SIDS, Exercise and Gender Difference Research

For Immediate Release

Contact: Donna Krupa: 703.527.7357 (direct dial)703.967.2751 (cell) or [email protected]

SIDS, EXERCISE, AND GENDER DIFFERENCE RESEARCH AMONG HIGHLIGHTS OF CURRENT EDITION OF JOURNAL OF APPLIED PHYSIOLOGY

Research reports on the relationship between body position and clothing in SIDS, effects of morning vs. evening exercise, bed rest for athletes, and physiological gender differences are part of the July edition of the Journal of Applied Physiology.

Bethesda, MD-- Factors such as heat, exercise, bed rest and even body position can all have a physiological effect on how a body reacts, whether it is an infant, a sedentary adult or a well-conditioned athlete. The July 2001 issue of the Journal of Applied Physiology, one of 14 monthly scientific journals published by the American Physiological Society (APS), spotlights several aspects of these factors.

SUDDEN INFANT DEATH SYNDROME (SIDS)Summary: Since guidelines have been issued to place infants on their back rather than on their stomach when going to sleep, the incidences of sudden infant death syndrome (SIDS) have decreased dramatically. Many researchers believe that the increased risk of SIDS could be attributed to heat stress associated with body position.

Methodology: In a study led by Elmountacer Billah Elabbassi of the Facultei' de Medecine, of the Unite de Recherches sur les Adaptations Physiologiques et Comportementales, in Amiens, France, the dry heat losses of small-for-gestational-age newborns, both nude and clothed, were assessed and compared to determine whether there is a difference in the body's ability to lose heat between the prone and the supine position. Elabbasi and his colleagues exposed an anthropomorphic thermal mannequin to six environmental temperatures, ranging between 25 and 37 degrees C, in a single-walled, air-heated incubator.

Conclusion: The researcher found that with clothing, body temperature was the same in prone and supine position and without clothing, body temperature was the same in the supine and prone position.

EXERCISECytokine (Protein) Levels Increase After Marathon Race Regardless of Age or Gender

Summary: In a study of 98 runners who competed in two marathon races, researchers from Appalachian State University and the Department of Exercise Science at the University of South Carolina examined the influence of carbohydrate ingestion, gender, and age on pro- and anti-inflammatory plasma cytokine (protein) and hormone changes.

Methodology: David C. Nieman and his colleagues randomly divided the marathon runners into two groups, one group receiving 1 l/h of a 6% carbohydrate beverage and the other a placebo. The beverages were administered during two competitive marathon races. Plasma glucose was higher and cortisol was lower in the group receiving the beverage. For all subjects combined, the plasma levels of interleukin (IL)-10, IL-1 receptor antagonist (IL-1ra), IL-6, and IL-8 rose significantly immediately after the races and remained above pre-race levels an hour and a half later. The pattern of change in all cytokines did not differ significantly between the 12 women and 86 men in the study and the 23 subjects who were older than 50 years of age and the 75 who were younger than 50.

Conclusion: The researchers concluded that plasma levels of IL-10, IL-1ra, IL-6, and IL-8 rose strongly in all runners after a competitive marathon, regardless of age or gender.

Morning Exercise Can Significantly Affect Body's Physiological Responses to Subsequent Afternoon Exercise

Summary: Pietro Galassetti and colleagues from the Departments of Medicine and Molecular Physiology and Biophysics at the Vanderbilt University School of Medicine and the Nashville Veterans Affairs Medical Center studied 16 healthy volunteers (8 men and 8 women), between the ages of 26 and 28, to determine whether a bout of morning exercise can alter neuroendocrine and metabolic responses to subsequent afternoon exercise and whether any such changes are influenced by gender.

Methodology: Participants were studied after an overnight fast. Each exercise session consisted of 90 minutesiXseparated by 3 hoursiXof cycling on a stationary bike at 48 (plus or minus 2 percent) of maximal oxygen uptake. To avoid the effects of hypoglycemia and glycogen depletion, which would have clouded the results, the subjects were given a carbohydrate after the morning exercise session, and plasma glucose was maintained at euglycemia during both exercise sessions by a modification of the glucose-clamp technique. Basal insulin levels and exercise-induced insulin decreases were similar during both sessions. The glucose infusion rate needed to maintain euglycemia during the last 30 minutes of exercise was increased during the second session, compared with the morning exercise. Although the increased need for glucose was similar in men and women, the differences in counter-regulatory responses between the genders were significant.

Conclusions: Compared with the morning session, epinephrine, norepinephrine, growth hormone, pancreatic polypeptide, and cortisol responses were blunted during the afternoon session in men, but neuroendocrine responses were preserved or increased in women. The researchers concluded that morning exercise significantly impairs the body's ability to maintain euglycemia during later exercise of similar intensity and duration.

Bed Rest Can Affect Physiological Responses to Exercise

Summary: To test the hypotheses that short-term bed rest influences metabolic, cardiorespiratory, and neurohormonal responses to exercise, and that these effects depend on the subjects' training status, researchers at the Academy of Physical Education and Medical Research Centre in Warsaw, Poland, and the National Aeronautics and Space Administration Ames Research Center in California studied 12 sedentary men, 10 endurance and 10 strength-trained athletes.

Methodology: The men were put on three days of bed rest. Before and after the bed rest, they performed incremental exercise tests until volitional exhaustion. Respiratory gas exchange and heart rate were recorded continuously, and stroke volume was measured at submaximal loads. Blood was taken for lactate concentration, epinephrine concentration, norepinephrine concentration, plasma renin activity, human growth hormone concentration, testosterone, and cortisol determination. Reduction of peak oxygen uptake after bed rest was greater in the endurance athletes than in the remaining groups. Resting and exercise respiratory exchange ratios were increased in athletes. Cardiac output was unchanged by bed rest in all groups, but exercise heart rate was increased and stroke volume diminished in the sedentary subjects.

Conclusions: The researchers concluded that the reduction of exercise performance and modifications in neurohormonal response to exercise after bed rest depend on the previous level and mode of physical training, being the most pronounced in the endurance athletes.

GENDER DIFFERENCES IN MUSCLE CONTRACTION, FATIGUEGender Alters the Impact of Hypobaric Hypoxia on Adductor Pollicis Muscle Performance

Summary: In a recent study, Charles S. Fulco and colleagues reported that at similar voluntary force development contractions of the adductor pollicis muscle, fatigue developed more slowly in women than in men. The researchers theorized that the slower fatigue in women was due, in part, to a greater capacity for muscle oxidative phosphorylation. In their current study, Fulco and colleagues examined whether a gender difference in adductor pollicis muscle performance also exists during acute exposure to hypobaric hypoxia.

Methodology: Healthy young men and women performed repeated static contractions at 50% of maximal voluntary contraction (MVC) force of rested muscle for 5 seconds followed by 5 seconds of rest until exhaustion. Exhaustion was defined as an MVC force decline to 50% of that of rested muscle. For both men and women, the maximal voluntary contraction force of rested muscle in hypobaric hypoxia was not significantly different from that in normobaric normoxia. MVC force tended to decline at a faster rate for men than for women. For men, endurance time to exhaustion was shorter. For women, endurance time to exhaustion was similar. In both environments, endurance time to exhaustion was longer for men than for women.

Conclusions: The researchers' findings were consistent with their hypothesis of a higher adductor pollicis muscle oxidative capacity in women than in men and the implication that isolated performance of the muscle with a higher oxidative capacity may be less impaired when the muscle is exposed to hypobaric hypoxia.

Gender Differences in Carbohydrate Loading Related to Energy Intake

Summary: Mark A. Tarnopolsky and researchers from the Departments of Medicine and Kinesiology at McMaster University and Human Biology and Nutrition from the University of Guelph, in Ontario, Canada, studied six men and six women to determine whether well-trained individuals increased their concentration of muscle glycogen after an increase in both the relative and absolute energy and carbohydrate intake and whether potential gender differences were related to muscle hexokinase enzyme activity.

Methodology: The subjects were randomly allocated three diets labeled habitual, high carbohydrate (75%), and high carbohydrate plus extra energy. The diets were given for a 4-day period before a muscle biopsy for analysis of total and pro- and macroglycogen and hexokinase activity. The total glycogen concentration was higher for the men on the high carbohydrate and high carbohydrate with energy trials compared with the habitual diet, whereas the women increased only on the high carbohydrate-with-energy diet compared with the habitual.

Conclusions: The researchers concluded that female endurance athletes did not increase their muscle glycogen concentration after an increase in their dietary carbohydrate intake, whereas men did. They also found that there were no gender differences in the proportion of pro- and macroglycogen or hexokinase activity. According to Tarnopolsky et al., a low-energy intake may explain a previously reported lower capacity for women to glycogen load compared with men.

Gender Differences Apparent in Leucine, but not Lysine, Kinetics

Summary: Controversy surrounding the effects of gender on leucine kinetics (the rate of turnover of an amino acid essential for nitrogen equilibrium in adults) has been found in physiological literature. Two research groups have found that men oxidize more leucine during exercise, whereas another group has shown no gender effects. To further test the effects of gender on leucine and, for comparison purposes, lysine kinetics, Linda Lamont, et. al., at the University of Rhode Island and The Schwartz Center for Metabolism and Nutrition at Case Western Reserve University School of Medicine studied seven matched pairs of men and women who were selected for their habits and age.

Methodology: After one week of a standardized diet, the subjects exercised at 40% of maximal oxygen uptake for one hour.

Conclusions: There was an effect of exercise in both genders: an increased leucine oxidation and an attenuation in non-oxidative leucine disposal compared with rest. The study further confirmed:

-- there are gender differences in leucine, but not lysine, kinetics; -- men had a higher rate of leucine oxidation and a lower rate of non-oxidative leucine disposal during exercise; -- for women, a larger proportion of their exercise energy needs come from fat; for men, a greater fraction come from carbohydrate; -- female exercisers rely to a greater extent on fat as an energy source, thereby using less carbohydrate, amino acid, and protein as a fuel source.

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The American Physiological Society (APS) was founded in l887 and since then has played a crucial role in the development of modern medicine. The American Journal of Physiology, one of 14 APS peer-reviewed journals, has been published continuously since l898. Today, nearly 3,800 articles are published each year, including articles in the most recently released journal, Physiological Genomics.

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Editor's Note: For the full text of the summaries cited above, or to set up an interview with a lead investigator, please contact Donna Krupa at 703.527.7357 (direct dial), 703.967.2751 (cell) or [email protected]. Or log on to www.aps.org/jap