Effect of Vitamin D Supplementation on Exercise Adaptations in Patients on Statin Therapy

Overview

Statins along with lifestyle modifications including exercise are commonly prescribed to patients with type 2 diabetes. American diabetes association recommends using moderate-intensity statin and lifestyle therapy for patients with diabetes aged ≥40 years, even without additional cardiovascular disease(CVD) risk factors.. Myopathy is a well known adverse effect of statins, which occurs in 1-7% of patients. The spectrum of statin-related myopathy ranges from common benign myalgia to rare but life threatening rhabdomyolysis. Being lipophilic, simvastatin diffuses nonselectively into extrahepatic tissues such as muscle, leading to higher incidence of myopathy among statin users. In addition, simvastatin attenuates the exercise-induced increase in cardiorespiratory fitness, and reduces the skeletal muscle mitochondrial content and oxidative capacity in humans. Impaired cardiorespiratory fitness and mitochondrial function is possibly due to reduction in Coenzyme Q10, which is a component of the electron transport chain and is indispensable for generation of adenosine triphosphate (ATP) during oxidative phosphorylation in mitochondria. Statins or hydroxyl-methylglutaryl coenzyme A (HMA CoA) reductase inhibitors interfere with the production of mevalonic acid, which is a precursor in the synthesis of coenzyme Q10. Mitochondrial dysfunction has also been reported in vitamin D deficient individuals which has been attributed to intra-mitochondrial calcium deficiency or deficient enzyme function of the oxidative pathway ( by direct effect of vitamin D on enzyme gene or protein expression). Thus, vitamin D may improve the statin-mediated changes in cardiorespiratory fitness and mitochondrial function by improving the enzymatic machinery involved in oxidative phosphorylation which is blocked by statin. This study is being done to look for the effect of vitamin D supplementation on simvastatin-mediated change in exercise-mediated cardiorespiratory fitness and skeletal muscle mitochondrial content in adults with type 2 diabetes

Statins, a class of hydroxyl methylglutaryl-coenzyme A reductase inhibitors that lower low-density lipoprotein cholesterol, are commonly prescribed to patients with the metabolic syndrome or those with multiple cardiovascular disease risk factors when lifestyle changes fail to achieve LDL targets to reduce the risk of coronary heart disease morbidity and mortality. American Diabetes Association (ADA) recommends moderate intensity statin for patients with diabetes without additional CVD risk factors aged \>40.Statins are widely prescribed in combination with exercise to lower risk of cardiovascular disease morbidity and mortality. Every 1millimole per liter reduction in LDL is associated with a 10-20% reduction in risk of cardiovascular events and all-cause mortality, while every 1 Metabolic equivalent \[MET\] (3.5 milliliters of oxygen per kilogram of body weight per minute) increase in fitness is associated with an 18% reduction in cardiovascular disease mortality and an 11-50% reduction in all-cause mortality.Statins are generally safe, but myotoxicity, including fatal rhabdomyolysis can occur. Although severe muscle-related side effects occur in \<0.1% of statin users, less severe symptoms, such as myalgia and muscle cramps, occur in 1-7% of users. The mechanisms mediating statin myopathy are unclear, but possibilities include decreased sarcolemmal or endoplasmic reticulum cholesterol, reduced production of prenylated proteins including the mitochondrial electron transport protein coenzyme Q10, reduced fat catabolism, increased myocellular concentrations of cholesterol and plant sterols, failure to repair damaged skeletal muscle, vitamin D deficiency, and inflammation. Increasingly, interest has focused on altered cellular energy use and mitochondrial dysfunction, with the dysfunction activating pathways leading to muscle atrophy. Although the mechanisms are poorly understood, some statins (simvastatin, atorvastatin, fluvastatin) have been shown to reduce skeletal muscle mitochondrial content and oxidative capacity in humans. Sirvent et al evaluated the mitochondrial function and calcium signaling in muscles of patients treated with statins, who present or not muscle symptoms, by oxygraphy and recording of calcium sparks, respectively. Patients treated with statins showed impairment of mitochondrial respiration that involved mainly the complex I of the respiratory chain and altered frequency and amplitude of calcium sparks. The muscle problems observed in statin-treated patients appear thus to be related to impairment of mitochondrial function and muscle calcium homeostasis. Mikus et al examined the effects of simvastatin on changes in cardiorespiratory fitness and skeletal muscle mitochondrial content in response to aerobic exercise training. The primary outcomes were cardiorespiratory fitness and skeletal muscle (vastus lateralis) mitochondrial content (citrate synthase enzyme activity). Thirty-seven participants (exercise plus statins; n=18; exercise only; n=19) completed the study. Cardiorespiratory fitness increased by 10% (P\<0.05) in response to exercise training alone, but was blunted by the addition of simvastatin resulting in only a 1.5% increase (P\<0.005 for group by time interaction). Similarly, skeletal muscle citrate synthase activity increased by 13% in the exercise only group (P \<0.05), but decreased by 4.5% in the simvastatin plus exercise group (P\<0.05 ) Impaired cardiorespiratory fitness and mitochondrial function is possibly due to reduction in Coenzyme Q10, which is a component of the electron transport chain and is indispensable for generation of ATP during oxidative phosphorylation in mitochondria. Statins or hydroxyl-methylglutaryl coenzyme A (HMA CoA) reductase inhibitors interfere with the production of mevalonic acid, which is a precursor in the synthesis of coenzyme Q10. Mitochondrial dysfunction has also been reported in vitamin D deficient individuals which has been attributed to intra-mitochondrial calcium deficiency or deficient enzyme function of the oxidative pathway ( by direct effect of vitamin D on enzyme gene or protein expression). Mukherjee et al conducted a study in which chicks were raised for 3 to 4 weeks either on a normal (vitamin D supplemented) or a rachitogenic diet. The Ca2+ content of the serum, heart tissue and heart mitochondria was significantly decreased in chicks raised on a rachitogenic diet. In mitochondria isolated from calcium deficient hearts, the rate of adenosine diphosphate induced state 3 respiration and 2,4-Dinitrophenol uncoupled respiration were significantly decreased.When vitamin D deficient chicks were orally dosed with vitamin D3, serum calcium level and state 3 respiration rate returned to normal indicating that the above changes are reversible In a longitudinal study, the effects of cholecalciferol therapy on skeletal mitochondrial oxidative function in vitamin D deficient subjects using 31Phosphorus magnetic resonance spectroscopy were examined.The phosphocreatine recovery half-time (t1/2PCr) was significantly reduced after cholecalciferol therapy in the subjects indicating an improvement in maximal oxidative phosphorylation (34.44 ±8.18 sec to 27.84 ±9.54 sec, P \<.001). Thus, vitamin D may improve the statin-mediated changes in cardiorespiratory fitness and mitochondrial function by improving the enzymatic machinery involved in oxidative phosphorylation which is blocked by statin. Another proposed mechanism of interaction between statin and vitamin D is inhibition of CYP3A4 by statins, which displays 25-hydroxylase activity in vitro. Vitamin D deficiency leads to 'preferential shunting' of CYP3A4 for hydroxylation of vitamin D, thus decreasing the availability of CYP3A4 for statin metabolism leading to statin-induced toxicity. This study describes the effect of vitamin D supplementation on simvastatin-mediated change in exercise-mediated cardiorespiratory fitness and skeletal muscle mitochondrial content in adults with type 2 diabetes.