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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.
Articles follow on:
Exercise
- Cytokine (Protein) Levels Increase After
Marathon Race Regardless of Age or Gender
- Morning Exercise Can Significantly Affect
Body's Physiological Responses to Subsequent Afternoon Exercise
- Bed Rest Can Affect Physiological Responses
to Exercise
Gender Differences
- Gender Alters the Impact of Hypobaric
Hypoxia on Adductor Pollicis Muscle Performance
- Gender Differences in Carbohydrate Loading
Related to Energy Intake
- Gender Differences Apparent in Leucine,
but not Lysine, Kinetics
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 Faculte’ 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.
EXERCISE
Cytokine (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 percent 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 minutes—separated by 3 hours—of
cycling on a stationary bike at 48 percent (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, FATIGUE
Gender 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 percent 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 percent 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 percent), 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 percent 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.
-end-
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.
***
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
djkrupa1@aol.com.
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