The Regulation of Fat Metabolism during Aerobic Exercise

Since the lipid pro le is altered by physical activity, the study of lipid metabolism is a remarkable element in understanding if and how physical activity a ects the health of both professional athletes and sedentary subjects. Although not fully de ned, it has become clear that resistance exercise uses fat as an energy source. The fatty acid oxidation rate is the result of the following processes: (a) triglycerides lipolysis, most abundant in fat adipocytes and intramuscular triacylglycerol (IMTG) stores, (b) fatty acid transport from blood plasma to muscle sarcoplasm, (c) availability and hydrolysis rate of intramuscular triglycerides, and (d) transport of fatty acids through the mitochondrial membrane. In this review, we report some studies concerning the relationship between exercise and the aforementioned processes also in light of hormonal controls and molecular regulations within fat and skeletal muscle cells. Keywords: lipid metabolism; endurance exercise; plasma fatty acids; lipoprotein; high-density lipoprotein (HDL); low-density lipoprotein (LDL) 1. Introduction Regular physical activity is important not only for mental health but also for physical health. Exercise training has implications in epigenetic regulation [1], aging [2], improvement of glycemic control in patients with type 2 diabetes mellitus and insulin sensitivity and resistance [3,4], prevention of cardiovascular diseases [5–7], and others such as multiple sclerosis, lung diseases, Parkinson's disease, and so on [8–13]. Thus, the study of lipid metabolism is a key element to understand how physical activity in uences our health and, in particular, that of professional athletes. Several studies have highlighted the di erences between athletes and sedentary subjects, although some di erences among sport disciplines exist [14]. Fat and carbohydrate provide the most important form of fuel for exercise and sports activities. During exercise, there are four major endogenous

element to understand how physical activity in uences our health and, in particular, that of professional athletes. Several studies have highlighted the di erences between athletes and sedentary subjects, although some di erences among sport disciplines exist [14]. Fat and carbohydrate provide the most important form of fuel for exercise and sports activities. During exercise, there are four major endogenous sources of energy: plasma glucose derived from liver glycogenolysis, free fatty acids (FFAs) released from adipose tissue lipolysis and from the hydrolysis of triacylglycerol (TG) in very low-density lipoproteins (VLDL-TG), and muscle glycogen and intramyocellular triacylglycerols (IMTGs) available within the skeletal muscle bers. Fats and carbohydrates are oxidized simultaneously, but their relative contribution depends on a variety of factors, exercise duration and intensity included. Substrate utilization as fuel sources during physical activity is also highly in uenced by the type of exercise. Endogenous triacylglycerols represent the largest energy reserve in the body, 60 times greater than the amount of energy stored as glycogen. In a lean adult man, most triacylglycerols are stored in adipose tissue ( 17,500 mmol), skeletal muscle ( 300 mmol), and plasma ( 0.5 mmol) [15]. Furthermore, Biomolecules2021,10, 1699; doi:10.3390 /biom10121699 /journal/biomolecules

Biomolecules2021,10, 1699 2 of 32 liver and pancreas, together with muscle, represent ectopic fat deposition sites [16]. Actually, the amount of FFA available from muscle triglycerides is not accurately known; in fact, it is not easy to discriminate between FFA coming from the lipid droplets inside the muscle bers or from the adipocytes present between the bers [17,18]. However, the signi cant quantity of FFA used during exercise comes solely from adipose tissue and muscle [17,19,20]. The substrate used to derive energy during exercise depends on the duration and intensity of the latter: glucose utilization is greater during high exercise intensity, while FAs oxidation increases during moderate exercise intensity [21,22]. In fact, there is a balance between carbohydrate and lipids that in uence their utilization: this phenomenon is called `Randle cycle' and consists of inhibiting glucose uptake and oxidation in muscle when FAs oxidation is intense. Conversely, `reverse Randle cycle' occurs when hyperglycemia can reduce FAs oxidation [23]. This review mainly concentrates on ndings in humans, and particular attention will be paid to lipid metabolism during aerobic exercise, particularly emphasizing hormonal controls and molecular regulations within fat and skeletal muscle cells. It is acceptable to imagine that understanding how and why lipid metabolism varies during physical performance can improve health through aerobic exercise. Articles included in the review are shown in Table. 2. Lipids as a Form of Energy during Exercise Fat digestion occurs in the duodenum, due to the pancreatic lipase that releases monoacylglycerols (MAG), diacylglycerols (DAG), and FFA. Long-chain FA (LCFA) are absorbed into duodenum and reformed in triglycerides which, together with cholesterol and proteins, constitute the chylomicrons and are transported into the blood. Muscle and fat cells receive FAs from chylomicrons [24]. Triglycerides and plasma cholesterol are transported in four main classes of lipoproteins: (1) chylomicrons, (2) very low-density lipoproteins (VLDL) rich in triglycerides, (2) intermediate-density lipoproteins (IDL), (3) low-density lipoproteins (LDL) rich in cholesterol, and (4) high-density lipoproteins (HDL). High-density lipoprotein plays an essential role in plasma lipid transport, providing to the metabolism of chylomicrons and VLDL and acting as a scavenger of surplus unesteri ed

transported in four main classes of lipoproteins: (1) chylomicrons, (2) very low-density lipoproteins (VLDL) rich in triglycerides, (2) intermediate-density lipoproteins (IDL), (3) low-density lipoproteins (LDL) rich in cholesterol, and (4) high-density lipoproteins (HDL). High-density lipoprotein plays an essential role in plasma lipid transport, providing to the metabolism of chylomicrons and VLDL and acting as a scavenger of surplus unesteri ed cholesterol from these lipoproteins. The chylomicron particle number remains unchanged following acute and chronic aerobic exercise [25,26]. Interestingly, after six months of resistance exercise intervention in diabetic adults, a signi cant reduction in the concentration of apo B48 is obtained, which is present in chylomicrons and in their remnants [27]. In addition, resistance exercise decreases triglycerides and cholesterol within chylomicrons, in healthy sedentary men [28], and also endogenous and meal-derived FA incorporation into chylomicron-TG and TRL-TG, in overweight/obese men with prediabetes [29]. The aerobic and/or resistance exercise decrease total cholesterol and LDL-C and increase high-density lipoprotein- cholesterol (HDL-C) [30,31]. High-density lipoprotein- cholesterol concentrations are inversely associated with risk for cardiovascular disease [32], thus exercise interventions are routinely prescribed to decrease the risk of cardio-metabolic complications by promoting an increase in HDL-C concentration [31,33–35]. Unfortunately, recent clinical trials aimed at reducing the risk of cardiovascular disease by increasing HDL-C levels have been unsuccessful [36]. However, several studies also showed that exercise training caused changes in HDL subclasses, favoring increases in larger HDL subclasses, independent of changes in body composition [31,37,38]. In addition, in adults with CVD, diabetes mellitus, and metabolic syndrome, regular exercise has bene cial e ects on various HDL functions, including endothelial protection [39], antioxidative [38,40], and anti-in ammatory properties [41,42]. Although VLDLs represent the main source of circulating triglycerides both in fasting and fed states, the FAs from labeled VLDL-TG were shown to comprise 3% of total energy utilization [43] or 13% of total FA oxidation [44] during moderate-intensity exercise in humans. VLDLs are converted to lipoproteins with intermediate (IDL) and low (LDL) densities, having low levels of triglycerides [19,45–48]. During fasting, FAs provide both local energy and ketone bodies that represent an energy source for heart and

labeled VLDL-TG were shown to comprise 3% of total energy utilization [43] or 13% of total FA oxidation [44] during moderate-intensity exercise in humans. VLDLs are converted to lipoproteins with intermediate (IDL) and low (LDL) densities, having low levels of triglycerides [19,45–48]. During fasting, FAs provide both local energy and ketone bodies that represent an energy source for heart and kidneys but are not regarded as part of the true triglyceride energy pool.

Biomolecules2021,10, 1699 3 of 32 2.1. Fat Metabolism and Endurance Training The contribution of carbohydrate and fats to the body's energy production depends on exercise duration and intensity, training condition, sex, body composition, and diet [49]. Since, at rest, the FAs released from adipose tissue surpass the quantity of FAs oxidized in the skeletal muscles, most of the FAs are re-esteri ed into liver triglycerides [15]. Then, fats are mainly oxidized at rest and at low aerobic exercise intensities, while carbohydrates are chie y used at high intensities of exercise. 2.1.1. The rate of Lipolysis is Modulated by Temperature Environmental temperature may also have some e ects. Environmental heat stress increases muscle glycogenolysis, hepatic glucose output, and whole-body carbohydrate oxidation rates, whilst it decreases fat oxidation rates at given intensities. From this, it could lead to the hypothesis that maximal fat oxidation (MFO) decreases in the heat compared to temperate conditions [50,51]. However, more recently, O'Hearn et al. [52] rated FFA concentration and oxidation in eight male subjects after passive heating at 42 C for 120 min and following exercise on a treadmill in the same temperature at 50% VO2maxfor 30 min. Plasma FFA concentration was signi cantly higher both following passive heating and exercise, compared to the control group (exercise at 23 C), whereas TG, cholesterol, and phospholipid levels did not di er. The high FFA concentration in the passively heated group was not related to a whole-body FA oxidation [52]. The e ect of cold environments on substrate metabolism during prolonged exercise is less certain. Some investigations have reported augmented carbohydrate utilization in cold vs. temperate conditions [53,54], whereas others suggested that fat utilization is augmented, and carbohydrate utilization is suppressed in the cold. The data disparities are probably due to interactions between the speci c environmental conditions and exercise modality (cycling vs. running) [55]. For example, during moderate-intensity cycling, greater fat oxidation rates at 11 C than at 21 C were reported, but this was suppressed at 4 C [53]. When carbohydrate and lipid oxidation were examined in six males rested for 3 h at 29 C

probably due to interactions between the speci c environmental conditions and exercise modality (cycling vs. running) [55]. For example, during moderate-intensity cycling, greater fat oxidation rates at 11 C than at 21 C were reported, but this was suppressed at 4 C [53]. When carbohydrate and lipid oxidation were examined in six males rested for 3 h at 29 C and at 5 C, it was observed that cold increased plasma glucose and plasma FFA ratios. In spite of enhanced lipolysis, only about half the rate of FFA is ultimately oxidized [56]. However, exercise performance could be in uenced by several factors that must be taken into account. For example, the surrounding medium (air or water), the exercise intensity, individual's anthropometric characteristics, body composition, and clothes can in uence results obtained at the same temperature [57]. 2.1.2. The Rate of Lipolysis Is Modulated by the Intensity of Physical Activities The metabolism of lipid includes lipolysis, their transport in the blood to the cytosol of the muscle, and the FAs transport to the mitochondria of running muscles to be oxidized in order to produce a great quantity of ATP. In the following sections, some studies regarding the relation between the mentioned phases and endurance training and physical tness are reported. Articles included in the review are shown in Table. Consistent with its central importance in lipid and energy homeostasis, lipolysis occurs in essentially all tissues and cell types. FAs derived from adipose tissue, muscle lipid droplets, and diet represent the main energy supply during exercise with intensities between 45% and 65% VO2max[58]. At a low to moderate intensity, as well as during prolonged exercise, most of the energy requirements for skeletal muscle can be met from predominantly FA oxidation, with a small contribution from glucose oxidation. On the contrary, glucose predominates as an energy substrate during short-term intense exercise [59]. Thus, when exercise intensity increases, the use of fat to total oxidative metabolism decreases [60,61]. The index that establishes the training load is the maximal oxygen consumption (also named as maximal oxygen uptake or maximal aerobic capacity, VO2max), which is the

small contribution from glucose oxidation. On the contrary, glucose predominates as an energy substrate during short-term intense exercise [59]. Thus, when exercise intensity increases, the use of fat to total oxidative metabolism decreases [60,61]. The index that establishes the training load is the maximal oxygen consumption (also named as maximal oxygen uptake or maximal aerobic capacity, VO2max), which is the maximum amount of oxygen that can be used in the unit of time by an individual, during a physical activity. VO2max varies over a wide range among individuals, depending on level of aerobic training, genetic makeup,

Biomolecules2021,10, 1699 4 of 32 age, health status, and sex. It de nes functional aerobic capacity of a single individual in a speci c exercise performance and re ects a person's cardiorespiratory tness level [62]. The source of FA changes during exercise: at 25% of VO2max, the oxidized fat derives from plasma FAs [20,60,61,63,64]. When exercise intensity increases, there is a shift from FA to glucose oxidation with a reduction in the percentage of the total energy requirement derived from fat oxidation and a reciprocal increase in carbohydrate oxidation, which becomes the main energy source when exercise reaches above ~80% of VO2max[58,65–67]. During exercise intensity at 65% of VO2max, the contribution of plasma FAs decreases and the rate of IMTG increases and provides about 50% of the FA for total fat oxidation [20,68,69]. Thus, peripheral lipolysis and, consequently, the release rate of FFA into plasma, is stimulated at maximum at the lowest exercise intensity and progressively decreases with increasing exercise intensity up to a point where the concentration of plasma FFA during exercise at 85% of VO2maxappears signi cantly suppressed. Fatty acids uptake from plasma lipoprotein triacylglycerols represents less than 3% of the energy consumed during prolonged exercise [67,83]. The increase of lipolysis and, consequently, the release rate of FFA in the plasma, is greater in endurance-trained, with respect to untrained subjects [63]. The majority of the studies have shown a decrement of TG after aerobic exercises, due to their mobilization from visceral and sub-cutaneous adipose tissues along with TG in the VLDL-C broken down to FFA by lipases [68,69,80,81]. It is well-known that marathon and middle-distance runners have di erent protocols of endurance training. While middle-distance runners adopt fast and discontinuous exercises, marathon runners execute most continuous running exercises. Then, the middle-distance runners, being faster, have a higher anaerobic capacity than marathon runners who have a higher VO2max, keeping in mind that di erent training histories and genetic di erences exist [84]. Muscle TG lipolysis is stimulated by high-intensity exercises. Therefore, after a high-intensity exercise, while lipolysis is immediately decreased, the release of FFA into the plasma increases, indicating

Then, the middle-distance runners, being faster, have a higher anaerobic capacity than marathon runners who have a higher VO2max, keeping in mind that di erent training histories and genetic di erences exist [84]. Muscle TG lipolysis is stimulated by high-intensity exercises. Therefore, after a high-intensity exercise, while lipolysis is immediately decreased, the release of FFA into the plasma increases, indicating that these derive from previously hydrolyzed triglycerides during the recovery [20]. Also, the marathon performance level correlates to a decrease of blood TG and to a proportional glycerol concentration increase, as revealed in a study performed on 14 top-class marathon runners, after a 10 km run at their individual marathon velocity [69]. In addition, in marathon runners, a signi cant glucose concentration increment, a longer and/or less unsaturated blood FA, and a higher aminoacidic production and blood release (resulting by catabolism of several proteins for amino acid supply to skeletal muscle), were also reported [69]. These results show that both carbohydrate, lipid, and amino acid metabolisms are necessary to improve energetic supply to skeletal muscle during runner exercise. Thus, the lipolytic response should not be di erent between endurance-trained and untrained men. In fact, plasma glycerol and FFA rate of appearance raised similar values in both ve endurance-trained (with 4 h of treadmill exercise) and ve control subjects [63]. A considerable blood FAs increment, during and after the race, was also measured in 18 non-professional, middle-aged runners of a 2-day ultramarathon (130 km). Conversely, plasma TG decreased on days 2 and 3, while HDL-C was elevated from day 2 to day 5 [81]. After moderate-intensity endurance exercise, lipolysis remains signi cantly elevated compared to rest for up to 24 h, thus even a single bout of exercise can in uence energy expenditure/balance over the next day [85]. Hetlelid et al. [86] demonstrated a three times higher fat oxidation in elite runners compared to non-elite runners during high-intensity exercise. Aslankeser and Balc [87] observed 17 times higher fat oxidation in an athlete group compared to an untrained group during high-intensity intermittent exercise (80% VO2max), while carbohydrate oxidation rate was the same

energy expenditure/balance over the next day [85]. Hetlelid et al. [86] demonstrated a three times higher fat oxidation in elite runners compared to non-elite runners during high-intensity exercise. Aslankeser and Balc [87] observed 17 times higher fat oxidation in an athlete group compared to an untrained group during high-intensity intermittent exercise (80% VO2max), while carbohydrate oxidation rate was the same in trained and untrained subjects. In 2017, Nieman et al. [72] conducted a study on twenty-four male runners in order to evaluate changes in metabolic pro le related to exercise intensity of 70% VO2max. After running, an increase in FA oxidation products (dicarboxylate and monohydroxy fatty acids, acylcarnitine) and ketone bodies as well as a decrease in muscle glycogen was found [70].

Biomolecules2021,10, 1699 5 of 32 Table 1.Articles included in the review. References Type of Exercise Type of Study Study Sample Results Low-Intensity Exercise Klein et al. (1994) [63] 4 h of treadmill exercise eliciting an oxygen uptake of 20 mL/kg/min. Glycerol and free fatty acid rate of appearance and lipid oxidation were evaluated during basal resting conditions and after 4 h of treadmill exercise and 1 h of recovery. n=5 endurance-trained men; n=5 untrained men. After 4 h of exercise, the average glycerol and free fatty acid values, was similar in both trained and untrained subjects; but during recovery, glycerol and free fatty acid values decreased more rapidly in trained than in untrained subjects. Triglyceride oxidation was greater during exercise in the trained than in the untrained group. Wolfe et al. (1990) [70] 4 h of treadmill exercise at 40% maximum O 2 consumption, and 2 h of recovery. Total fat oxidation was quanti ed by indirect calorimetry in response to exercise and in recovery from exercise. n=5 healthy male subjects. Rate of appearance of glycerol and free fatty acids increased after 30 min and 4 h of exercise. Lipolysis decreased rapidly from the rst 20 min to 2 h of recovery. Verboven et al. (2018) [71] 12-week exercise training Abdominal subcutaneous adipose tissue (SCAT) extracellular glycerol concentration and blood ow were measured using microdialysis at rest, during low-intensity endurance-type exercise and post-exercise recovery; at the same time, the response to -/ -adrenoceptor was evaluated. n=10 healthy lean insulin-sensitive menn=10 obese insulin-sensitive menn=10 obese insulin-resistant men. Exercise induce an increase of extracellular glycerol in SCAT in obese IS versus lean IS men: this could be the result of a lower blood ow in subcutaneous adipose tissue in obese IS men. Nonetheless, extracellular glycerol was blunted in obese IR versus obese IS men, despite comparable local blood ow after exercise. SCAT extracellular glycerol was reduced by 60% following local -/ -adrenoceptor blockade in obese IS but not in obese IR men; in the latter, exercise training did not a ect non- adrenergically-mediated lipolysis, despite an improved metabolic pro le and body

The review explores the relationship between exercise and fat metabolism.