Biomedical subjects

C N Meredith

Publications and source records attributed to C N Meredith.

26 records · Page 2Linked to original sources

PubMed1987

Body composition and aerobic capacity in young and middle-aged endurance-trained men.

Aging is associated with increased body fat, decreased muscle mass, lower maximal O2 uptake, and lower energy intake. It has been asserted that these changes are caused in part by decreased activity. In order to determine the effects of aging in men who are habitually active, 6 young men (26.8 +/- 1.2 yr) and 6 middle-aged men (52.0 +/- 1.9 yr) who had been running, cycling, and/or rowing for at least 2 yr were studied in a metabolic ward. The men consumed a diet to maintain constant body weight while keeping to their usual exercise schedule and intensity. Both groups had a similar body mass index and muscle mass, but the middle-aged men had a 77% greater fat mass (P = 0.028). Daily energy requirement was 17% lower in the middle-aged men (P = 0.029) although basal metabolic rate was similar in both groups. Aerobic capacity was 15% lower in the middle-aged men per unit body weight (P = 0.048) but not per unit of lean body mass. The young men exercised for an average 12.3 hr.wk-1 and the middle-aged men for 7.5 hr.wk-1 (P = 0.018). For both young and middle-aged men, weekly hours of exercise were negatively correlated with fat mass, and positively correlated with daily energy requirements and maximal O2 uptake. These findings show that in both young and middle-aged men who habitually exercise aerobically at 65 to 80% maximal O2 uptake, the time spent training was associated with body composition, energy requirements, and aerobic capacity.

Adult↗

PubMed1986

Leucine kinetics at graded leucine intakes in young men.

A study was carried out with 12 young men to examine the relationships between the intake of leucine and indices of leucine kinetics, using L-[1-13C]leucine as a tracer. Six subjects received L-amino acid diets during 7-day periods supplying leucine in the range of 79 to 20 mg.kg-1.day-1 (Group I) and another six subjects (Group II) received leucine intakes ranging from 20 to 4 mg.kg-1.day-1. Estimations were made of leucine kinetics, at the end of each diet period, when subjects were receiving small isonitrogenous, isocaloric meals during the isotope infusion period. Leucine flux declined with reduced leucine intake and leucine oxidation tended not to change at intakes below 20 mg.kg-1.day-1 (slope not statistically different than zero). Plasma valine increased markedly with further restriction in leucine intake below this level. The daily mass balance of leucine, estimated from the difference between intake and oxidation, became negative at an intake of about 20 mg.kg-1.day-1. These findings are discussed in relation to the published mean and upper range of requirement for leucine in healthy adults, currently taken to be 11 mg.kg-1.day-1 and 14 mg.kg-1.day-1, respectively.

Adolescent↗

PubMed1986

Valine kinetics at graded valine intakes in young men.

Twelve young men, six subjects in each group studied in two phases, participated in an experiment to explore the relationships between valine intake, plasma valine concentrations, and valine kinetics, using 1-[13C]valine as a tracer. Below a valine intake of about 20 mg.kg-1.day-1 plasma valine concentrations reached a low and relatively constant level. The rate of valine oxidation fell with the decline in the intake of amino acid. Below valine intakes of 16 mg.kg-1.day-1, the mean daily rate of oxidation was estimated to be generally higher than the intake level, implying a negative valine balance during the 24 h day. These findings indicate that an intake of 10 mg valine kg-1.day-1 would not be adequate to maintain protein nutritional status. Our results are discussed in relation to the currently accepted 1973 FAO/WHO value of 10 mg.kg-1.day-1 as being the upper range of the valine requirement in healthy adult humans.

Adolescent↗

PubMed1986

Lysine kinetics at graded lysine intakes in young men.

Healthy young men participated in a study designed to explore the effects of decreasing dietary lysine content on plasma amino acid concentrations and lysine kinetics, studied with L-[1-13C]lysine as tracer. Diets provided adequate energy and the equivalent (N X 6.25) of 0.8 g protein kg-1.day-1 as a synthetic L-amino acid mixture simulating egg protein. Lysine intake was reduced every 7 days. Changes in plasma amino acids suggested that effects characteristic of a dietary lysine inadequacy were prevented by consuming more than 32 mg lysine.kg-1 day-1. Primed, continuous intravenous infusions of L-[1-13C]lysine, at the end of each diet period while subjects were in the fed state, showed that as dietary lysine was reduced, 13C enrichment increased in plasma lysine and decreased in expired CO2. It was estimated that lysine oxidation exceeded, by 4.4 mg kg-1 day-1, the lysine intake of 20 mg kg-1.day-1 indicating that the lysine required for body protein maintenance would probably exceed this latter value. These results are discussed in relation to the physiological requirement in adults for lysine, currently accepted to be met by an intake of 12 mg kg-1.day-1. which is assumed to be the upper range of the lysine requirement for this population group.

Adolescent↗

PubMed1986

Threonine kinetics at graded threonine intakes in young men.

A study was undertaken in eight healthy young men to examine the effects of varying intakes of threonine on plasma free threonine concentrations and threonine kinetics, using a 3 h constant intravenous infusion of L-[1-13C]threonine. Subjects consumed diets based on an L-amino acid mixture, in which the quality of threonine was reduced every 7 days. On the last day of each diet period, determinations of plasma threonine flux and threonine oxidation were carried out while subjects consumed small meals, each supplying 1/12 daily intake, at hourly intervals. Threonine oxidation rates fell with reduced threonine intake, reaching a relatively constant level at intakes of 20 mg.kg-1.day-1 and below. These metabolic data are discussed in relation to the currently established value of 7 mg.kg-1.day-1 as the upper range of the threonine requirement for healthy young adults. It is concluded that actual threonine requirements may be considerably higher for this age group.

Adolescent↗

PubMed1986

Diet and bone status in amenorrheic runners.

Our purpose was to determine the relationship of diet, exercise, and amenorrhea with bone mineral status in trained young women. Bone mineral density of the lumbar spine was significantly lower in amenorrheic compared to normally menstruating runners. Circulating estradiol was also significantly lower. No significant differences between the two groups were found in body composition, maximum aerobic power, or amount of training per week. A 3-day dietary record showed that the amenorrheic women reported a significantly lower daily energy intake, but no difference in the calcium intake. Protein intake was less than the US Recommended Dietary Allowance in 82% of amenorrheic women and 35% of eumenorrheic women. Abnormal eating behaviors may be associated with the development of amenorrhea. Our results show that when weight-bearing exercise and a low energy intake are associated with amenorrhea, the accretion of a large bone mass in young trained women is not favored.

Adult↗

PubMed1986

Metabolic changes following eccentric exercise in trained and untrained men.

The effects of one 45-min bout of high-intensity eccentric exercise (250 W) were studied in four male runners and five untrained men. Plasma creatine kinase (CK) activity in these runners was higher (P less than 0.001) than in the untrained men before exercise and peaked at 207 IU/ml 1 day after exercise, whereas in untrained men the maximum was 2,143 IU/ml 5 days after exercise. Plasma interleukin-1 (IL-1) in the trained men was also higher (P less than 0.001) than in the untrained men before exercise but did not significantly increase after exercise. In the untrained men, IL-1 was significantly elevated 3 h after exercise (P less than 0.001). In the untrained group only, 24-h urines were collected before and after exercise while the men consumed a meat-free diet. Urinary 3-methylhistidine/creatinine in the untrained group rose significantly from 127 mumol/g before exercise to 180 mumol/g 10 days after exercise. The results suggest that in untrained men eccentric exercise leads to a metabolic response indicative of delayed muscle damage. Regularly performed long distance running was associated with chronically elevated plasma IL-1 levels and serum CK activities without acute increases after an eccentric exercise bout.

Creatine Kinase↗

PubMed1986

Physiological mechanisms contributing to increased interleukin-1 secretion.

Interleukin-1 (IL-1) is a monocyte-derived polypeptide that mediates many host defense adaptations to environmental and infectious stresses. This investigation was intended to characterize further IL-1 activity found in human plasma following exercise (3) and to identify physiological initiators of IL-1 secretion. IL-1 activity was measured by the ability of plasma fractions to stimulate lymphocyte proliferation. This activity appeared in plasma several hours after exercise on a cycle ergometer (1 h at 60% of aerobic capacity, n = 8 subjects) and was neutralized with a specific antiserum to human IL-1. The hypothesis that IL-1 release from monocytes was initiated by phagocytosis of material from cells damaged by exercise was tested. The increase in IL-1 activity did not correlate significantly (r = 0.55) with creatine kinase activity, a marker for release of intracellular proteins into the circulation, and IL-1 secretion by monocytes was not stimulated by incubation with red blood cell lysates in vitro. Thus the stimulus for IL-1 secretion did not appear to be related to a scavenging function of monocytes. The possibility that IL-1 secretion may be mediated by stress hormones associated with exercise was examined. IL-1 secretion by monocytes was increased up to 48 +/- 18% (P less than 0.01) by addition of physiological concentrations of epinephrine in vitro. Low concentrations of hydrocortisone (1 ng/ml) also augmented IL-1 secretion by 58 +/- 20%. Higher concentrations in the physiological range had no effect, and combinations of epinephrine and hydrocortisone suppressed IL-1 secretion.(ABSTRACT TRUNCATED AT 250 WORDS)

Adult↗