Enhanced metabolic response to caffeine in exercise-trained human subjects

The effect of caffeine on resting metabolic rate (RMR) was investigated in eight trained and eight nontrained young male subjects. The ingestion of 4 mg/kg caffeine produced a greater increase of RMR in trained subjects. This effect was associated with a greater increase in plasma free fatty acids and a larger fall in respiratory quotient, indicating an enhanced lipid oxidation following caffeine in exercise-trained subjects. An initial fall in plasma glucose was observed but only in trained subjects, and caffeine did not change plasma insulin in either group studied. Caffeine caused a significant fall in plasma norepinephrine and an increase in plasma epinephrine in both groups of subjects, but this action was significantly greater in trained subjects. It is suggested that the greater increase in RMR observed in trained subjects following caffeine ingestion is related to an enhanced lipid mobilization, possibly produced by a greater epinephrine secretion and by subsequent increased lipid oxidation.     

Characteristics of norepinephrine stimulated thermogenesis in undernourished subjects

Thermoregulatory thermogenesis was evaluated in chronically undernourished subjects as well as normally nourished controls by 60 min infusions of norepinephrine at a calculated dose of 0.15 μg/kg fat free mass/min. There was a significant (P< 0.05) reduction in the thermogenic response to norepinephrine in the undernourished labourers. When the characteristics of response were evaluated, the baseline and peak responses were comparable in both groups. However, the steepness of the response was greater (P< 0.05) in the undernourished subjects. The time to register a threshold response of 5 ml per min increase in oxygen consumption over the baseline after the start of infusion was longer in undernourished (about 20 min) as compared to the controls (about 10 min). It appears that, while the undernourished subjects have a reduced thermogenic component in their energy output, their peak capacity or potential for regulatory thermogenesis is the same as that in control subjects, since their thermogenic response has a greater slope. It is possible that chronically undernourished subjects may have a suppressed response over shorter periods of stimulation (less than 20 min), that is, before steady state thermogenic responses are achieved.     

Impaired fat-induced thermogenesis in obese subjects: the NUGENOB study

Objectives: To study energy expenditure before and 3 hours after a high‐fat load in a large cohort of obese subjects (n = 701) and a lean reference group (n = 113). Research Methods and Procedures: Subjects from seven European countries underwent a 1‐day clinical study with a liquid test meal challenge containing 95% fat (energy content was 50% of estimated resting energy expenditure). Fasting and 3‐hour postprandial energy expenditures, as well as metabolites and hormones, were determined. Results: Obese subjects had a reduced postprandial energy expenditure after the high‐fat load, independent of body composition, age, sex, research center, and resting energy expenditure, whereas within the obese group, thermogenesis increased again with increasing BMI category. Additionally, insulin resistance, habitual physical activity, postprandial plasma triacylglycerols, and insulin were all independently positively related to the postprandial energy expenditure. Resting energy expenditure, adjusted for fat‐free mass, increased with degree of obesity, a difference that disappeared after adjustment for fat mass. Furthermore, insulin resistance, fasting plasma free fatty acids, and cortisol were positively associated, whereas fasting plasma leptin and insulin‐like growth factor‐1 were negatively associated, with resting energy expenditure. Discussion: The 3‐hour fat‐induced thermogenic response is reduced in obesity. It remains to be determined whether this blunted thermogenic response is a contributory factor or an adaptive response to the obese state.     

Lipolytic response of adipocytes to epinephrine in sedentary and exercise-trained subjects: sex-related differences

Adipose tissue lipolytic activity is increased in endurance-trained subjects, but little is known about the mechanisms of this increase. To understand more fully the mechanisms involved and to discover whether sex-related differences exist, biopsies of fat were performed in the periumbilical region of 20 sedentary subjects (10 women (W) and 10 men (M)) and 20 trained subjects (10 W, 10 M); the in vitro response to epinephrine of the collagenase-isolated fat cells was studied. Glycerol release, chosen as an adipocyte lipolysis indicator, was measured by bioluminescence. Dose-response curves with epinephrine (alpha 2 and beta agonist), with isoproterenol (beta agonist) and epinephrine + propranolol and adenosine deaminase, were studied. Epinephrine-induced lipolysis was enhanced in trained subjects and this was due to an increased efficiency of the beta-adrenergic pathway. However, differences were found between the two sexes. In trained men, the lipolysis increase resulted from the enhancement of the beta-adrenergic pathway efficiency without any significant decrease in the alpha 2-adrenergic pathway efficiency. In trained women, the lipolysis increase was not only due to the enhancement of the beta-adrenergic pathway efficiency (which was greater than in trained men), but also to a significant decrease in the alpha 2-adrenergic pathway efficiency. Despite the decrease, the alpha 2-adrenergic pathway remained more efficient in trained women than in trained men, as was the case in sedentary subjects. It is concluded that endurance training led to better lipid mobilization and that this effect seemed greater in women than in men.     

Cholesterol absorption efficiency declines at moderate dietary doses in normal human subjects.

While unphysiologically large cholesterol doses are known to reduce percent cholesterol absorption, smaller amounts are reported to have no effect in human subjects. To determine the dose;-response relation between dietary cholesterol consumed and the efficiency of intestinal cholesterol absorption, we fed 18 normal subjects two test meals containing different amounts of natural cholesterol. In each test pentadeuterated cholesterol tracer was given orally, hexadeuterated cholesterol tracer was given intravenously, and the tracer ratio was measured in plasma 4 days later by gas chromatography/negative ion mass spectrometry. Baseline cholesterol absorption in the presence of 26 mg cholesterol tracer was 40.7 +/- 2.3%. This decreased by 4.9 percentage points (P = 0.05) when a total of 188 mg cholesterol was included in the meal and by 15.6 percentage points (P = 0.006) when 421 mg cholesterol was given, showing that the efficiency of cholesterol absorption declines appreciably even with modest increases in cholesterol dose. Considerable variation was noted in the response of different subjects and, on the higher cholesterol dose, dietary cholesterol absorption varied 5-fold from 40 mg to 212 mg. Fasting plasma insulin was correlated with the ability to absorb higher cholesterol doses without loss of efficiency (r(s) = 0.700, P = 0.036). Percent cholesterol absorption in a single meal is significantly influenced by the amount of cholesterol in that meal, suggesting that acute dietary factors influencing cholesterol absorption need further study.     

5'-AMP-activated protein kinase activity and subunit expression in exercise-trained human skeletal muscle.

5'-AMP-activated protein kinase (AMPK) has been proposed to be a pivotal factor in cellular responses to both acute exercise and exercise training. To investigate whether protein levels and gene expression of catalytic (alpha(1), alpha(2)) and regulatory (beta(1), beta(2), gamma(1), gamma(2), gamma(3)) AMPK subunits and exercise-induced AMPK activity are influenced by exercise training status, muscle biopsies were obtained from seven endurance exercise-trained and seven sedentary young healthy men. The alpha(1)- and alpha(2)-AMPK mRNA contents in trained subjects were both 117 +/- 2% of that in sedentary subjects (not significant), whereas mRNA for gamma(3) was 61 +/- 1% of that in sedentary subjects (not significant). The level of alpha(1)-AMPK protein in trained subjects was 185 +/- 34% of that in sedentary subjects (P < 0.05), whereas the levels of the remaining subunits (alpha(2), beta(1), beta(2), gamma(1), gamma(2), gamma(3)) were similar in trained and sedentary subjects. At the end of 20 min of cycle exercise at 80% of peak O(2) uptake, the increase in phosphorylation of alpha-AMPK (Thr(172)) was blunted in the trained group (138 +/- 38% above rest) compared with the sedentary group (353 +/- 63% above rest) (P < 0.05). Acetyl CoA-carboxylase beta-phosphorylation (Ser(221)), which is a marker for in vivo AMPK activity, was increased by exercise in both groups but to a lower level in trained subjects (32 +/- 5 arbitrary units) than in sedentary controls (45 +/- 1 arbitrary units) (P < 0.01). In conclusion, trained human skeletal muscle has increased alpha(1)-AMPK protein levels and blunted AMPK activation during exercise.     

Diminished NO release in chronic hypoxic human endothelial cells

The present study addressed whether chronic hypoxia is associated with reduced nitric oxide (NO) release due to decreased activation of endothelial NO synthase (eNOS). Primary cultures of endothelial cells from human umbilical veins (HUVECs) were used and exposed to different oxygen levels for 24 h, after which NO release, intracellular calcium, and eNOS activity and phosphorylation were measured after 24 h. Direct measurements using a NO microsensor showed that in contrast to 1-h exposure to 5% and 1% oxygen (acute hypoxia), histamine-evoked (10 microM) NO release from endothelial cells exposed to 5% and 1% oxygen for 24 h (chronic hypoxia) was reduced by, respectively, 58% and 40%. Furthermore, chronic hypoxia also lowered the amount and activity of eNOS enzyme. The decrease in activity could be accounted for by reduced intracellular calcium and altered eNOS phosphorylation. eNOS Ser(1177) and eNOS Thr(495) phosphorylations were reduced and increased, respectively, consistent with lowered enzyme activity. Akt kinase, which can phosphorylate eNOS Ser(1177), was also decreased by hypoxia, regarding both total protein content and the phosphorylated (active) form. Moreover, the protein content of beta- actin, which is known to influence the activity of eNOS, was almost halved by hypoxia, further supporting the fall in eNOS activity. In conclusion, chronic hypoxia in HUVECs reduces histamine-induced NO release as well as eNOS expression and activity. The decreased activity is most likely due to changed eNOS phosphorylation, which is supported by decreases in Akt expression and phosphorylation. By reducing NO, chronic hypoxia may accentuate endothelial dysfunction in cardiovascular disease.     

Muscle mitochondrial ultrastructure in exercise-trained iron-deficient rats

To investigate effects of endurance training and iron deficiency, as well as the combination of these two conditions, on mitochondrial ultrastructure, weanling rats at 3 wk of age were assigned to iron-deficient (Fe-) and iron-sufficient (Fe+) groups. Subsequently, groups were subdivided into exercise-trained (T) and sedentary (S) groups. Electron microscopy showed subsarcolemmal and intrafibrillar mitochondria in the Fe-T animals to be enlarged with sparse cristae and vacuole-like areas compared with the other groups. An increase in the number of lipid droplets in both Fe- groups was observed. Stereological measurements revealed a 99% increase in the volume occupied by muscle mitochondria in the Fe-T animals (11.9 +/- 0.8%) over the Fe+T (5.9 +/- 0.4%) and Fe+S (6.0 +/- 0.3%) groups and a 55% increase over the Fe-S groups (7.7 +/- 0.3%). The ratio of mitochondrial surface area to tissue volume was significantly decreased only in the Fe-T group. These results indicate that the combined stresses of iron deficiency and training produce mitochondrial ultrastructural changes far greater than those of iron deficiency or training alone. Because this is also the case with the disproportion among mitochondrial enzymes, it is possible that the ultrastructural changes are indicative of morphological responses that maintain ATP turnover during exercise in iron deficiency when oxygen transport and electron transport chain activities are reduced.     

Disruption of estrous cycles in exercise-trained rats

The female Sprague-Dawley rat was evaluated as an animal model for the menstrual irregularities that are common in women athletes. Daily vaginal smears revealed that estrous cycles were markedly disrupted in rats during a 10-week exercise training program, while cycles remained normal in sedentary rats. Compared to 9 sedentary rats, the 10 exercise-trained rats had longer mean cycle lengths and fewer estrus smears. Six of the exercise-trained rats, but none of the sedentary rats, had an "anestrus period" with more than twice the normal interval between estrus smears; one exercise-trained rat became essentially acyclic. Weight gain during the 10-week training program was lower in exercise-trained rats than in sedentary rats. Colonic temperatures, monitored at rest and during 30 min of exercise, were slightly lower in exercise-trained rats with irregular estrous cycles than in exercise-trained rats with regular cycles, indicating that unusually elevated body temperatures during exercise are not responsible for exercise-related reproductive acyclicity. It is concluded that the female Sprague-Dawley rat may be a useful animal model for the study of menstrual irregularities associated with exercise training.     

Recombinant human uncoupling protein-3 increases thermogenesis in yeast cells

The long form of human uncoupling protein-3 (hUCP3L) is highly homologous to thermogenin (UCPI), the uncoupling protein of brown fat mitochondria, but is expressed predominantly in skeletal muscle. Its putative role is to regulate the coupling efficiency of oxidative phosphorylation and thus thermogenesis in skeletal muscle, a major thermogenic tissue in higher mammals. To study the functional relevance of hUCP3L, the protein was expressed in yeast cells under the control of the galactose promoter. Expression of hUCP3L induced a series of phenotype changes in the yeast cells. The cellular growth and the mitochondrial membrane potential were both diminished. The portion of cellular respiration coupled to oxidative phosphorylation decreased from 57% to 11% (P<0.001) and the cellular heat production, as measured by direct microcalorimetry, was increased by 33.3 +/- 3.2% (P<0.001) after induction of UCP3L. These observations demonstrate for the first time the intrinsic thermogenic properties of hUCP3L in intact cells.     

Evidence of O2 supply-dependent VO2 max in the exercise-trained human quadriceps.

Maximal O2 delivery and O2 uptake (VO2) per 100 g of active muscle mass are far greater during knee extensor (KE) than during cycle exercise: 73 and 60 ml. min-1. 100 g-1 (2.4 kg of muscle) (R. S. Richardson, D. R. Knight, D. C. Poole, S. S. Kurdak, M. C. Hogan, B. Grassi, and P. D. Wagner. Am. J. Physiol. 268 (Heart Circ. Physiol. 37): H1453-H1461, 1995) and 28 and 25 ml. min-1. 100 g-1 (7.5 kg of muscle) (D. R. Knight, W. Schaffartzik, H. J. Guy, R. Predilleto, M. C. Hogan, and P. D. Wagner. J. Appl. Physiol. 75: 2586-2593, 1993), respectively. Although this is evidence of muscle O2 supply dependence in itself, it raises the following question: With such high O2 delivery in KE, are the quadriceps still O2 supply dependent at maximal exercise? To answer this question, seven trained subjects performed maximum KE exercise in hypoxia [0.12 inspired O2 fraction (FIO2)], normoxia (0.21 FIO2), and hyperoxia (1.0 FIO2) in a balanced order. The protocol (after warm-up) was a square wave to a previously determined maximum work rate followed by incremental stages to ensure that a true maximum was achieved under each condition. Direct measures of arterial and venous blood O2 concentration in combination with a thermodilution blood flow technique allowed the determination of O2 delivery and muscle VO2. Maximal O2 delivery increased with inspired O2: 1.3 +/- 0.1, 1.6 +/- 0.2, and 1.9 +/- 0.2 l/min at 0.12, 0.21, and 1.0 FIO2, respectively (P < 0.05). Maximal work rate was affected by variations in inspired O2 (-25 and +14% at 0.12 and 1.0 FIO2, respectively, compared with normoxia, P < 0.05) as was maximal VO2 (VO2 max): 1.04 +/- 0.13, 1. 24 +/- 0.16, and 1.45 +/- 0.19 l/min at 0.12, 0.21, and 1.0 FIO2, respectively (P < 0.05). Calculated mean capillary PO2 also varied with FIO2 (28.3 +/- 1.0, 34.8 +/- 2.0, and 40.7 +/- 1.9 Torr at 0.12, 0.21, and 1.0 FIO2, respectively, P < 0.05) and was proportionally related to changes in VO2 max, supporting our previous finding that a decrease in O2 supply will proportionately decrease muscle VO2 max. As even in the isolated quadriceps (where normoxic O2 delivery is the highest recorded in humans) an increase in O2 supply by hyperoxia allows the achievement of a greater VO2 max, we conclude that, in normoxic conditions of isolated KE exercise, KE VO2 max in trained subjects is not limited by mitochondrial metabolic rate but, rather, by O2 supply.     

Baroreflex function in endurance- and static exercise-trained men

The effect of exercise training mode on reflex cardiovascular control was studied in a cross-sectional design. We examined the cardiovascular responses to progressive incremental phenylephrine (PE) infusion to maximal rates of 120 micrograms/min and the delta heart rate/delta blood pressure responses to lower body negative pressure (LBNP) to -50 Torr in 30 men who were either endurance exercise trained (ET), untrained (UT), or weight trained (WT). During PE infusion, measures of blood pressures, forearm blood flow, heart rate and cardiac output, and calculations of forearm vascular resistance, stroke volume, and peripheral vascular resistance were made at each infusion rate when steady-state blood pressure was attained. No significant differences (P less than 0.05) in forearm blood flow or resistance were observed between the groups at any dose of PE, suggesting that the vasoconstrictor response was similar among the groups. Regression analyses of heart rate against mean blood pressure during the PE infusion were performed to evaluate baroreflex function. A linear model was used and correlation coefficients ranging from 0.82 to 0.96 were obtained (P less than 0.05). The slope of the line of best fit for the ET subjects (-0.57) was significantly less (P less than 0.05) than the slopes obtained for either the UT (-0.91) or WT (-0.88) subjects. In addition, the delta heart rate/delta blood pressure measurements obtained during LBNP reflected a similarly significant attenuation of reflex chronotropic control in the ET subjects.(ABSTRACT TRUNCATED AT 250 WORDS)     

Adaptive thermogenesis by dietary n-3 polyunsaturated fatty acids: Emerging evidence and mechanisms

Brown/beige fat plays a crucial role in maintaining energy homeostasis through non-shivering thermogenesis in response to cold temperature and excess nutrition (adaptive thermogenesis). Although numerous molecular and genetic regulators have been identified, relatively little information is available regarding thermogenic dietary molecules. Recently, a growing body of evidence suggests that high consumption of n-3 polyunsaturated fatty acids (PUFA) or activation of GPR120, a membrane receptor of n-3 PUFA, stimulate adaptive thermogenesis. In this review, we summarize the emerging evidence that n-3 PUFA promote brown/beige fat formation and highlight the potential mechanisms whereby n-3 PUFA require GPR120 as a signaling platform or act independently. Human clinical trials are revisited in the context of energy expenditure. Additionally, we explore some future perspective that n-3 PUFA intake might be a useful strategy to boost or sustain metabolic activities of brown/beige fat at different lifecycle stages of pregnancy and senescence. Given that a high ratio of n-6/n-3 PUFA intake is associated with the development of obesity and type 2 diabetes, understanding the impact of n-6/n-3 ratio on energy expenditure and adaptive thermogenesis will inform the implementation of a novel nutritional strategy for preventing obesity.