Differences between observed and predicted energy costs at rest and during exercise in three subsistence-level populations

Estimates of daily energy expenditure are important for many areas of research in human ecology and adaptability. The most common technique for estimating human energy expenditure under field conditions, the factorial method, generally relies on activity-specific energy costs derived from published sources, based largely on North American and European subjects. There is concern that such data may not be appropriate for non-Western populations because of differences in metabolic costs. The present study addresses this concern by comparing measured vs. predicted energy costs at rest and during sub-maximal exercise in 83 subjects (52 males, 31 females) from three subsistence-level populations (Siberian herders and high-land and coastal Ecuadorian farmers). Energy costs at rest (i.e., lying, sitting and standing) and while performing a standard stepping exercise did not significantly differ among the three groups. However, resting energy costs were significantly elevated over predicted levels (+16% in men, +11% in women), whereas exercising costs were comparable to predicted values (?6% in men, +3% in women). Elevations in resting energy needs appear to reflect responses to thermal stress. These results indicate that temperature adjustments of resting energy costs are critical for accurately predicting daily energy needs among traditionally living populations. o 1996 Wiley-Liss, Inc.     

Norepinephrine spillover at rest and during submaximal exercise in young and old subjects

Mazzeo, Robert S., Chakravarthi Rajkumar, Garry Jennings,and Murray Esler. Norepinephrine spillover at rest and during submaximal exercise in young and old subjects. J. Appl. Physiol. 82(6): 1869-1874, 1997.Aging isassociated with elevations in plasma norepinephrineconcentrations. The purpose of this investigation was toexamine total body and regional norepinephrine spillover as anindicator of sympathetic nerve activity. Eight young (26 ± 3 yr)and seven old (69 ± 5 yr) male subjects were studied at rest andduring 20 min of submaximal cycling exercise at 50% of peak workcapacity. Norepinephrine spillover was determined by continuousintravenous infusion of[³H]norepinephrine.Arterial norepinephrine concentrations were significantly greater atrest for old vs. young subjects (280 ± 36 vs. 196 ± 27 ng/ml,respectively). Whereas total norepinephrine spillover did not differbetween groups at rest, hepatomesenteric norepinephrine spillover was50% greater in old subjects compared with their young counterparts (51 ± 7 vs. 34 ± 5 ng/min, respectively). Additionally,norepinephrine clearance rates at rest were significantly lower for theold subjects (23%). During exercise, plasmanorepinephrine concentrations increased compared with rest, with oldsubjects again demonstrating greater values than the young group.Hepatomesenteric norepinephrine spillover was significantly greater(+36%) during exercise for old subjects compared with young; however,no difference was found for whole body spillover rates between agegroups. Norepinephrine clearance rates remained depressed(30%) in the old subjects during exercise. Clearance ofepinephrine mirrored that for norepinephrine both at rest and duringexercise across age groups. It was concluded that in old subjects, areduction in norepinephrine clearance and an increase in regionalnorepinephrine spillover can account for the higher plasmanorepinephrine concentrations observed at rest. This relationship isnot exacerbated by the stress imposed during an acute bout of exercise.

     

Thermoregulation at rest and during exercise in prepubertal boys

Thermal balance was studied in 11 boys, aged 10-12 years, with various values for maximal oxygen uptake (VO2max), during two standardized sweating tests performed in a climatic chamber in randomized order. One of the tests consisted in a 90-min passive heat exposure [dry bulb temperature (Tdb) 45 degrees C] at rest. The second test was represented by a 60-min ergocycle exercise at 60% of individual VO2max (Tdb 20 degrees C). At rest, rectal temperature increased during heat exposure similar to observations made in adults, but the combined heat transfer coefficient reached higher values, reflecting greater radiative and convective heat gains in the children. Children also exhibited a greater increase in mean skin temperature, and a greater heat dissipation through sweating. Conversely, during the exercise sweating-test, although the increase in rectal temperature did not differ from that of adults for similar levels of exercise, evaporative heat loss was much lower in children, suggesting a greater radiative and convective heat loss due to the relatively greater body surface area. Thermophysiological reactions were not related to VO2max in children, in contrast to adults.     

Effects of acute hypoxia on renal and endocrine function at rest and during graded exercise in hydrated subjects.

Renal effects of altitude hypoxia are unclear. Renal and hormonal function was investigated in eight males at rest and during graded exercise at sea level (SL) and 48 h after rapid ascent to 4,350 m (HA). HA did not change resting values of effective renal plasma flow (ERPF), glomerular filtration rate (GFR), sodium clearance (CNa), urine flow, or lithium clearance (CLi), which was used as an index of proximal tubular outflow. At rest, HA increased plasma norepinephrine concentration and decreased plasma concentrations of renin and aldosterone. Exercise decreased ERPF similarly in both environments. Normoxic exercise progressively reduced GFR, but at HA GFR only decreased during heavy exercise. This resulted in a higher filtration fraction during light and moderate hypoxic exercise. However, calculated absolute proximal reabsorption rate (GFR-CLi) at HA was higher during low-intensity exercise, and there were no significant differences between exercise-induced decreases in CLi, CNa, and urine flow at HA compared with SL. Exercise gradually increased plasma norepinephrine, but values were higher at HA during light and moderate exercise. The small changes in the renal response to low-intensity hypoxic exercise may be secondary to increased adrenosympathetic activity. However, antidiuretic and antinatriuretic effects of exercise were maintained in hypoxia and in both environments seemed to be the consequence of decreased proximal tubular outflow. The results demonstrate that renal glomerular and tubular function is well preserved in acute hypoxia despite marked hormonal changes.     

Ventilatory and PaCO2 responses to voluntary and electrically induced leg exercise

We studied the role of central command mediation of exercise hyperpnea by comparing the ventilatory and arterial CO2 partial pressure (PaCO2) responses to voluntary (ExV) and electrically induced (ExE) muscle contractions in normal, awake human subjects. We hypothesized that if central command signals are critical to a normal ventilatory response, then ExE should cause a slower ventilatory response resulting in hypercapnia at the onset of exercise. ExE was induced through surface electrodes placed over the quadriceps and hamstring muscles. ExE and ExV produced leg extension (40/min) against a spring load that increased CO2 production (VCO2) 100-1,000 ml/min above resting level. PaCO2 and arterial pH during work transitions and in the steady state did not differ significantly from rest (P greater than 0.05) or between ExE and ExV. The temporal pattern of ventilation, tidal volume, breathing frequency, and inspired and expired times, and the ventilation-VCO2 relationship were similar between ExE and ExV. We conclude that since central command was reduced and/or eliminated by ExE, central command is not requisite for the precise matching of alveolar ventilation to increases in VCO2 during low-intensity muscle contractions.     

Effect of altitude relocations upon AaDo2 at rest and during exercise.

The supine pulmonary venous admixture (shunt) has been measured at Cerro de Pasco, 4,350 m altitude in eight subjects native to high altitude (HAN) under resting condition. Alveolar-arterial O2 tension difference (AaDO2) was also determined at rest and during exercise. The same subjects were studied again after 10 days' sojourn at sea level in Lima at 150 m altitude. They were compared with four subjects from sea level (SLN) who were studied first at Lima and after 2 and 10 days at Cerro de Pasco. At altitude, AaDO2 was smaller in HAN than SLN both at rest and during exercise. Shunt was the same in both groups. It is concluded that HAN show more even ventilation/perfusion relationship (VA/Q) at altitude, probably due to their high pulmonary artery pressure. On the contrary, SLN show less even VA/Q on altitude exposure, since their shunt decreased 37%. At sea level, HAN increased their AaDO2 due partially to an increase of 110% in their shunt, and in part due to less even VA/Q as shown by augmented VD/VT ratios. Each group tended to have a more effective gas exchange in its own environment.     

Endurance training increases gluconeogenesis during rest and exercise in men

Plasma albumin is well known to decrease in response to inflammation. The rate of albumin synthesis from both liver and plasma was measured in vivo by use of a large dose of L-[(2)H(3)-(14)C]valine in rats injected intravenously with live Escherichia coli and in pair-fed control rats during the acute-phase period (2 days postinfection). The plasma albumin concentration was reduced by 50% in infected rats compared with pair-fed animals. Infection induced a fall in both liver albumin mRNA levels and albumin synthesis relative to total liver protein synthesis. However, absolute liver albumin synthesis rate (ASR) was not affected by infection. In plasma, albumin fractional synthesis rate was increased by 50% in infected animals compared with pair-fed animals. The albumin ASR estimated in the plasma was similar in the two groups. These results suggest that hypoalbuminemia is not due to reduced albumin synthesis during sepsis. Moreover, liver and plasma albumin ASR were similar. Therefore, albumin synthesis measured in the plasma is a good indicator of liver albumin synthesis.     

Differences in lower-extremity muscular activation during walking between healthy older and young adults

Previous studies have identified differences in gait kinetics between healthy older and young adults. However, the underlying factors that cause these changes are not well understood. The objective of this study was to assess the effects of age and speed on the activation of lower-extremity muscles during human walking. We recorded electromyography (EMG) signals of the soleus, gastrocnemius, biceps femoris, medial hamstrings, tibialis anterior, vastus lateralis, and rectus femoris as healthy young and older adults walked over ground at slow, preferred and fast walking speeds. Nineteen healthy older adults (age, 73 ± 5 years) and 18 healthy young adults (age, 26 ± 3 years) participated. Rectified EMG signals were normalized to mean activities over a gait cycle at the preferred speed, allowing for an assessment of how the activity was distributed over the gait cycle and modulated with speed. Compared to the young adults, the older adults exhibited greater activation of the tibialis anterior and soleus during mid-stance at all walking speeds and greater activation of the vastus lateralis and medial hamstrings during loading and mid-stance at the fast walking speed, suggesting increased coactivation across the ankle and knee. In addition, older adults depend less on soleus muscle activation to push off at faster walking speeds. We conclude that age-related changes in neuromuscular activity reflect a strategy of stiffening the limb during single support and likely contribute to reduced push off power at fast walking speeds.