 second, and diffusing lung capacity. Univariate and multiple correlation and regression statistical analyses were used to determine relationships between histogram features and results of PFTs. RESULTS: Moderate correlations existed between histogram features and PFT results. Kurtosis showed the greatest degree of correlation with physiologic abnormality (r = 0.53, P <.01). Strength of correlation increased with exclusion of suboptimal scans but did not change significantly after application of an attenuation correction algorithm. Attenuations for lungs, gas, and soft tissue varied considerably between scanner manufacturers. Kurtosis alone provided predictions of pulmonary function that were virtually as good as those from all histogram features combined. CONCLUSION: Thin-section CT histograms of the lungs were found to correlate with results of PFTs in patients with IPF, which supports the claim that histogram features can be used as valid indexes of IPF in a multiinstitutional nonspirometrically controlled study.

Algorithms↗

PubMed2003

Association of anthropometric measures with risk of diabetes and cardiovascular disease in Hispanic and Caucasian adolescents.

BACKGROUND: Hispanics are disproportionately affected by the obesity epidemic in the United States. Obesity is a primary risk factor for the development of cardiovascular disease (CVD) and Type 2 diabetes, which are problematic in Hispanic adults. There are limited data relating obesity status in Hispanic adolescents to diabetes and CVD risk. METHODS: We studied 115 lean and obese adolescents (89 Hispanic, 26 Caucasian), ranging in body mass index (BMI) from 15 to 52 kg/m(2). We assessed the relationships between four anthropometric indices of obesity and risk factors for Type 2 diabetes (insulin (INS), glucose (GLU)), and CVD (total cholesterol, triglycerides, HDL, and systolic (SBP) and diastolic (DBP) blood pressure). RESULTS: All anthropometric indices were positively correlated with total cholesterol, triglycerides, log(INS), GLU, SBP, and DBP, and negatively correlated with HDL. Correlations and multiple regression analyses indicated that weight and waist circumference (WC) were generally the best single predictors of disease risk. Using more than one anthropometric measure in multiple regression did not improve predictions of risk over using a single predictor. CONCLUSIONS: These results indicate that overweight adolescents (particularly Hispanics) are at risk for developing Type 2 diabetes and CVD, and that WC and weight are useful for identifying those at particular risk.

Adolescent↗

PubMed2003

Variation in mortality risk factors with time after coronary artery bypass graft operation.

BACKGROUND: Differences in mortality risk factor sets during different time periods (eg, short-term versus intermediate-term) after coronary artery bypass grafting have been reported. However, little is known about the time-varying effects of mortality risk factors after the operation. METHODS: We analyzed 11,815 veterans who had coronary artery bypass grafting at any of the 43 Veterans Affairs cardiac surgery centers from October 1997 to September 1999. Time-varying effects of 14 mortality risk factors during the 210 days after coronary artery bypass grafting were evaluated using Cox B-spline regression, which provides an estimate of risk for each variable for each day after operation. RESULTS: Eight variables showed significant time-varying effects after operation. The effect of prior heart operation was very high immediately after operation, but disappeared within 1 week. Three other cardiac variables (prior myocardial infarction, preoperative intraaortic balloon pump, and Canadian Cardiovascular Society anginal class III or IV) also conferred the highest risk on the day of operation and decreased thereafter. In contrast, the four time-varying noncardiac risk variables (age, impaired functional status, chronic obstructive pulmonary disease, and renal dysfunction) showed little or no association with mortality immediately after operation, but had increasing impact during the several months after operation. CONCLUSIONS: A sizable number of mortality risk factors have time-varying effects after coronary artery bypass grafting. Several cardiac risk factors have peak impact immediately after operation but dissipate thereafter. Several noncardiac risk factors confer little risk immediately after operation, but these risks increase during several months. This information may help clinicians focus management strategies for patients during the 7 months after operation.

Age Factors↗

PubMed2003

Comparison of methods for achieving 24-hour energy balance in a whole-room indirect calorimeter.

OBJECTIVE: To evaluate and compare methods for achieving 24-hour energy balance in a whole-room indirect calorimeter. RESEARCH METHODS AND PROCEDURES: Twenty-four-hour energy expenditure (EE) for 34 healthy adults (16 women, 18 men) was measured in a calorimeter during a prestudy day and on a subsequent nonconsecutive assessment day (AD). Several methods for estimating EE on the AD using activity factors or regression equations with data available before the AD [anthropometrics, body composition, resting metabolic rate (RMR), sleeping metabolic rate (SMR) on prestudy day, 24-hour EE on prestudy day] were compared for predictive accuracy. RESULTS: Use of a 24-hour calorimeter stay gave the smallest mean absolute error (119 +/- 16 kcal/d) and smallest single maximum error (361 kcal/d). However, several other methods were only slightly, and not significantly, less accurate (e.g., mean absolute error = 131 +/- 17, 140 +/- 20, and 141 +/- 22 kcal/d and greatest error = 384, 370, and 593 kcal/d for anthropometric, RMR, and SMR regression equations, respectively). Fat-free mass alone and SMR with a simple activity factor were seen to be less accurate. DISCUSSION: Our results indicate that there may be some improvement in achieving 24-hour energy balance in a metabolic chamber by using a preceding 24-hour calorimeter stay; that only slightly less accurate predictions can be obtained using a combination of anthropometric, body composition, and/or RMR measurements; and that there is little or no advantage in using SMR from a previous overnight calorimeter stay.

Adult↗

PubMed2003

Total energy expenditure and carbohydrate oxidation are increased in the human immunodeficiency virus lipodystrophy syndrome.

To determine whether total energy expenditure (TEE) is increased in the human immunodeficiency virus (HIV) lipodystrophy syndrome, we compared energy expenditure (EE) and substrate oxidation rates in 12 HIV-infected men with lipodystrophy, 7 HIV-infected men without lipodystrophy, and 14 healthy controls. TEE and nutrient oxidation rates were assessed by whole-room indirect calorimetry. Resting energy expenditure (REE) was measured by indirect calorimetry using the open-circuit technique. Body composition was assessed by dual-energy x-ray absorptiometry (DEXA). Insulin sensitivity was measured using the insulin-modified frequently sampled intravenous glucose tolerance test. TEE adjusted for lean body mass (LBM) was significantly higher in the HIV-infected group with lipodystrophy compared to HIV-infected patients without lipodystrophy (2,873.3 +/- 69 v 2,573.9 +/- 92 kcal/d, P =.02) and compared to healthy controls (2,873.3 +/- 69 v 2,404.0 +/- 64 kcal/d, P <.001). REE and sleeping metabolic rate (SMR) adjusted for LBM were also significantly higher in the HIV-infected group with lipodystrophy compared to both HIV-infected and healthy controls. Carbohydrate oxidation rates adjusted for LBM were higher in men with HIV lipodystrophy as compared to healthy controls (362.5 +/- 23 v 250.0 +/- 22 g/d, P = <.01) and tended to be higher as compared to HIV-infected controls (362.5 +/- 23.6 v 297.3 +/- 31 g/d, P =.1). In conclusion, TEE and carbohydrate oxidation are increased in the HIV lipodystrophy syndrome. The increase in TEE appears to be due to increases in REE. The pathogenesis of elevated EE in HIV lipodystrophy and other forms of lipodystrophy remains to be determined.

Adult↗

PubMed2003

History of depression, angina, and quality of life after acute coronary syndromes.

BACKGROUND: Depression has been associated with higher mortality and morbidity rates after acute coronary syndromes (ACS), but little is known about the association between depression, angina burden, and quality of life. We evaluated the association between a history of depression and patient-reported angina frequency, physical limitation, and quality of life 7 months after discharge from the hospital for ACS. METHODS: Patients were enrolled in the Department of Veterans Affairs Access to Cardiology Study, a cohort study of all patients with acute myocardial infarction or unstable angina who were discharged from 24 Veterans Affairs medical centers between March 1998 and February 1999. Data from 1957 patients who completed a 7-month postdischarge Seattle Angina Questionnaire were analyzed. Multivariate logistic regression was used to evaluate a history of depression as an independent predictor of angina frequency, physical limitation, and quality of life 7 months after ACS, as measured with the Seattle Angina Questionnaire. RESULTS: A total of 526 patients (26.7%) had a history of depression. After adjustment for a wide array of demographic, cardiac, and comorbid factors, a history of depression was significantly associated with more frequent angina (odds ratio [OR] 2.40, 95% CI 1.86-3.10, P <.001), greater physical limitation (OR 2.89, 95% CI 2.17-3.86, P <.001), and worse quality of life (OR 2.84, 95% CI 2.16-3.72, P <.001) after ACS. CONCLUSION: We found a strong association between a history of depression and both heavier angina burden and worse health status after ACS. These findings further support the importance of depression as a risk marker for adverse outcomes after ACS.

Acute Disease↗

PubMed2003

Number of days, number of subjects, and sources of variation in longitudinal intervention or crossover feeding trials with multiple days of measurement.

Dietary studies are often conducted as longitudinal intervention or crossover trials using multiple days of measurement on each subject during each of several measurement periods, and determining the required numbers of days and subjects is important in designing these studies. Linear mixed statistical models were used to derive equations for precision, statistical power and sample size (number of days and number of subjects) and to obtain estimates of between-subject, period-to-period, and day-to-day variation needed to apply the equations. Two cohorts of an on-going exercise intervention study, and a crossover study of Olestra, each with 14 d of measurement/subject per period, were used to obtain estimates of variability for energy and macronutrient intake. Numerical examples illustrate how the equations for calculating the number of days or number of subjects are applied in typical situations, and sample SAS code is given. It was found that between-subject, period-to-period, and day-to-day variation all contributed significantly to the variation in energy and macronutrient intake. The ratio of period-to-period and day-to-day standard deviations controls the trade-off between the number of days and the number of subjects, and this remained relatively stable across studies and energy and macronutrient intake variables. The greatest gains in precision were seen over the first few measurement days. Greater precision and fewer required days were noted in the study (Olestra) that exerted greater control over the subjects and diets during the feeding protocol.

Adult↗

PubMed2002

The impact of ethnicity on outcomes following coronary artery bypass graft surgery in the Veterans Health Administration.

OBJECTIVES: We evaluated the effect of African American (AA) and Hispanic American (HA) ethnicity on mortality and complications following coronary artery bypass graft (CABG) surgery in the Veterans Health Administration (VHA). BACKGROUND: Few studies have examined the impact of ethnicity on outcomes following cardiovascular procedures. METHODS: This study included all 29,333 Caucasian, 2,570 AA, and 1,525 HA patients who underwent CABG surgery at any one of the 43 VHA cardiac surgery centers from January 1995 through March 2001. We evaluated the relationship between ethnicity (AA vs. Caucasian and HA vs. Caucasian) and 30-day mortality, 6-month mortality, and 30-day complications, adjusting for a wide array of demographic, cardiac, and noncardiac variables. RESULTS: After adjustment for baseline characteristics, AA and Caucasian patients had similar 30-day (AA/Caucasian odds ratio [OR] 1.07; 95% confidence interval [CI] 0.84 to 1.35; p = 0.59) and 6-month mortality risk (AA/Caucasian OR 1.10; 95% CI 0.91 to 1.34; p = 0.31). However, among patients with low surgical risk, AA ethnicity was associated with higher mortality (OR 1.52, CI 1.10 to 2.11, p = 0.01), and AA patients were more likely to experience complications following surgery (OR 1.28; 95% CI 1.14 to 1.45; p < 0.01). In contrast, HA patients had lower 30-day (HA/Caucasian OR 0.70; 95% CI 0.49 to 0.98; p = 0.04) and 6-month mortality risk (HA/Caucasian OR 0.66; 95% CI 0.50 to 0.88; p < 0.01) than Caucasian patients. CONCLUSIONS: Ethnicity does not appear to be a strong risk factor for adverse outcomes following CABG surgery in the VHA. Future studies are needed to determine why AA patients have more complications, but ethnicity should not affect the decision to offer the operation.

Black or African American↗

PubMed2002

Simvastatin lowers C-reactive protein within 14 days: an effect independent of low-density lipoprotein cholesterol reduction.

BACKGROUND: The early response of C-reactive protein to initiation of a hydroxymethylglutaryl coenzyme A reductase inhibitor (statin) is not known. The purpose of this study was to determine the rate at which highly sensitive C-reactive protein (hsCRP) levels change after initiation of simvastatin and whether this occurs independently of the change in LDL cholesterol. METHODS AND RESULTS: The study was a crossover, double-blind design including 40 subjects with elevated LDL cholesterol. Subjects were randomly assigned to 1 of 2 groups: simvastatin 40 mg for 14 days, then placebo for 14 days, or placebo first, then simvastatin. Simvastatin decreased LDL cholesterol by 56+/-4 mg/dL (P<0.0001) at day 7 and by an additional 8+/-3 mg/dL (P=0.02) at day 14. Baseline log(hsCRP) levels were similar in the 2 groups. By day 14, log(hsCRP) was significantly lower in patients on simvastatin when compared with placebo (P=0.011). Although there was no significant difference in fibrinogen levels, simvastatin produced a modest increase in log[lipoprotein(a)] (P=0.03) at days 7 and 14. There were no relationships between the decrease in LDL cholesterol and the decrease in hsCRP. CONCLUSIONS: Simvastatin lowers hsCRP by 14 days, independent of its effect on LDL cholesterol. This rapid impact of a statin on hsCRP has potential implications in the management of acute coronary syndromes.

Adult↗

PubMed2002

Long-term weight loss and breakfast in subjects in the National Weight Control Registry.

OBJECTIVE: To examine breakfast consumption in subjects maintaining a weight loss in the National Weight Control Registry (NWCR). RESEARCH METHODS AND PROCEDURES: A cross-sectional study in which 2959 subjects in the NWCR completed demographic and weight history questionnaires as well as questions about their current breakfast consumption. All subjects had maintained a weight loss of at least 13.6 kg (30 lb) for at least 1 year; on average these subjects had lost 32 kg and kept it off for 6 years. RESULTS: A large proportion of NWCR subjects (2313 or 78%) reported regularly eating breakfast every day of the week. Only 114 subjects (4%) reported never eating breakfast. There was no difference in reported energy intake between breakfast eaters and non-eaters, but breakfast eaters reported slightly more physical activity than non-breakfast eaters (p = 0.05). DISCUSSION: Eating breakfast is a characteristic common to successful weight loss maintainers and may be a factor in their success.

Adult↗

PubMed2002

Relationship between waist circumference, body mass index, and medical care costs.

OBJECTIVE: Excessive visceral adiposity as measured by anthropomorphic measures may be more closely associated with adverse health consequences than body weight or body mass index (BMI), the more commonly obtained clinical measures. Waist circumference (WC) provides information about regional adiposity and may correlate with health care costs better than body weight or BMI. RESEARCH METHODS AND PROCEDURES: A total of 424 men (37%) and women (63%) were identified as they were seen in outpatient medical clinics at Denver Health, an integrated health care system serving a largely indigent population. Height, weight, and WC were measured by one examiner. Information on outpatient, laboratory, pharmacy, inpatient, and total charges attributable to each subject for the preceding year were obtained from computerized databases. Data on health care charges were divided into quartiles based on WC and BMI. RESULTS: Total annual health care charges were significantly greater in the highest WC quartile (WC < 83.3 cm: $6062 +/- $784; 83.3 to 93.5 cm: $5968 +/- $812; 93.7 to 103.5 cm: $6369 +/- $1015; >103.5 cm: $8699 +/- $1092; p = 0.047). Those with a WC >103.5 cm generated 85% more inpatient charges than the group with a WC <83.3 cm. Although there was a positive trend, BMI was not found to significantly correlate with total health care charges in this population sample. DISCUSSION: These results suggest that abdominal adiposity as assessed by WC is associated with increased total health care charges and may be a better predictor of health care charges than the more widely used BMI.

Adolescent↗

PubMed2002

Differences in resting metabolic rate between white and African-American young adults.

OBJECTIVE: A reported lower resting metabolic rate (RMR) in African-American women than in white women could explain the higher prevalence of obesity in the former group. Little information is available on RMR in African-American men. RESEARCH METHODS AND PROCEDURES: We assessed RMR by indirect calorimetry and body composition by DXA in 395 adults ages 28 to 40 years (100 African-American men, 95 white men, 94 African-American women, and 106 white women), recruited from participants in the Coronary Artery Risk Development in Young Adults (CARDIA), Birmingham, Alabama, and Oakland, California, field centers. RESULTS: Using linear models, fat-free mass, fat mass, visceral fat, and age were significantly related to RMR, but the usual level of physical activity was not. After adjustment for these variables, mean RMR was significantly higher in whites (1665.07 +/- 10.78 kcal/d) than in African Americans (1585.05 +/- 11.02 kcal/d) by 80 +/- 16 kcal/d (p < 0.0001). The ethnic x gender interaction was not significant (p = 0.9512), indicating that the difference in RMR between African-American and white subjects was similar for men and women. DISCUSSION: RMR is approximately 5% higher in white than in African-American participants in CARDIA. The difference was the same for men and women and for lean and obese individuals. The prevalence of obesity is not higher in African-American men than in white men. Because of these reasons, we believe that RMR differences are unlikely to be a primary explanation for why African-American women are more prone to obesity than white women.

Adipose Tissue↗

PubMed2002

Modest serum creatinine elevation affects adverse outcome after general surgery.

BACKGROUND: Modest preoperative serum creatinine elevation (1.5 to 3.0 mg/dL) has been recently shown to be independently associated with morbidity and mortality after cardiac surgery. It is important to know if this association can be applied more broadly to general surgery cases. METHODS: Multivariable logistic regression analyses of 46 risk variables in 49,081 cases from the Veterans Affairs National Surgical Quality Improvement Program, undergoing major general surgery from 10/1/96 through 9/30/98. RESULTS: Thirty day mortality and several cardiac, respiratory, infectious and hemorrhagic morbidities were significantly (P < 0.001) higher in patients with a serum creatinine>1.5 mg/dL. With multivariable analysis, the adjusted odds ratio for mortality for patients with a serum creatinine of 1.5 to 3.0 mg/dL was 1.44 [95% confidence interval (95% CI) 1.22 to 1.71] and for creatinine>3.0 mg/dL was 1.93 (95% CI 1.51 to 2.46). The adjusted odds ratio for morbidity (one or more postoperative complications) for patients with a serum creatinine of 1.5 to 3.0 mg/dL was 1.18 (95% CI 1.06 to 1.32) and for creatinine>3.0 mg/dL was 1.19 (95% CI 0.99 to 1.43). Further stratification and recursive partitioning of creatinine levels revealed that a serum creatinine level>1.5 mg/dL was the approximate threshold for both increased morbidity and mortality. CONCLUSIONS: Modest preoperative serum creatinine elevation (>1.5 mg/dL) is a significant predictor of risk-adjusted morbidity and mortality after general surgery. A preoperative serum creatinine of 1.5 mg/dL or higher is a readily available marker for potential adverse outcomes after general surgery.

Creatinine↗

PubMed2002

Disease severity at time of referral for pediatric failure to thrive and obesity: time for a paradigm shift?

The percent of ideal body weight at first visit to pediatric referral clinics was compared by chart review between patients with failure to thrive and obese patients. Results indicated that underweight children were mildly underweight, whereas the overweight children were severely so. In primary care settings, underweight children may be referred more aggressively than overweight children.

Adolescent↗

PubMed2002

Standard versus fiberoptic pulmonary artery catheterization for cardiac surgery in the Department of Veterans Affairs: a prospective, observational, multicenter analysis.

BACKGROUND: Controversy exists regarding the utility of continuous monitoring of mixed venous oxygen saturation (STvo2) during cardiac surgery. During a multicenter, prospective, observational study in the Department of Veterans Affairs (Cooperative Study #5), frequency of use of standard pulmonary artery catheterization (PAC) and STvo2-PAC was recorded. Here the authors relate these data to clinical outcomes. METHODS: Logistic and Cox regression models evaluating the association of PAC type with mortality, one or more postoperative complications, cardiac complications, time to extubation, and intensive care unit length of stay were constructed. The number of thermodilution cardiac outputs and arterial blood gas analyses performed in the first 24 h postoperatively were compared. RESULTS: Data from 3,265 patients undergoing myocardial revascularization (81.7%) or valve replacement-repair (18.3%) were considered. STvo2-PAC was used in 49% and PAC in 51% of patients. In the 14 hospitals, STvo2-PAC was used in all patients in four, in some patients in four, and never in six. No association of STvo2-PAC use with outcome were observed aside from unexplained hospital level effects. A small but statistically significant reduction in the number of arterial blood gas analyses (8 +/- 3 vs. 10 +/- 4, P < 0.0001, STvo2-PAC vs. PAC, respectively) and thermodilution cardiac outputs (14 +/- 8 vs. 15 +/- 9, P < 0.0001, STvo2-PAC vs. PAC, respectively) was observed with use of STvo2-PAC. CONCLUSIONS: Despite higher cost, STvo2-PAC was commonly used in this cohort. Our analysis failed to detect associations with improved outcomes aside from a small reduction in resource utilization. The precise role of STvo2-PAC remains uncertain.

Adult↗

PubMed2002

Resistance and aerobic exercise have similar effects on 24-h nutrient oxidation.

BACKGROUND: Whether resistance exercise is as effective as aerobic exercise for body-weight management is debated. PURPOSE: To compare 24-h energy expenditure (EE) and macronutrient oxidation elicited by comparable bouts of stationary cycling (BK) and weightlifting (WTS). METHODS: 24-h EE and macronutrient oxidation were measured in 10 nonobese male subjects on three occasions using whole-room indirect calorimetry. BK and WTS days were compared with a nonexercise control day (Con). RESULTS: During BK, subjects exercised for 49 +/- 7 min (mean +/- SEM) at 70% of OV(2max) and expended 546 +/- 16 kcal. During WTS, subjects performed a 70-min circuit consisting of four sets of 10 different exercises at 70% of exercise-specific 1-repetition maximum and expended 448 +/- 21 kcal (P < 0.001 vs BK). 24-h EE on BK and WTS days (2,787 +/- 76 kcal x d(-1), 2,730 +/- 106 kcal x d(-1), respectively, P > 0.05) was elevated compared with Con (2,260 +/- 96 kcal x d(-1), P < 0.001), but 24-h respiratory exchange ratio (RER) was not different. 24-h carbohydrate oxidation was significantly elevated on the exercise days (BK = 370 +/- 18 g x d(-1), WTS = 349 +/- 23 g x d(-1), P > 0.05) compared with Con (249 +/- 29 g x d(-1), P = 0.04). 24-h fat and protein oxidation were the same on BK, WTS, and Con days. EE and macronutrient oxidation in the periods after exercise also did not differ across conditions. CONCLUSION: In men, resistance exercise has a similar effect on 24-h EE and macronutrient oxidation as a comparable bout of aerobic exercise. Neither exercise produced an increase in 24-h fat oxidation above that observed on a nonexercise control day.

Adult↗

PubMed2002

Effect of exercise intensity on 24-h energy expenditure and nutrient oxidation.

The aim of this study was to determine the effects of exercise at different intensities on 24-h energy expenditure (EE) and substrate oxidation. Sixteen adults (8 men and 8 women) were studied on three occasions [sedentary day (Con), a low-intensity exercise day (LI; 400 kcal at 40% of maximal oxygen consumption) and a high-intensity exercise day (HI; 400 kcal at 70% of maximal oxygen consumption)] by using whole room indirect calorimetry. Both 24-h EE and carbohydrate oxidation were significantly elevated on the exercise days (Con < LI = HI), but 24-h fat oxidation was not different across conditions. Muscle enzymatic profile was not consistently related to 24-h fat or carbohydrate oxidation. With further analysis, it was found that, compared with men, women sustained slightly higher rates of 24-h fat oxidation (mg x kg FFM(-1) x min(-1)) and had a muscle enzymatic profile favoring fat oxidation. It is concluded that exercise intensity has no effect on 24-h EE or nutrient oxidation. Additionally, it appears that women may sustain slightly greater 24-h fat oxidation rates during waking and active periods of the day.

Adult↗

PubMed2002

Analysis of repeated measures data with clumping at zero.

Longitudinal or repeated measures data with clumping at zero occur in many applications in biometrics, including health policy research, epidemiology, nutrition, and meteorology. These data exhibit correlation because they are measured on the same subject over time or because subjects may be considered repeated measures within a larger unit such as a family. They present special challenges because of the extreme non-normality of the distributions involved. A model for repeated measures data with clumping at zero, using a mixed-effects mixed-distribution model with correlated random effects, is presented. The model contains components to model the probability of a nonzero value and the mean of nonzero values, allowing for repeated measurements using random effects and allowing for correlation between the two components. Methods for describing the effect of predictor variables on the probability of nonzero values, on the mean of nonzero values, and on the overall mean amount are given. This interpretation also applies to the mixed-distribution model for cross-sectional data. The proposed methods are illustrated with analyses of effects of several covariates on medical expenditures in 1996 for subjects clustered within households using data from the Medical Expenditure Panel Survey.

Biometry↗