CPTII muscle deficiency is not easily recognized and patients may have to wait for decades just to get a proper diagnosis, generally based on referred symptoms (Sigauke et al., 2003). Molecular genetic investigation, regarded as the gold standard for the diagnosis, is often performed only after several years of clinical misunderstandings and attempts at treatment. The muscle CPTII disease may be often confused with other, clinically very similar myopathies, such as McArdle disease (MD). MD is an autosomal recessive disorder resulting in myophosphorylase deficiency leading to the inability to use glycogen storages (McArdle, 1951; Llavero et al., 2019). This condition represents the most common cause of acute exercise intolerance in young adults; it is associated with muscle cramps, soreness and recurrent myoglobinuria. However, CPTII deficiency and MD can be functionally differentiated in relation to the onset of some exercise-related signs and symptoms: (1) post-exercise lactate levels are generally higher in CPTII deficiency (>2 mmol/L) (Orngreen et al., 2003) than in MD (<1 mmol/L) (Godfrey et al., 2009); (2) following exercise, muscle disorders appear later in CPTII deficiency and very early in MD (Faigel, 1995); (3) the so-called “second-wind” effect (i.e., an improvement in exercise tolerance with a decrease in perceived exertion and heart rate after 7–8 min of a constant-load exercise protocol) is typically observed in MD patients (Vissing and Haller, 2003; Santalla et al., 2014) and not in CPTII deficiency individuals (Arélin et al., 2020).

An extensive description of the methods for diagnosing CPTII deficiency (i.e., clinical presentation, pathobiochemical characteristics, molecular genetic aspects, detectable biomarkers and genotype-phenotype analysis) has been recently published by Joshi and Zierz (2020). Other possible assessments may consider the electromyography exam, able to demonstrate dysfunction of myoelectric signals related to muscle damage (Sigauke et al., 2003) or functional evaluations, able to show abnormalities in lactate levels, VO₂ max capacity and respiratory exchange ratio (R) during an ergometric incremental test, which may underline the defective mitochondrial oxidative functionality (Tarnopolsky, 2016).

Patients with CPTII deficiency experience recurrent episodes of severe muscle pain associated with rhabdomyolysis (Joshi and Zierz, 2020), in relation to the presence of triggering factors which can produce dramatic and extensive muscle damage due to the altered mitochondrial muscle lipid bioenergetics (Wieser et al., 2003). In particular, these factors include endurance exercise, fasting, exposure to cold and infections, as well as low fluid intake, psychological stress and lack of sleep (Joshi et al., 2019; Joshi and Zierz, 2020). Rhabdomyolysis consists in the partial rupture of myocytes, followed by a dramatic release of several muscle molecules (e.g., creatine kinase (CK), lactate dehydrogenase (LDH), myoglobin (Mb), aminotransferases) in the extra cellular space and then into the blood stream. These molecules are used as blood biomarkers and usually measured when rhabdomyolysis occurs (Bonnefont et al., 1999), even though CK level is considered the most sensitive biomarker (Kim et al., 2016). The normal level of CK is 22–198 U/L. Depending on the degree of rhabdomyolysis, and in relation to the muscle effort sustained and the extension of muscle damage, the level of CK could increase up to 10,000–200,000 U/L observed 24–48 h after the exercise (Criddle, 2003). In addition, muscle pain, electrolyte balance, urine color, arterial blood gas examination, muscle biopsy, and/or electrocardiogram may be used to complete the assessment for the diagnosis of rhabdomyolysis (Kim et al., 2016). The mechanisms involved in the progression of rhabdomyolytic events were elucidated by several authors (Bagley et al., 2007; Kim et al., 2016; Parimbelli et al., 2021). The most severe potential complication of rhabdomyolysis is an acute renal failure due to the dramatic release of Mb by the damaged muscle, resulting in its precipitation and accumulation in renal tubules with an impairment of the filtering efficiency (Panizo et al., 2015).

Physical inactivity is highly harmful and can negatively impair whole-body homeostasis (Schnyder and Handschin, 2015; Booth et al., 2017; Rezuş et al., 2020; Romanello and Sandri, 2021): (1) by increasing chronic systemic inflammation and oxidative stress, which are correlated with the development of many degenerative diseases (e.g., atherosclerosis and osteoarthritis); (2) by decreasing muscle protein synthesis and neuromuscular remodeling, associated with age-related disabling conditions (e.g., sarcopenia and frailty); (3) by compromising the endocrine system and the immune response to infections, thus leading to a decline of the capacity to sustain adequate responses to stress (e.g., injury, acute phases of disease, etc.). Furthermore, muscle disuse leads to a decline of the efficiency of essential processes (i.e., mitochondrial biogenesis, fusion and fission, degradation) necessary for the control of mitochondrial homeostasis (number, morphology and distribution) (Romanello and Sandri, 2021). A loss of mitochondrial homeostasis impairs metabolic adaptation of the mitochondrial network to the cellular bioenergetic requirements and negatively affects skeletal muscle mass, quality and performance, representing a critical factor in the pathophysiology of several muscle-wasting conditions (e.g., aging, intensive care unit-acquired weakness, diabetes, obesity, chronic obstructive pulmonary disease (COPD), cancer cachexia and neuromuscular disorders) (Romanello and Sandri, 2021).

Both inactivity on the one hand and exercise on the other can impact on mitochondrial homeostasis and metabolism very quickly, and this has been recently established comparing the effects of muscle disuse with those of physical rehabilitation on the expression of peroxisome proliferator-activated receptor gamma coactivator-1alpha (PGC-1α) and Sirtuin 3 (SIRT 3) (Buso et al., 2019). PGC-1α is the key factor in the control of mitochondrial biogenesis, morphological and functional integrity, both in physiological conditions and during pathophysiological processes of muscle atrophy and aging (Finck, 2006). SIRT3 is recognized as one of the main mitochondrial activity regulators, with a central role in skeletal muscle (Jing et al., 2011, 2013; Vassilopoulos et al., 2014). In the study by Buso et al. (2019), after 14 days of bed rest a significant decrease in the expression of PGC-1α and SIRT3 was observed, both in the young and the elderly, with an up-regulation of the glycolytic metabolism as a consequence of the mitochondrial impairment. Conversely, in the subsequent 2 weeks of exercise rehabilitation program (3 sessions per week of strength training and high-intensity interval training) the levels of PGC-1α and SIRT3 protein expression showed a remarkable increase in relation to the energy needs, with PGC-1α levels higher than baseline (4.7 times and 2.6 times for young and elderly, respectively) and a shift back to a prevalent mitochondrial oxidative metabolism (Buso et al., 2019). These data are particularly relevant considering that PGC-1α stimulates not only the mitochondrial function but also the peroxisomal biogenesis/activity (Bagattin et al., 2010; Huang et al., 2017), and this effect may positively affect the mitochondria-peroxisome biochemical interplay, a potential alternative pathway to sustaining the energy requirements of exercise in CPTII deficiency patients.

Strong evidence supports a close functional crosstalk between mitochondria and peroxisomes in the process of oxidation of fatty acids (Delille et al., 2010; Dixit et al., 2010; Fransen et al., 2012, 2017; Schrader et al., 2015; Islinger et al., 2018). The β-oxidation (βOx) of fatty acids (FAs) takes place both in mitochondria and peroxisomes, involving, however, different substrates, respectively (Schrader et al., 2015; Wanders et al., 2016): short, medium and long-chain fatty acids (SCFAs, MCFAs and LCFAs) are mainly oxidized in mitochondria (Fransen et al., 2017), whilst very-long-chain fatty acids (VLCFAs), dicarboxylic acids (DCAs) and branched-chain fatty acids (BCFAs) are typically oxidized in peroxisomes (Fransen et al., 2017). Mitochondrial and peroxisomial βOx also lead to the formation of different end-products with relative different metabolic destiny: in the first case, acetyl-CoA is exclusively produced and afterwards directly processed by the tricarboxylic acid cycle (TCA); in the second case, besides acetyl-CoA, other acyl-CoAs (i.e., propionil-CoA, 4,8-dimethylnonanoyl-CoA, adipoyl-CoA, hexanoyl-CoA) are formed, converted in carnitine molecules by carnitine octanoyltransferase (COT) and carnitine acetyltransferase (CAT), then transported across the inner mitochondrial membrane by the carnitine acylcarnitine translocase (CACT) and reconverted in CoAs compounds by a mitochondrial CAT before reaching their final biochemical targets (i.e., mitochondrial βOx or TCA) (Figure 1).

Although less studied and known, the peroxisomial βOx seems to be particularly important considering that its contribution to the total cellular βOx was estimated in mammals from <5% to up to 30% (Thomas et al., 1980; Kondrup and Lazarow, 1985) allowing peroxisomes to perform a pivotal metabolic role in the oxidation of a wide variety of specific FAs. Furthermore, authors suggested that the peroxisome may also be able to oxidize substrates that are typically handled by mitochondria (Vamecq and Draye, 1989; Vamecq et al., 1989; Ferdinandusse et al., 2004), especially when specific mitochondrial pathways are inhibited (Violante et al., 2013, 2019). On this topic remarkable studies have been carried out by Violante et al. (2013, 2019), which demonstrated, in vitro and in vivo, that peroxisomes can accept and oxidize MCFAs and LCFAs, typically oxidized in mitochondria, becoming metabolically relevant in presence of mitochondrial fatty acid oxidation disorders, including CPTII deficiency.

An extensive description of the metabolic interactions between mitochondria and peroxisomes in the management of fatty acid oxidation has recently been published by Houten et al. (2020). Although the magnitude of mitochondria-peroxisomes interaction in the CPTII deficiency is yet to be established in humans, we can speculate that this “unconsidered” cooperative role of peroxisomal βOx may represent a key process and be potentially trainable by an appropriate program of physical exercise.

Since patients carrying the myopathic form of CPTII deficiency display normal fat turnover at rest (Orngreen et al., 2002) and physical exercise was recognized as the most common trigger factor involved in myalgia and/or rhabdomyolytic events, current recommendations exclude the practice of all types of physical activities, in particular prolonged and strenuous exercise, and complete rest is still recommended (Joshi and Zierz, 2020). However, this approach does not properly take into account the variability of metabolic requests that occur in relation to different exercise intensities and/or durations, and how these metabolic requests can affect macronutrient utilization. Indeed, during submaximal exercise intensities (<65% VO₂ max), especially if maintained beyond 90 min (Purdom et al., 2018), FAs are the prevalent fuel source of muscle energy, with a maximal FAs utilization (maximal fat oxidation, MFO) between 45 and 65% of VO₂ max. At these intensities, as in endurance activities, the contribution of carbohydrates oxidation is relatively low. On the other hand, for exercise intensities that exceed MFO, with duration typically lower than 90 min (as in intermittent training, stop-and-go activities and/or resistance exercise), the utilization of carbohydrates is predominant, mainly as a muscle glycogen (Purdom et al., 2018), phosphocreatine (PC) resynthesis is involved for a rapid ATP regeneration (Hargreaves and Spriet, 2020), and the use of FAs to maintain energy availability is sharply reduced (Purdom et al., 2018). Considering this physiological background, traditional endurance activities (e.g., running, cycling, swimming, etc.) should certainly be avoided in most CPTII deficiency patients, especially if unconditioned to exercise, as they are unable to properly satisfy the increase of muscle energy demand from FAs oxidation. Conversely, high intensity interval training (HIIT) or resistance training (RT) can be safely performed in CPTII disorders because the ATP turnover of these activities is mainly based on PC/glycogen pathways, which are totally preserved and optimizable in these patients.

HIIT is commonly defined as relatively intense bouts of exercise that elicit ≥80% of maximal heart rate (MHR), interspersed by periods of lower intensity exercise or rest for recovery (Weston et al., 2014). HIIT has received considerable attention in recent years for its metabolic effects, which are higher than in aerobic continuous training, with clinical relevance both in healthy and diseased subjects (Alkahtani et al., 2013; Jiménez-Pavón and Lavie, 2017; Gibala, 2018). The capacity of interval training to elicit greater health benefits than continuous exercise, matched for duration and intensity, seems to be related to the intrinsic nature of the intermittent mechanical work (Jiménez-Pavón and Lavie, 2017; Gibala, 2018). Notably, on this topic authors suggest that some cellular signaling pathways that regulate skeletal muscle mitochondrial biogenesis and function (i.e., AMPK, p38-MAPK, CaMKII and PGC-1α) are activated to a greater extent after intermittent activity compared with continuous exercise, even when relative intensity is moderate (Combes et al., 2015). However, since HIIT exercise mode can discourage physically inactive individuals and/or can be inappropriate in certain conditions (e.g., untrained obese), a moderate-intensity intermittent training (MIIT) may be preferred. Furthermore, MIIT does not require supervision and can generally be easier to put into practice for unfit categories (Jiménez-Pavón and Lavie, 2017). Comparative studies have shown that HIIT and MIIT have similar effects on several metabolic outcomes, such as fats oxidation (Alkahtani et al., 2013), vascular function (Rakobowchuk et al., 2012), body composition, insulin resistance and low-density lipoprotein cholesterol (LDL) (Racil et al., 2013).

Based on the specific involvement of type II muscle fibers, RT plays a key role in the maintenance of adequate muscle mass, strength and power, which are fundamental to supporting movement control, walking speed and functional independence (Westcott, 2012). RT has a strong clinical background for the prevention and management of type 2 diabetes, improving visceral fat quantity, HbA1c blood levels, glucose transporter type 4 distribution, and insulin sensitivity (Westcott, 2012; Codella et al., 2018; Mcleod et al., 2019). RT also produces cardiovascular benefits, by reducing resting blood pressure, LDL and triglycerides, and increasing high-density lipoprotein cholesterol (HDL). Furthermore, RT may promote bone development and prevent muscle loss due to aging, decreasing pain sensibility typically associated with arthritis or other inflammatory diseases that can even affect CPTII deficiency patients, especially during the advanced stage of life. Less clear are the effects of RT on mitochondrial biogenesis or oxidative capacity, with mixed results (Parry et al., 2020). However, given that RT may increase VO₂ max in untrained individuals, it is plausible that some aspects of the mitochondrial function may be enhanced with this paradigm of exercise (Parry et al., 2020), even though this assertion is only speculative at this stage and more investigation should be run in this area. No information is available on the effect of HIIT/MIIT or RT and peroxisomal biogenesis/oxidative capacity or mitochondria-peroxisomes interaction.

The first trial that successfully applied MIIT and RT on CPTII deficiency patients has recently been reported by our lab (Parimbelli et al., 2021), with promising results. In the study, a 14-year-old female with CPTII deficiency was involved in an exercise program of 6-months (3-days per week, 1 h/session each) based on MIIT at 70% VO₂ max and resistance upper/lower split workouts. After 6 months, indirect calorimetry revealed improved resting metabolic rate (RMR) and reduced respiratory quotient (RQ). Furthermore, the cardiopulmonary exercise test (CPET) revealed remarkable increases in peak power output and isocapnic buffering period. Importantly, this study also showed that the training program was absolutely safe and sustainable, with no significant post-exercise changes in blood CK, and no muscle pain and/or rhabdomyolysis attacks were experienced by the patient during the experimental protocol. Improvement of cardiovascular and muscular adaptations (including mitochondrial content and biogenesis) were only speculated by the authors at this stage of knowledge, and this requires further examination.

Based on a biochemical background, the use of carnitine has been speculated for CPTII deficiency by several authors in the past, even though this practice has never been investigated in these patients. However, considering that to date no scientific basis supports significant improvement of muscle carnitine content and/or performance in healthy subjects or athletes after a long lasting supplementation (Gnoni et al., 2020) and that carnitine can be partly degraded to trimethylamine-N-oxide (TMAO), which is considered a novel risk factor for cardiovascular diseases (Arsenian, 1997; Randrianarisoa et al., 2016; Tang and Hazen, 2017; Ding et al., 2018; Gao et al., 2019), a chronic supplementation of carnitine should be discouraged in these patients. In addition, in muscle CPTII deficiency acyl-carnitine is only partially transported across the inner mitochondrial membrane and the conversion of acylcarnitine into acyl-CoA is anyway insufficient to justify carnitine supplementation (Gnoni et al., 2020; Figure 1).

Medium chain triglycerides (MCTs) are able to cross the mitochondrial membranes independently of CPT transporters and CAT (Jeukendrup et al., 1998), and this has led to speculation on a possible use of MCTs in CPTII deficiency. Despite this background, conflicting opinions emerge on this topic, with some authors suggesting their use to sustain muscle bioenergetics and reduce plasma triglycerides (Scott et al., 1991; Bonnefont, 2004; Deschauer et al., 2005), while others do not consider MCTs effective in controlling muscle pain, avoiding rhabdomyolysis, or improving exercise performance and muscle recovery (Carroll et al., 1978; Roe et al., 2008). An interesting contribution on MCTs comes more recently from Violante et al. who, by using CPTII- and CACT-deficient cell lines, demonstrated how the majority of MCTs require carnitine shuttles to access the mitochondrial matrix and only 25% may cross the mitochondrial membranes by diffusion, underlining that when the CPTII is defective MCTs can be partially oxidized in peroxisomes (Violante et al., 2013). However, currently we don’t know how the use of MCTs may modulate the mitochondria-peroxisome interplay during different conditions of diet and exercise, and how this may regulate muscle metabolism in CPTII deficiency patients. At present, therefore, the potential use of MCTs to counter oxidative requests and prevent muscle symptoms in these subjects needs further evaluation.

Creatine is one of the most popular nutritional ergogenic aids for athletes (Kreider et al., 2017). Studies have consistently shown that creatine supplementation can improve exercise performance, through an increase of intramuscular PC concentration, and may enhance training adaptation/muscle hypertrophy by controlling molecular pathways (e.g., MRF-4, Myf-5, Myo-D, and myogenin) involved in muscle protein expression (Negro et al., 2019). Furthermore, creatine supplementation might influence post-exercise muscle recovery by enhancing muscle satellite cell proliferation (Vierck et al., 2003; Olsen et al., 2006). Creatine monohydrate (CM) is currently considered a safe and effective supplement to support resistance activities in the healthy, and several therapeutic benefits in diseased populations (ranging from the young to the elderly) have been reported (Smith et al., 2014; Candow et al., 2019; Stares and Bains, 2020; Jagim and Kerksick, 2021; Kreider and Stout, 2021). Considering its strong background, a periodical use of CM can be speculated in CPTII deficiency patients with different aims (e.g., at the beginning of a training program, to support muscle bioenergetics or to reduce fatigue when muscle recovery/adaptation needs to be boosted).

An important body of evidence suggests that a higher amount of proteins is required by active individuals to optimize exercise training adaptations and muscle remodeling (Cermak et al., 2012; Devries and Phillips, 2015; Churchward-Venne et al., 2016; Jäger et al., 2017; Morton et al., 2018). Dietary proteins and/or essential amino acids (EAAs), positively acting on the balance between protein synthesis and breakdown, can increase muscle mass or attenuate muscle loss, even during immobilization and aging (Cholewa et al., 2017). Furthermore, considering the capacity of EAAs to regulate ATP production/utilization (Dioguardi, 2004; Pasini et al., 2004) a possible role of these compounds in energy delivery during fatiguing tasks has recently been speculated by our lab (Negro et al., 2018). Based on the above, to satisfy the daily proteins intake to partly sustain the energy cost of strength activities and to promote muscle recovery after exercise sessions, a periodical supplementation with protein powders (e.g., whey proteins) or EAAs may be suggested in these subjects. This recommendation could be even more important in the presence of structural alteration of fibers as mirrored by increased blood CK levels, to prevent possible progression to rhabdomyolysis.

There is a strong association between the nutritional status of vitamin D and an optimal muscle function (Bhattoa et al., 2017). Indeed, through its binding to muscle vitamin D receptors, vitamin D mediates genomic and non-genomic effects in muscle cells, promoting muscle contractility through calcium uptake, myoblast differentiation, and the insulin sensitivity (Dirks-Naylor and Lennon-Edwards, 2011; Christakos et al., 2016; Bhattoa et al., 2017). Large cross-sectional studies underline a relationship between an insufficient level of serum 25(OH)D (<50 nmol/L) and low physical performance (Bischoff-Ferrari et al., 2004; Houston et al., 2007, 2011, 2012; Dirks-Naylor and Lennon-Edwards, 2011; Toffanello et al., 2012), mobility (Bischoff-Ferrari et al., 2004; Houston et al., 2011, 2012; Toffanello et al., 2012), muscle strength (Mowé et al., 1999; Zamboni et al., 2002; Houston et al., 2007, 2011, 2012; Toffanello et al., 2012), and greater disability (Zamboni et al., 2002; Houston et al., 2011). The importance of considering baseline serum 25(OH)D concentrations has been emphasized, since individuals with vitamin D deficiency appear to be more responsive to supplementation (Annweiler et al., 2009). Considering these findings, CPTII deficiency patients need to be screened to evaluate vitamin D blood levels, and in relation to the concentration detected a dietary supplementation should be suggested.

Exercise-induced muscle damage (EIMD) is a physiological regulated process of degradation and repairing, closely related to muscle adaptation and remodeling, characterized by muscle soreness and transient lower muscle function, in which several cell types are involved (i.e., neutrophils, macrophages, lymphocytes) along with an increase of reactive oxygen spices (ROS), the serum rise of various inflammatory cytokines (e.g., IL-1β, IL-6), and a release of intramuscular proteins (e.g., CK, LDH) due to damage (Byrne et al., 2001; Marcora and Bosio, 2007). EIMD, within certain limits, must also take place in CPTII deficiency subjects and is essential for their muscle recovery and adaptation to mechanical work. However, since the muscle bioenergetics is compromised in these patients, their degree of EIMD can be greater than in normal subjects, with higher levels of oxidative stress and inflammation, and this also results in a longer muscle repair time. To reduce the impact of EIMD and to favor faster muscle recovery, the use of anti-inflammatory supplements, such as omega-3 polyunsaturated fatty acids (PUFA), and/or functional foods rich in antioxidants (e.g., juices rich in polyphenols) can be beneficial in these patients. PUFA have a well-established capacity to reduce markers of EIMD (CK, LDH, IL-1β, IL-6) measured after high-damaging protocols (eccentric exercise), and after different duration times of supplementation, from 30 days (Tartibian et al., 2011) to 10 weeks (Ramos-Campo et al., 2020). Concentrated juices (e.g., red beetroot, pomegranate and tart “Montmorency” cherries) can exert positive effects on the magnitude of EIMD and its resolution (Owens et al., 2019; Bongiovanni et al., 2020) through several mechanisms that involve polyphenols effects: (1) by lowering free radical production and lipid peroxidation (Ammar et al., 2018); (2) by reducing inflammation transcripts and molecules related to perceived soreness after damage (e.g., NF-κB, TNF-α and cyclo-oxygenase-2 (COX-2) (Ammar et al., 2018; Kashi et al., 2019); (3) by improving muscle perfusion and oxygen extraction due to their vascular function (Kashi et al., 2019).

Single or multiple attacks of mild to severe myalgia, even not exercise-induced, are common symptoms in CPTII deficiency, with at least 50 attacks per year (Joshi et al., 2019; Joshi and Zierz, 2020). Based on this, pain management is another important clinical target in these patients, with the aim to improve the quality of their life. Nutritional compounds, such as curcumin and ginger, may improve pain symptoms through several mechanisms (Perna et al., 2020) and, therefore, they may be potentially used in these patients if necessary. Curcumin has a significant effect in reducing the production of recognized algogenic substances, such as COX-2 molecules (Kang et al., 2004), prostaglandin E2 (Clutterbuck et al., 2013) and histamine (Nugroho et al., 2009) and can decrease pain sensitivity both by interacting with nociceptor response (Yeon et al., 2010; Leamy et al., 2011), and by modulating pro-inflammatory cytokines production, such as TNF-α, IL-6, and IL-8 (Fernández-Lázaro et al., 2020). Likewise, ginger can exhibit pain-reducing effects by interacting with COX-2 pathways, by inhibiting inflammatory cytokines transcripts (via NF-κB), and by acting as an agonist of vanilloid nociceptors (Rondanelli et al., 2020).