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Biomedical subjects

T E Duncan

Publications and source records attributed to T E Duncan.

31 records · Page 2Linked to original sources
PubMed1995

Peer and parental influences on adolescent tobacco use.

Longitudinal models of the development of adolescent smoking and smokeless tobacco (ST) use were tested for a sample of 643 adolescents, age 14 to 17. The sample was assessed at three time points. Smoking, smokeless tobacco, and other problem behaviors formed a single problem behavior factor. Structural equation modeling indicated that inadequate parental monitoring and association with deviant peers at Time 2 predicted tobacco use at Time 3. When parental and peer smoking at Time 2 were added to the model, each accounted for significant variance in predicting Time 3 smoking, but inadequate parental monitoring and association with deviant peers still accounted for some of the variance in Time 3 smoking. In predicting boys' smokeless tobacco use, monitoring at Time 2 predicted smokeless tobacco use, but only when parental approval of ST use was not included. Fathers approval of ST use at Time 2 predicted ST use at Time 3, while maternal disapproval predicted its use.

Adolescent↗
PubMed1995

The consistency of family and peer influences on the development of substance use in adolescence.

Latent growth modeling (LGM) was used to analyse longitudinal data for adolescent substance use from five overlapping age cohorts (11, 12, 13, 14, and 15 years at first assessment) measured at four annual time points. An associative cohort-sequential model was tested for alcohol, cigarette and marijuana use with a sample of 345 adolescents (11-18 years old) from an urban area in the Pacific Northwestern region of the United States. Hypotheses concerning the shape of the growth curve, the extent of individual differences in the common trajectory over time, and the influence of family cohesion, peer encouragement and gender on initial substance use and shape of the growth curve were tested. Results indicated similarities between alcohol, cigarette and marijuana initial use and development, with peer encouragement and family cohesion predictive of initial levels of use, and changes in peer encouragement influencing the developmental trajectories of the three substances. Females were higher than males in initial status and developed less rapidly in their use of the substances than did males. Findings are discussed in terms of the similarities and differences in the developmental trajectories of the three substances and the importance of family and peer influences on these trends.

Adolescent↗
PubMed1994

The effects of family cohesiveness and peer encouragement on the development of adolescent alcohol use: a cohort-sequential approach to the analysis of longitudinal data.

This article demonstrates a latent growth curve methodology for analyzing longitudinal data for adolescent alcohol use by combining information from different overlapping age cohorts to form a single developmental trajectory. Hypotheses concerning the form of growth in alcohol use, the extent of individual differences in the common trajectory over time, and covariates influencing both initial status and the form of growth were tested. Utilizing five separate age cohorts each measured over the same 4-year period, results suggested a common trajectory existed across the 8 years represented by the cohort-sequential analysis, with alcohol use increasing more rapidly during the adolescents' transition to high school. Family cohesion and peer encouragement for alcohol use were hypothesized to influence both initial status and the trajectory of alcohol consumption during adolescence. While family cohesion served to suppress initial levels of consumption delaying the upward trajectory of alcohol use, peer encouragement was related not only to initial, and elevated, levels of use, but was predictive of those changes that occurred during adolescence. Discussion involves the importance of family and peer influences in the development of adolescent alcohol use and the utility of the cohort-sequential approach in the analysis of longitudinal data.

Adolescent↗
PubMed1993

Social support and efficacy cognitions in exercise adherence: a latent growth curve analysis.

Although a considerable amount of research has established the beneficial effects of social support on health and well-being, relatively little work has focused on identifying those processes by which social support influences various health promoting behaviors. Various lines of research demonstrate converging evidence which suggests that self-efficacy may operate as one possible mediator linking psychosocial influences to positive health functioning associated with regular habitual exercise. The present study examined the relationships among social support, self-efficacy, and the ongoing exercise behavior of sedentary adults. Latent growth modeling techniques were utilized to determine whether self-efficacy served a mediational role in the influence of social support on exercise behaviors. Findings are discussed with reference to the role of cognitive and behavioral mechanisms in the relationship between supportive functions of social networks and improvements in health and well-being associated with regular exercise.

Exercise↗
PubMed1993

Modeling social and psychological determinants of exercise behaviors via structural equation systems.

Recent advances in structural modeling techniques allow for the testing of complex models representing social and behavioral processes. However, most reported applications in sport and physical activity have been limited to simple models involving variables measured at a single point in time. Therefore, the purpose of this article is to demonstrate the use of both cross-sectional and longitudinal latent variable modeling techniques by examining the relationships among efficacy cognitions, social support, and the exercise behaviors of sedentary adults. Results revealed a good fit for the re-specified model, suggesting the existence of a lagged feedback mechanism in which exercise behaviors influenced residual change in social support. In turn, efficacy cognitions appeared to serve a mediational function in the synchronous relationship between social support and exercise behavior. Findings are discussed in terms of the utility of structural equation modeling techniques for understanding the complex social and cognitive processes involved in exercise behavior.

Cognition↗
PubMed1993

Long-term maintenance of exercise, self-efficacy, and physiological change in older adults.

The present study documents the maintenance of exercise participation, self-efficacy, and physiological change in older adults at 9-month follow-up to a 5-month structured exercise program. Males and females (mean age = 54 years) completed graded exercise testing, body composition, and physical performance testing at the end of and 9 months after cessation of an exercise program. Self-efficacy assessments were also conducted prior to and following each graded exercise test and in the last week of the program. Results indicated that whereas some general reductions in physiological conditioning occurred, significant declines in physical performance and self-efficacy measures were evidenced at follow-up. However, exposure to the acute bout of physical testing increased the efficacy expectations to the point were they were no longer significantly different from postexercise program levels. Moreover, exercise self-efficacy was the only variable to significantly predict adherence to exercise during follow-up. However, previous attendance in the program, aerobic capacity, and self-efficacy all significantly discriminated between compliers and noncompliers to exercise prescription. Results are discussed with respect to the role played by acute exercise bouts in enhancing perceptions of personal efficacy in older adults and the utility of self-efficacy as a predictor of exercise behavior at various stages of the exercise process.

Aged↗
PubMed1993

Liver vitamin A levels in rats are predicted by a modified isotope dilution technique.

We tested the hypothesis that the fraction of an injected dose (FDp) of [3H]retinol remaining in plasma could be used to predict liver vitamin A mass. Rats (n = 36) with liver vitamin A ranging from 1.4 to 23,000 nmol received an intravenous dose of [3H]retinol-labeled plasma. Plasma samples obtained at 4, 4.4, 5, 5.4, 6, 7 and 8 d were analyzed for radioactivity; liver vitamin A levels were determined at 8 d. Using a regression equation developed previously [liver vitamin A (nmol) = 7493.exp(-148.FDp)], there was a log linear relationship between FDp and liver vitamin A mass from 1.4 to 5200 nmol retinol; the equation did not predict levels > or = 7500 nmol. By expanding the regression equation [liver vitamin A (nmol) = 88,928.exp(-1347.FDp) + 5606.exp(-120.FDp)] and using the FDp at 4.4 d, liver vitamin A levels ranging from < 4 to 23,000 nmol were well predicted. Satisfactory equations were also developed for data obtained at 4, 5 and 5.4 d. We conclude that this modified isotope dilution technique is a sensitive and reliable method for assessing vitamin A status in rats over a wide range of vitamin A nutriture. It is likely that this method could be modified for use in humans.

Animals↗
PubMed1991

A latent growth curve approach to investigating developmental dynamics and correlates of change in children's perceptions of physical competence.

In both the physical and academic domains interest has recently grown regarding developmental change in children's perceptions of competence. Recent advances in statistical methodology, such as latent growth modeling, enable the researcher to analyze developmental trends from both an inter- and intraindividual perspective, thus serving to increase our understanding of correlates of change in perceived competence. Using latent growth modeling, the purpose of the present study was to investigate developmental change in children's perceived competence at physical skills, examining maturational factors and chronological age as possible correlates of change. Subjects were male adolescents (N = 94). Data were collected during the child's sixth, eighth, and ninth grade school years. Results revealed a good fit for the hypothesized model, which specified mean level changes in the variables over time and an interrelationship among the latent chronometric common factors. Although a significant relationship existed between maturation and perceived competence, no significant relationship was evident between chronological age and perceived competence. Findings are discussed with particular reference to the utility of latent growth curve models for assessing developmental processes at both the inter- and intraindividual level.

Adolescent↗
PubMed1986

Photically-induced retinal damage in diabetic rats.

The present study examines the interaction of light damage to the retina and streptozotocin (SZ)-induced diabetes in male and female rats during the early development of the disease, when changes occur in the blood-retinal barrier and in pigment cell membranes. Exposure of rats to low illuminance was used to determine the relationship between photically-induced cell death and diabetes. Other groups of animals were exposed to a greater illuminance for shorter time periods (24 hours) in attempts to identify a specific post-treatment day for the effect of diabetes. Blood glucose levels were monitored to indicate the severity of the diabetes. Morphometric analyses and histopathologic observations demonstrated that the outer nuclear layer (ONL, photoreceptor nuclei) was reduced significantly in thickness in female rats exposed to light during a 9 day period after SZ injection, but was unchanged from the control groups when exposed beginning at 12 days after SZ treatment. Removal of the pituitary gland prior to SZ treatment and light exposure resulted in the survival of more photoreceptor cells and prevented the differential in ONL thickness observed between control and diabetic intact animals. Attempts to establish a period of greatest susceptibility of the diabetic retina to photic damage were unsuccessful, but results indicate that prior light history and/or shipment stress might be related to retinal damage from light exposure.

Animals↗
PubMed1985

The diurnal susceptibility of rat retinal photoreceptors to light-induced damage.

Exposure of albino rats to high intensity light results in rapid, graded loss of photoreceptors. The hormonal status and age of an animal at the time of exposure affect the severity of light-induced retinal damage. The adrenal axis and pituitary hormones (prolactin) have been demonstrated previously to affect the degree of cell death in the retina. Because circadian rhythms for adrenal and pituitary secretion have been demonstrated in the rat, a series of experiments was undertaken to determine if a diurnal pattern of retinal susceptibility to light damage exists which might be related to endogenous endocrine rhythms. Male Sprague-Dawley rats were exposed to 4 hr of high intensity fluorescent light for 8 consecutive days during different phases of the 14:10 hr light: dark animal room light cycle. Morphometric analysis performed at the light microscopic level 2 weeks after exposure demonstrated a differential susceptibility to light-induced cell death depending upon the period during the light-dark cycle when animals received their daily light exposure. Neuronal cell death was confined to the outer nuclear layer as previously described. The retinas of animals exposed during the middle of the dark period or during the first 5 hr of the light period were significantly more damaged than the retinas of animals exposed during the last 9 hr of the light period. Control groups for the relative amounts of dark-adaptation between groups suggested that the diurnal susceptibility to light damage was not solely dependent upon the degree of dark adaptation. These results demonstrate a diurnal susceptibility of photoreceptors to light-induced cell death.

Animals↗