**2. Lifestyle**

tory pathways of metabolic status. One of the most important parameters of human metabolic homeostasis is glycemia that results from the availability and the utilization of nutrient sources. In this way, the cellular uptake and glycogenesis in skeletal muscle fibers have important implications in the regulation of blood glucose. This metabolic process was regulated by insulin that acts on muscle promoting the translocation of glucose transporter-4 (GLUT4), which is the most abundant insulin-dependent transporter in the cell membranes of the skeletal muscle, heart muscle, and adipose tissue, which leads to the uptake of glucose into the cell.

In this scenario, the muscle can be considered an important organ in glucose metabolism regulation, functioning at the same time as locus of start- and end-point of metabolic disorders related to glucose metabolism, and also the key organ of intervention strategies against insulin resistance. This chapter proposes an overview about effects of physical activity and exercise in the muscle as a strategy against insulin resistance. The physiological approach in this chapter is based on two major signaling pathways and biomarkers of muscle function, as well as its interaction with other cells and tissues—the cytokines and heat shock proteins (HSPs).

Skeletal muscle contains anatomic and physiological characteristics that represent different possibilities and functions in terms of metabolic properties and also in contractility and mitochondrial activity. There are three important factors that influence these characteristics: age, the dietary behavior, and the levels of physical activity. Together, these factors determine the muscle health status and capacity of physical performance, then, consequently, the whole body homeostasis. Aging, high caloric diet consumption, and sedentary behavior leads the progression of the muscle dysfunction that culminates in the loss of metabolic homeostasis,

However, these outcomes are presented in the established metabolic disease, while many subclinical processes precede the onset of disease. Subclinical modifications also accom panied the undesired progression of metabolic disease, increased the incidence of comorbidities and increased hospital admissions. These silent subclinical effects, such as inflammation, oxidative stress, and molecular alterations, decreased gradually the individual health status and

**Figure 1.** Muscle as the target of subclinical modification related to metabolic disease development.

promoting dyslipidemia and glycemic alterations.

decreased the quality of life (Figure 1).

86 Muscle Cell and Tissue

A high dietary fat intake and low levels of physical activity characterizes much of the overall lifestyle. Surplus of fat intake is stored in many human tissues and these intracellular lipids serve as a rapidly available energy source during, for example, physical activity. Mainly in the sedentary condition, lipid excess leads to the development of modern diseases such as obesity and insulin resistance [1].

The consumption of high-fat diets (HFD) is associated with an excessive storage of fatty acids in the skeletal muscle [1]. The human (and also laboratory animals) body are composed of several muscles that contain slow-twitch (type I) fibers, which contain a high number of mitochondria and use oxidative metabolism as an energy source, and fast-twitch (type II) fibers, which generate energy mainly through glycolysis. High intake of hypercaloric or highfat diets promotes a series of structural and metabolic changes that affect muscle capacity. An inadequate diet, such as HFD, induces muscle adaptations at molecular levels, promoting an increase in the proportion of oxidative fibers (type I fiber) by increasing the levels of the myosin heavy chain, slow fiber type protein, complexes of the oxidative phosphorylation, and the mitochondrial membrane composition. However, despite the increased oxidative fibers proportion, these modifications are insufficient to prevent impairments in oxidative metabo‐ lism [2]. The long-term HFD consumption promotes a decrease in the muscle mass and an increase in muscle triglyceride accumulation in parallel to the increased expression of bio‐ markers of mitochondrial metabolism such as succinate dehydrogenase complex subunitmyocytes and within the fascia surrounding skeletal muscle (increase in the intra and enzyme of β-oxidation), and the phosphorylation of acetyl-CoA carboxylase (ACC) (regulation of lipid synthesis). These alterations contribute to the morphological impairment known as myostea‐ tosis or the ectopic skeletal muscle adiposity that represents fat infiltration within myocytes and within the fascia surrounding skeletal muscle (increase in the intra- and intermuscular fat content, respectively) [3]. Mounting evidence indicates that elevated intramyocellular lipid deposition is associated with diminished insulin sensitivity in the skeletal muscle, promoting insulin resistance. Since fiber type I have a higher capacity for "fat burning", studies have reported a negative association between adiposity and the relative percentage of type I fibers. In other words, more muscle oxidative capacity results in less adiposity [4]. The increase in type I fiber proportion represent an attempt to restore the energy homeostasis between the source and energy demand. If this adaptive response is insufficient, the human body is susceptible to metabolic dysfunction.

Insulin resistance in the skeletal muscle in humans is associated with decreased oxidative capacity of ATP synthesis and also related to the decrease of many genes expression. Genes that control mitochondrial activity, including peroxisome proliferator-activated receptor gamma coactivator 1-alpha (PGC-1α), may indeed play a crucial role in the development of mitochondrial dysfunction, insulin resistance, and diabetes mellitus type 2 (T2DM) through the western lifestyle that is rich in hypercaloric or high-fat diets. Three days of HFD reduces PGC-1α protein levels by approximately 20% in humans and 40% in C57B1/6J mice after three weeks on an HFD treatment [5]. Sparks et al. [5] emphasized that HFDs in both humans and mice were associated with the reduction in the expression of genes involved in electron transport chain, nuclear genes encoding mitochondrial proteins (e.g., mitochondrial carrier proteins), and those involved in mitochondrial biogenesis (e.g., PGC1 and PGC), supporting the hypothesis that HFDs or high-fat flux explain the reduction in oxidative phosphorylation pathway (OXPHOS) genes seen in aging, the prediabetic state, and in overt diabetes.

Ciapaite et al. [6] suggest that the consumption of unhealthy obesogenic HFDs in combination with a sedentary lifestyle may create a vicious cycle by impairing skeletal muscle function and decreasing exercise potential, which may lead to further aggravation of obesity and skeletal muscle dysfunction. The adaptation response to dietary lipid overload occurs by fiber-typespecific mechanisms, leading to differential impairment of fast-twitch and slow-twitch skeletal muscle contractile function. Fast-twitch fibers suffered impairment in mitochondrial ATP production and Ca2+ homeostasis, and slow-twitch fibers have changed the sarcomere composition and force production. Together, changes in both types of fiber related to the consumption of HFD affect the functionality and muscular performance [6].

Muscle metabolic function can also be impaired by increasing visceral fat accumulation. This fat is more lipolytic (rapidly turned over) than subcutaneous fat and less sensitive to the antilipolytic effect of insulin. As abdominal fat develops in obesity related T2DM, the adipocytes release non-esterified fatty acids, many inflammatory products and reactive oxygen species (ROS). When non-esterified fatty acids accumulate in cells they undergo β-oxidation, forming acetyl-CoA that enters the Krebs cycle. The excessive amount of free radical formed in this situation requires a protective response against oxidative stress—decreased entry of glucose into the cell to avoid more free radical formation by glucose metabolism. Thus, indirectly, whole body adiposity inhibit the phosphorylation of tyrosine in insulin receptor substrate 1 (IRS-1) as a 'protective mechanism' that down-regulates insulin sensitivity in the muscle [7]. In the same direction, fat accumulation inside muscle cells may lead to the entry of fatty acids into the mitochondria where they are prone to ROS production [1]. Considering that the muscle tissue plays a key role in the regulation of metabolism, especially concerning glucose levels, disturbances on functionality and in the redox state in the muscle might be related to diabe‐ togenic effects.

Adipose tissue insulin resistance and dysfunctional lipid storage in adipocytes are sentinel events in the progression toward metabolic dysregulation with obesity [8]. Many of the complications of obesity are due to a chronic subclinical inflammation. Gene expression profiling of the obese phenotype revealed a differential regulation of many pro-inflammatory genes. In part, a significantly higher number of macrophages are present in obese adipose tissue explain the increase in pro-inflammatory status [9]. Thus, a major determinant for many obesity-induced implications is the low-grade inflammation of the enlarged adipose tissue and the persistent release of inflammatory adipokines such as tumor necrosis factor-α (TNF-α) and interleukin-6 (IL-6) [10, 11]. The basis for this view is that increased circulating levels of several markers of inflammation, both pro-inflammatory cytokines and acute-phase proteins, are elevated in the obese [11].

gamma coactivator 1-alpha (PGC-1α), may indeed play a crucial role in the development of mitochondrial dysfunction, insulin resistance, and diabetes mellitus type 2 (T2DM) through the western lifestyle that is rich in hypercaloric or high-fat diets. Three days of HFD reduces PGC-1α protein levels by approximately 20% in humans and 40% in C57B1/6J mice after three weeks on an HFD treatment [5]. Sparks et al. [5] emphasized that HFDs in both humans and mice were associated with the reduction in the expression of genes involved in electron transport chain, nuclear genes encoding mitochondrial proteins (e.g., mitochondrial carrier proteins), and those involved in mitochondrial biogenesis (e.g., PGC1 and PGC), supporting the hypothesis that HFDs or high-fat flux explain the reduction in oxidative phosphorylation

pathway (OXPHOS) genes seen in aging, the prediabetic state, and in overt diabetes.

consumption of HFD affect the functionality and muscular performance [6].

togenic effects.

88 Muscle Cell and Tissue

Ciapaite et al. [6] suggest that the consumption of unhealthy obesogenic HFDs in combination with a sedentary lifestyle may create a vicious cycle by impairing skeletal muscle function and decreasing exercise potential, which may lead to further aggravation of obesity and skeletal muscle dysfunction. The adaptation response to dietary lipid overload occurs by fiber-typespecific mechanisms, leading to differential impairment of fast-twitch and slow-twitch skeletal muscle contractile function. Fast-twitch fibers suffered impairment in mitochondrial ATP production and Ca2+ homeostasis, and slow-twitch fibers have changed the sarcomere composition and force production. Together, changes in both types of fiber related to the

Muscle metabolic function can also be impaired by increasing visceral fat accumulation. This fat is more lipolytic (rapidly turned over) than subcutaneous fat and less sensitive to the antilipolytic effect of insulin. As abdominal fat develops in obesity related T2DM, the adipocytes release non-esterified fatty acids, many inflammatory products and reactive oxygen species (ROS). When non-esterified fatty acids accumulate in cells they undergo β-oxidation, forming acetyl-CoA that enters the Krebs cycle. The excessive amount of free radical formed in this situation requires a protective response against oxidative stress—decreased entry of glucose into the cell to avoid more free radical formation by glucose metabolism. Thus, indirectly, whole body adiposity inhibit the phosphorylation of tyrosine in insulin receptor substrate 1 (IRS-1) as a 'protective mechanism' that down-regulates insulin sensitivity in the muscle [7]. In the same direction, fat accumulation inside muscle cells may lead to the entry of fatty acids into the mitochondria where they are prone to ROS production [1]. Considering that the muscle tissue plays a key role in the regulation of metabolism, especially concerning glucose levels, disturbances on functionality and in the redox state in the muscle might be related to diabe‐

Adipose tissue insulin resistance and dysfunctional lipid storage in adipocytes are sentinel events in the progression toward metabolic dysregulation with obesity [8]. Many of the complications of obesity are due to a chronic subclinical inflammation. Gene expression profiling of the obese phenotype revealed a differential regulation of many pro-inflammatory genes. In part, a significantly higher number of macrophages are present in obese adipose tissue explain the increase in pro-inflammatory status [9]. Thus, a major determinant for many obesity-induced implications is the low-grade inflammation of the enlarged adipose tissue and the persistent release of inflammatory adipokines such as tumor necrosis factor-α (TNF-α) and interleukin-6 (IL-6) [10, 11]. The basis for this view is that increased circulating levels of several The most crucial step in insulin signal transduction, the phosphorylation of the IRS-1 can be blunted by both TNF-α and free fat acids conferring insulin resistance in target tissues. Also, several serine kinases are involved in this impaired insulin signaling as c-jun amino terminal kinase (JNK) that is potently induced by TNF-α and free fat acids. Interestingly, increased intramyocellular lipid levels is correlated to insulin resistance with no significant changes in TNF-α, IL-6 or adiponectin concentrations, suggesting that a dysregulation in muscular fatty acid oxidation per se may mediate insulin resistance by mitochondrial defect in oxidative phosphorylation [9]. Thus, systemic inflammation may participate in insulin resistance development but muscle metabolism impairment can be crucial to T2DM installation.

Conditions of tissue stress, such as oxidative stress, inflammation, and molecular alterations, could develop initial compensatory responses of cytoprotection, such as the expression of HSP70. The severity of the metabolic state (measured by glycemia, glucose intolerance, obesity, or insulin resistance) promotes modification in the muscle and adipose HSP70 expression. The initial impairment and moderate glucose intolerance promotes an increase in HSP70 content in adipose tissue and no modification in the muscle [12] as an initial adaptative response, while obesity plus T2DM have an decrease in both muscle and adipose HSP70 content [13]. Since HSP70 expression can inhibit nuclear factor kappa-light-chain-enhancer of activated B cells (NF-κB) and JNK dependent mechanisms that promote insulin resistance, blunted heat shock response can be interpreted as an additional silent effect of HFD consumption and sedentar‐ ism. These results support the hypothesis that an increase in visceral fat, closely associated with the lifestyle (high-fat intake and sedentarism) promotes subclinical effects that is associated with the development of muscle insulin resistance [14] (Figure 2).

**Figure 2.** Lifestyle promotes molecular and metabolic alterations in muscle and adipose tissue, promoting insulin re‐ sistance.

Another fact is that increased extracellular HSPs levels (mainly 70kDa isoforms as eHSP72) are correlated with oxidative damage and stress in diabetes and in obesity. Moreover, the content of the plasma eHSP72 is higher in T2DM obesity compared to DM or only obese subjects, suggesting eHSP70 levels as biomarker of glucose homeostasis unbalance [15]. Together, eHSP70 and pro-inflammatory cytokines represent a link between metabolic and immune related events. During severe stress response that can cause insulin resistance, we can observe the action of inflammatory cytokines such as IL-1 and TNF-α [16]. Under hypoglyce‐ mic conditions, as a part of the homeostatic stress response, HSP70 is secreted to the blood‐ stream and may be purely a danger signal to all the tissues of the body for the enhancement of immune and metabolic surveillance state or actively participate in glycemic control under stressful situations [17]. Additionally, eHSP70 can bind receptors in immune cells that induce pro-inflammatory cytokine release.
