Impact of sex on the heart's metabolic and functional responses to diabetic therapies

Increased myocardial lipid delivery is a determinant of myocardial substrate metabolism and function in animal models of type 2 diabetes (T2DM). Sex also has major effects on myocardial metabolism in the human heart. Our aims were to determine whether 1) sex affects the myocardial metabolic response to lipid lowering in T2DM, 2) altering lipid [fatty acid (FA) or triglyceride] delivery to the heart would lower the elevated myocardial lipid metabolism associated with T2DM, and 3) decreasing lipid delivery improves diastolic dysfunction in T2DM. To this end, we studied 78 T2DM patients (43 women) with positron emission tomography, echocardiography, and whole body tracer studies before and 3 mo after randomization to metformin (MET), metformin + rosiglitazone (ROSI), or metformin + Lovaza (LOV). No treatment main effects were found for myocardial substrate metabolism, partly because men and women often had different responses to a given treatment. In men, MET decreased FA clearance, which was linked to increased plasma FA levels, myocardial FA utilization and oxidation, and lower myocardial glucose utilization. In women, ROSI increased FA clearance, thereby decreasing plasma FA levels and myocardial FA utilization. Although LOV did not change triglyceride levels, it improved diastolic function, particularly in men. Group and sex also interacted in determining myocardial glucose uptake. Thus, in T2DM, different therapeutic regimens impact myocardial metabolism and diastolic function in a sex-specific manner. This suggests that sex should be taken into account when designing a patient's diabetes treatment.

Noncontrast skeletal muscle oximetry

PURPOSE

The objective of this study was to develop a new noncontrast method to directly quantify regional skeletal muscle oxygenation.

METHODS

The feasibility of the method was examined in five healthy volunteers using a 3 T clinical MRI scanner, at rest and during a sustained isometric contraction. The perfusion of skeletal muscle of the calf was measured using an arterial spin labeling method, whereas the oxygen extraction fraction of the muscle was measured using a susceptibility-based MRI technique.

RESULTS

In all volunteers, the perfusion in soleus muscle increased significantly from 6.5 ± 2.0 mL (100 g min)(-1) at rest to 47.9 ± 7.7 mL (100 g min)(-1) during exercise (P < 0.05). Although the corresponding oxygen extraction fraction did not change significantly, the rate of oxygen consumption increased from 0.43 ± 0.13 to 4.2 ± 1.5 mL (100 g min)(-1) (P < 0.05). Similar results were observed in gastrocnemius muscle but with greater oxygen extraction fraction increase than the soleus muscle.

CONCLUSION

This is the first MR oximetry developed for quantification of regional skeletal muscle oxygenation. A broad range of medical conditions could benefit from these techniques, including cardiology, gerontology, kinesiology, and physical therapy.

Sex and type 2 diabetes: obesity-independent effects on left ventricular substrate metabolism and relaxation in humans

Patients with type 2 diabetes (T2DM), particularly women, are at risk for heart failure. Myocardial substrate metabolism derangements contribute to cardiac dysfunction in diabetic animal models. The purpose of this study was to determine the effects of diabetes and sex on myocardial metabolism and diastolic function in humans, separate from those of obesity. Thirty-six diabetic subjects (22 women) and 36 nondiabetic, BMI-matched subjects (21 women) underwent positron emission tomography (myocardial metabolism) and echocardiography (structure, function). Myocardial blood flow and oxygen consumption (MVO(2)) were higher in women than men (P = 0.003 and <0.0001, respectively). Plasma fatty acid (FA) levels were higher in diabetics (vs. obese, P < 0.003) and sex and diabetes status interacted in its prediction (P = 0.03). Myocardial FA utilization, oxidation, and esterification were higher and percent FA oxidation lower in diabetics (vs. obese, P = 0.0004, P = 0.007, P = 0.002, P = 0.02). FA utilization and esterification were higher and percent FA oxidation lower in women (vs. men, P = 0.03, P = 0.01, P = 0.03). Diabetes and sex did not affect myocardial glucose utilization, but myocardial glucose uptake/plasma insulin was lower in the diabetics (P = 0.04). Left ventricular relaxation was lower in diabetics (P < 0.0001) and in men (P = 0.001), and diabetes and sex interacted in its prediction (P = 0.03). Sex, T2DM, or their interaction affect myocardial blood flow, MVO(2), FA metabolism, and relaxation separate from obesity's effects. Sexually dimorphic myocardial metabolic and relaxation responses to diabetes may play a role in the known cardiovascular differences between men and women with diabetes.

Potentiation of abnormalities in myocardial metabolism with the development of diabetes in women with obesity and insulin resistance

BACKGROUND

Because studies in animal models of type-2 diabetes mellitus (DM) show that excessive myocardial fatty acid (FA) metabolism (at the expense of glucose metabolism) cause cardiac dysfunction, we hypothesized that women with DM would have more FA and less glucose myocardial metabolism than normal or even obese (OB) women.

RESEARCH DESIGN AND METHODS

Women who were lean volunteers (NV) (N = 14; age 35 ± 17 years, body mass index 23 ± 1 kg/m(2)), OB (N = 28;31 ± 6 years, BMI 39 ± 7 kg/m2), and DM (n = 22; 54 ± 11 years, BMI 38 ± 5 kg/m2) were studied. Cardiac positron emission tomography was performed for the determination of myocardial blood flow, oxygen consumption, FA and glucose metabolism. Cardiac work was measured by echocardiography and efficiency by the ratio of work to myocardial oxygen consumption.

RESULTS

Fractional glucose uptake was comparable between NV and OB but lower in DM (P < .05 versus NV). Myocardial FA utilization and oxidation were both higher in DM compared with NV and OB (P < .0001). Myocardial FA utilization and oxidation had positive correlations with HOMA (R = 0.35, P = .005 and R = 0.40, P = .001, respectively) whereas fractional glucose uptake exhibited an inverse correlation (R = -.31, P = .01). Cardiac work and efficiency were similar among the three groups.

CONCLUSIONS

In women, the presence of OB and DM compared with OB alone is associated with a greater reliance on myocardial FA metabolism at the expense of glucose metabolism. These perturbations in myocardial metabolism are not associated in a decline left ventricular efficiency or function suggesting that the metabolic perturbations may precede an eventual decline left ventricular function as is seen in animal models of DM.

Measurement of myocardial fatty acid esterification using [1-11C]palmitate and PET: comparison with direct measurements of myocardial triglyceride synthesis

BACKGROUND

The purpose of the present study was to assess the accuracy of rates of myocardial fatty acid esterification (MFAE) obtained using positron emission tomography (PET).

METHODS AND RESULTS

Sixteen dogs were studied after an overnight fast (FAST), during a euglycemic hyperinsulinemic clamp (CLAMP), or during infusion of intralipid (IL) or IL plus dobutamine (IL/DOB). MFAE was quantified using [1-(11)C]palmitate and PET and compared to the rate of triglyceride (TG) synthesis measured using [1-(13)C]palmitate and tissue sampling. Plasma free fatty acid (FFA) concentration varied approximately 20-fold across groups, with this variation in FFA availability accompanied by a approximately 20-fold range in TG synthesis. MFAE varied approximately 12-fold across groups, and was significantly correlated with TG synthesis (R = 0.80, P < .001). MFAE, however, was 3- to 4-fold higher than TG synthesis in FAST, CLAMP, and IL, but only approximately 50% higher when cardiac work was increased in IL/DOB, suggesting that MFAE reflects, in part, the incorporation of label into amino acids via TCA cycle exchange reactions.

CONCLUSIONS

Changes in MFAE parallel changes in TG synthesis, at least in the basal state. Although the data need to be interpreted cautiously, such measurements should be useful for quantifying acute changes in FFA storage by the heart in various pathophysiological states.

Assessment of myocardial triglyceride oxidation with PET and 11C-palmitate

BACKGROUND

The goal of this study was to test whether myocardial triglyceride (TG) turnover including oxidation of TG-derived fatty acids (FA) could be assessed with PET and (11)C-palmitate.

METHODS AND RESULTS

A total of 26 dogs were studied fasted (FAST), during Intralipid infusion (IL), during a hyperinsulinemic-euglycemic clamp without (HIEG), or with Intralipid infusion (HIEG + IL). (11)C-palmitate was injected, and 45 minutes were allowed for labeling of myocardial TG pool. 3D PET data were then acquired for 60 minutes, with first 15 minutes at baseline followed by 45 minutes during cardiac work stimulated with constant infusion of either phenylephrine (FAST, n = 6; IL, n = 6; HIEG + IL, n = 6) or dobutamine (FAST, n = 4; HIEG, n = 4). Myocardial (11)C washout during adrenergic stimulation (AS) was fitted to a mono-exponential function (Km(PET)). To determine the source of this (11)C clearance, Km(PET) was compared to direct coronary sinus-arterial measurements of total (11)C activity, (11)C-palmitate, and (11)CO(2). Before AS, PET curves in all groups were flat indicating absence of net clearance of (11)C activity from heart. In both FAST groups, AS resulted in negligible net (11)C activity and (11)CO(2) production higher than net (11)C-palmitate uptake. AS with phenylephrine resulted in net myocardial uptake of total (11)C activity and (11)C-palmitate in IL and HIEG + IL, and (11)CO(2) production lower than (11)C-palmitate uptake. In contrast, AS with dobutamine in HIEG resulted in net clearance of all (11)C metabolites (total (11)C activity, (11)C-palmitate and (11)CO(2)) with (11)CO(2) contributing 66% to endogenous FA oxidation. The AS resulted in significant Km(PET) in all the groups, except HIEG + IL. However, positive correlation between Km(PET) and (11)CO(2) was observed only in HIEG (R (2) = 0.83, P = .09).

CONCLUSIONS

This is the first study to demonstrate that using PET and pre-labeling of intracardiac TG pool with (11)C-palmitate, noninvasive assessment of myocardial TG use is feasible under metabolic conditions that favor endogenous TG use such as increased metabolic demand (beta-adrenergic stimulation of cardiac work) with limited availability of exogenous substrate (HIEG).

PET measurements of myocardial glucose metabolism with 1-11C-glucose and kinetic modeling

The aim of this study was to investigate whether compartmental modeling of 1-(11)C-glucose PET kinetics can be used for noninvasive measurements of myocardial glucose metabolism beyond its initial extraction.

METHODS

1-(11)C-Glucose and U-(13)C-glucose were injected simultaneously into 22 mongrel dogs under a wide range of metabolic states; this was followed by 1 h of PET data acquisition. Heart tissue samples were analyzed for (13)C-glycogen content (nmol/g). Arterial and coronary sinus blood samples (ART/CS) were analyzed for glucose (mumol/mL), (11)C-glucose, (11)CO(2), and (11)C-total acidic metabolites ((11)C-lactate [LA] + (11)CO(2)) (counts/min/mL) and were used to calculate myocardial fractions of (a) glucose and 1-(11)C-glucose extractions, EF(GLU) and EF((11)C-GLU); (b) (11)C-GLU and (11)C-LA oxidation, OF((11)C-GLU) and OF((11)C-LA); (c) (11)C-glycolsysis, GCF((11)C-GLU); and (d) (11)C-glycogen content, GNF((11)C-GLU). On the basis of these measurements, a compartmental model (M) that accounts for the contribution of exogenous (11)C-LA to myocardial (11)C activity was implemented to measure M-EF(GLU), M-GCF(GLU), M-OF(GLU), M-GNF(GLU), and the fraction of myocardial glucose stored as glycogen M-GNF(GLU)/M-EF(GLU)).

RESULTS

ART/CS data showed the following: (a) A strong correlation was found between EF((11)C-GLU) and EF(GLU) (r = 0.92, P < 0.0001; slope = 0.95, P = not significantly different from 1). (b) In interventions with high glucose extraction and oxidation, the contribution of OF((11)C-GLU) to total oxidation was higher than that of OF((11)C-LA) (P < 0.01). In contrast, in interventions in which glucose uptake and oxidation were inhibited, OF((11)C-LA) was higher than OF((11)C-GLU) (P < 0.05). (c) A strong correlation was found between GNF((11)C-GLU)/EF(GLU) and direct measurements of fractional (13)C-glycogen content, (r = 0.96, P < 0.0001). Model-derived PET measurements of M-EF(GLU), M-GCF(GLU), and M-OF(GLU) strongly correlated with EF(GLU) (slope = 0.92, r = 0.95, P < 0.0001), GCF((11)C-GLU) (slope = 0.79, r = 0.97, P < 0.0001), and OF((11)C-GLU) (slope = 0.70, r = 0.96, P < 0.0001), respectively. M-GNF(GLU)/M-EF(GLU) strongly correlated with fractional (13)C-content (r = 0.92, P < 0.0001).

CONCLUSION

Under nonischemic conditions, it is feasible to measure myocardial glucose metabolism noninvasively beyond its initial extraction with PET using 1-(11)C-glucose and a compartmental modeling approach that takes into account uptake and oxidation of secondarily labeled exogenous (11)C-lactate.

Are peristaltic pumps as reliable as syringe pumps for metabolic research? Assessment of accuracy, precision, and metabolic kinetics

Syringe pumps are traditionally used to infuse tracers in metabolic research because they are perceived to be more accurate and precise than peristaltic pumps. This study evaluated the accuracy (actual v programmed infusion rate) and precision (reproducibility of infusion) of a peristaltic pump (Gemini PC 2; IMED, San Diego, CA) and a syringe pump (Model 22; Harvard Apparatus, Natick, MA) for metabolic research. In one protocol, saline delivery was measured in vitro in 5 trials at 4 flow rates: 3, 30, 150, and 300 mL/h. In the second protocol, basal glycerol rate of appearance (Ra) was determined in vivo in 5 women on 2 consecutive days. On day 1, [2-(13C)]glycerol was infused with 1 pump and [1,1,2,3,3-(2H5)]glycerol with the other. On day 2, the opposite pattern was used. The accuracy of the 2 pumps was the same (error approximately 2%). In addition, both the syringe and the peristaltic pumps were very precise, with coefficients of variation (CV) <1% at all flow rates. Glycerol Ra values were the same when tracer was infused with either a syringe or peristaltic pump on day 1 and day 2: 4.1 +/- 1.7 (syringe pump) and 4.2 +/- 1.9 (peristaltic pump) micromol. kg fat mass (FM)(-1). min(-1) on day 1; 4.2 +/- 1.2 (syringe pump) and 4.2 +/- 1.3 (peristaltic pump) micromol. kg FM(-1). min(-1) on day 2. These data demonstrate that both syringe and peristaltic pumps are very accurate and precise across a large range of flow rates. Moreover, the assessment of in vivo substrate kinetics in human subjects is the same when either pump is used to infuse isotope tracers.

Absence of an effect of liposuction on insulin action and risk factors for coronary heart disease

BACKGROUND

Liposuction has been proposed as a potential treatment for the metabolic complications of obesity. We evaluated the effect of large-volume abdominal liposuction on metabolic risk factors for coronary heart disease in women with abdominal obesity.

METHODS

We evaluated the insulin sensitivity of liver, skeletal muscle, and adipose tissue (with a euglycemic-hyperinsulinemic clamp procedure and isotope-tracer infusions) as well as levels of inflammatory mediators and other risk factors for coronary heart disease in 15 obese women before and 10 to 12 weeks after abdominal liposuction. Eight of the women had normal glucose tolerance (mean [+/-SD] body-mass index, 35.1+/-2.4), and seven had type 2 diabetes (body-mass index, 39.9+/-5.6).

RESULTS

Liposuction decreased the volume of subcutaneous abdominal adipose tissue by 44 percent in the subjects with normal glucose tolerance and 28 percent in those with diabetes; those with normal oral glucose tolerance lost 9.1+/-3.7 kg of fat (18+/-3 percent decrease in total fat, P=0.002), and those with type 2 diabetes lost 10.5+/-3.3 kg of fat (19+/-2 percent decrease in total fat, P<0.001). Liposuction did not significantly alter the insulin sensitivity of muscle, liver, or adipose tissue (assessed by the stimulation of glucose disposal, the suppression of glucose production, and the suppression of lipolysis, respectively); did not significantly alter plasma concentrations of C-reactive protein, interleukin-6, tumor necrosis factor alpha, and adiponectin; and did not significantly affect other risk factors for coronary heart disease (blood pressure and plasma glucose, insulin, and lipid concentrations) in either group.

CONCLUSIONS

Abdominal liposuction does not significantly improve obesity-associated metabolic abnormalities. Decreasing adipose tissue mass alone will not achieve the metabolic benefits of weight loss.

Gender differences in glucose kinetics and substrate oxidation during exercise near the lactate threshold

The purpose of this investigation was to determine plasma glucose kinetics and substrate oxidation in men and women during exercise relative to the lactate threshold (LT). Subjects cycled for 25 min at 70 and 90% of O(2) uptake (VO(2)) at LT (70 and 90% LT, respectively). Plasma glucose appearance (R(a)) and disappearance (R(d)) were determined with a primed constant infusion of [6,6-(2)H]glucose. There were no significant differences in glucose R(a) between men [22.6 +/- 1.9 and 39.9 +/- 3.9 micromol x kg fat-free mass (FFM)(-1) x min(-1) for 70 and 90% LT, respectively] and women (22.3 +/- 2.7 and 33.9 +/- 5.7 micromol x kg FFM(-1) x min(-1) for 70 and 90% LT, respectively). Similarly, there were no significant differences in glucose R(d) between men (21.2 +/- 1.9 and 38.1 +/- 3.7 micromol x kg FFM(-1) x min(-1) for 70 and 90% LT, respectively) and women (21.3 +/- 2.8 and 33.3 +/- 5.6 micromol x kg FFM(-1) x min(-1) for 70 and 90% LT, respectively). Although there were no differences between genders in the relative contribution of carbohydrate (CHO) to total energy expenditure, the relative contribution of muscle glycogen to total CHO oxidation (75.8 +/- 3.2 and 64.2 +/- 8.0% for men and women, respectively, at 70% LT and 75.1 +/- 2.6 and 60.1 +/- 11.2% for men and women, respectively, at 90% LT) was lower in women. Consequently, the relative contribution of blood glucose to total CHO oxidation was significantly higher in women. These results indicate that although plasma glucose R(a) and R(d) are similar in men and women, the relative contribution of muscle glycogen and blood glucose is significantly different in women during moderate-intensity exercise relative to LT.

Glucose kinetics and substrate oxidation during exercise in the follicular and luteal phases

The purpose of this investigation was to determine whether plasma glucose kinetics and substrate oxidation during exercise are dependent on the phase of the menstrual cycle. Once during the follicular (F) and luteal (L) phases, moderately trained subjects [peak O(2) uptake (V(O(2))) = 48.2 +/- 1.1 ml. min(-1). kg(-1); n = 6] cycled for 25 min at approximately 70% of the V(O(2)) at their respective lactate threshold (70%LT), followed immediately by 25 min at 90%LT. Rates of plasma glucose appearance (R(a)) and disappearance (R(d)) were determined with a primed constant infusion of [6,6-(2)H]glucose, and total carbohydrate (CHO) and fat oxidation were determined with indirect calorimetry. At rest and during exercise at 70%LT, there were no differences in glucose R(a) or R(d) between phases. CHO and fat oxidation were not different between phases at 70%LT. At 90%LT, glucose R(a) (28.8 +/- 4.8 vs. 33.7 +/- 4.5 micromol. min(-1). kg(-1); P < 0.05) and R(d) (28.4 +/- 4.8 vs. 34.0 +/- 4.1 micromol. min(-1). kg(-1); P < 0.05) were lower during the L phase. In addition, at 90%LT, CHO oxidation was lower during the L compared with the F phase (82.0 +/- 12.3 vs. 93.8 +/- 9.7 micromol. min(-1) .kg(-1); P < 0.05). Conversely, total fat oxidation was greater during the L phase at 90%LT (7.46 +/- 1.01 vs. 6.05 +/- 0.89 micromol. min(-1). kg(-1); P < 0.05). Plasma lactate concentration was also lower during the L phase at 90%LT concentrations (2.48 +/- 0.41 vs. 3.08 +/- 0.39 mmol/l; P < 0.05). The lower CHO utilization during the L phase was associated with an elevated resting estradiol (P < 0.05). These results indicate that plasma glucose kinetics and CHO oxidation during moderate-intensity exercise are lower during the L compared with the F phase in women. These differences may have been due to differences in circulating estradiol.

Fat metabolism during high-intensity exercise in endurance-trained and untrained men

To determine whether trained individuals rely more on fat than untrained persons during high-intensity exercise, six endurance-trained men and six untrained men were studied during 30 minutes of exercise at 75% to 80% maximal oxygen consumption (VO2max). The rates of appearance (Ra) and disappearance (Rd) of glycerol and free fatty acids (FFAs) were determined using [1,1,2,3,3-2H]glycerol and [1-13C]palmitate, respectively, whereas the overall rate of fatty acid oxidation was determined using indirect calorimetry. During exercise, the whole-body rate of lipolysis (ie, glycerol Ra) was higher in the trained group (7.1 +/- 1.2 v 4.5 +/- 0.7 micromol x min(-1) x kg(-1), P < .05), as was the Ra (approximately Rd) of FFA (9.0 +/- 0.9 v 5.0 +/- 1.0 micromol x min(-1) x kg(-1), P < .001). FFA utilization was higher in trained subjects even when expressed as a percentage of total energy expenditure (10% +/- 1% v 7% +/- 1%, P < .05). However, this difference in plasma FFA flux could not account for all of the difference in fatty acid oxidation between trained and untrained subjects (20.8 +/- 3.3 v 7.9 +/- 1.6 micromol x min(-1) x kg(-1), or 23% +/- 3% v 13% +/- 2% of total energy expenditure, both P < .05). Thus, the oxidation of fatty acids derived from some other source also must have been greater in the trained men. We conclude that trained athletes use more fat than untrained individuals even during intense exercise performed at the same percentage of VO2max. The additional fatty acids appear to be derived from both adipose tissue and, presumably, intramuscular triglyceride stores.

Assessment of methods for improving tracer estimation of non-steady-state rate of appearance

The most common approach for estimating substrate rate of appearance (R(a)) is use of the single-pool model first proposed by R. W. Steele, J. S. Wall, R. C. DeBodo, and N. Altszuler. (Am. J. Physiol. 187: 15-24, 1956). To overcome the model error during highly non-steady-state conditions due to the assumption of a constant volume of distribution (V), two strategies have been proposed: 1) use of a variable tracer infusion rate to minimize tracer-to-tracee ratio (TTR) variations (fixed-volume approach) or 2) use of two tracers of the same substrate with one infused at a constant rate and the other at a variable rate (variable-volume approach or approach of T. Issekutz, R. Issekutz, and D. Elahi. Can. J. Physiol. Pharmacol. 52: 215-224, 1974). The goal of this study was to compare the results of these two strategies for the analysis of the kinetics of glycerol and glucose under the non-steady-state condition created by a constant infusion of epinephrine (50 ng. kg(-1). min(-1)) with the traditional approach of Steele et al., which uses a constant infusion and fixed volume. The results showed that for glucose and glycerol the estimates of R(a) obtained with the constant and the variable tracer infusion rate and the equation of Steele et al. were comparable. The variable tracer infusion approach was less sensitive to the choice of V in estimating R(a) for glycerol and glucose, although the advantage of changing the tracer infusion rate was greater for glucose than for glycerol. The model of Issekutz et al. showed instability when the ratio TTR(1)/TTR(2) approaches a constant value, and the model is more sensitive to measurement error than the constant-volume model for glucose and glycerol. We conclude that the one-tracer constant-infusion technique is sufficient in most cases for glycerol, whereas the one-tracer variable-infusion technique is preferable for glucose. Reasonable values for glucose R(a) can be obtained with the constant-infusion technique if V = 145 ml/kg.

Use of stable isotopes to study carbohydrate and fat metabolism at the whole-body level

The present review discusses the advantages and limitations of using stable-isotope tracers to assess carbohydrate and fat metabolism at the whole-body level. One advantage of stable- (v. radioactive-) isotope tracers is the relative ease with which the location of a label within a molecule can be determined using selected-ion-monitoring GC-mass spectrometry (SIM-GC-MS). This technique minimizes potential problems due to label recycling, allows the use of multiple-labelled compounds simultaneously (e.g. to quantify glucose cycling), and perhaps most importantly, has led to the development of unique stable-isotope methods for, for example, quantifying gluconeogenesis. However, the limited sensitivity of SIM-GC-MS sometimes requires that relatively large amounts of a stable-isotope tracer be used, thus increasing cost and potentially altering metabolism. At least theoretically, stable- (or radioactive-) isotope tracers can also be used in conjunction with indirect calorimetry to estimate utilization of muscle glycogen or triacylglycerol stores, thus potentially circumventing the need to obtain muscle biopsies. These calculations, however, require certain critical assumptions, which if incorrect could lead to major errors in the values obtained. Despite such limitations, stable-isotope tracers provide a powerful and sometimes unique tool for investigating carbohydrate and fat metabolism at the whole-body level. With continuing advances in availability, instrumentation and methods, it is likely that stable-isotope tracers will become increasingly important in the immediate future.

Training-induced alterations in fat and carbohydrate metabolism during exercise in elderly subjects

Compared with young adults, fat oxidation is lower in elderly persons during endurance exercise performed at either the same absolute or relative intensity. We evaluated the effect of 16 wk of endurance training on fat and glucose metabolism during 60 min of moderate intensity exercise [50% of pretraining peak oxygen consumption (VO2peak)] in six elderly men and women (74 +/- 2 yr). Training caused a 21% increase in mean VO2peak. The average rate of fat oxidation during exercise was greater after (221 +/- 28 mumol/min) than before (166 +/- 17 mumol/min) training (P = 0.002), and the average rate of carbohydrate oxidation during exercise was lower after (3,180 +/- 461 mumol/min) than before (3,937 +/- 483 mumol/min) training (P = 0.003). Training did not cause a significant change in glycerol rate of appearance (Ra), free fatty acid (FFA) Ra, and FFA rate of disappearance during exercise. However, glucose Ra during exercise was lower after (1,027 +/- 95 mumol/min) than before (1,157 +/- 69 mumol/min) training (P = 0.01). These results demonstrate that a 16-wk period of endurance training increases fat oxidation without a significant change in lipolysis (glycerol Ra) or FFA availability (FFA Ra) during exercise in elderly subjects. Therefore, the training-induced increase in fat oxidation during exercise is likely related to alterations in skeletal muscle fatty acid metabolism.

Regulation of fatty acid oxidation in untrained vs. trained men during exercise

We have recently shown that increased carbohydrate flux decreases fat oxidation during exercise by inhibition of fatty acid entry into the mitochondria. Because endurance training reduces the rate of carbohydrate flux during exercise, we hypothesized that training increases fat oxidation by relieving this inhibition. To test this hypothesis, five sedentary and five endurance-trained men exercised on a cycle ergometer at an oxygen consumption (VO2) of approximately 2.0 l/min, representing 80 and 40% peak VO2, respectively. [1-13C]oleate and [1-14C]octanoate, long- and medium-chain fatty acids, respectively, were infused for the duration of the studies. Carbohydrate oxidation was significantly higher in the sedentary group (196 +/- 9 vs. 102 +/- 17 mumol.kg-1.min-1, P < 0.05). Oleate oxidation was higher in the trained group (3.8 +/- 0.6 vs. 1.9 +/- 0.3 mumol.kg-1.min-1, P < 0.05), whereas octanoate oxidation was not different between the two groups. The percentage of oleate that was taken up by tissues and oxidized was higher in the trained group (76 +/- 7 vs. 58 +/- 3%, P < 0.05). However, the percentage of octanoate taken up and oxidized was not different (82 +/- 3 vs. 85 +/- 4%, not significant). Because octanoate, unlike oleate, can freely diffuse across the mitochondrial membrane, the present results suggest that the difference in fatty acid oxidation between trained and untrained individuals may be due to enhanced fatty acid entry into the mitochondria.

The glucose crossover concept is not an important new concept in exercise metabolism

1. The basic premise of the 'crossover' concept (i.e. that the balance of carbohydrate (CHO) and fat utilization during exercise depends on the interaction between exercise intensity and the individual's endurance training status) has been accepted since at least the 1930s. 2. The crossover concept differs from earlier perspectives mostly in its greater emphasis on the absolute exercise intensity as an important determinant of substrate selection during exercise. Because of this emphasis, it is argued that while trained subjects may utilize less CHO than their untrained counterparts during low- or moderate-intensity exercise, this is not true during high-intensity exercise, because during such exercise even trained persons must 'crossover' to CHO dependency. In fact, the crossover concept predicts that utilization of at least one CHO source (i.e. plasma-borne glucose) should be greater in trained subjects during intense exercise. This increase in glucose utilization is hypothesized to be supported by an enhanced rate of gluconeogenesis. 3. In direct contradiction of the crossover concept, the literature consistently shows that, compared with untrained individuals, trained subjects rely less on CHO for fuel, even during high-intensity exercise. In particular, it has been shown that the rate of glucose utilization is lower in trained subjects under these conditions. Recent data from Dr Brooks' own laboratory support this conclusion and also show that this reduction in glucose use is associated with a decrease in the rate of gluconeogenesis. These recent observations confirm prior studies of moderate-intensity exercise. 4. Based on the above, it is clear that the crossover concept cannot be considered an important new concept in exercise metabolism. Instead, the crossover concept actually serves to hinder understanding in this area.

Regulation of glucose production during exercise at 80% of VO2peak in untrained humans

To determine whether alterations in insulin and/or glucagon secretion play an important role in stimulating glucose production (Ra) during intense but submaximal exercise, we studied six untrained subjects during 30 min of cycling at 80% of peak oxygen uptake on two occasions: once under control conditions and once when alterations in insulin and glucagon secretion were prevented with the use of the pancreatic islet clamp technique. In the latter experiments, glucose was infused during exercise to match glycemia with control levels. Glucose kinetics were measured in both trials using a primed, continuous infusion of [6,6-2H]glucose. In the control trial, glucose Ra rose from 11.9 +/- 0.8 mumol.min-1.kg-1 at rest to 42.5 +/- 4.3 mumol.min-1.kg-1 by the end of exercise. A similar increment was observed in the islet clamp experiments, with endogenous Ra peaking at 37.2 +/- 7.9 mumol.min-1.kg-1. This was true even through glucagon concentration did not change from basal and insulin concentration actually rose (the latter apparently due to a decrease in insulin clearance during intense exercise). Thus neither decrements in insulin or increments in glucagon are apparently required to stimulate glucose Ra under the present conditions. Because epinephrine levels rose only slightly, it appears that either neurally released norepinephrine or some other, as yet unidentified, factor is responsible for stimulating glucose Ra during intense but submaximal exercise.

Plasma glucose metabolism during exercise: effect of endurance training in humans

It has long been recognized that endurance training reduces the reliance on carbohydrate as a source of energy during submaximal exercise. Historically, this has been ascribed to a decrease in muscle glycogen utilization. However, recent studies have demonstrated that, at least in humans, training also reduces the production and utilization of plasma-borne glucose during exercise. The latter is true not only during moderate exercise performed at the same absolute intensity before and after training, but also during intense exercise performed at the same relative intensity in the trained and untrained states. Moreover, this adaptation is often quantitatively just as important as the decline in muscle glycogen utilization in accounting for the overall carbohydrate-sparing effect of training. This reduced reliance on plasma glucose, which appears to result from a decrease in muscle glucose transport, seems to be related to the training-induced increase in muscle mitochondrial respiratory capacity. On the other hand the training-induced decrease in glucose production (which is the result of reductions in both hepatic glycogenolysis and gluconeogenesis) is probably largely due to alterations in the glucoregulatory hormone response to exercise, although other factors (such as changes in hepatic hormone sensitivity and/or responsiveness) may also play a role. By minimizing the possibility of hypoglycemia, these adaptations in glucose production and utilization likely contribute to the increased endurance that results from exercise training.

Fat and carbohydrate metabolism during exercise in elderly and young subjects

We evaluated the effect of aging on fat and carbohydrate metabolism during moderate intensity exercise. Glycerol, free fatty acid (FFA), and glucose rate of appearance (Ra) in plasma and substrate oxidation were determined during 60 min of cycle ergometer exercise in six elderly (73 +/- 2 yr) and six young adults (26 +/- 2 yr) matched by gender and lean body mass. The elderly group was studied during exercise performed at 56 +/- 3% of maximum oxygen uptake, whereas the young adults were studied during exercise performed at the same absolute and at a similar relative intensity as the elderly subjects. Mean fat oxidation during exercise was 25-35% lower in the elderly subjects than in the young adults exercising at either the same absolute or similar relative intensities (P < 0.05). Mean carbohydrate oxidation in the elderly group was 35% higher than the young adults exercising at the same absolute intensity (P < 0.001) but 40% lower than the young adults exercising at the same relative intensity (P < 0.001). Average FFA Ra in the elderly subjects was 85% higher than in the young adults exercising at the same absolute intensity (P < 0.05) but 35% lower than the young adults exercising at a similar relative intensity (P < 0.05). We conclude that fat oxidation is decreased while carbohydrate oxidation is increased during moderate intensity exercise in elderly men and women. The shift in substrate oxidation was caused by age-related changes in skeletal muscle respiratory capacity because lipolytic rates and FFA availability were not rate limiting in the older subjects.