Role of pulsatile growth hormone (GH) secretion in the regulation of lipolysis in fasting humans

Background

The increase in growth hormone (GH) secretion during a prolonged fast stimulates lipolytic rate, thereby augmenting the mobilization of endogenous energy at a time when fuel availability is very low.

Study aim

To identify the specific component of GH secretory pattern responsible for the stimulation of lipolytic rate during fasting in humans.

Study protocol

We measured lipolytic rate (using stable isotope dilution technique) after an overnight fast in 15 young, healthy, non-obese subjects (11 men and 4 women), and again on four separate occasions after a 59 h fast. These four prolonged fasting trials differed only by the contents of an infusion solution provided throughout the 59 h fasting period. Subjects were infused either with normal saline (“Control”; n = 15) or with graded doses of a GH Releasing Hormone Receptor Antagonist (GHRHa):10 μg/kg/h (“High”; n = 15), 1 μg /kg/h (“Medium”; n = 8), or 0.5 μg /kg/h (“Low”; n = 6).

Results

As expected, the 59 h fast completely suppressed plasma insulin levels and markedly increased endogenous GH concentrations (12 h vs 59 h Fast; p = 0.0044). Administration of GHRHa induced dose-dependent reduction in GH concentrations in response to the 59 h fast (p < 0.05). We found a strong correlation between the rate of lipolysis and GH mean peak amplitude (R = 0.471; p = 0.0019), and total GH pulse area under the curve (AUC) (R = 0.49; p = 0.0015), but not the GH peak frequency (R = 0.044; p = 0.8) or interpulse GH concentrations (R = 0.25; p = 0.115).

Conclusion

During prolonged fasting (i.e., 2–3 days), when insulin secretion is abolished, the pulsatile component of GH secretion becomes a key metabolic regulator of the increase in lipolytic rate.

Inflammation and metabolism gene sets in subcutaneous abdominal adipose tissue are altered 1 hour after exercise in adults with obesity

Although the health benefits of exercise in adults with obesity are well described, the direct effects of exercise on adipose tissue that may lead to improved metabolic health are poorly understood. The primary aims of this study were to perform an unbiased analysis of the subcutaneous abdominal adipose tissue transcriptomic response to acute exercise in adults with obesity, and to compare the effects of moderate-intensity continuous exercise versus high-intensity interval exercise on this response. Twenty-nine adults with obesity performed a session of either high-intensity interval exercise (HI; 10 × 1 min at 90%HRpeak, 1 min recovery between intervals; n = 14) or moderate-intensity continuous exercise (MI; 45 min at 70%HRpeak; n = 15). Groups were well matched for BMI (HI 33 ± 3 vs. MI 33 ± 4 kg/m²), sex (HI: 9 women vs. MI: 10 women), and age (HI: 32 ± 6 vs. MI: 29 ± 5). Subcutaneous adipose tissue was collected before and 1 h after the session of HI or MI, and samples were processed for RNA sequencing. Gene set enrichment analysis revealed 7 of 21 gene sets enriched postexercise overlapped between HI and MI. Interestingly, both HI and MI upregulated gene sets involved in inflammation (IL6-JAK-STAT3 signaling, allograft rejection, TNFα signaling via NFκB, and inflammatory response; FDR q value < 0.25). Exercise also downregulated adipogenic and oxidative metabolism gene sets in both groups. Overall, these data suggest genes involved in subcutaneous adipose tissue metabolism and inflammation may be an important part of the initial response after a session of exercise.NEW & NOTEWORTHY This study compared the effects of a single session of high-intensity interval exercise versus moderate-intensity continuous exercise on transcriptional changes in subcutaneous abdominal adipose tissue collected from adults with obesity. Our novel findings indicate exercise upregulated inflammation-related gene sets, while it downregulated metabolism-related gene sets - after both high-intensity and moderate-intensity exercise. These data suggest exercise can alter the adipose tissue transcriptome 1 h after exercise in ways that may impact inflammation and metabolism.

Relatively low endogenous fatty acid mobilization and uptake helps preserve insulin sensitivity in obese women

Background

Although obesity is commonly linked with metabolic disease risk, some obese adults do not develop metabolic abnormalities, such as insulin resistance.

Objectives

The primary aim of this study was to determine whether alterations in fatty acid mobilization and uptake underlie differences in insulin sensitivity (S_i) among a seemingly homogeneous cohort of obese women.

Methods

Insulin sensitivity (FSIVGTT), basal fatty acid rate of disappearance from plasma (Rd), resting whole-body fat oxidation, intramyocellular triacylglycerol (IMTG) concentration, and markers of skeletal muscle inflammation were measured in 21 obese women. Participants were divided into tertiles based on their S_i. The subset of participants with the lowest S_i (LOW-S_i; S_i ≤2.1 (mU/L)⁻¹·min⁻¹; n=7) was compared with the subset of participants with the highest S_i, who exhibited relatively normal insulin sensitivity (NORM-S_i; S_i ≥3.4 (mU/L)⁻¹·min⁻¹; n=8).

Results

Despite nearly identical physical characteristics in LOW-S_ivs. NORM-S_i (BMI: 34±2 vs. 34±1 kg/m²; %body fat: 48±1% vs. 47±1%; waist circumference: 104±2 vs. 104±2 cm; VO₂max: 2.2±0.2 vs. 2.3±0.1 L/min), fatty acid Rd was nearly 30% lower in NORM (P=0.02). Importantly, the greater rate of fatty acid uptake in LOW-S_ivs. NORMS_i did not translate to higher rate of fat oxidation (3.5±0.2 vs. 3.7±0.2 μmol/kg/min) or to a measureable difference in IMTG content, (68.3±12.7 vs. 63.7±6.7 μmol/g dry weight). In conjunction with the lower fatty acid Rd in NORM-S_ivs. LOW-S_i, activation of inflammatory pathways known to impair insulin action in skeletal muscle was also lower (i.e. lower phosphorylated JNK, higher IκBα abundance). In contrast, LOW-S_i and NORM-S_i exhibited no differences in plasma markers of inflammation (i.e. TNFα, IL-6, MCP-1).

Conclusion

These findings suggest that obese women who maintain a relatively low rate of endogenous fatty acid uptake may be somewhat “protected” against the development of insulin resistance potentially by less activation of inflammatory pathways within skeletal muscle.

Regulation of plasma ceramide levels with fatty acid oversupply: evidence that the liver detects and secretes de novo synthesised ceramide

AIMS/HYPOTHESIS

Plasma ceramide concentrations correlate with insulin sensitivity, inflammation and atherosclerotic risk. We hypothesised that plasma ceramide concentrations are increased in the presence of elevated fatty acid levels and are regulated by increased liver serine C-palmitoyltransferase (SPT) activity.

METHODS

Lean humans and rats underwent an acute lipid infusion and plasma ceramide levels were determined. One group of lipid-infused rats was administered myriocin to inhibit SPT activity. Liver SPT activity was determined in lipid-infused rats, and obese, insulin resistant mice. The time and palmitate dose-dependent synthesis of intracellular and secreted ceramide was determined in HepG2 liver cells.

RESULTS

Plasma ceramide levels were increased during lipid infusion in humans and rats, and in obese, insulin-resistant mice. The increase in plasma ceramide was not associated with changes in liver SPT activity, and inhibiting SPT activity by ~50% did not alter plasma ceramide levels in lipid-infused rats. In HepG2 liver cells, palmitate incorporation into extracellular ceramide was both dose- and time-dependent, suggesting the liver cells rapidly secreted the newly synthesised ceramide.

CONCLUSIONS/INTERPRETATION

Elevated systemic fatty acid availability increased plasma ceramide but this was not associated with changes in hepatic SPT activity, suggesting that liver ceramide synthesis is driven by substrate availability rather than increased SPT activity. This report also provides evidence that the liver is sensitive to the intracellular ceramide concentration, and an increase in liver ceramide secretion may help protect the liver from the deleterious effects of intracellular ceramide accumulation.

Gender differences in lipid and glucose kinetics during short-term fasting

Data obtained from studies conducted in animal models and humans suggest that gender affects the metabolic response to fasting. However, differences in body composition between males and females confound the interpretation of these studies, because increased adiposity itself alters the metabolic response to short-term fasting. We evaluated whole body lipid and glucose kinetics during basal (14-h fast) and short-term fasting (22-h fast) conditions in six women and six men who were matched for adiposity (24 +/- 2 and 23 +/- 2% body wt as fat, respectively). Substrate kinetics were measured by infusing stable isotope labeled tracers of glucose ([(2)H(2)]glucose) and glycerol ([(2)H(5)]glycerol). Basal glycerol rate of appearance (R(a)) in plasma, an indicator of whole body lipolytic rate, was greater in women than in men (2.1 +/- 0.2 vs. 1.5 +/- 0.1 micromol x kg body wt(-1) x min(-1); P < 0.05). However, the relative increase in glycerol R(a) with continued fasting was blunted in women compared with men (40 +/- 7 vs. 80 +/- 4% increase; P < 0.05), resulting in similar lipolytic rates in both genders at 22 h (2.8 +/- 0.2 and 2.6 +/- 0.1 micromol x kg body wt(-1) x min(-1) for women and men, respectively). In contrast, glucose R(a) was similar in men and women at 14 h (11 +/- 0.6 vs. 12 +/- 0.7 micromol x kg body wt(-1) x min(-1) and 22 h of fasting (9 +/- 0.6 vs 10 +/- 0.6 micromol x kg body wt(-1) x min(-1). These data demonstrate the presence of sexual dimorphism in lipid, but not glucose, metabolism during both basal and short-term fasting conditions.

Effect of short-term fasting on free and bound leptin concentrations in lean and obese women

Plasma leptin exists in protein-bound and free forms, which may affect its hormonal bioactivity. Therefore, the relationship between bound and free leptin may be particularly important during physiological conditions that cause rapid alterations in total plasma leptin concentration, such as fasting. The purpose of this study was to evaluate the effect of short-term fasting on bound and free plasma leptin concentrations and leptin binding capacity (a measure of plasma leptin-binding protein content) in lean and obese women. Six lean (body mass index, 21 +/- 1 kg/m2) and 6 abdominally obese (BMI, 36 +/- 1 kg/m2) women were studied after 14 h and 22 h of fasting. Although total plasma leptin concentration was more than 6-fold greater in obese (45.4 +/- 7.6 microg/liter) compared with lean (7.4 +/- 1.0 microg/liter) women at 14 h of fasting (P < 0.05), the percentage of leptin in the bound form was greater in lean than obese subjects (29 +/- 2% vs. 12 +/- 3%; P < 0.05). Arterial total, free, and bound plasma leptin concentrations all declined between 14 h and 22 h of fasting in both lean and obese groups, but the relative decline of these fractions was greater in lean (36 +/- 4%, 60 +/- 9%, and 51 +/- 13%, respectively) than in obese (19 +/- 5%, 21 +/- 8%, and 12 +/- 7%, respectively) subjects (all P < 0.05). In contrast, leptin binding capacity was unchanged. The percentage of total plasma leptin present in bound form was constant between 14 h and 22 h of fasting in lean subjects and increased slightly but significantly in obese subjects. These data demonstrate that both free and bound fractions of leptin in plasma decrease quickly in response to energy restriction, but the decline is blunted in abdominally obese compared with lean women. In addition, the equilibrium between bound and free leptin fractions is maintained during brief fasting and is not regulated by leptin binding capacity.

Regulation of lipid mobilization and oxidation during exercise in obesity

Regulation of lipid mobilization and oxidation during exercise in obesity. Exerc. Sport Sci. Rev. Vol. 29, No. 1, pp 42-46, 2001. Obesity is associated with metabolic disorders that may be related to alterations in lipid mobilization and oxidation. Although exercise is essential for successful weight management, the regulation of fatty acid metabolism during exercise in obesity is unclear. This review discusses factors that regulate fat metabolism during exercise and the effects of endurance training on these responses.

Whole-body and adipose tissue glucose metabolism in response to short-term fasting in lean and obese women

BACKGROUND

Alterations in glucose metabolism during early fasting may be an important trigger of the hormonal and metabolic responses to fasting.

OBJECTIVE

The purpose of this study was to determine whether glucose metabolism in response to brief starvation differs in lean and abdominally obese women.

DESIGN

We evaluated whole-body glucose metabolism by use of stable-isotope tracer methods and glucose uptake in subcutaneous abdominal adipose tissue by use of arteriovenous balance in 7 lean [58 +/- 2 kg; body mass index (BMI; in kg/m(2)): 21 +/- 5] and 6 abdominally obese (96 +/- 2 kg; BMI: 36 +/- 1) women after 14 and 22 h of fasting.

RESULTS

Between 14 and 22 h of fasting, whole-body glucose production and disposal declined in both groups (P < 0.05), but the reduction was 50% greater in lean than in obese women (P < 0.05). The decline in glucose uptake at 22 h of fasting was also lower in obese (0.11 +/- 0.04 micromol*100 g(-1) x min(-1)) than in lean (0.26 +/- 0.03 micromol x 100 g(-1) x min(-1)) women (P < 0.05). Decreases in plasma insulin and leptin concentrations between 14 and 22 h of fasting were also lower in obese than in lean women (insulin: 20 +/- 3% and 32 +/- 5%; leptin: 18 +/- 3% and 37 +/- 6%; both P < 0.05).

CONCLUSIONS

The normal decline in glucose production and uptake that occurs during early fasting is blunted in women with abdominal obesity. These alterations in glucose metabolism are associated with a blunted decline in circulating concentrations of both insulin and leptin, which may explain some of the differences in the metabolic response to fasting observed between lean and abdominally obese persons.

Oxidation of nonplasma fatty acids during exercise is increased in women with abdominal obesity

We evaluated plasma fatty acid availability and plasma and whole body fatty acid oxidation during exercise in five lean and five abdominally obese women (body mass index = 21 +/- 1 vs. 38 +/- 1 kg/m(2)), who were matched on aerobic fitness, to test the hypothesis that obesity alters the relative contribution of plasma and nonplasma fatty acids to total energy production during exercise. Subjects exercised on a recumbent cycle ergometer for 90 min at 54% of their peak oxygen consumption. Stable isotope tracer methods ([(13)C]palmitate) were used to measure fatty acid rate of appearance in plasma and the rate of plasma fatty acid oxidation, and indirect calorimetry was used to measure whole body substrate oxidation. During exercise, palmitate rate of appearance increased progressively and was similar in obese and lean groups between 60 and 90 min of exercise [3.9 +/- 0.4 vs. 4.0 +/- 0.3 micromol. kg fat free mass (FFM)(-1). min(-1)]. The rate of plasma fatty acid oxidation was also similar in obese and lean subjects (12.8 +/- 1.7 vs. 14.5 +/- 1.8 micromol. kg FFM(-1). min(-1); P = not significant). However, whole body fatty acid oxidation during exercise was 25% greater in obese than in lean subjects (21.9 +/- 1.2 vs. 17.5 +/- 1.6 micromol. kg FFM(-1). min(-1); P < 0.05). These results demonstrate that, although plasma fatty acid availability and oxidation are similar during exercise in lean and obese women, women with abdominal obesity use more fat as a fuel by oxidizing more nonplasma fatty acids.

Racial differences in lipid metabolism in women with abdominal obesity

We evaluated palmitate rate of appearance (R(a)) in plasma during basal conditions and during a four-stage epinephrine infusion plus pancreatic hormonal clamp in nine white and nine black women with abdominal obesity, who were matched on fat-free mass, total and percent body fat, and waist-to-hip circumference ratio. On the basis of single-slice magnetic resonance imaging analysis, black women had the same amount of subcutaneous abdominal fat but less intra-abdominal fat than white women (68 +/- 9 vs. 170 +/- 14 cm(2), P < 0.05). Basal palmitate R(a) was lower in black than in white women (1.95 +/- 0.26 vs. 2.88 +/- 0.23 micromol. kg fat-free mass(-1). min(-1), P < 0.005), even though plasma insulin and catecholamine concentrations were the same in both groups. Palmitate R(a) across a physiological range of plasma epinephrine concentrations remained lower in black women, because the increase in palmitate R(a) during epinephrine infusion was the same in both groups. We conclude that basal and epinephrine-stimulated palmitate R(a) is lower in black than in white women with abdominal obesity. The differences in basal palmitate kinetics are not caused by alterations in plasma insulin or catecholamine concentrations or lipolytic sensitivity to epinephrine. The lower rate of whole body fatty acid flux and smaller intra-abdominal fat mass may have clinical benefits because of the relationship between excessive fatty acid availability and metabolic diseases.

Lipid metabolism during endurance exercise

Endogenous triacylglycerols represent an important source of fuel for endurance exercise. Triacylglycerol oxidation increases progressively during exercise; the specific rate is determined by energy requirements of working muscles, fatty acid delivery to muscle mitochondria, and the oxidation of other substrates. The catecholamine response to exercise increases lipolysis of adipose tissue triacylglycerols and, presumably, intramuscular triacylglycerols. In addition, increases in adipose tissue and muscle blood flow decrease fatty acid reesterification and facilitate the delivery of released fatty acids to skeletal muscle. Alterations in fatty acid mobilization and the relative use of adipose and intramuscular triacylglycerols during exercise depend, in large part, on degree of fitness and exercise intensity. Compared with untrained persons exercising at the same absolute intensity, persons who have undergone endurance training have greater fat oxidation during exercise without increased lipolysis. Available evidence suggests that the training-induced increase in fat oxidation is due primarily to increased oxidation of non-plasma-derived fatty acids, perhaps from intramuscular triacylglycerol stores. Fat oxidation is lower in high-intensity exercise than in moderate-intensity exercise, in part because of decreased fatty acid delivery to exercising muscles. Parenteral lipid supplementation during high-intensity exercise increases fat oxidation, but the effect of ingesting long-chain or medium-chain triacylglycerols on substrate metabolism during exercise is less clear. This review discusses the relation between fatty acid mobilization and oxidation during exercise and the effect of endurance training, exercise intensity, and lipid supplementation on these responses.

Effect of endurance training on lipid metabolism in women: a potential role for PPARalpha in the metabolic response to training

Endurance training increases fatty acid oxidation (FAO) and skeletal muscle oxidative capacity. However, the source of the additional fat and the mechanisms for increasing FAO capacity in muscle are not clear. We measured whole body and regional lipolytic activity and whole body and plasma FAO in six lean women during 90 min of bicycling exercise (50% pretraining peak O(2) consumption) before and after 12 wk of endurance training. We also assessed skeletal muscle content of peroxisome proliferator-activated receptor-alpha (PPARalpha) and its target proteins that regulate FAO [medium-chain and very long chain acyl-CoA dehydrogenase (MCAD and VLCAD)]. Despite a 25% increase in whole body FAO during exercise after training (P < 0.05), training did not alter regional adipose tissue lipolysis (abdominal: 0.56 +/- 0.26 and 0.57 +/- 0.10 micromol x 100 g(-1) x min(-1); femoral: 0.13 +/- 0.07 and 0.09 +/- 0.02 micromol x 100 g(-1) x min(-1)), whole body palmitate rate of appearance in plasma (168 +/- 18 and 150 +/- 25 micromol/min), and plasma FAO (554 +/- 61 and 601 +/- 45 micromol/min). However, training doubled the levels of muscle PPARalpha, MCAD, and VLCAD. We conclude that training increases the use of nonplasma fatty acids and may enhance skeletal muscle oxidative capacity by PPARalpha regulation of gene expression.

Effect of pinitol treatment on insulin action in subjects with insulin resistance

OBJECTIVE

Endogenous low-molecular-weight glycans containing pinitol (3-O-methyl-D-chiro-inositol) and D-chiro-inositol are thought to mediate certain actions of insulin. We tested the hypothesis that oral administration of soybean-derived pinitol would improve insulin sensitivity in obese subjects (BMI = 36.6 kg/m2) with diet-treated type 2 diabetes or glucose intolerance (HbA1c = 6.8%).

RESEARCH DESIGN AND METHODS

There were 22 subjects randomized to receive either pinitol 20 mg x kg(-1) x day(-1) (n = 12) or placebo (n = 10) in a 28-day double-blinded trial.

RESULTS

No toxicity due to the pinitol was observed during the study The sensitivity of glucose and lipid metabolism to insulin were assessed by measurement of whole-body glucose, palmitate, and glycerol kinetics during basal conditions and a hyperinsulinemic-euglycemic clamp. Metabolic measurements were made at baseline and again at the end of the study Final plasma levels of pinitol were 48-fold (1.06 +/- 0.15 vs. 0.02 +/- 0.01 micromol/l, P < 0.0001) and D-chiro-inositol levels 14-fold (0.56 +/- 0.08 vs. 0.04 +/- 0.02 micromol/l, P < 0.0001) greater in the pinitol group compared with placebo.

CONCLUSIONS

Four weeks of pinitol treatment did not alter baseline glucose production, insulin-mediated glucose disposal, or rates of appearance of free fatty acids and glycerol in plasma. We conclude that plasma levels of both pinitol and D-chiro-inositol are very responsive to pinitol ingestion, but insulin sensitivity does not increase after pinitol treatment in individuals with obesity and mild type 2 diabetes.

Whole body and abdominal lipolytic sensitivity to epinephrine is suppressed in upper body obese women

We measured whole body and regional lipolytic and adipose tissue blood flow (ATBF) sensitivity to epinephrine in 8 lean [body mass index (BMI): 21 +/- 1 kg/m(2)] and 10 upper body obese (UBO) women (BMI: 38 +/- 1 kg/m(2); waist circumference >100 cm). All subjects underwent a four-stage epinephrine infusion (0.00125, 0.005, 0.0125, and 0.025 microgram. kg fat-free mass(-1). min(-1)) plus pancreatic hormonal clamp. Whole body free fatty acid (FFA) and glycerol rates of appearance (R(a)) in plasma were determined by stable isotope tracer methodology. Abdominal and femoral subcutaneous adipose tissue lipolytic activity was determined by microdialysis and (133)Xe clearance methods. Basal whole body FFA R(a) and glycerol R(a) were both greater (P < 0.05) in obese (449 +/- 31 and 220 +/- 12 micromol/min, respectively) compared with lean subjects (323 +/- 44 and 167 +/- 21 micromol/min, respectively). Epinephrine infusion significantly increased FFA R(a) and glycerol R(a) in lean (71 +/- 21 and 122 +/- 52%, respectively; P < 0.05) but not obese subjects (7 +/- 6 and 39 +/- 10%, respectively; P = not significant). In addition, lipolytic and ATBF sensitivity to epinephrine was blunted in abdominal but not femoral subcutaneous adipose tissue of obese compared with lean subjects. We conclude that whole body lipolytic sensitivity to epinephrine is blunted in women with UBO because of decreased sensitivity in upper body but not lower body subcutaneous adipose tissue.

Leptin production during early starvation in lean and obese women

We evaluated abdominal adipose tissue leptin production during short-term fasting in nine lean [body mass index (BMI) 21 +/- 1 kg/m(2)] and nine upper body obese (BMI 36 +/- 1 kg/m(2)) women. Leptin kinetics were determined by arteriovenous balance across abdominal subcutaneous adipose tissue at 14 and 22 h of fasting. At 14 h of fasting, net leptin release from abdominal adipose tissue in obese subjects (10.9 +/- 1.9 ng x 100 g tissue x (-1) x min(-1)) was not significantly greater than the values observed in the lean group (7.6 +/- 2.1 ng x 100 g(-1) x min(-1)). Estimated whole body leptin production was approximately fivefold greater in obese (6.97 +/- 1.18 microg/min) than lean subjects (1.25 +/- 0.28 microg/min) (P < 0.005). At 22 h of fasting, leptin production rates decreased in both lean and obese groups (to 3.10 +/- 1.31 and 10.5 +/- 2.3 ng x 100 g adipose tissue(-1) x min(-1), respectively). However, the relative declines in both arterial leptin concentration and local leptin production in obese women (arterial concentration 13.8 +/- 4.4%, local production 10.0 +/- 12.3%) were less (P < 0.05 for both) than the relative decline in lean women (arterial concentration 39.0 +/- 5.5%, local production 56.9 +/- 13.0%). This study demonstrates that decreased leptin production accounts for the decline in plasma leptin concentration observed after fasting. However, compared with lean women, the fasting-induced decline in leptin production is blunted in women with upper body obesity. Differences in leptin production during fasting may be responsible for differences in the neuroendocrine response to fasting previously observed in lean and obese women.

Preexercise medium-chain triglyceride ingestion does not alter muscle glycogen use during exercise

This investigation determined whether ingestion of a tolerable amount of medium-chain triglycerides (MCT; approximately 25 g) reduces the rate of muscle glycogen use during high-intensity exercise. On two occasions, seven well-trained men cycled for 30 min at 84% maximal O(2) uptake. Exactly 1 h before exercise, they ingested either 1) carbohydrate (CHO; 0.72 g sucrose/kg) or 2) MCT+CHO [0.36 g tricaprin (C10:0)/kg plus 0.72 g sucrose/kg]. The change in glycogen concentration was measured in biopsies taken from the vastus lateralis before and after exercise. Additionally, glycogen oxidation was calculated as the difference between total carbohydrate oxidation and the rate of glucose disappearance from plasma (R(d) glucose), as measured by stable isotope dilution techniques. The change in muscle glycogen concentration was not different during MCT+CHO and CHO (42.0 +/- 4.6 vs. 38.8 +/- 4.0 micromol glucosyl units/g wet wt). Furthermore, calculated glycogen oxidation was also similar (331 +/- 18 vs. 329 +/- 15 micromol. kg(-1). min(-1)). The coingestion of MCT+CHO did increase (P < 0.05) R(d) glucose at rest compared with CHO (26.9 +/- 1.5 vs. 20.7 +/- 0. 7 micromol.kg(-1). min(-1)), yet during exercise R(d) glucose was not different during the two trials. Therefore, the addition of a small amount of MCT to a preexercise CHO meal did not reduce muscle glycogen oxidation during high-intensity exercise, but it did increase glucose uptake at rest.

Endurance exercise training does not alter lipolytic or adipose tissue blood flow sensitivity to epinephrine

We evaluated the relationship between lipolysis and adipose tissue blood flow (ATBF) in response to epinephrine and the effect of endurance exercise training on these responses. Five healthy untrained men underwent a four-stage incremental epinephrine infusion (0.00125, 0.005, 0.0125, and 0.025 microgram. kg fat free mass(-1). min(-1)) plus hormonal clamp before and after 16 wk of cycle ergometry exercise training. Whole body glycerol and free fatty acid (FFA) rates of appearance (R(a)) in plasma were determined by stable isotope methodology, and ATBF was assessed by (133)Xe clearance. After each training session, subjects were fed the approximate number of calories expended during exercise to prevent changes in body weight. Glycerol R(a), FFA R(a), and ATBF increased when plasma epinephrine concentration reached 0.8 nM, but at plasma epinephrine concentrations >1.6 nM ATBF plateaued, whereas lipolysis continued to increase. Exercise training increased peak oxygen uptake by 24 +/- 7% (2.9 +/- 0.2 vs. 3.6 +/- 0.1 l/min; P < 0. 05) but did not alter body weight [70.5 +/- 3.8 vs. 72.0 +/- 3.8 kg; P = nonsignificant (NS)] or percent body fat (18.4 +/- 1.6 vs. 17.8 +/- 1.9%; P = NS). Lipolytic and ATBF responses to epinephrine were also the same before and after training. We conclude that the lipolytic and ATBF responses to epinephrine are coordinated when plasma epinephrine concentration is </=1.6 nM, but that at higher epinephrine concentrations, lipolysis continues to increase while ATBF remains constant. Endurance exercise training does not change lipolytic or ATBF sensitivity to epinephrine infusion in vivo during resting conditions.

Substrate metabolism when subjects are fed carbohydrate during exercise

This study determined the effect of carbohydrate ingestion during exercise on the lipolytic rate, glucose disappearance from plasma (Rd Glc), and fat oxidation. Six moderately trained men cycled for 2 h on four separate occasions. During two trials, they were fed a high-glycemic carbohydrate meal during exercise at 30 min (0.8 g/kg), 60 min (0.4 g/kg), and 90 min (0.4 g/kg); once during low-intensity exercise [25% peak oxygen consumption (VO2 peak)] and once during moderate-intensity exercise (68% VO2 peak). During two additional trials, the subjects remained fasted (12-14 h) throughout exercise at each intensity. After 55 min of low-intensity exercise in fed subjects, hyperglycemia (30% increase) and a threefold elevation in plasma insulin concentration (P < 0.05) were associated with a 22% suppression of lipolysis compared with when subjects were fasted (5.2 +/- 0.5 vs. 6.7 +/- 1.2 micromol. kg-1. min-1, P < 0.05), but fat oxidation was not different from fasted levels at this time. Fat oxidation when subjects were fed carbohydrate was not reduced below fasting levels until 80-90 min of exercise, and lipolysis was in excess of fat oxidation at this time. The reduction in fat oxidation corresponded in time with the increase in Rd Glc. During moderate-intensity exercise, the very small elevation in plasma insulin concentration (approximately 3 microU/ml; P < 0.05) during the second hour of exercise when subjects were fed vs. when they were fasted slightly attenuated lipolysis (P < 0.05) but did not increase Rd Glc or suppress fat oxidation. These findings indicate that despite a suppression of lipolysis after carbohydrate ingestion during exercise, the lipolytic rate remained in excess and thus did not limit fat oxidation. Under these conditions, a reduction in fat oxidation was associated in time with an increase in glucose uptake.

Effect of short-term fasting on lipid kinetics in lean and obese women

We evaluated whole body and regional adipose tissue lipid kinetics and norepinephrine (NE) spillover during brief fasting in six lean [body mass index (BMI) 21 +/- 1 kg/m2] and six upper-body obese (UBO; BMI 36 +/- 1 kg/m2) women. At 14 h of fasting, abdominal adipose tissue glycerol and free fatty acid (FFA) release rates were lower (P = 0.07), but whole body glycerol and FFA rates of appearance (Ra) were greater (P < 0.05) in obese than in lean subjects. At 22 h of fasting, glycerol and FFA Ra increased less in obese (19.8 +/- 7.0 and 87.1 +/- 30.3 micromol/min, respectively) than in lean (44.2 +/- 6.6 and 137.4 +/- 30.4 micromol/min, respectively; P < 0.05) women. The percent increase in glycerol Ra correlated closely with the percent decline in plasma insulin in both groups (r2 = 0.85; P < 0.05). Whole body NE spillover declined in lean (P < 0.05) but not obese subjects with continued fasting, whereas regional adipose tissue NE spillover did not change in either group. We conclude that, compared with lean women, in UBO women 1) basal adipose tissue lipolysis is lower, but whole body lipid kinetics is higher because of their greater fat mass; 2) the increase in lipolysis during fasting is blunted because of an attenuated decline in circulating insulin; and 3) downregulation of whole body sympathetic nervous system activity is impaired during fasting.

Whole body, adipose tissue, and forearm norepinephrine kinetics in lean and obese women

We evaluated whole body and regional (subcutaneous abdominal adipose tissue and forearm) norepinephrine (NE) kinetics in seven lean (body mass index 21.3 +/- 0.5 kg/m2) and six upper body obese (body mass index 36.4 +/- 0.4 kg/m2) women who were matched on fat-free mass. NE kinetics were determined by infusing [3H]NE and obtaining blood samples from a radial artery, a deep forearm vein draining mostly skeletal muscle, and an abdominal vein draining subcutaneous abdominal fat. Mean systemic NE spillover tended to be higher in obese (2.82 +/- 0.49 nmol/min) than in lean (2.53 +/- 0.40 nmol/min) subjects, but the differences were not statistically significant. Adipose tissue and forearm NE spillover rates into plasma were greater in lean (0.91 +/- 0.08 pmol. 100 g tissue-1. min-1 and 1.01 +/- 0.09 pmol. 100 ml tissue-1. min-1, respectively) than in obese (0.26 +/- 0.05 pmol. 100 g tissue-1. min-1 and 0.58 +/- 0.11 pmol. 100 ml tissue-1. min-1, respectively) subjects (P < 0.01). These results demonstrate that adipose tissue is an active site for NE metabolism in humans. Adipose tissue NE spillover is considerably lower in obese than in lean women, which may contribute to the lower rate of lipolysis per kilogram of fat mass observed in obesity.

Lipolytic suppression following carbohydrate ingestion limits fat oxidation during exercise

This study determined if the suppression of lipolysis after preexercise carbohydrate ingestion reduces fat oxidation during exercise. Six healthy, active men cycled 60 min at 44 +/- 2% peak oxygen consumption, exactly 1 h after ingesting 0.8 g/kg of glucose (Glc) or fructose (Fru) or after an overnight fast (Fast). The mean plasma insulin concentration during the 50 min before exercise was different among Fast, Fru, and Glc (8 +/- 1, 17 +/- 1, and 38 +/- 5 microU/ml, respectively; P < 0.05). After 25 min of exercise, whole body lipolysis was 6.9 +/- 0.2, 4.3 +/- 0.3, and 3.2 +/- 0.5 micromol x kg(-1) x min(-1) and fat oxidation was 6.1 +/- 0.2, 4.2 +/- 0.5, and 3.1 +/- 0.3 micromol x kg(-1) x min(-1) during Fast, Fru, and Glc, respectively (all P < 0.05). During Fast, fat oxidation was less than lipolysis (P < 0.05), whereas fat oxidation approximately equaled lipolysis during Fru and Glc. In an additional trial, the same subjects ingested glucose (0.8 g/kg) 1 h before exercise and lipolysis was simultaneously increased by infusing Intralipid and heparin throughout the resting and exercise periods (Glc+Lipid). This elevation of lipolysis during Glc+Lipid increased fat oxidation 30% above Glc (4.0 +/- 0.4 vs. 3.1 +/- 0.3 micromol x kg(-1) x min(-1); P < 0.05), confirming that lipolysis limited fat oxidation. In summary, small elevations in plasma insulin before exercise suppressed lipolysis during exercise to the point at which it equaled and appeared to limit fat oxidation.

High efficiency of type I muscle fibers improves performance

We have recently demonstrated that people with a high percentage of Type I muscle fibers display a relatively high muscular efficiency when cycling. These individuals generate a relatively high muscular power output at a given steady-state level of oxygen consumption and caloric expenditure. The purpose of this study was to directly determine the extent to which differences in muscle fiber composition and efficiency influence endurance performance in competitive cyclists. The percentage of Type I and II muscle fibers was determined from several biopsies from the vastus lateralis which were histochemically stained for ATPase activity. During a laboratory performance test, 14 endurance trained cyclists (mean +/- SE; VO2max, 5.2 +/- 0.11/min; body weight, 74 +/- 1 kg) cycled an ergometer for 1 h at the highest work rate they could tolerate. VO2 and RER were simultaneously measured using open circuit spirometry for calculating caloric expenditure. Subjects were divided into two groups of seven according to their muscle fiber type composition: High % Type I Group (> 56% Type I fibers); Normal % Type I Group (38-55% Type I fibers). Each subject from High % Type I Group was paired with a subject from the Normal % Type I Group according to their similarity in VO2max, blood lactate threshold and average VO2 maintained during the 1 h performance test. Both groups averaged 4.5 +/- 0.11/min during the 1 h performance test (i.e., 86-88% VO2max).(ABSTRACT TRUNCATED AT 250 WORDS)

Regulation of endogenous fat and carbohydrate metabolism in relation to exercise intensity and duration

Stable isotope tracers and indirect calorimetry were used to evaluate the regulation of endogenous fat and glucose metabolism in relation to exercise intensity and duration. Five trained subjects were studied during exercise intensities of 25, 65, and 85% of maximal oxygen consumption (VO2max). Plasma glucose tissue uptake and muscle glycogen oxidation increased in relation to exercise intensity. In contrast, peripheral lipolysis was stimulated maximally at the lowest exercise intensity, and fatty acid release into plasma decreased with increasing exercise intensity. Muscle triglyceride lipolysis was stimulated only at higher intensities. During 2 h of exercise at 65% VO2max plasma-derived substrate oxidation progressively increased over time, whereas muscle glycogen and triglyceride oxidation decreased. In recovery from high-intensity exercise, although the rate of lipolysis immediately decreased, the rate of release of fatty acids into plasma increased, indicating release of fatty acids from previously hydrolyzed triglycerides. We conclude that, whereas carbohydrate availability is regulated directly in relation to exercise intensity, the regulation of lipid metabolism seems to be more complex.

Metabolic responses to preexercise meals containing various carbohydrates and fat

After a 12-h fast six different meals (potatoes, rice, sucrose solution, potatoes+margarine, rice+margarine, and a confectionery bar), each containing 0.7 g carbohydrate/kg body wt, were consumed 30 min before exercise (n = 9). During the 30 min immediately after ingestion, the glycemic and insulinemic responses to the meals distinguished these foods into two groups: 1) high glycemic (ie, potatoes, sucrose, confectionery bar) and 2) moderate glycemic (ie, rice, potatoes+margarine, rice+margarine). After 20 min of moderate-intensity exercise, plasma glucose declined in all carbohydrate trials to values of 3.4 +/- 0.1 mmol/L, which were significantly (P < 0.05) lower than values after subjects fasted (4.2 +/- 0.2 mmol/L). Therefore, the glycemic and insulinemic responses to these meals were independent of their carbohydrate classification of simple or complex and the addition of fat reduced the glycemic and insulinemic responses. However, despite these different responses before exercise, all of the meals caused plasma glucose to decline to equally low concentrations early in exercise.

Effects of carbohydrate loading and weight-lifting on muscle girth

Bodybuilders have used different carbohydrate loading regimens in conjunction with resistance exercise prior to competition in the belief that this would result in increased muscle size. To investigate this possibility, muscle girth measurements were obtained from nine weight-trained males before and after a control (standard isocaloric diet) and an experimental trial (carbohydrate loading). The latter regimen consisted of 3 days of intense weight-lifting while the subjects ingested a diet of 10% carbohydrate (CHO), 57% fat (F), and 33% protein (P), followed by 3 days of light weight-lifting and a day of rest while ingesting a diet of 80% CHO, 5% F, and 15% P. The control trial consisted of an identical weight-lifting regimen while subjects ingested an isocaloric (45 kcal/kg BW/day) diet. Body weight and girths (forearm, upper arm, chest, thigh, waist, and calf) were obtained before and after each trial in a relaxed and flexed state. The results indicate that an exercise/carbohydrate loading regimen had no significant effect on muscle girth as compared to the control trial. It is concluded that CHO loading has no additional advantage to enhancing muscle girth in bodybuilders over weight-lifting alone.

Cycling efficiency is related to the percentage of type I muscle fibers

We determined that the variability in the oxygen cost and thus the caloric expenditure of cycling at a given work rate (i.e., cycling economy) observed among highly endurance-trained cyclists (N = 19; mean +/- SE; VO2max, 4.9 +/- 0.1 l.min-1; body weight, 71 +/- 1 kg) is related to differences in their % Type I muscle fibers. The percentage of Type I and II muscle fibers was determined from biopsies of the vastus lateralis muscle that were histochemically stained for ATPase activity. When cycling a Monark ergometer at 80 RPM at work rates eliciting 52 +/- 1, 61 +/- 1, and 71 +/- 1% VO2max, efficiency was determined from the caloric expenditure responses (VO2 and RER using open circuit spirometry) to steady-state exercise. Gross efficiency (GE) was calculated as the ratio of work accomplished.min-1 to caloric expenditure.min-1, whereas delta efficiency (DE) was calculated as the slope of this relationship between approximately 50 and 70% VO2max. The % Type I fibers ranged from 32 to 76%, and DE when cycling ranged from 18.3 to 25.6% in these subjects. The % Type I fibers was positively correlated with both DE (r = 0.85; P less than 0.001; N = 19) and GE (r = 0.75; P less than 0.001; N = 19) during cycling. Additionally, % Type I fibers was positively correlated with GE (r = 0.74; P less than 0.001; N = 13) measured during the novel task of two-legged knee extension; performed at a velocity of 177 +/- 6 degrees.s-1 and intensity of 50 and 70% of peak VO2 for that activity.(ABSTRACT TRUNCATED AT 250 WORDS)

Load and velocity of contraction influence gross and delta mechanical efficiency

The effects of three different cadences and five different work rates on Gross (GE) and Delta Efficiency (DE) during cycle ergometry were studied. Fifteen well-trained cyclists exercised for 30 minutes at 60, 80, or 100 RPM on three different occasions. On each occasion, the load was increased every five minutes and corresponded to approximately 50, 60, 70, 80 and 90% of VO2max. During the last three minutes of each stage, steady-state energy expenditure was calculated while work rate was recorded. In addition, the oxygen cost of unloaded cycling (CUC) was also measured. GE was calculated as the ratio of work rate to the rate of energy expenditure, whereas DE was calculated as the reciprocal of the slope of this relationship at work rates between 50 and 90% of VO2max. The CUC corresponded to 0.66 +/- 0.03 l/min, 0.77 +/- 0.04 l/min and 1.04 +/- 0.04 l/min at 60 RPM, 80 RPM and 100 RPM, respectively (p less than 0.01 for all comparisons). GE was similar at all cadences when cycling at 80 and 90% VO2max. DE increased with increasing rpm and corresponded to 20.6 +/- 0.4%, 21.8 +/- 0.6%, and 23.8 +/- 0.4% at 60 RPM, 80 RPM and 100 RPM, respectively (p less than 0.01 for all comparisons). Therefore, when trained cyclists exercise intensely (80-90% VO2max), GE is similar at cadences of 60, 80 and 100 RPM, despite the significant increase in the CUC. Thus, it is possible that delta efficiency increases with increasing cadence.