The main aim of the present study was to evaluate, in LOPD patients long term treated with ERT, the effects of exercise alone (exercise) or combined to a high-protein diet (exercise + diet) on indices of exercise tolerance, motor and respiratory functions and quality of life. A control period of no intervention was also present in the protocol.
Interestingly, we observed that the V̇O2 peak decreased in control, whereas it increased in exercise and more significantly so in exercise + diet. V̇O2 peak, a close estimate of maximal aerobic power, is traditionally considered an index evaluating the maximal performance of the integrated respiratory, cardiovascular and muscular factors governing oxidative metabolism during exercise, in other words the maximal flux of O2 from ambient air to the mitochondria of skeletal muscles [20]. In more functional terms, V̇O2 peak estimates the maximal power which can be sustained by oxidative metabolism, and therefore the maximal power which can be sustained for a relatively long period of time (several minutes) without incurring in fatigue. Besides its obvious relationship with exercise tolerance, and therefore with the patients’ quality of life, it has been demonstrated that in different pathological conditions (such as cardiovascular or respiratory diseases) V̇O2 peak also possesses a strong prognostic relevance [21]. It is noteworthy that during the 6-month period of control V̇O2 peak showed a significant decrease. This seems to be compatible with the clinical observation that many patients long term treated with ERT show disease progression despite the therapy [9, 10]. Our data suggest that regular exercise, in particular in association with a high-protein diet, can counteract the progressive loss of V̇O2 peak.
Rather surprisingly, the observed changes in V̇O2 peak were not associated with changes in the results obtained during the 6MWT, which did not change following any experimental conditions. This discrepancy further supports data from a previous study by our group [6], suggesting that in LOPD patients V̇O2 peak is a more sensitive parameter than the distance walked during 6MWT in the assessment of the patients’ exercise tolerance. It is also possible that the 6MWT performance is more related that the V̇O2 peak to the strength of the proximal muscle of the legs. Indeed, no changes in muscle strength were observed in all experimental conditions. This is presumably attributable to the very moderate resistance to movements imposed by the elastic bands. The very moderate resistance was chosen to avoid risks of inducing muscle damage.
The results in terms of V̇O2 peak are compatible with those found in relation to the quality of life evaluation, which showed a slight amelioration after exercise, and significant improvements for different aspects after exercise + diet. Interestingly, the association of the high-protein diet with exercise improved also other clinically relevant outcomes, such as the blood levels of muscular enzymes (which showed a decrease, especially of LDH), and variables of respiratory function (VC and FEV1); the improvements of blood levels of muscular enzymes and of spirometry variables were not observed after exercise alone. Based on these results, it can be hypothesized that high protein diet has an effect in restoring not only skeletal muscle function but also respiratory muscles.
The rationale to combine exercise + diet in LOPD patients comes from cell pathology. The damage of muscle fibers in these patients is indeed not only related to glycogen accumulation, but also to secondary derangements such as increased proteolysis and aberrant autophagy [22]. Exercise and diet could counteract this process, acting on different mechanisms. In fact, in muscle fibers aerobic exercise can modify energy metabolism, increasing the use of fatty acids as an alternative source of energy, thus reducing proteolysis, autophagy and muscle damage [17]. Moreover, exercise training can counteract the general deconditioning typical of chronic diseases, as well as the chronic inflammatory condition associated with inactivity [23]. On the other hand, the high-protein -low-carbohydrate diet can reduce glycogen deposition in muscle fibers and increase intracellular protein synthesis, thereby reducing glycogen accumulation, proteolysis, muscle autophagy and damage [24, 17].
Before the approval of ERT, many clinicians prescribed diet and exercise to LOPD patients, based on data by Slonim et al. obtained in a 10 years retrospective study on 34 patients with LOPD, treated with the so-called NET (nutrition and exercise therapy) [11]. The nutritional approach adopted by Slonim et al. consisted of 25–30% protein, 30–35% carbohydrate, and 35–40% fat, which is the same approach used in the present study. Patients on NET also took supplements of the aminoacid L-alanine, which was not used in the present research since further studies failed to demonstrate the clinical efficacy of this supplementation [25]. In NET the exercise intervention consisted of a daily treadmill program for 45–50 min, followed by aerobic upper-body exercise for 10–15 min. In the present study we chose to use exercise on a cycle-ergometer to allow the participation of patients who had ambulatory difficulties and proposed only for 4 sessions a week in order to make the intervention more compatible with the patients’ everyday life, and to increase their compliance. The rate of clinical deterioration (as estimated by the Walton Scale) during NET was compared with the rate of deterioration observed before starting NET: the data showed that the more compliant patients responded far better than those who became less active and did not follow the diet [11]. No differences were found for the Walton scale score in our study, probably since the control and intervention periods of 6 months were too short to observe modifications on this scale.
After the advent of ERT for Pompe disease, only a few studies investigated the effects of exercise on ERT-treated patients [12, 13, 26] and, to the best of our knowledge, no studies investigated the effects of a high-protein diet associated with exercise in these patients. The advantages of associating physical exercise with ERT were first demonstrated in a study performed in the mouse model of Pompe disease, in which the animals underwent treadmill training and showed improved aerobic capacity, strength and motor functions [27]. In 2011 Terzis et al. [12] reported the effects of a 20-week exercise training in 5 LOPD patients receiving ERT. The prescribed exercise consisted in 3 sessions a week, including 30 minutes of cycling, stretching and resistance training. Significant increases in muscular strength and in the distance walked during the 6MWT were observed after training [12]. More recently, Van der Berg et al. [13] performed a larger study, including 25 patients with LOPD who performed 12 weeks of exercise training while continuing their ERT infusions. The prescribed exercise consisted in 3 sessions a week of training combining aerobic, resistance and core stability exercises, utilizing a cycle ergometer and other gym instruments such as chest press, biceps curl, leg press and leg curl. After training V̇O2 peak and maximum workload capacity improved significantly, while pulmonary function (VC) did not change. Increases in strength of the hip flexors and the shoulder abductors were also observed. The distance walked during the 6MWT increased by 6%. Two patients experienced significant increases of CK but the exercise was generally well tolerated [13]. Compared to the study of Van der Berg et al. [13] our population of LOPD patients seems to be more severely affected at baseline, with 11 out of 14 patients having walking difficulties; this might explain the lack of significant changes in the 6MWT and in muscle strength in our patients after exercise. Overall, a comparison with previous studies carried out prescribing exercise in patients receiving chronic ERT seems to confirm the concept that exercise alone does not ameliorate blood levels of muscle enzymes (CK actually tends to increase) and variables of pulmonary function, which, can ameliorate when an high-protein diet is introduced in addition to exercise. (see Slonim et al. [11] and the present study).
The lack of previous studies on nutritional interventions in LOPD patients receiving chronic ERT could be linked to the difficulty of guaranteeing compliance to high-protein diets, as a consequence of the lower palatability of many high-protein foods compared to those with high carbohydrate content, or of the higher satiety power of proteins, which may lead to a reduction in food intake [28]. A review [29] published in the pre-ERT era, dealing with LOPD patients on high-protein diets, reported that only 25% of patients showed improvements in muscle or respiratory function. However, the compliance to the diet in most of the studies mentioned in that review was very poor. The authors attributed the problems of compliance to the large amount of prescribed proteins and to the associated perceived risk of weight gain [29]. Conversely, in our study we obtained a very high compliance to the prescribed diet, presumably as a consequence of the personalization of the food composition of the diet carried out by experienced nutritionists who took in consideration the patients’ preferences and habits. A strict contact between patients and dieticians was assured throughout the exercise + diet intervention. The risk of weight gain was not confirmed by our data, which on the contrary showed a significant reduction of BMI during exercise + diet, even in patients following normocaloric diets. A high-protein diet, as the one prescribed in the present study, should be carefully prescribed and monitored in underweight patients, and the different dietary sources of proteins should be wisely chosen, assuring great variety between meat, fish, eggs, dairy products and vegetables proteins (as beans or nuts). Some experimental and observational human studies have suggested that a high-protein intake may increase kidney diseases, but only a few randomized trials with an observation time longer than 6 months have been carried out on this topic; most of these studies, moreover, were conducted in patients with pre-existing diseases that predispose them to the development of kidney diseases [30]. In any case, a screening for kidney diseases and a strict monitoring of creatinine and urinary parameters should be considered before and during a long-term treatment with a high-protein diet. Of note, no changes in creatinine plasma values were observed throughout the study.
The present study presents several limitations. In particular, the total number of patients who completed the 3 periods was relatively limited. This could be explained, at least in part, by considering the rarity of the disease and the length of the study (overall duration 24 months). Despite the relatively low number of patients, however, the obtained data showed statistically significant differences in some of the main variables of interest, following some of the proposed interventions. This could be attributed to the elevated compliance by the patients to the proposed life style interventions, facilitated by the constant motivational contacts between researchers and patients. Eight out of the 10 patients who completed the exercise + diet period were keen on following the same prescriptions also after the end of the study. Long term follow-up data of these patients are necessary to see if the compliance and the beneficial effects of the interventions will be maintained.