1. Effects of Antioxidants in Skeletal Muscle Work

ROS are generated from either mitochondrial or nonmitochondrial sources during skeletal muscle work. These include NADPH, xanthine oxidases, phospholipase A2, and nitric oxide from NO synthase. During moderate exercise, oxidative balance is kept within physiological limits to minimize the effects of oxidative damage. It contains a complicated antioxidant defense system: antioxidant enzymes like glutathione peroxidases, superoxide dismutase (SOD), thioredoxins, peroxiredoxins, and catalase. They are capable of reducing ROS, while endogenous antioxidant substrates such as glutathione can scavenge ROS/RNS. Physical exercise has antioxidant effects by enhancing these endogenous antioxidant defenses.

In pathological conditions like diabetes or cancer, this endogenous antioxidant effect is probably the most efficient health-promoting mechanism. Despite this, antioxidant treatment is very popular and widely used in medical treatment and among individuals doing recreational or professional sports to enhance activity. These treatments can modify skeletal muscle signaling like force production, glucose uptake, insulin sensitivity, ion pump functions, and mitochondrial biogenesis.

Mitochondria have an essential role in skeletal muscle contraction. It is where ATP synthesizes; redox status modulates, and pH is controlled. Since, without ATP and calcium, muscle fibers do not contract, any alteration in mitochondrial status could lead to myopathy or muscle-related disease like diabetes.

Regular exercise increased mitochondrial size and density and cardiolipin content in type-2 diabetes. In Barth syndrome, which is an X-linked recessive disorder manifesting in muscle weakness and cardiomyopathy, dysfunction of tafazzin (a mitochondrial acyltransferase) reduces cardiolipin content and alters mitochondrial function. Treatment with mito-Tempo (a mitochondria-specific antioxidant) in cardiac myocytes lacking tafazzin normalized its level, decreased mitochondrial ROS production, and increased cellular ATP content.

Unfortunately, not all mitochondrial-targeted treatments have beneficial effects. A recent study shows in Barth Syndrome again that targeted overexpression of catalase in mitochondria did not prevent the development of myopathy in mice. Next to the pathological conditions, aging also decreases mitochondrial functions.

In a particular animal aging model, mtDNA mutator mice were treated with the antioxidant SkQ1 (10-(6′-plastoquinonyl)decyltri-phenylphosphonium cation) the phosphorylation capacity of mitochondria in skeletal muscle was improved. This positive effect of SkQ1 was partly because the treatment restored the cardiolipin amount in the mtDNA mutator mice to a wild-type level.

2. Effects of Astaxanthin during Physical Exercise and Muscle Injury

During heavy exercise, training, and competition, the elevation of reactive oxygen and nitrogen species evolves, causing damage to lipid, protein, and nucleic acid molecules. That is why special nutritional strategy, like supplementation with antioxidant compounds, is now essential for active individuals and athletes.

Based on mouse exercise experiments, supplementation with astaxanthin can effectively improve the side effects of exercise metabolism and the individual’s performance and recovery.

Four weeks of astaxanthin treatment in mice prolonged the running to exhaustion. During exercise, astaxanthin administration facilitated lipid metabolism instead of glucose utilization, improving endurance and reducing adipose tissue. The same group showed the effect of astaxanthin on ROS-targeted proteins involved in skeletal muscle metabolism during exercise.

They found that the oxidative stress-induced modification of lipid peroxidase carnitine palmitoyltransferase I (CPT I) was reduced by applying the antioxidant astaxanthin. Astaxanthin intake increases the PGC-1α level in skeletal muscle leading to the acceleration of lipid utilization by activating mitochondrial aerobic metabolism during exercise. In oxidative-type soleus muscle, 45 days of astaxanthin supplementation resulted in mitochondrial-targeted action, as the treatment increased glutathione content in the mitochondria during exercise, limited oxidative stress, and delayed exhaustion Wistar rats.

Unlike in the exercising mouse model, where astaxanthin supplementation enhanced mainly the utilization of fat and depleted muscle glycogen stores during endurance exercise, four weeks of treatment did not significantly influence the carbohydrate and fat oxidation rate in exercising humans.

In this study, astaxanthin supplementation has no significant effect on performance during endurance training, not even on more extended training periods or in a higher dose (20 mg/day, four weeks) in young, trained individuals.

Moreover, a high carotene-containing diet also proved to moderate some of the negative outcomes of sarcopenia on a low physical performance by reducing DNA damage in aged humans. It has been proved that an astaxanthin-containing diet modified the expression level of PGC-1α, thereby inducing mitochondrial biogenesis in vivo.

It was also shown that the prolonged supplementation had not modified the lipid oxidation to spare glycogen stores during training, as it already proved in animal studies, which can be due to the increased fitness levels of the inspected subjects. Krill oil treatment also activated the mTORC1 signaling pathway as it was shown in C2C12 myoblasts; however, in young, untrained, healthy individuals, 3 g of krill oil (0.5 g astaxanthin content) administration during eight weeks did not elevate the muscle force significantly in resistance exercise.

In elder subjects (between 65 and 85 years), an astaxanthin-containing (12 mg) diet with additional antioxidative properties (10 mg tocotrienol, 6 mg zinc) significantly improved the performance in endurance training. Additionally, it enhanced the force and muscle mass compared to the control group with placebo and exercise alone.

A new study showed that astaxanthin treatment helps preserve mitochondrial integrity and function in heat-induced skeletal muscle injury examined in cultured C2C12 cells and isolated rat skeletal muscle fibers. The supplementation prevented mitochondrial fragmentation and depolarization, reduced apoptotic cell death, and increased PGC-1α and mitochondrial transcription factor A expression following heat stress.

Human investigations have also been carried out to show the effect of astaxanthin application on exercise-induced muscle injury. Eccentric loading was applied for three weeks in resistance-trained men, and different markers of muscle injury (muscle soreness, creatine kinase level, and muscle performance) were tested. According to the results, the antioxidant supplementation did not favorably affect the markers above.

In another study, cardiac troponin release was examined after endurance-type exercise in cyclists. In this experiment, astaxanthin treatment did not affect antioxidant capacity (uric acid, malondialdehyde) and inflammation (high-sensitivity C-reactive protein) markers. It did not change creatine kinase release induced by exercise.

However, a positive effect of antioxidant astaxanthin was suggested in untrained healthy men, where the supplementation significantly increased carbohydrate oxidation and oxygen consumption during exercise and decreased the plasma insulin level. These results indicate that astaxanthin-rich foods can positively affect the aerobic metabolism of carbohydrates and fat during rest and exercise.

However, several studies using a variety of animal models of myocardial ischemia and reperfusion demonstrated the efficiency of astaxanthin supplementation by reducing markers of oxidative stress and inflammation; one has to consider several aspects of the use of antioxidant supplementation for attenuating muscle injury. These supplements seem to attenuate certain signs of muscle injury during exercise.

However, the optimal dose and treatment period is unclear, and whether the effect is specific to nonresistance-trained individuals. It is also urgent to find the best markers of skeletal muscle injury and more suitable analytical methods so that a more reliable conclusion can be generated regarding the effect of antioxidant agents like astaxanthin in exercise-induced muscle damage. A recent review has collected and discussed all the questioned aspects of astaxanthin supplementation on exercise performance and recovery.

3. Muscle Atrophy Is Improved by Astaxanthin Treatment

Skeletal muscle atrophy can occur in the case of physiological and several pathological conditions such as immobilization, aging, chronic diseases (e.g., heart failure and renal failure), or cancer. A correlation between oxidative stress and muscle mass has already been observed. The increased production of reactive oxygen species has an important role in disusing muscle atrophy by increasing protease activation. The activation of oxidative stress pathways in atrophying muscles has been suggested to cause apoptosis.

Oxidative stress activates lysosomal proteases (e.g., cathepsin L), calcium-activated proteases (calpain), and the ubiquitin-proteasome pathway during disuse muscle atrophy leading to the activation of proteolysis. The effects of antioxidants in disuse muscular atrophy have been investigated, and the antioxidant astaxanthin comes to the front as an effective molecule to prevent inactivity-induced muscle atrophy.

In different animal models, astaxanthin supplementation before and during hind limb unloading prevented muscle atrophy. Dietary astaxanthin intake for 14 days before and during hind limb immobilization attenuated muscle atrophy in rats. It interfered with the increased expression of CuZn-SOD (CuZn-superoxide dismutase), cathepsin L, calpain, and ubiquitin caused by immobilization.

In another rat model, the dietary astaxanthin supplementation for two weeks before unloading and during 7-day extended immobilization attenuated soleus muscle atrophy. It suppressed myonuclear apoptosis measured by the number of TUNEL-positive nuclei. The capillary number is related to the loading and activity of a skeletal muscle; the unloading results in capillary regression.

Administration of astaxanthin decreased the ROS production, decreased the level of SOD-1, and increased the expression of VEGF (vascular endothelial growth factor) in the soleus of hind limb unloaded rats; furthermore, the 7-day long astaxanthin administration reduced the capillary regression during unloading, while the 2-week long treatment maintained the capillary network near control levels. Interestingly, the 2-week astaxanthin diet affected soleus muscle mass during unloading.

Further study showed that the combinatory treatment with dietary astaxanthin supplementation and heat stress prevented disuse muscle atrophy in the soleus muscle. This protective effect may be partially due to the higher number of satellite cells (skeletal muscle stem cells). Increased ROS production within immobilization-induced skeletal muscle mediates TGF-β1-induced fibrosis by promoting fibroblasts’ differentiation and increasing collagen synthesis, where astaxanthin application attenuated skeletal muscle fibrosis. Also, in combination with a functional training program, astaxanthin formulation increased the tibialis anterior muscle size determined from magnetic resonance imaging in the elderly.