after an overnight fast, subjects underwent an isotope infusion test. Teflon catheters were inserted in an antecubital vein for isotope infusion and retrogradely into a contralateral dorsal hand vein for sampling of arterialized venous blood.

After placement of the catheters, subjects rested on a bed, and the cannulated hand was placed in a hotbox, in which air was circulated at 60°C to obtain arterialized venous blood.

After 30 min, baseline oxygen consumption and carbon dioxide production was measured, and breath and blood samples were collected. Immediately thereafter, subjects were given an intravenous dose of 0.

Then, at time zero, a constant intravenous infusion of either [U- 13 C]palmitate 0. With these infusion rates, the amount of 13 C infused during palmitate and acetate infusion are similar.

Blood samples and breath samples were taken at 0, , , and min at rest and , , and min during exercise. At rest, V o 2 and V CO 2 were measured continuously during the first 90 min using open circuit spirometry Oxycon-β. During exercise, V o 2 and V CO 2 were measured immediately before the measurement of breath 13 CO 2 enrichment.

To determine the exact infusion rate, the concentration of palmitate in the infusate was measured for each experiment using analytical gas chromatography GC using heptadecanoic acid as internal standard see sample analysis.

The acetate concentration was measured in each infusate with an enzymatic method Boehringer Mannheim, Mannheim, Germany. Muscle biopsies were taken from the mid-thigh region from M.

vastus lateralis according to the technique of Bergstrom et al. The subjects were required to abstain from training or vigorous exercise 48 h before the biopsy. The biopsy was used for isolation of total RNA using the acid phenol method of Chomozynski and Sacchi 28 , with an additional DNAse digestion step with concomitant acid phenol extraction and ethanol precipitation.

The mRNA levels of LPL, hexokinase II, GLUT4, ACC2, and UCP3 were quantified by RT-competitive PCR For the assays, the RT reaction was performed from 0. The competitive PCR assays were performed as previously described 30 — To improve the quantification of the amplified products, fluorescent dye-labeled sense oligonucleotides were used.

The PCR products were separated and analyzed on an ALFexpress DNA sequencer Pharmacia with the Fragment Manager Software. Total RNA preparations and RT-competitive PCR assays of the two skeletal muscle samples from the same individual before and after weight loss were performed simultaneously.

Oxygen saturation Hemoximeter OSM2; Radiometer, Copenhagen, Denmark was determined immediately after sampling in heparinized blood and used to check arterialization.

Fifteen milliliters of arterialized venous blood was sampled in tubes containing EDTA to prevent clotting and immediately centrifuged at 3, rpm 1, g for 10 min at 4°C.

Plasma substrates were determined using the hexokinase method Roche, Basel for glucose, the Wako NEFA nonesterified fatty acid C test kit Wako Chemicals, Neuss, Germany for FFAs, and the glycerolkinase-lipase method Boehringer Mannheim for glycerol and triglycerides.

For determination of plasma palmitate, FFAs were extracted from plasma, isolated by thin-layer chromatography, and derivated to their methyl esters.

From palmitate oxidation, plasma-derived fatty acid oxidation was then calculated by dividing palmitate oxidation rate by the fractional contribution of palmitate to the total FFA concentration.

Differences in measured variables before and after training were tested using paired t tests. Repeated measures one-way ANOVA were used to detect differences in variables in time.

For testing differences in blood parameters between treatments, areas under the concentration versus time curve where calculated for 0— min at rest and — during exercise.

On average, subjects completed a total of 31 ± 1. Therefore, the average exercise duration per week was 2. The week training program had no influence on percentage body fat or V o 2max Table 1. At rest, total fat oxidation was not significantly influenced by the week training program ± 18 vs.

Similarly, plasma-derived fatty acid oxidation was not significantly influenced by the week training program ± 24 vs. Plasma-derived fatty acid oxidation during exercise was not significantly influenced by the training program ± 88 vs.

Rate of appearance of FFA was not influenced by the training program, neither at rest ± 41 vs. The percentage of R a that was oxidized was also not influenced by the training program, neither at rest 40 ± 4 vs.

At rest, carbohydrate oxidation was not significantly affected by the training program ± 9 vs. Carbohydrate oxidation during exercise tended to be lower after training 1, ± vs. Energy expenditure, both at rest 4. Acetate recovery, both at rest Plasma triglyceride concentrations Fig.

Both at rest and during exercise, the average concentrations for plasma glucose at rest: 4. The week training program had no effect on two genes involved in the transport and oxidation of blood glucose: hexokinase II 2.

However, the expression of two genes encoding for key enzymes in fatty acid metabolism were affected by the training program: skeletal muscle ACC2 was significantly lower after training ± 24 vs. The expression of UCP3 The effect of endurance training on the contribution of different fat sources to total fat oxidation after endurance training is under debate.

Part of this controversy could be explained by the methodological difficulties in using [ 13 C]- and [ 14 C]-fatty acid tracers to estimate the oxidation of plasma fatty acids, especially in the resting state However, Sidossis et al.

We showed that this acetate recovery is reproducible 25 but has a high interindividual variation and is influenced by infusion period, metabolic rate, respiratory quotient, and body composition 21 and therefore needs to be determined in every individual under similar conditions and at similar time points as the measurement of plasma-derived fatty acid oxidation.

In the present study, we therefore measured the acetate recovery factor at all time points in each individual both before and after the training program at least 7 days separated from the last training session to exclude the influence of the last exercise bout on the measurements and were therefore able to correct plasma-derived fatty acid oxidation rate for loss of label in the TCA cycle.

With the available stable isotope tracer methodology, we cannot distinguish between IMTG- or VLDL-derived fatty acid oxidation. Using electron microscopy, it has previously been shown that endurance-trained athletes have increased IMTG concentrations 36 , and because endurance athletes have an increased fat oxidation capacity, it seems logical that this increased IMTG storage after endurance training is an adaptation mechanism to allow IMTG oxidation during exercise.

The localization of the IMTG near the mitochondria would make these triglyceride pools an efficient source of substrate, especially during exercise. However, biochemical analysis of IMTGs is problematic, and therefore the use of IMTG remains controversial.

On the other hand, the contribution of VLDL-derived fatty acids to fat oxidation during exercise is also still under debate 18 , The increased expression of LPL mRNA after training, as observed in our study, which is in accordance with previous studies showing increased LPL activity after endurance training in rodents 38 , 39 , and the reduced plasma triglyceride levels after the training program suggest that VLDL-derived fatty acids contribute significantly to total fat oxidation.

Alternatively, an increase in LPL after training might serve to provide fatty acids for the replenishment of IMTGs that have been oxidized during exercise Certainly, further studies are needed to clarify the contribution of IMTG- and VLDL-derived fatty acid oxidation to total fat oxidation.

Another important aspect of the present study is that we have examined the effect of a low-intensity training program for only 2 h per week. Because endurance training has been shown to increase the capacity to oxidize fatty acids, it has been proposed to be beneficial in overcoming the disturbances in fat oxidation often observed in obesity and diabetes 9.

To investigate the mechanisms behind the changes in substrate oxidation after the endurance-training program, we measured mRNA levels of several genes involved in glucose and fatty acid metabolism. A muscle biopsy was taken 6—7 days before the training program and 6—7 days after the last training session to exclude the influence of acute exercise on mRNA expression.

The expression of two genes involved in regulatory steps of glucose metabolism, i. As mentioned above, mRNA expression of LPL, which hydrolyzes plasma triglycerides and directs the released FFAs into the tissue 22 , tended to increase after training, suggesting that the capacity of skeletal muscle to hydrolyze VLDL triglycerides may be improved by the training program.

Inside the muscle cell, ACC2 activity has recently been suggested to control the rate of fatty acid oxidation and triglyceride storage ACC2 catalyzes the carboxylation of acetyl-CoA to form malonyl-CoA, an intermediate that inhibits the activity of CPT1.

CPT1 catalyzes the rate-limiting step in the transfer of fatty acyl-CoA into mitochondria, where they undergo oxidation. Although we were not able to measure ACC2 enzyme activity, it is tempting to speculate that a decrease in ACC2 activity after training was responsible for the observed training-induced increase in fat oxidation.

Because high levels of malonyl-CoA have been associated with insulin resistance 42 , the reduction of ACC2 with endurance training could possibly be beneficial in the treatment of type 2 diabetes.

Finally, we determined the expression of the human UCP3, which has recently also been implicated in the transport of fatty acids across the inner mitochondrial membrane In a cross-sectional study, we have previously found that UCP3 mRNA was lower in trained than in untrained subjects In the present study, we did not find a significant effect of the training program on UCP3 mRNA expression, suggesting that the training program was not severe enough to result in changes in UCP3 mRNA.

Remarkably, we recently found that, in the same study, UCP3 protein content was significantly decreased after training in all subjects The reason for the discrepancy between the effect of training on UCP3 mRNA expression and protein cannot be deduced from the present study but might involve posttranslational regulation, although the number of subjects is too limited to make such a conclusion.

The mechanism behind this adaptation seems to involve a chronic upregulation of LPL mRNA expression and a chronic downregulation of ACC2, potentially leading to lower malonyl-CoA concentration and less inhibition of CPT1. In contrast to moderate- to high-intensity endurance training, the mild training protocol did not increase hexokinase II and GLUT4 expression, indicating that specifically fat oxidation was improved.

This study was supported by a grant from the Netherlands Organization for Scientific Research NWO to P. and a grant from the Netherlands Heart Foundation to D. The laboratories are members of the Concerted Action FATLINK FAIR-CT , supported by the European Commission.

The authors thank Paulette Vallier for help in mRNA analysis and Dr. Diraison for making and validating the ACC2 competitor. Address correspondence and reprint requests to Dr. Schrauwen, Department of Human Biology, Maastricht University, P.

Box , MD Maastricht, the Netherlands. E-mail: p. schrauwen hb. Sign In or Create an Account. Search Dropdown Menu. header search search input Search input auto suggest. filter your search All Content All Journals Diabetes.

Advanced Search. User Tools Dropdown. Sign In. Skip Nav Destination Close navigation menu Article navigation. Volume 51, Issue 7. Previous Article Next Article. RESEARCH DESIGN AND METHODS.

Article Information. Article Navigation. Pathophysiology July 01 The Effect of a 3-Month Low-Intensity Endurance Training Program on Fat Oxidation and Acetyl-CoA Carboxylase-2 Expression Patrick Schrauwen ; Patrick Schrauwen.

This Site. Google Scholar. Dorien P. van Aggel-Leijssen ; Dorien P. van Aggel-Leijssen. Gabby Hul ; Gabby Hul. Anton J. Wagenmakers ; Anton J. Hubert Vidal ; Hubert Vidal.

Wim H. Saris ; Wim H. Marleen A. van Baak Marleen A. van Baak. Diabetes ;51 7 — Get Permissions. toolbar search Search Dropdown Menu. toolbar search search input Search input auto suggest. View large Download slide. TABLE 1 Subject characteristics. Age years View Large.

TABLE 2 Palmitate and breath CO 2 enrichment before and after training. Time min. Breath 13 CO 2 enrichment TTR × 1, Physical Activity and Health: A Report of the Surgeon General. Schrauwen P, Westerterp KR: The role of high-fat diets and physical activity in the regulation of body weight.

Br J Nutr. Zurlo F, Larson K, Bogardus C, Ravussin E: Skeletal muscle metabolism is a major determinant of resting energy expenditure. J Clin Invest. Blaak EE, van Aggel-Leijssen DP, Wagenmakers AJ, Saris WH, van Baak MA: Impaired oxidation of plasma-derived fatty acids in type 2 diabetic subjects during moderate-intensity exercise.

Colberg SR, Simoneau J-A, Thaete FL, Kelley DE: Skeletal muscle utilization of free fatty acids in women with visceral obesity. He J, Watkins S, Kelley DE: Skeletal muscle lipid content and oxidative enzyme activity in relation to muscle fiber type in type 2 diabetes and obesity.

For skeletal muscle, it has been suggested that mitochondria that are in contact with lipid droplets have a greater capacity for ATP production than non-lipid-droplet-interacting mitochondria [ 19 ]. Thus, lipid droplet—mitochondrial tethering may facilitate high fat oxidation by liberating fatty acids in the direct vicinity of mitochondria with a high capacity to oxidise fatty acids, thereby contributing to ATP maintenance during exercise.

At present, experimental proof in humans for these functional processes is lacking. It should be noted, though, that trained individuals possess higher PLIN5 levels, have more PLIN5-coated lipid droplets [ 6 ] and may, thus, have more lipid droplet—mitochondrial interaction sites than individuals with type 2 diabetes.

Lipid droplet—mitochondria interactions are not different between healthy lean and healthy obese participants [ 21 , 22 ], but these data are lacking for individuals with type 2 diabetes in comparison with endurance-trained athletes. Data on changes in lipid droplet—mitochondria tethering during exercise are only available for endurance-trained athletes.

In male elite cross-country skiers, lipid droplet—mitochondria interactions increase upon an acute exercise bout despite unaltered IMCL content [ 16 ]. In endurance-trained women, lipid droplet—mitochondria tethering increases during exercise, with a concomitant reduction in IMCL content [ 23 ].

The latter study suggests that lipid droplet—mitochondrial interaction upon exercise promotes fatty acid oxidation. The seemingly contradictory finding that an exercise-mediated increase in lipid droplet—mitochondria interaction is paralleled by reduced IMCL content in women [ 23 ] but not in men [ 16 ] might originate from sex differences, as reviewed recently [ 24 ].

A lack of a reduction in IMCL upon exercise as observed in the male elite cross-country skiers may also be reflective of a high IMCL turnover IMCL utilisation during exercise matches fatty acid incorporation into lipid droplets. The underlying mechanism for increased mitochondria—lipid droplet tethering during exercise and whether PLIN5 is important for the capacity to increase lipid droplet—mitochondrial tethering are so far unknown.

Furthermore, it is not clear whether lipid droplet—mitochondrial tethering is disturbed in individuals with type 2 diabetes. The literature indicates that PLIN5 is important for lipid droplet—mitochondrial tethering [ 18 , 20 ] in oxidative tissues.

PLIN5 protein quantification in individual lipid droplets should be performed concomitantly with lipid droplet—mitochondrial interaction analyses in athletes and in those with type 2 diabetes upon an acute exercise bout to gain a better understanding of how lipid droplet—mitochondrial tethering works and if the capacity to tether additional mitochondria to lipid droplets upon exercise is compromised in individuals with type 2 diabetes Fig.

Compromised mitochondrial respiratory capacity is frequently reported in type 2 diabetes [ 25 , 26 , 27 ] and obesity [ 26 ], albeit not always confirmed [ 28 ].

A potent way to increase mitochondrial respiratory capacity and a concomitant increase in fat oxidation is endurance training. Several studies have shown that mitochondrial respiratory capacity and fat oxidation increases upon endurance exercise training, even in type 2 diabetic [ 25 , 29 ] and obese [ 25 , 30 ] participants.

As well as increasing mitochondrial capacity, endurance training also is an effective intervention to improve fat oxidation and modulate fat storage in the skeletal muscle of lean sedentary participants [ 31 ].

Several studies have shown that endurance training 4—16 weeks may affect lipid droplet characteristics without major changes in total IMCL content in type 2 diabetic [ 5 , 11 , 25 , 29 , 32 ], obese [ 21 , 25 , 33 ], and healthy lean, sedentary [ 21 , 34 , 35 ] participants.

In most of these studies, however, insulin sensitivity improved. To understand this seemingly paradoxical observation, we need to focus on what happens at the lipid droplet level, rather than at the total IMCL content level. Upon exercise training, lipid droplet size [ 5 , 22 , 32 ] and subsarcolemmal lipid droplet content [ 11 , 21 , 22 ] reduces, while intramyofibrillar lipid droplet content increases [ 22 ].

These exercise-mediated changes, in previously untrained insulin-resistant individuals, resembles the IMCL storage pattern observed in insulin-sensitive endurance-trained athletes. In contrast, in individuals with type 2 diabetes, fewer but larger lipid droplets are observed, with a higher fraction of lipid droplets in the subsarcolemmal region of type II muscle fibres [ 5 ].

Lipid droplet—mitochondrial tethering increases upon endurance training in obese participants [ 21 , 22 ], while no such effect was observed in individuals with type 2 diabetes [ 36 ]. All of these athlete-like changes were observed in training programmes that were carried out for more than 10 weeks Fig.

Short-term training 4 weeks in obese participants did not change lipid droplet size and number, but lipid droplet—mitochondrial interaction was increased [ 33 ].

This indicates that an athlete-like shift in lipid droplet phenotype permits storage of IMCL without impeding insulin sensitivity. A training-induced improvement in lipid droplet—mitochondrial tethering appears to be an early adaptation of endurance training that is crucial for remodelling of the IMCL storage pattern.

Training studies in healthy lean participants show that endurance training for 6 weeks promotes IMCL utilisation during exercise [ 14 , 35 , 37 ]. While in the untrained state PLIN2- and PLIN5-coated lipid droplets are preferentially used during exercise, 6 weeks of endurance training resulted in preferred utilisation of PLIN5-coated lipid droplets during exercise [ 14 ].

While the effect of exercise training on proteins involved in lipid-droplet turnover, such as PLIN2, PLIN5 and ATGL, has been measured, data on the effect of endurance training on IMCL utilisation and lipid-droplet turnover during an exercise bout in obese participants and individuals with type 2 diabetes is lacking Fig.

PLIN5 gene expression and protein content upon an endurance training intervention increases in obese participants and individuals with type 2 diabetes [ 5 , 33 , 38 , 39 ].

For PLIN2 [ 5 , 33 , 38 , 39 , 40 ], PLIN3 [ 5 , 33 , 38 ] and ATGL [ 5 , 38 ] the training effects are less consistent, either showing an increase or no change in the general population.

Increased PLIN5 protein content upon endurance training indicates that IMCL use during exercise is facilitated and that lipolysis rates of lipid droplets are better matched to mitochondrial fatty acid oxidation rates in individuals with type 2 diabetes vs baseline.

To test these mechanisms in a human setting, acute exercise studies in participants with type 2 diabetes are needed and should include fatty acid tracers and muscle biopsies to study IMCL utilisation during exercise, and changes in PLIN5 protein content at the lipid droplet surface before and after training.

Additionally, in vitro studies in human primary myotubes obtained from endurance-trained athletes and individuals with type 2 diabetes, in combination with imaging of fatty acid tracers with live-cell imaging, can give important insights into turnover of individual lipid droplets upon exposure to different stimuli resembling exercise.

Moreover, to study the direct role of PLIN5 in lipid-droplet turnover, these in vitro studies should be combined with overexpression of fluorescently tagged PLIN5 to test whether PLIN5-coated lipid droplets indeed have a higher lipid-droplet turnover. In most of the studies discussed above, the timing of meal intake relative to the training sessions was not monitored strictly or intentionally timed so that participants trained fasted.

Interestingly, training in the overnight fasted state has gained popularity to promote fat oxidative capacity. Upon fasting, adipose tissue lipolysis and plasma NEFA levels increase. The increase in NEFA drives myocellular uptake of fatty acids and, thus, can promote IMCL storage and oxidation of fatty acids.

Indeed, fat oxidation rates during acute exercise in the fasted state are higher than in the fed state [ 41 , 42 ]. Also, the sustained increase in NEFA levels upon exercise in the fasted state can hypothetically provide ligands for peroxisome proliferator-activated receptor PPAR -mediated gene expression and, thereby, promote an adaptive response in regard to fat metabolism.

Interestingly, endurance training in the fasted state improves glucose tolerance to a greater extent than training in the fed state [ 43 ].

Data on functional adaptations like increased fat oxidative capacity following training in the fasted state are inconsistent [ 35 , 37 , 44 , 45 ]. Acute exercise studies measuring IMCL utilisation with fatty acid tracers and in muscle biopsies have been performed in the fasted state and show IMCL utilisation during exercise [ 1 , 14 , 15 ].

Compared with exercise in the fed state, exercising in the fasted state results in higher NEFA levels, higher fat oxidation rates and a drop in IMCL content [ 42 ]. We previously observed that, over a wide range of interventions, elevated plasma fatty acids promote IMCL storage.

Whether this also occurs during exercise in the fasted state and translates into a higher flux of fatty acids in lipid droplets during exercise remains to be studied.

Upon 6 weeks of endurance training, IMCL content drops during a single exercise bout in the fasted state. This drop in IMCL content upon acute exercise was similar if the training was performed in the carbohydrate-fed state vs that fasted state [ 35 , 37 ].

Currently, most training interventions under fasted conditions have only been performed in healthy lean participants and translation towards the type 2 diabetes population should be done carefully.

Based on the results in healthy lean individuals, training while fasted may induce more IMCL remodelling due to a higher stimulus for lipid-droplet turnover in individuals with type 2 diabetes. Before drawing these conclusions, training interventions in the fasted vs fed state should be performed in individuals with type 2 diabetes.

Intrahepatic lipid IHL storage is associated with type 2 diabetes and cardiovascular diseases. The poor accessibility of the liver in healthy individuals means that most studies towards the effect of acute exercise and exercise training on IHLs and lipid metabolism in humans are based upon non-invasive techniques, such as MRI and tracer studies.

Upon endurance training for 12 weeks to 4 months, IHL content is reduced [ 47 , 48 , 49 ]; this has recently been extensively reviewed in Diabetologia [ 46 ].

While a drop in IHL levels after endurance training generally occurs in the absence of changes in body weight, we observed that the training-mediated drop in IHL correlated with a drop in body fat mass [ 46 , 47 ].

Increased IHL storage is, in general, not associated with disturbed VLDL-triacylglycerol secretion rates [ 46 ], and data on VLDL -triacylglycerol secretion rates upon endurance training is contradictory, either showing no change [ 49 ] or a decrease [ 50 ] Table 1.

It is tempting to speculate that exercise-mediated improvements in whole-body insulin sensitivity include reduced de novo lipogenesis in the liver, thereby contributing to a lower IHL content. While we are not aware of any studies underpinning this notion, it is interesting to note that a short-term 7 day training programme resulted in altered composition but not content of IHL.

After training, IHL contained more polyunsaturated fatty acids [ 51 ]; this is in line with lower de novo lipogenesis, which gives rise to saturated fat Fig.

Liver lipid metabolism: acute exercise and endurance training effects. IHL content is lower in healthy lean individuals than in those who are metabolically compromised. This may be a consequence of lower plasma NEFA levels and lower rates of de novo lipogenesis in lean vs metabolically compromised individuals.

a Upon acute endurance exercise, especially in the fasted state, IHL content rises, most likely due to increased plasma NEFA levels. Furthermore, VLDL-triacylglycerol secretion rates drop during acute exercise, and de novo lipogenesis is blunted due to higher postprandial glycogen synthesis by the muscle, thereby reducing glucose availability for lipid synthesis by the liver.

b The underlying mechanisms that are hypothetically involved during endurance training in metabolically compromised individuals are shown exercise training depicted by the calendar ; these include reduced de novo lipogenesis, and improved postprandial glucose and NEFA uptake by the muscle and, thus, lower availability of glucose and NEFA for the liver to synthesise lipids.

In addition, VLDL-triacylglycerol secretion rate upon endurance training in metabolically compromised individuals drops or is unchanged.

As exercise training reduces IHL content [ 47 , 48 ], one could suggest that IHL also drops upon acute exercise. We observed that, upon 2 h of endurance exercise, IHL content was unaffected, irrespective of participants being in the fed or fasted stated.

After exercise and upon recovery in the fasted state, however, we observed an increase in IHL [ 41 ]. Additionally, IHL increases upon an exercise bout in active lean participants who consumed a light meal before the start of the exercise [ 52 ].

Interestingly, in both studies [ 41 , 52 ], increased IHL content after exercise occurred in the presence of elevated plasma NEFA levels. If this rise in plasma NEFAs is prevented by providing a glucose drink every half hour during and after exercise, IHL does not increase. This indicates that the rise in plasma NEFA levels upon exercise drives the increased IHL content after an exercise bout.

IHL can be used during exercise, upon secretion of VLDL-triacylglycerols into the bloodstream. VLDL-triacylglycerol kinetic analyses during an acute exercise bout in the fasted state show that VLDL-triacylglycerol secretion rates drop during exercise and that the contribution of these particles to total energy expenditure is decreased [ 53 ].

Thus, besides the increase in NEFA influx, the lower VLDL-triacylglycerol secretion rates during exercise may also contribute to the increase in IHL content after acute exercise in the fasted state Fig. In lean, normoglycaemic but insulin-resistant individuals, postprandial IHL synthesis and de novo lipogenesis is lower after a single bout of exercise compared with rest [ 54 ].

Overall, IHL may increase upon acute exercise, but is lower after training, possibly due to lower postprandial de novo lipogenesis during recovery.

It is also lower in endurance-trained individuals. It is currently unknown how the apparent increase in IHL after acute exercise turns into reduced IHL content after endurance training. We cannot exclude that training, per se, is not the major determinant of IHL but that the dietary habits of trained individuals may also make an important contribution.

IMCL and IHL content are increased, and fat oxidative capacity decreased in metabolically compromised individuals, such as obese individuals and those with type 2 diabetes.

While endurance exercise training reduces total intracellular fat content in the liver, the effects in muscle indicate remodelling rather than lowering of the myocellular lipid droplet pool.

In fact, in most populations and under most conditions, endurance exercise training augments IMCL content. Thus, the ability of exercise to modulate lipid droplet dynamics in the liver and muscle contributes to differences in fat oxidative metabolism. Endurance training in individuals with type 2 diabetes remodels IMCL content towards an athlete-like phenotype, while IHL content is reduced.

While many training intervention studies have been performed in metabolically compromised individuals, the effects of acute exercise have not been extensively studied, particularly not in participants with type 2 diabetes.

Thus, it is unclear why IMCL utilisation during exercise is lower in individuals with type 2 diabetes and whether the observed IMCL remodelling towards the athlete-like phenotype in these individuals also translates into the anticipated increase in IMCL utilisation during exercise.

Study findings on the effects of sex differences and exercise intensity on IMCL use during exercise or lipid droplet remodelling upon training are either contradictory or lacking. Compared with skeletal muscle, the underlying mechanisms of the effects of exercise and training on IHL are even more poorly understood.

The reduction in IHL content upon training that is observed in metabolically compromised individuals may partly originate from reduced postprandial de novo lipogenesis. Since diurnal rhythms are present in lipid metabolism, future studies should also focus on the effect of timing of exercise on the parameters discussed in this review in order to elucidate the optimal conditions for exercise-induced improvements in insulin sensitivity in individuals with type 2 diabetes.

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Their total mass, LBM, fat mass and body fat percentage were measured with dual-energy x-ray absorptiometry DXA DXA Prodigy, GE Lunar Corp. Similar to the exercise test, the same Vmax Encore 29 metabolic cart was used and calibrated accordingly. First, the participants rested 10 min in a supine position.

Then, their gas exchange was recorded for 16 min using the ventilated canopy method, and their VO 2 and VCO 2 were averaged at 1-min intervals. First 5 min measurement data were excluded. The average steady-state duration was 9.

A protein correction factor of 0. A standard 2-h OGTT followed the resting metabolism measurement. After the collection of their fasted blood samples, the participants ingested a g glucose solution GlucosePro, Comed LLC, Tampere, Finland.

Next, their blood samples were collected at min, 1-h and 2-h intervals post-ingestion. The plasma glucose concentration was analysed with Konelab 20 XT Thermo Fisher Scientific, Vantaa, Finland and the serum insulin concentration was analysed using IMMULITE® Siemens Medical Solution Diagnostics, Los Angeles, CA, USA.

Additionally, the area under the curve AUC was calculated for insulin and glucose with the trapezoidal method. Good clinical and scientific practices and guidelines, as well as the Declaration of Helsinki, were followed while conducting the study. All participants provided their written informed consent before the laboratory measurements.

Statistical analysis was carried out with IBM SPSS Statistics A one-way random model was used to calculate the intraclass correlation coefficients ICCs between the MZ co-twins. An ICC compares within-pair variation with between-pair variation and thus explains how similar the co-twins are when compared with the other pairs.

Pairwise correlations and differences were analysed with Pearson correlation coefficient and paired-sample t test, respectively. Twin individual-based correlations were analysed with simple linear regression, and the within-pair dependency was taken into account Williams with the clustering option of Stata.

In all regression analyses, RFO or PFO was treated as the dependent variable. All the variables or the regression analysis residuals were determined normally distributed with the Shapiro—Wilk test or with the visual inspection of the histograms and the normality plots.

The p value 0. For clarity, RFO or PFO without a unit symbol is used in the text when the statistical significance persists both when using absolute or LBM relative values in the analysis. Table 1 presents the participant characteristics. Overall, the study population consisted of healthy men aged 32—37 years with varying physical activity, body composition and cardiorespiratory fitness levels.

The calculated ICCs of the resting metabolism variables and PFO showed significant resemblance between co-twins Table 2. We also categorised the co-twins as more active or less active based on their month LTMET index to calculate pairwise correlations Figs.

This division did not lead to significant mean differences between the groups in RFO 0. Pairwise correlations of a absolute and b lean body mass LBM relative resting fat oxidation RFO in 21 MZ twin pairs.

Pairwise correlations of a absolute and b lean body mass LBM relative peak fat oxidation PFO during exercise in 19 MZ twin pairs. Figure 3 illustrates individual RFO and PFO results and within-pair relationships.

As reported earlier Rottensteiner et al. However, there were no differences in REE, RER at rest or RFO between active and inactive co-twins. On average, the active co-twins tended to have higher PFO rates and lower FAT MAX when compared with the inactive co-twins, but the differences were not statistically significant.

Figures include group means and standard deviations. Colours represent the same twin pairs in both charts. Note the different scale in the y -axis. RFO or PFO were not correlated with fasting glucose, fasting insulin or the Matsuda index in the twin individual-based analysis Table 4.

For the first time, our study data showed that fat oxidation rates at rest and during exercise were similar between MZ co-twins, even though the study group was enriched with pairs who had discordant LTPA habits. The co-twins also exhibited similar FAT MAX values and thus tended to reach PFO at the same absolute exercise intensities.

The finding supports those of Toubro et al. In a study involving male MZ twin pairs Bouchard et al. As the researchers also investigated the substrate use of dizygotic twins, they were able to control their analysis for the common environmental effect.

Their calculated heritability estimates ranged from 0. However, as RER only describes the relative use of energy substrates, this study broadens the concept by showing that absolute fat oxidation rates behave accordingly and supports the earlier suggestion that genes play a role in determining fat oxidation capacity during exercise Jeukendrup and Wallis ; Randell et al.

This assumption seems evident, as the large cross-sectional studies investigating fat oxidation during exercise have been able to describe only partly the observed inter-individual variability in PFO Venables et al. We identified a subpopulation of MZ twin pairs, where the co-twins differed in their past 3-year LTPA.

In this study, we found no differences between the co-twins in their systemic energy metabolism at rest or during exercise. In previous observational studies, PFO was associated with self-reported physical activity Venables et al.

However, it is highly likely that physical activity participation and fat oxidation capacity have shared genetic factors, and the relationship noted in observational studies is partly genetically mediated.

In experimental studies, endurance-training interventions commonly increased PFO, at least in untrained populations reviewed by Maunder et al. Earlier mechanistic evidence from our laboratory also supports the role of physical activity as a modulator of PFO.

In same-sex twin pairs, an over year long physical activity discordance led to significant differences in myocellular gene expression related to oxidative phosphorylation and lipid metabolism Leskinen et al.

The effects of physical activity on RFO have been investigated less, with mixed results. A modest increase in fat oxidation rates at rest has been reported in some Barwell et al.

When the current scientific evidence is taken together with our results, physical activity seems to be able to influence PFO, while its effect on RFO is questionable.

However, we found no association between PFO and the Matsuda index, our main surrogate of insulin sensitivity. As explained in the methods section, the Matsuda index is influenced by fasting values, which were not associated with PFO in our study.

Previously, Robinson et al. As Robinson et al. However, it should be mentioned that PFO does not always seem to be associated with a healthier metabolic phenotype because an obesity-related increase in fatty acid availability has also been linked to higher PFO Ara et al.

In contrary to PFO, RFO was not associated with a healthy metabolic response to the OGTT. Previous studies have noted mixed findings. Rosenkilde et al.

However, there were no differences in fasting glucose or insulin levels between the groups. Some case—control studies Perseghin et al.

An elevated RFO could potentially function as a protective mechanism against insulin resistance Perseghing et al. Overall, further research is needed to clarify the interaction between systemic fat oxidation and metabolic health. Our study has both strengths and limitations.

A key strength was our ability to measure RFO and PFO in 21 and 19 MZ twin pairs, respectively. This enabled us to investigate the influence of hereditary factors on RFO and PFO in a reasonably sized study group. The calculated ICCs represent the upper bound of heritability, as differences between MZ twins are due to non-genetic factors.

However, as MZ twin pairs share also many aspects of their development and environment, the actual heritability of the trait may be lower.

A more precise estimation of heritability would require several kinds of relatives for quantitative trait modeling or very large study population for measurement of all genetic variation by whole genome sequencing. Additionally, since our study included only males, the results cannot be generalised to females.

This enabled us to conduct a more in-depth examination of the possible associations between fat oxidation and metabolic health. However, our study protocol was not optimal for PFO determination, which should be considered when interpreting the results.

Nutrition intake the day before Støa et al. In this study, we did not control for the nutrition intake before the exercise test. For example, this could partially explain why we did not find any association between RFO and PFO, as previously shown by Robinson et al.

Moreover, we used 2-min exercise stages during PFO testing. The 2-min stages might be too short to reach a steady-state, especially for the subjects with lower cardiorespiratory fitness Dandanell et al.

To assess whether the stage duration excessively affected the results, we compared VO 2 and VCO 2 between intervals 90— s and — s of the PFO-stage. There were no systematic differences in VO 2 or VCO 2 between the intervals. Removing these participants from the analyses did not materially change the results.

Therefore, the influence of the stage duration was considered acceptable. Thus, the measurements seemed to reflect the PFO of our study participants.

In conclusion, we show that fat oxidation rates at rest and during exercise are similar between MZ co-twins. Our results support the suggestion that hereditary factors influence fat oxidation capacity.

The internal factors likely set the baseline for fat oxidation capacity that the external factors can modulate. In our study, the role of physical activity seemed smaller, especially concerning RFO.

Furthermore, we observed that only higher capacity to utilize fatty acids during exercise associated with better metabolic health. Aaltonen S, Ortega-Alonso A, Kujala UM, Kaprio J Genetic and environmental influences on longitudinal changes in leisure-time physical activity from adolescence to young adulthood.

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Appl Physiol Nutr Metab 42 4 — Fat oxidation rates increase from low to moderate intensities and then decrease when the intensity becomes high. The mode of exercise can also affect fat oxidation, with fat oxidation being higher during running than cycling.

Endurance training induces a multitude of adaptations that result in increased fat oxidation. The duration and intensity of exercise training required to induce changes in fat oxidation is currently unknown. GENUD Toledo Research Group Growth, Exercise, NUtrition and Development , University of Castilla-La Mancha, Toledo, Spain.

Department of Physiatry and Nursing, University of Zaragoza, Zaragoza, Spain. Department of Biomedical Sciences, Center for Healthy Aging, University of Copenhagen, Copenhagen, Denmark. Department of Physical Education, University of Las Palmas de Gran Canaria, Canary Island, Spain.

Institute of Sports Medicine Copenhagen, Bispebjerg Hospital, Copenhagen, Denmark. Department of Rheumatology, Rigshospitalet, University of Copenhagen, Copenhagen, Denmark.

You can also search for this author in PubMed Google Scholar. Correspondence to I Ara. Reprints and permissions. Ara, I. et al. Normal mitochondrial function and increased fat oxidation capacity in leg and arm muscles in obese humans.

Int J Obes 35 , 99— Download citation. Received : 21 December Revised : 06 April Accepted : 03 May Published : 15 June Issue Date : January Anyone you share the following link with will be able to read this content:.

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European Journal of Applied Physiology Skip to main content Thank you for visiting nature. nature international journal of obesity original article article. Subjects Fat metabolism Mitochondria Obesity. Methods: Indirect calorimetry was used to calculate fat and carbohydrate oxidation during both progressive arm-cranking and leg-cycling exercises.

Results: During the graded exercise tests, peak fat oxidation during leg cycling and the relative workload at which it occurred FatMax were higher in PO and O than in C. Conclusions: In O subjects, maximal fat oxidation during exercise and the eliciting relative exercise intensity are increased.

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This article is cited by Toward Exercise Guidelines for Optimizing Fat Oxidation During Exercise in Obesity: A Systematic Review and Meta-Regression Isaac A.