Katarina T. Borer

Physical activity in the prevention and amelioration of osteoporosis in women: Interaction of mechanical, hormonal and dietary factors

AbstractOsteoporosis is a serious health problem that diminishes quality of life and levies a financial burden on those who fear and experience bone fractures. Physical activity as a way to prevent osteoporosis is based on evidence that it can regulate bone maintenance and stimulate bone formation including the accumulation of mineral, in addition to strengthening muscles, improving balance, and thus reducing the overall risk of falls and fractures. Currently, our understanding of how to use exercise effectively in the prevention of osteoporosis is incomplete. It is uncertain whether exercise will help accumulate more overall peak bone mass during childhood, adolescence and young adulthood. Also, the consistent effectiveness of exercise to increase bone mass, or at least arrest the loss of bone mass after menopause, is also in question. Within this framework, section 1 introduces mechanical characteristics of bones to assist the reader in understanding their responses to physical activity.Section 2 reviews hormonal, nutritional and mechanical factors necessary for the growth of bones in length, width and mineral content that produce peak bone mass in the course of childhood and adolescence using a large sample of healthy Caucasian girls and female adolescents for reference. Effectiveness of exercise is evaluated throughout using absolute changes in bone with the underlying assumption that useful exercise should produce changes that approximate or exceed the absolute magnitude of bone parameters in a healthy reference population. Physical activity increases growth in width and mineral content of bones in girls and adolescent females, particularly when it is initiated before puberty, carried out in volumes and at intensities seen in athletes, and accompanied by adequate caloric and calcium intakes. Similar increases are seen in young women following the termination of statural growth in response to athletic training, but not to more limited levels of physical activity characteristic of longitudinal training studies. After 9–12 months of regular exercise, young adult women often show very small benefits to bone health, possibly because of large subject attrition rates, inadequate exercise intensity, duration or frequency, or because at this stage of life accumulation of bone mass may be at its natural peak. The important influence of hormones as well as dietary and specific nutrient abundance on bone growth and health are emphasised, and premature bone loss associated with dietary restriction and estradiol withdrawal in exercise-induced amenorrhoea is described.In section 3, the same assessment is applied to the effects of physical activity in postmenopausal women. Studies of postmenopausal women are presented from the perspective of limitations of the capacity of the skeleton to adapt to mechanical stress of exercise due to altered hormonal status and inadequate intake of specific nutrients. After menopause, effectiveness of exercise to increase bone mineral depends heavily on adequate availability of dietary calcium. Relatively infrequent evidence that physical activity prevents bone loss or increases bone mineral after menopause may be a consequence of inadequate calcium availability or low intensity of exercise in training studies. Several studies with postmenopausal women show modest increases in bone mineral toward the norm seen in a healthy population in response to high-intensity training. Physical activities continue to stimulate increases in bone diameter throughout the lifespan. These exercise-stimulated increases in bone diameter diminish the risk of fractures by mechanically counteracting the thinning of bones and increases in bone porosity.Seven principles of bone adaptation to mechanical stress are reviewed in section 4 to suggest how exercise by human subjects could be made more effective. They posit that exercise should: (i) be dynamic, not static; (ii) exceed a threshold intensity; (iii) exceed a threshold strain frequency; (iv) be relatively brief but intermittent; (v) impose an unusual loading pattern on the bones; (vi) be supported by unlimited nutrient energy; and (vii) include adequate calcium and cholecalciferol (vitamin D3) availability.

The Effects of Exercise on Growth

SummaryThe way in which exercise influences statural, hypertrophic and reparative growth is examined from the perspective of the human lifespan.Statural growth depends on a neuroendocrine programme which channels nutrient energy towards increments in lean body mass. Exercise can facilitate statural growth and is a necessary stimulus for reparative growth through its stimulatory effects on secretion of growth hormone (GH) and other anabolic hormones. An exercise-associated increase in GH secretion is a response to acute or prolonged exercise-induced fuel shortage that directs metabolism towards utilisation of lipids and promotes growth. Exercise can transiently block the expression of statural growth by competitively removing the necessary nutritional support for growth. Statural growth retardation can be corrected by catch-up growth, but stunting may also be permanent (depending on the timing and magnitude of the energy drain).Hypertrophic growth is less dependent on hormonal and nutritional support than statural growth, and exercise provides the necessary mechanical stresses for growth and remodelling of the musculoskeletal system. Excessive mechanical strain may suppress hypertrophic growth. The intermittent nature of exercise provides temporal organisation that is necessary for the normal operation of cellular growth processes.Exercise by pregnant women does not appear to influence fetal growth. Evaluation of the effect of exercise on growth of children and adolescents is complicated by nonrandom selection of individuals for participation in organised sports, and by lack of information on the magnitude of exercise-induced energy drain. Exercise is essential for regulation of body composition in adulthood. It provides mechanical and metabolic stimuli that are necessary for hypertrophy of the musculoskeletal system and increased GH secretion for reparative growth.

Body temperature rhythm and response to pyrogen in exercising and sedentary hamsters.

In this study, we tested the hypothesis that daily voluntary exercise results in a chronic elevation in core temperature in the female golden hamster. Temperature and activity were measured by biotelemetry. Hamsters ran 6-7 km per night (12:12 L:D) when permitted access to wheels. No running occurred during the light periods. During the 3rd wk of running, temperatures of exercising hamsters were significantly elevated by 0.5 degree C (P less than 0.001) during the dark period and by 0.3 degree C (P less than 0.003) during the light period compared with sedentary hamsters. Cessation of running removed the difference between groups, and resumption of running restored it. Both the injection of endotoxin and the psychological stress of cage switch resulted in similar peak temperatures in exercising and sedentary hamsters despite higher pre-treatment temperatures in the exercise group. We interpret these results to support the hypothesis that regular exercise may cause an upward resetting of the set-point for body temperature.

Absence of weight regulation in exercising hamsters

BORER, K. T. Absence of weight regulation in exercising hamsters. PHYSIOL. BEHAV. 12(4) 589-597, 1974. Spontaneous activity on horizontal discs causes a permanent upward displacement of hamster weight, length, and, under some circumstances, percentage body fat. This phenomenon manifests itself through an immediate increase in the rate of weight gain, a disappearance of a preference for sunflower seeds, as well as in a delayed and gradual increase in food intake during and immediately following the activity period. A week after termination of spontaneous disc running normal regulation of body weight is reinstated and maintained at the new elevated weight level. These results suggest that some concomitant of spontaneous disc running in hamsters renders the weight-regulatory mechanisms inoperative. Spontaneous exercise Weight gain Growth Body fat Food consumption Food selection Weight regulation

Appetite Responds to Changes in Meal Content, Whereas Ghrelin, Leptin, and Insulin Track Changes in Energy Availability

CONTEXT It is uncertain how between-meal variations in energy availability and physiological changes in ghrelin, leptin, and insulin affect appetite. OBJECTIVE The aim of the study was to examine the influence on human appetite of the meal size and its nutrient content or changes in energy availability and concentrations of ghrelin, leptin, and insulin. DESIGN We conducted a crossover study manipulating meal size and energy availability through exercise energy expenditure and iv nutrient replacement (TPN). SETTING The study was performed at a Clinical Research Center. PARTICIPANTS Ten healthy postmenopausal women (age, 59.7 +/- 1.5 yr; mean body mass index, 26 kg/m(2)) were studied. INTERVENTIONS We conducted trials based on different morning meal size (418 vs. 2090 KJ), presence or absence of exercise energy expenditure (2273 to 2361 KJ), energy replacement by TPN (1521 to 1538 KJ), and a midday ad libitum meal. MAIN OUTCOME MEASURES Changes in hunger, fullness, midday ad libitum food consumption, and concentrations of ghrelin, leptin, insulin, and metabolic fuels were measured. We also performed midday meal tests for the presence of caloric compensation. RESULTS Appetite was influenced by the size and energy content of the meals, but not by variation in energy availability which also did not trigger consummatory compensation. Exercise reduced hunger and increased fullness. Ghrelin, leptin, and insulin responded to changes in energy availability but not to meal size. Appetite was unaffected by physiological changes in ghrelin, leptin, or insulin. CONCLUSIONS During rest, appetite is influenced by the size and energy content of meals, but it bears no homeostatic relationship to between-meal changes in energy availability due to small meals, exercise, or TPN, or concentrations of ghrelin, leptin, and insulin.

Hamster Prolactin: Physiological Changes in Blood and Pituitary Concentrations as Measured by a Homologous Radioimmunoassay

A specific and sensitive homologous radioimmunoassay (RIA) for determination of hamster prolactin is described and compared to heterologous hamster prolactin RIA and to the homologous and heterologous RIAs for rat prolactin. With this method, we have determined that, in the golden hamster, diurnal plasma prolactin fluctuations have a mean interpeak interval of 1.5 h, that serum prolactin concentrations are influenced by day length, by ether vapors, by animals gender, and by refeeding following a fast but not by neurosurgical procedures which increase somatic growth and serum GH concentration in adult hamsters. Variations in day length influenced pituitary prolactin content and concentration, and pituitary removal abolished prolactin secretory responses to ether stress. A heterologous RIA for hamster prolactin which utilizes radioiodinated rat prolactin and rabbit antihamster prolactin serum (RK1-15) represents the most advantageous method for the measurement of hamster prolactin because of its high sensitivity (0.14 ng/ml), high antiserum binding to iodinated rat prolactin, and the ability to measure both hamster and rat prolactin.

Characteristics of growth-inducing exercise.

Abstract Characteristics of growth-inducing exercise in hamsters were studied by examining (a) dependence of this phenomenon on a specific activity device, (b) its species-specificity, and (c) features of the running pattern which produce optimal growth acceleration. Acceleration of growth by exercise occurs in hamsters running on either horizontal discs or in vertical wheels, and in gerbils running in rotating wheels, and is therefore neither device- or species-specific phenomenon. In contrast to the two rodent species demonstrating increases in the rate of weight gain at levels of voluntary activity between 10,000 and 30,000 RPD, rats running on either activity device, and gerbils and ground squirrels running on horizontal discs, generated low levels of activity and no evidence of increased somatic growth. At weight ranges associated with maximal acceleration of growth by disc exercise, hamsters ran at moderately high speeds of between 35 and 51 cm/sec for up to one hour without a pause. Their total daily activity exceeded 15,000 RPDs (5.9–8.5 km) and lasted between 5 and 10 hours. Prolonged voluntary activity at relatively low speed constitutes a sufficient condition for acceleration of somatic growth in two rodent species.

Exercise-induced growth in golden hamsters: effects of age, weight, and activity level.

Abstract The relationship between the exercise-induced growth and the endogenous controls of growth was examined in 356 golden hamsters of which equal numbers were given access to horizontal disc exercisers and maintained sedentary at different ages (17 and 42 days) and body weights (25–180 g). Hamsters started exercising around Day 35, increased their activity levels to 30,000 revolutions per day (RPD) by Day 35, and ran progressively less as their weight increased above 65 g. Exercise accelerated growth only in hamsters which have entered the slow asymptotic phase of growth, which weighed between 60–180 g and generated over 15,000 RPD. Disc exercise appears to reinstate higher rates of ponderal and linear growth after hamsters have entered the slow asymptotic phase of growth. Sustained inhibition of exponential growth may participate in the long-term regulation of body size in adult rodents.

Stimulation by Voluntary Exercise of Adrenal Glucocorticoid Secretion in Mature Female Hamsters

The possibility that habitual voluntary running induces a chronic change in adrenal glucocorticoid synthesis and secretion was examined in freely running mature female hamsters, in whom this behavior accelerates growth, reduces body fat levels, and elevates core temperature. Hamsters were free to run on horizontal discs or in vertical wheels between 32 and 80 days, in 14L:10D or in 10L:14D photoperiods, and at the end of this period, corticosterone and cortisol steroidogenesis and serial plasma corticosterone concentrations during day and night were used as measures of the chronic stimulation of adrenal cortical activity. Habitual voluntary running significantly increased steroidogenesis of both glucocorticoids and plasma corticosterone concentrations and alone accounted for all the variance in enhanced synthesis and secretion of corticosterone. Acute exercise and/or the nocturnal phase of circadian period enhanced the chronic stimulatory effects of exercise on cortisol. Despite its voluntary and apparently stress-free nature, running induces chronic increases in basal glucocorticoid secretion in mature female hamsters. Putative oversecretion of corticotropin releasing factor in freely running hamsters could account for increased steroidogenesis, acceleration of growth, reduced body fat levels, and core temperature elevation.

Exercise energy expenditure is not consciously detected due to oro-gastric, not metabolic, basis of hunger sensation

We tested the hypothesis that exercise energy expenditure (EEE) will elicit reflex metabolic compensations but no increases in hunger. Exercise expended 800 kcal once when fasted and at another time in a post-prandial state. During fasting exercise, pre-meal ratings of hunger were unaffected by EEE, but plasma concentrations of ghrelin, growth hormone and free fatty acids were higher than in the absence of EEE. We propose that the perception of hunger is based on the vagal, lateral hypothalamic and cortical projections of oral and gastro-intestinal (GI) stimuli and that EEE triggers neuroendocrine compensations and influences hunger indirectly by affecting GI functions.