Baseline subcellular lipid droplet distributions in arm (m. triceps brachii) and leg (m. vastus lateralis) muscle fibres (n = 20 observations) A, subcellular lipid droplet volume fractions. * P < 0.0001 main limb effect in intermyofibrillar and subsarcolemmal regions; †P = 0.0002 main fibre type effect in intermyofibrillar region. B, subcellular lipid droplet size in diameter. * P < 0.001 main limb effect in type 2 fibres; †P = 0.007 main fibre type effect in leg muscle. C, subcellular lipid droplet numbers. * P < 0.0001 main limb effect in intermyofibrillar and subsarcolemmal regions; †P < 0.0001 main fibre type effect in intermyofibrillar region. Bars and lines represent medians with interquartile range (A and C), or means with 95% confidence intervals (B).

Baseline subcellular lipid droplet distributions in arm (m. triceps brachii) and leg (m. vastus lateralis) muscle fibres (n = 20 observations) A, subcellular lipid droplet volume fractions. * P < 0.0001 main limb effect in intermyofibrillar and subsarcolemmal regions; †P = 0.0002 main fibre type effect in intermyofibrillar region. B, subcellular lipid droplet size in diameter. * P < 0.001 main limb effect in type 2 fibres; †P = 0.007 main fibre type effect in leg muscle. C, subcellular lipid droplet numbers. * P < 0.0001 main limb effect in intermyofibrillar and subsarcolemmal regions; †P < 0.0001 main fibre type effect in intermyofibrillar region. Bars and lines represent medians with interquartile range (A and C), or means with 95% confidence intervals (B).

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Key points: Although lipid droplets in skeletal muscle are an important energy source during endurance exercise, our understanding of lipid metabolism in this context remains incomplete. Using transmission electron microscopy, two distinct subcellular pools of lipid droplets can be observed in skeletal muscle - one beneath the sarcolemma and the o...

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Context 1
... fraction. The baseline lipid droplet volume fraction, size and density in the different locations are shown in Fig. 2, with the absolute values of the lipid droplet volume fractions shown in Table 1. The intermyofibrillar and total lipid droplet volume fractions were 4-fold higher in leg than in arm muscles, and the subsarcolemmal lipid droplet volume fraction was 6-fold higher in leg than in arm muscles (P < 0.0001). Furthermore, the inter- ...
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... droplet size. At baseline, there was no difference in lipid droplet size between the intermyofibrillar and sub- sarcolemmal regions (Fig. 2B). However, in type 2 muscle fibres, lipid droplet diameter was found to be 24% larger in the legs than in the arms (P < 0.001), while in type 1 muscle fibres, lipid droplet diameter tended to be 12% larger in the legs than in the arms (P = 0.05). Furthermore, lipid droplet diameter was 8% larger in type 2 than in type 1 fibres (P = ...
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... droplet numbers. Intermyofibrillar and sub- sarcolemmal lipid droplet numbers were found to be 59-60% higher in leg than in arm muscles (P < 0.001) (Fig. 2C). Comparing fibre types in upper and lower limbs, intermyofibrillar lipid droplet numbers were observed to be 68% higher in type 1 than in type 2 fibres (P < 0.0001), while there was no difference between subsarcolemmal lipid droplet numbers (P = ...

Citations

... It has recently been reported that~35% of isolated rat cortical astrocytes 24 h after the seeding (~6 × 10 4 cells/coverslip) contain multiple LDs with an average diameter of~450 nm (ranging between 0.2 and 1 µm; [37]), comparable with the measurements performed on hepatocytes [22,43], skeletal muscle [44,45], and tumor cell lines [46]. In contrast, adipocytes normally contain a single LD, occupying most of the cytoplasm and ranging up to 100 µm in diameter [47]. ...
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In recent years, increasing evidence regarding the functional importance of lipid droplets (LDs), cytoplasmic storage organelles in the central nervous system (CNS), has emerged. Although not abundantly present in the CNS under normal conditions in adulthood, LDs accumulate in the CNS during development and aging, as well as in some neurologic disorders. LDs are actively involved in cellular lipid turnover and stress response. By regulating the storage of excess fatty acids, cholesterol, and ceramides in addition to their subsequent release in response to cell needs and/or environmental stressors, LDs are involved in energy production, in the synthesis of membranes and signaling molecules, and in the protection of cells against lipotoxicity and free radicals. Accumulation of LDs in the CNS appears predominantly in neuroglia (astrocytes, microglia, oligodendrocytes, ependymal cells), which provide trophic, metabolic, and immune support to neuronal networks. Here we review the most recent findings on the characteristics and functions of LDs in neuroglia, focusing on astrocytes, the key homeostasis-providing cells in the CNS. We discuss the molecular mechanisms affecting LD turnover in neuroglia under stress and how this may protect neural cell function. We also highlight the role (and potential contribution) of neuroglial LDs in aging and in neurologic disorders.
... There is growing interest in elucidating how contracting muscles use lipid droplets (LD) in order to sustain exercise metabolism and/or for unknown purposes. For instance, during high-volume, high-intensity exercise (i.e., 57 min and 11 mmol/L blood lactate), LD within myofibrils but not those LD located close to the sarcolemma are used [58]. This is surprising as this kind of exercise mostly relies on oxidative metabolism mainly through fatty acid and glucose oxidation [59]. ...
... Furthermore, during innate immune response, LD increase their size [62]. This may also reveal different effects of LDs within skeletal muscle fibers, for instance, type 2 fibers have lower LD density, but they seems to have higher size than those LD form type 1 fibers in the leg skeletal muscle [58]. Although the current evidence is scarce, it would be interesting to test whether exercise impacts innate immune function by altering LD-mitochondria contact. ...
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The incidence and severity of metabolic diseases can be reduced by introducing healthy lifestyle habits including moderate exercise. A common observation in age-related metabolic diseases is an increment in systemic inflammation (the so-called inflammaging) where mitochondrial reactive oxygen species (ROS) production may have a key role. Exercise prevents these metabolic pathologies, at least in part, due to its ability to alter immunometabolism, e.g., reducing systemic inflammation and by improving immune cell metabolism. Here, we review how exercise regulates immunometabolism within contracting muscles. In fact, we discuss how circulating and resident macrophages alter their function due to mitochondrial signaling, and we propose how these effects can be triggered within skeletal muscle in response to exercise. Finally, we also describe how exercise-induced mitochondrial adaptations can help to fight against virus infection. Moreover, the fact that moderate exercise increases circulating immune cells must be taken into account by public health agencies, as it may help prevent virus spread. This is of interest in order to face not only acute respiratory-related coronavirus (SARS-CoV) responsible for the COVID-19 pandemic but also for future virus infection challenges.
... To the best of our knowledge, a relationship between PFO or Fat max and performance has not been elucidated in endurance sports dominated by upper body exercise. Interestingly, there are limb-specific differences in fiber-type composition, glycolytic and oxidative capacity, oxygen extraction, and storage of intramuscular triacylglycerol in highly trained cross-country (CC) skiers with equally trained upper and lower bodies, [13][14][15][16] which implies that the relationship between fat oxidation and performance may be different in upper body compared with lower body exercise. Furthermore, even at similar exercise intensities different fat oxidation rates have been reported between whole-body exercise modalities, [17][18][19] but it remains unknown whether this affects the potential relationship between peak fat oxidation and prolonged endurance performance. ...
... The reliance on intramuscular triacylglycerol (IMTG) is higher in trained compared with untrained individuals, 47,48 and in a study on elite CC-skiers, a 53% reduction in IMTG volume fraction in m. triceps brachii together with a solid positive correlation between IMTG degradation during a 20-km CC-skiing time trial and baseline IMTG volume fraction was reported. 16 This implies that the higher peak fat oxidation in ES compared with RS might derive from superior IMTG utilization. ...
Article
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The peak fat oxidation rate (PFO) and the exercise intensity that elicits PFO (Fatmax) is associated with endurance performance during exercise primarily involving lower body musculature, but it remains elusive whether these associations are present during predominant upper body exercise. The aim was to investigate the relationship between PFO and Fatmax determined during a graded exercise test on a ski‐ergometer using double poling (GET‐DP) and performance in the long‐distance cross‐country skiing race, Vasaloppet. Forty‐three healthy men completed GET‐DP and Vasaloppet and were divided into two subgroups; recreational (RS, n=35) and elite skiers (ES, n=8). Additionally, RS completed a cycle‐ergometer GET (GET‐Cycling) to elucidate whether the potential relationships were specific to exercise modality. PFO (r²=0.10, p=0.044) and Fatmax (r²=0.26, p<0.001) were correlated with performance, however, V̇O2peak was the only independent predictor of performance (adj. R²=0.36) across all participants. In ES, Fatmax was the only variable associated with performance (r²=0.54, p=0.038). Within RS, DP V̇O2peak (r²=0.11, p=0.047) and ski specific training background (r²=0.30, p=0.001) were associated with performance. Between the two GETs Fatmax (r²=0.20, p=0.006) but not PFO (r²=0.07, p=0.135) was correlated. Independent of exercise mode, neither PFO nor Fatmax were associated with performance in RS (p>0.05). These findings suggest that prolonged endurance performance is related to PFO and Fatmax but foremost to V̇O2peak during predominant upper body exercise. Interestingly, Fatmax may be an important determinant of performance among ES. Among RS, DP V̇O2peak and skiing experience appeared as performance predictors. Additionally, whole‐body fat oxidation seemed specifically coupled to exercise modality.
... First, we investigated fibre type and region (peripheral and central)-specific differences in IMTG content detected using BODIPY and ORO in muscle biopsies obtained after an overnight fast (study 1). In this context, subcellular distribution is an important consideration because IMTG use during exercise occurs primarily in the central (intermyofibrillar) region of type I fibres of the cell (Jevons et al. 2020;Koh et al. 2017), and IMTG deposited in the peripheral (subsarcolemmal) region is inversely related to insulin sensitivity Nielsen et al. 2010). We also critically reviewed several steps in the immunohistochemical protocol and analytical procedure that can affect the visualisation and quantification of IMTG content. ...
... White bar = 30 µm be considered to more accurately reflect subcellular differences in IMTG content. In this regard, studies employing transmission electron microscopy to investigate subcellular lipid content support this assertion, because they also demonstrate IMTG content to be greater in the central (or intermyofibrillar) region (Koh et al. 2017). Thus, it appears that the choice of lipid dye will have implications for the conclusions that are drawn from studies investigating regional differences in IMTG content and LD morphology in human skeletal muscle in relation to health and disease. ...
... While IMTG utilisation was of a similar magnitude in the central and peripheral regions when using BODIPY, utilisation of IMTG was specific to the peripheral region when examined using ORO. This latter finding is in direct contrast with a recent study employing transmission electron microscopy which reported that IMTG utilisation during high-intensity exercise occurred exclusively in the intermyofibrillar region (Koh et al. 2017). Whilst we acknowledge that using BODIPY may also not entirely replicate the observations made using high-resolution electron microscopy (because we observed exercise-induced decreases in IMTG content in both subcellular regions), our data suggest subcellularspecific IMTG utilisation during exercise should not be investigated using ORO. ...
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Despite over 50 years of research, a comprehensive understanding of how intramuscular triglyceride (IMTG) is stored in skeletal muscle and its contribution as a fuel during exercise is lacking. Immunohistochemical techniques provide information on IMTG content and lipid droplet (LD) morphology on a fibre type and subcellular-specific basis, and the lipid dye Oil Red O (ORO) is commonly used to achieve this. BODIPY 493/503 (BODIPY) is an alternative lipid dye with lower background staining and narrower emission spectra. Here we provide the first quantitative comparison of BODIPY and ORO for investigating exercise-induced changes in IMTG content and LD morphology on a fibre type and subcellular-specific basis. Estimates of IMTG content were greater when using BODIPY, which was predominantly due to BODIPY detecting a larger number of LDs, compared to ORO. The subcellular distribution of intramuscular lipid was also dependent on the lipid dye used; ORO detects a greater proportion of IMTG in the periphery (5 μm below cell membrane) of the fibre, whereas IMTG content was higher in the central region using BODIPY. In response to 60 min moderate-intensity cycling exercise, IMTG content was reduced in both the peripheral (- 24%) and central region (- 29%) of type I fibres (P < 0.05) using BODIPY, whereas using ORO, IMTG content was only reduced in the peripheral region of type I fibres (- 31%; P < 0.05). As well as highlighting some methodological considerations herein, our investigation demonstrates that important differences exist between BODIPY and ORO for detecting and quantifying IMTG on a fibre type and subcellular-specific basis.
... In addition, the site of lipid storage, with athletes having more lipid droplets in the intramyofibrillar area than individuals with type 2 diabetes, spatially and functionally matches a high lipid dropletderived fat oxidative capacity. Indeed, reduction in lipid droplet number and content in the intramyofibrillar area upon acute exercise is observed [8,16], suggesting a preferential utilisation of intramyofibrillar lipid droplets during exercise. ...
... Data on changes in lipid droplet-mitochondria tethering during exercise are only available for endurance-trained athletes. In male elite cross-country skiers, lipid dropletmitochondria interactions increase upon an acute exercise bout despite unaltered IMCL content [16]. In endurancetrained 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 dropletmitochondria 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). ...
Article
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Fatty acids are an important energy source during exercise. Training status and substrate availability are determinants of the relative and absolute contribution of fatty acids and glucose to total energy expenditure. Endurance-trained athletes have a high oxidative capacity, while, in insulin-resistant individuals, fat oxidation is compromised. Fatty acids that are oxidised during exercise originate from the circulation (white adipose tissue lipolysis), as well as from lipolysis of intramyocellular lipid droplets. Moreover, hepatic fat may contribute to fat oxidation during exercise. Nowadays, it is clear that myocellular lipid droplets are dynamic organelles and that number, size, subcellular distribution, lipid droplet coat proteins and mitochondrial tethering of lipid droplets are determinants of fat oxidation during exercise. This review summarises recent insights into exercise-mediated changes in lipid metabolism and insulin sensitivity in relation to lipid droplet characteristics in human liver and muscle. Graphical abstract
... Muscle contains two distinct types of mitochondria, intermyofibular and subsarcolemmal, each with a corresponding pool of LDs. In highly trained skiers, LDs positioned in the intermyofibrillar region, but not LDs positioned in the subsarcolemmal region, were consumed during 1 hour of exhaustive exercise (Koh et al., 2017). Another study obtained similar results, showing that trained individuals store the bulk of their LDs in the intermyofibrillar region while individuals with type 2 diabetes had a higher portion in the subsarcolemmal region. ...
Article
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Lipid droplets (LDs) are fat storage organelles integral to energy homeostasis and a wide range of cellular processes. LDs physically and functionally interact with many partner organelles, including the endoplasmic reticulum, mitochondria, lysosomes, and peroxisomes. Recent findings suggest that the dynamics of LD interorganelle contacts is in part controlled by LD intracellular motility. LDs can be transported directly by motor proteins along either actin filaments or microtubules, via Kinesin-1, Cytoplasmic Dynein, and type V Myosins. LDs can also be propelled indirectly, by hitchhiking on other organelles, cytoplasmic flows, and potentially actin polymerization. Although the anchors that attach motors to LDs remain elusive, other regulators of LD motility have been identified, ranging from modification of the tracks to motor cofactors to members of the perilipin family of LD proteins. Manipulating these regulatory pathways provides a tool to probe whether altered motility affects organelle contacts and has revealed that LD motility can promote interactions with numerous partners, with profound consequences for metabolism. LD motility can cause dramatic redistribution of LDs between a clustered and a dispersed state, resulting in altered organelle contacts and LD turnover. We propose that LD motility can thus promote switches in the metabolic state of a cell. Finally, LD motility is also important for LD allocation during cell division. In a number of animal embryos, uneven allocation results in a large difference in LD content in distinct daughter cells, suggesting cell-type specific LD needs.
... Interestingly, lower fat oxidation rates were reported during double poling (predominantly upper body exercise) compared with leg skiing in highly trained CC skiers. 12 Lower fat oxidation rates during double poling in highly trained CC skiers may be a consequence of limb-specific differences in the capacities for free fatty acid deliverance and uptake in the skeletal muscle 13 and the utilization of intramuscular triacylglycerol, 14 which could be attributed to limb differences in fiber type composition, glycolytic and oxidative activity, oxygen extraction, and intramuscular triacylglycerol storage. [14][15][16] These findings indicate differences in substrate utilization between limbs, which may reflect a difference in the relationship between fat oxidation rates and performance during prolonged upper body and lower body exercise (≥4 hours). ...
... 12 Lower fat oxidation rates during double poling in highly trained CC skiers may be a consequence of limb-specific differences in the capacities for free fatty acid deliverance and uptake in the skeletal muscle 13 and the utilization of intramuscular triacylglycerol, 14 which could be attributed to limb differences in fiber type composition, glycolytic and oxidative activity, oxygen extraction, and intramuscular triacylglycerol storage. [14][15][16] These findings indicate differences in substrate utilization between limbs, which may reflect a difference in the relationship between fat oxidation rates and performance during prolonged upper body and lower body exercise (≥4 hours). In order to further elucidate potential differences in fat oxidation rates between limbs, a GE protocol assessing upper body PFO and Fat max should be suitable to CC skiers. ...
Article
Full-text available
Peak fat oxidation rate (PFO) and the intensity that elicits PFO (Fatmax) are commonly determined by a validated graded exercise test (GE) on a cycling‐ergometer with indirect calorimetry. However, for upper body exercise fat oxidation rates are not well elucidated and no protocol has been validated. Thus, our aim was to test validity and inter‐method reliability for determination of PFO and Fatmax in trained men using a GE protocol applying double poling on a ski‐ergometer. PFO and Fatmax was assessed during two identical GE tests (GE1 and GE2) and validated against separated short continuous exercise bouts (SCE) at 35%, 50% and 65% of V̇O2peak on the ski‐ergometer in 10 trained men (V̇O2peak: 65.1±1.0 ml·min−1·kg−1, mean±SEM). Between GE tests no differences were found in PFO (GE1: 0.42±0.03; GE2: 0.45±0.03 g·min−1, p=0.256) or Fatmax (GE1: 41±2%; GE2: 43±3% of V̇O2peak, p=0.457) and the intra‐individual coefficient of variation (CV) was 8±2% and 11±2% for PFO and Fatmax, respectively. Between GE and SCE tests PFO (GEavg: 0.44±0.03; SCE; 0.47±0.06 g·min−1, p=0.510) was not different, whereas a difference in Fatmax (GEavg: 42±2%; SCE: 52±4% of V̇O2peak, p=0.030) was observed with a CV of 17±4% and 15±4% for PFO and Fatmax, respectively. In conclusion, GE has a high day‐to‐day reliability in determination of PFO and Fatmax in trained men, whereas it is unclear if PFO and Fatmax determined by GE reflect continuous exercise in general. This article is protected by copyright. All rights reserved.
... LDs are found in two distinct locations in muscle, just beneath the cell membrane (subsarcolemmal LDs) and between myofibrils (intermyofibrillar LDs) [44]. The total cellular volume of intermyofibrillar LDs greatly exceeds that of the subsarcolemmal pool [44]. ...
... LDs are found in two distinct locations in muscle, just beneath the cell membrane (subsarcolemmal LDs) and between myofibrils (intermyofibrillar LDs) [44]. The total cellular volume of intermyofibrillar LDs greatly exceeds that of the subsarcolemmal pool [44]. There is a substantial drop in total IMCL following acute exercise suggesting the use of IMCL as an energy reservoir for the muscle [11]. ...
... An analysis of LDs in muscle tissue before and after exhaustive exercise found a measurable drop in the cellular fraction of the intermyofibrillar but not the subsarcolemmal LDs [44]. Similarly, in another study, a reduction in the intermyofibrillar lipid pools was seen in endurance athletes after moderate and high-intensity exercise [45]. ...
Article
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The lipid droplet (LD) is an organelle enveloped by a monolayer phospholipid membrane with a core of neutral lipids, which is conserved from bacteria to humans. The available evidence suggests that the LD is essential to maintaining lipid homeostasis in almost all organisms. As a consequence, LDs also play an important role in pathological metabolic processes involving the ectopic storage of neutral lipids, including type 2 diabetes mellitus (T2DM), atherosclerosis, steatosis, and obesity. The degree of insulin resistance in T2DM patients is positively correlated with the size of skeletal muscle LDs. Aerobic exercise can reduce the occurrence and development of various metabolic diseases. However, trained athletes accumulate lipids in their skeletal muscle, and LD size in their muscle tissue is positively correlated with insulin sensitivity. This phenomenon is called the athlete’s paradox. This review will summarize previous studies on the relationship between LDs in skeletal muscle and metabolic diseases and will discuss the paradox at the level of LDs.
... Ten elite male Norwegian cross-country skiers participated in the study, as part of a larger project and related data from the project has already been published Ørtenblad et al., 2011;Koh et al., 2017). Their mean (±SD) age, height, weight, andVO 2max were 22 ± 1 yr, 181 ± 2 cm, 79 ± 8 kg, and 5.37 ± 0.46 L·min −1 (69 ± 5 ml·kg −1 ·min −1 ), respectively ( Table 1) and a hematocrit of 47 ± 1% and hemoglobin of 155 ± 2 mmol/l. ...
... Weighing the different fiber type distribution in leg and arm muscle, the mitochondrial volume fraction was equal in both ( Figure 3D). This suggests that arm muscles, despite lower fat oxidation capacity (Helge, 2010), HAD activity (present data), lower IMCL content (Koh et al., 2017), and higher lactate release during exercise (Van Hall et al., 2003), still require a high mitochondrial oxidative capacity. Indeed, there was a tendency (P = 0.095) toward a 10% higher mitochondrial volume fraction in the fibers from the arms compared with the legs (Figure 3C), predominantly due to a tendency to higher volume fraction in type 2 fibers in the arms ( Figure 3C). ...
... We have recently reported in a companion paper (Koh et al., 2017) that, in these subjects with highly trained upper and lower body, the IMCL volume fraction was fourfold higher in leg muscle than in the arm muscle. The higher content of IMCL content was apparent in both the IMF and the SS regions. ...
Article
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As one of the most physically demanding sports in the Olympic Games, cross-country skiing poses considerable challenges with respect to both force generation and endurance during the combined upper- and lower-body effort of varying intensity and duration. The isoforms of myosin in skeletal muscle have long been considered not only to define the contractile properties, but also to determine metabolic capacities. The current investigation was designed to explore the relationship between these isoforms and metabolic profiles in the arms (triceps brachii) and legs (vastus lateralis) as well as the range of training responses in the muscle fibers of elite cross-country skiers with equally and exceptionally well-trained upper and lower bodies. The proportion of myosin heavy chain (MHC)-1 was higher in the leg (58 ± 2% [34–69%]) than arm (40 ± 3% [24–57%]), although the mitochondrial volume percentages [8.6 ± 1.6 (leg) and 9.0 ± 2.0 (arm)], and average number of capillaries per fiber [5.8 ± 0.8 (leg) and 6.3 ± 0.3 (arm)] were the same. In these comparable highly trained leg and arm muscles, the maximal citrate synthase (CS) activity was the same. Still, 3-hydroxy-acyl-CoA-dehydrogenase (HAD) capacity was 52% higher (P < 0.05) in the leg compared to arm muscles, suggesting a relatively higher capacity for lipid oxidation in leg muscle, which cannot be explained by the different fiber type distributions. For both limbs combined, HAD activity was correlated with the content of MHC-1 (r2 = 0.32, P = 0.011), whereas CS activity was not. Thus, in these highly trained cross-country skiers capillarization of and mitochondrial volume in type 2 fiber can be at least as high as in type 1 fibers, indicating a divergence between fiber type pattern and aerobic metabolic capacity. The considerable variability in oxidative metabolism with similar MHC profiles provides a new perspective on exercise training. Furthermore, the clear differences between equally well-trained arm and leg muscles regarding HAD activity cannot be explained by training status or MHC distribution, thereby indicating an intrinsic metabolic difference between the upper and lower body. Moreover, trained type 1 and type 2A muscle fibers exhibited similar aerobic capacity regardless of whether they were located in an arm or leg muscle.
... At the microscopic level (shown by the lower limb of the 'Absorption-H-Storage-Micro' pathway in Figure 2), storage can be visualised as both glycogen particles and lipid droplets, especially in myocytes from striated muscles where the latter may occupy upwards of 15% of sub-sarcolemmal volume. 32 The total enthalpy contribution from these two sources is modest -albeit not without consequences to health if excessive. Of more immediate concern is macroscopic storage, which results when ΔH supply (in the form of food intake) exceeds ΔH demand (in the form of work or heat production), as shown schematically by the thick dark lines in Figure 2. ...
Article
We exploit the detail-independence feature of thermodynamics to examine issues related to the development of obesity. We adopt a ‘global’ approach consistent with focus on the First Law – namely that the metabolic energy provided by dietary foodstuffs has only three possible fates: the performance of work (be it microscopic or macroscopic), the generation of heat, or storage - primarily in the form of adipose tissue. Quantification of the energy expended, in the form of fat metabolised, during selected endurance events, reveals the inherent limitation of over-reliance on exercise as a primary agent of weight-loss. This result prompts examination of various (non-exercise based) possibilities of increasing the rate of heat loss. Since these, too, give little cause for optimism, we are obliged to conclude that obesity can be prevented, or weight-loss achieved, only if exercise is supplemented by reduction of food intake.