Effect of Exercise Training on 1,25(OH)₂D Levels: The FIT-AGEING Randomized Controlled Trial

Abstract

Background:

Vitamin D deficiency is currently endemic worldwide and is considered as an important factor in the development of several chronic conditions. Physical exercise has been postulated as an auspicious strategy to counteract age-related disorders preventing premature mortality. However, the effects of chronic exercise training on 1,25-dihydroxyvitamin D [1,25(OH)₂D] is unclear. This 12-week randomized controlled trial aimed to investigate the effects of different training modalities on 1,25(OH)₂D in healthy sedentary adults.

Hypothesis:

Exercise training will increase 1,25(OH)₂D in the study cohort.

Study Design:

Randomized controlled clinical trial.

Level of Evidence:

Level 1.

Methods:

A total of 89 healthy sedentary adults (53% women; 53.5 ± 4.9 years old) were enrolled in the FIT-AGEING study. The participants were randomized to (1) a control group (no exercise), (2) physical activity recommendation from the World Health Organization (PAR group), (3) high-intensity interval training (HIIT group), and (4) HIIT adding whole-body electromyostimulation training (HIIT + EMS). 1,25(OH)₂D plasma levels were measured using a DiaSorin Liaison immunochemiluminometric analyzer.

Results:

Compared with the control group, 1,25(OH)₂D increased in PAR (Δ = 10.99 ± 3.44 pg/mL; P = 0.01), HIIT (Δ = 11.63 ± 3.51 pg/mL; P = 0.009), and HIIT + EMS groups (Δ = 14.01 ± 3.59 pg/mL; P = 0.001) without statistical differences between them (all Ps > 0.1).

Conclusion:

In summary, the results show that a 12-week exercise intervention produced an increment of 1,25(OH)₂D independently of age, sex, and exercise modality in healthy sedentary adults.

Clinical Relevance:

The implementation of physical exercise could be considered a strategy not only aiming to reverse the seasonal decrease of 1,25(OH)₂D in winter explained by low sunlight exposure but also for obtaining subsequent increases of this hormone even in these a priori adverse conditions.

Vitamin D deficiency is currently endemic worldwide, not only in underdeveloped countries but also in developing and developed countries. ¹⁹ Indeed, this metabolic problem affects all age-groups—especially elderly adults—and its incidence is independent of the individual’s sex. ¹⁹ Previous studies have reported that the vitamin D status plays a key role in the aging process in addition to its important implications on bone and skeletal muscle metabolism. ²⁰ Given that vitamin D receptors have been found in more than 30 different types of cells, vitamin D also exerts additional functions in further tissues and organs (eg, immunological or digestive functions, among others). ²⁵ This argument is supported by the notion that poor levels of vitamin D are associated with increased prevalence of several age-related diseases and mortality in both healthy individuals and patients.^39,43 Serum 25-hydroxyvitamin D levels [25(OH)D] have traditionally been used as an indicator of vitamin D status. However, considerably less attention has been paid to 1,25-dihydroxyvitamin D [1,25(OH)₂D] (ie, calcitriol), which is principally responsible of its metabolic properties.^19,27

In addition to medical treatment, physical exercise has been postulated as an auspicious strategy to counteract both mental and physical chronic disorders. ⁹ Concretely, Pedersen and Saltin ²⁸ proposed physical exercise as the most important instrument to prevent and treat more than 35 chronic diseases (eg, psychiatric disorders, metabolic pathologies, cardiovascular diseases, or cancer). Moreover, a 30% reduced risk of mortality can be attained when a well-designed and structured physical exercise program is performed, thus increasing longevity and quality of life during the aging process. ¹⁶

Previous scientific evidence has demonstrated the efficacy of physical exercise to prevent and/or delay the deleterious effects of the ageing process on physiological functions. ² Nevertheless, the molecular and biological pathways remain partially unknown that explain the exercise-related positive influence on human health in old age. In this regard, considerably little attention has been focused on physical exercise as a regulator of vitamin D status. Interestingly, an aerobic training intervention ranging from 5 to 8 weeks induced a significant increment of 25(OH)D in elderly adults.^24,35 The problem of studying aging metabolic biomarkers in elderly individuals is that most of them suffer from age-related illnesses. ¹⁵ From a preventive perspective, it is, therefore, of scientific and clinical interest to study these metabolites in relatively young individuals free of chronic disorders. ¹⁴ To the best of our knowledge, there is no study investigating the effects of physical exercise on the active form of vitamin D. Furthermore, no data are yet available concerning whether different exercise training modalities could induce contrasting effects on 1,25(OH)₂D. Indeed, although a concurrent training based on physical activity recommendation from the World Health Organization is usually implemented for health promotion, ⁴⁰ its adherence rates are relatively low, and emerging training programs (eg, high-intensity interval training [HIIT] or whole-body electromyostimulation training [EMS]) could be attractive strategies to reach similar or even higher health-related improvements. Therefore, this study aimed to investigate the effects of different exercise training modalities on 1,25(OH)₂D in healthy sedentary adults. A further aim of this study was to determine whether changes in other health-related parameters (ie, body composition and physical fitness) were associated with these hypothetical changes in 1,25(OH)₂D in this study cohort. It is hypothesized that all training modalities will increase 1,25(OH)₂D in the present study cohort.

Methods

Participants

Eighty-nine healthy sedentary adults (53% women) were voluntarily enrolled in the FIT-AGEING study, an exercise-based randomized controlled trial. ⁴ Figure 1 shows the flowchart of this study. The participants were recruited via local media, social networks, and posters. Interested individuals were screened via telephone and/or email. A research medical staff conducted a medical examination to determine whether potentially eligible participants met the following inclusion criteria: (1) adults aged between 45 and 65 years, (2) be sedentary (<20 minutes of moderate-intensity physical activity on 3 days in a week over the past 3 months), and (3) have a stable weight over the past 3 months. The exclusion criteria were the following: (1) suffering from chronic cardiometabolic diseases, (2) taking any medication, and (3) having a major illness (acute or chronic), including any that would limit the ability to perform the necessary exercises. The study protocols and experimental design followed the principles of the last revised Declaration of Helsinki ⁴¹ and was approved by the Regional Ethics Committee on Human Research (0838-N-2017). All participants provided oral and written informed consent after having read and understood the details of the exercise programs and the experimental procedures. All the baseline and follow-up examinations were performed in the same setting.

Figure 1.

Flowchart of the study. BMI, body mass index; CDV, cardiovascular; ECG, electrocardiogram; HIIT, high-intensity interval training group; HIIT + EMS, high-intensity interval training plus whole-body electromyostimulation group; PAR, physical activity recommendations for adults proposed by the World Health Organization group.

Study Design

A randomized controlled trial with a parallel-group design was performed over 12 weeks according to the CONSORT (Consolidated Standards of Reporting Trials) statement for transparent reporting ³¹ and CERT (Consensus on Exercise Reporting Template) guidelines (Appendix Tables S1 and S2 available in the online version of this article). The randomized controlled trial was conducted in 2 waves (September-December 2016 and September-December 2017) of 45 participants maximum. After the baseline assessment was performed, participants were linked to a study identification number and were then randomized into 4 groups using a computer-generated simple randomization software ³² in the following groups: (1) a control group (no exercise), (2) a concurrent training based on physical activity recommendation from the World Health Organization (PAR) group, (3) a HIIT group, and (4) HIIT plus EMS (HIIT + EMS) group. The assessment staff was blinded to the group allocation.

Exercise Training Modalities

A full description of each exercise training modality has been previously published. ⁴

Briefly, the participants allocated to the PAR group completed 3 concurrent training (ie, aerobic plus resistance training) sessions per week for 12 consecutive weeks with at least 48 hours of recovery between sessions. This training program was based on the minimum physical activity recommended by the World Health Organization. ⁴⁰ The training volume was 150 min/wk at an intensity of 60% to 65% of the heart rate reserve for the aerobic training, while ∼60 min/wk at an intensity of 40% to 50% of one-repetition maximum was established for the resistant training. The aerobic training section was performed using a treadmill, cycle-ergometer, and elliptical ergometers. For the resistance training section, weightbearing and guided pneumatic machines were used. To reduce the risk of injuries as well as to promote exercise adherence, compensatory exercises (ie, flexibility, stabilizer muscles, and core stability) were included.

The participants allocated into the HIIT group completed 2 sessions per week for 12 weeks with at least 72 hours of recovery between sessions. This training program involved 2 different and alternative HIIT protocols,^11,12 which included an HIIT with long intervals (type A session), and an HIIT with short intervals (type B sessions). Type A session was characterized by a training volume of 40 to 65 min/wk at >95% of the maximum oxygen uptake (VO₂max). The participants completed type A session walking on a treadmill with a personalized slope. Type B session was composed by a training volume of 40 to 65 min/wk at level 6 to 9 on a perceived maximum effort scale (ranging from 0 to 10). ¹⁰ Eight weightbearing programmed exercises in a circuit form (ie, squat, deadlift, high knees up, high heels up, push up, horizontal row, lateral plank, and frontal plank) were performed in type B sessions.

The participants allocated in the HIIT + EMS group completed a training program following the same methodology that the HIIT group (ie, periodization, training frequency, volume, intensity, and type of exercise) combined with electrical pulses with a whole-body electromyostimulation wireless device (Wiemspro). The programmed electric pulse was characterized by (1) a frequency of 15 to 20 Hz, an intensity of 100 mA, an impulse breadth of 200 to 400 µs, and a duty cycle of 99% in the type A sessions and (2) a frequency of 35 to 75 Hz, an intensity of 80 mA, an impulse breadth of 200 to 400 µs, and a duty cycle of 50% to 63% in the type B sessions.

All training sessions were supervised by a qualified and certified graduate in sport sciences, who constantly motivated the participants and instructed them to reach the specific target intensity in all sessions. Before each training session, all participants underwent a dynamic standardized warm-up, including general mobility exercises. Training sessions concluded with a cooling-down protocol (ie, active global stretching). Heart rate was continuously monitored during exercise and the rated perceived exertion scale was also registered in all sessions. Furthermore, a gradual progression was also proposed to control the exercise dose in each training group. ⁴ To be included in the final analysis, participants were required to attend at least 90% of sessions.

The participants of the control group were instructed to maintain their lifestyle and not being enrolled in any structured exercise program during the intervention. For ethical reasons, participants were invited to an information meeting where the research staff provided general advice about a healthy lifestyle.

Blood Sample Collection and 1,25-Dihydroxyvitamin D Assessment

A 10-mL peripheral blood samples were collected using the Vacutainer SST system (Becton Dickinson). They were obtained from an antecubital vein of the forearm after a 12-hour overnight fast, and then centrifuged at 4000 rpm for 7 minutes at 4°C. All samples were collected at baseline and after the 12-week intervention. Aliquots of plasma were stored at −80°C until further analysis. 1,25(OH)₂D plasma levels were determined using a Liaison immunochemiluminometric analyzer (DiaSorin; Saluggia, Italy) and expressed as picograms per milliliter (pg/mL). All participants were previously requested to abstain from drugs, alcohol, and/or caffeine, to eat a standardized dinner, and to avoid any physical activity of moderate (24 hours before) and/or vigorous intensity (48 hours before). Blood samples were obtained after 72 to 96 hours of the last bout of exercise in the postintervention assessment.

Anthropometric and Body Composition Assessment

Anthropometric and body composition measurements were performed before and after the intervention program. Height and body mass were measured through a prevalidated Seca model 799 scale and stadiometer (Seca) with light clothing and without shoes. Body mass index (BMI) was subsequently calculated by dividing body mass (kg) by the square of the height in meters (m ² ). Lean mass, fat mass, and bone mineral density assessment were performed by dual x-ray absorptiometry (Discovery Wi, Hologic, Inc) under the manufacture’s recommendations. Fat mass was also expressed as a percentage of the total body mass.

Cardiorespiratory Fitness Assessment

VO₂max was measured through a maximum treadmill (H/P/Cosmos Pulsar treadmill, H/P/Cosmos Sport & Medical GmbH) exercise test applying the modified Balke protocol. ⁸ Briefly, the incremental protocol began walking at 3 km/h at 0% grade for the first minute followed by 2 minutes at 4 km/h. Treadmill speed was increased to 5.3 km/h at 0% grade for 1 minute with subsequent increases of 1% of the grade every minute until the participants reached their volitional exhaustion. Gas exchange was continuously measured (CPX Ultima CardiO2, Medical Graphics Corp). O₂ uptake and CO₂ production were averaged every 5 seconds with the Breeze Suite Software (version 8.1.0.54 SP7, MGC Diagnostic). Flow calibration was performed by a 3-L syringe, while gas calibration was performed with 2 standard gas concentrations before each use according to the manufacturer’s instructions. The 6-20 Borg scale rating of perceived exertion was applied during the last 15 seconds of each stage and at exhaustion, while heart rate was measured continuously (every 5 seconds) (Polar RS300). VO₂max was defined by the following criteria: (1) to attain a respiratory exchange ratio ≥1.1, (2) to show a plateau in the VO₂ curve despite increasing the exercise intensity (defined as a change of <100 mL/min in the last 30 seconds), and (3) to reach the age-predicted maximal heart rate (209 − 0.73 × age) ± 10 beats/min. ³⁶ Peak oxygen consumption value was considered when these criteria were not met. ²⁶ The participants were previously instructed to meet the following preconditions: (1) to refrain from stimulant substances at least 24 hours before the test, (2) to fast for 3 hours, and (3) to avoid any physical activity of moderate and/or vigorous intensity for 24 and 48 hours before the test, respectively.

Muscular Strength Assessment

Isokinetic knee extension strength was measured with a Gymnex Iso-2 dynamometer (Easytech s.r.l.) applying the same preconditions as in the cardiorespiratory fitness measurement. The isokinetic dynamometer was calibrated following the manufacturer’s instructions before starting the data collection. The isokinetic peak torque of extensor muscles was measured at an angular velocity of 60 deg/s. The participants were positioned on the seat, securing the upper leg, hips, and shoulder to the chair using safety belts limiting any extra movement of the body during the test. For safety reasons, a knee joint of motion angle ranged from 90° to 170° was set for each participant. All of them performed 5 submaximal repetitions (ie, familiarization protocol), followed by a 1-minute rest interval. Then, 3 maximal repetitions were completed to evaluate the isokinetic knee strength. ⁷ The peak concentric torque (N·m) was determined as the single repetition with the highest muscular force output. Constant verbal motivation encouragement was given to participants during the tests to generate maximum effort in each repetition, and the same trained researcher conducted all the isokinetic tests.

Handgrip strength was measured using a digital hand dynamometer with an adjustable grip (T.K.K. 5401 Grip-D; Takey). The participants alternately completed 2 attempts with each hand and were asked to generate the maximum isometric force during 2 to 3 seconds. Following previous studies, the optimal grip span of the dynamometer was adjusted at 5.5 cm for men and calculated with a validated equation for women. ³⁰ Total hand grip strength was considered as the sum of the highest value (in kg) of each hand.

Dietary Intake Assessment

Dietary intake was obtained by using the average of three 24-hour food records. They were obtained during an interview performed by a qualified and experimented research dietitian on nonconsecutive days (1 day on the weekend). The recalls were processed on the EvalFINUT software, which is based on US Department of Agriculture and “Base de Datos Española de Composición de Alimentos” databases, to obtain data related to energy, macronutrients, and micronutrients intake. The interviews were supported by the use of a guide with colored photographs of different food portions sizes to facilitate the estimation of the quantity of food consumed. ²³

Statistical Analysis

The sample size and power calculations were based on a pilot sample (n = 30). A minimum predicted change in 1,25(OH)₂D of 10% was observed between the intervention and the control group with a standard deviation of 10%. A sample size of 16 participants per group was predicted to provide a statistical power of 85% considering a type 1 error = 0.05. ³³ A minimum of 20 participants per group were recruited assuming a maximum loss at follow-up of 25%.

Data normality was checked using the Shapiro-Wilk test, visual check of histograms, and Q-Q plots. The descriptive parameters are reported as mean and standard deviation. Student t tests for unpaired values were conducted to examine differences in dependent variables at the baseline between groups.

Analysis of covariance (ANCOVA) was performed to study the effect of the groups (fixed factor) on dependent outcomes, adjusting for the baseline values (ie, post-1,25(OH)₂D minus pre-1,25(OH)₂D). ANCOVA was also conducted to investigate whether the aforementioned changes were independent of sex and age and energy/macronutrient intake. We performed Bonferroni post hoc test with adjustment for multiple comparisons to determine differences between all exercise training modalities groups.

To examine the relationship of changes in body composition variables (ie, body mass, lean mass, fat mass percentage) and physical fitness variables (ie, VO₂max in absolute values and in relation to body mass, extension peak torque, and handgrip strength) with changes in 1,25(OH)₂D, we conducted simple linear regression. Multiple linear regressions were also performed adjust by sex (model 1) and age (model 2).

The level of significance was set at P ≤ 0.05. Statistical analyses were performed using the Statistical Package for the Social Sciences (SPSS, Version 22.0, IBM SPSS Statistics, IBM Corporation). Graphical presentations were prepared using GraphPad Prism 5 (GraphPad Software Inc).

Results

Figure 1 shows the flowchart for enrollment and analysis. A total of 66 participants (n = 15 in the control group, n = 17 in the PAR group, n = 16 in the HIIT group, and n = 18 in the HIIT + EMS group) completed the study obtaining a global attendance of 99% to the supervised exercised sessions in the intervention groups.

Table 1 shows the descriptive characteristics of the study participants at baseline. No significant differences at the baseline were shown in age, sex, body composition, blood parameters, physical fitness, or dietary intake. There were roughly equal number of men and women in each group.

Table 1.

Descriptive baseline parameters ^a

	All (n = 66)	Control (n = 15)	PAR (n = 17)	HIIT (n = 16)	HIIT + EMS (n = 18)
Age, y	53.5 ± 4.9	51.7 ± 4.1	54.9 ± 4.5	53.9 ± 5.5	53.3 ± 5.3
Sex, n (%)
Men	30 (45.5)	6 (40.0)	8 (47.1)	7 (43.8)	9 (50.0)
Women	36 (54.5)	9 (60.0)	9 (52.9)	9 (56.3)	9 (50.0)
1,25-Dihydroxyvitamin D, pg/mL	40.1 ± 13.7	45.5 ± 13.8	42.5 ± 11.6	40.5 ± 9.6	32.8 ± 16.2
Anthropometry and body composition
Body mass, kg	75.8 ± 14.9	74.1 ± 13.8	72.6 ± 11.3	76.3 ± 17.9	79.8 ± 16.3
Body mass index, kg/m2	26.7 ± 3.9	26.7 ± 3.9	25.4 ± 2.9	26.1 ± 3.2	28.5 ± 4.7
Lean mass, kg	43.7 ± 11.3	44.3 ± 11.6	43.6 ± 10.8	42.3 ± 12.7	44.7 ± 11.1
Fat mass, %	39.5 ± 8.5	37.7 ± 8.2	37.4 ± 8.8	42.0 ± 8.1	40.8 ± 8.8
Bone mineral density, g/cm2	1.10 ± 0.10	1.10 ± 0.12	1.08 ± 0.08	1.10 ± 0.10	1.12 ± 0.10
Physical fitness
VO2max, mL/min	2307.2 ± 623.4	2203.0 ± 610.9	2320.4 ± 649.7	2313.9 ± 599.7	2375.5 ± 670.4
VO2max, mL/kg/min	30.4 ± 5.4	29.5 ± 4.8	31.6 ± 6.1	30.6 ± 5.9	29.6 ± 5.0
Handgrip strength, kg	70.6 ± 23.7	70.1 ± 24.4	72.0 ± 25.0	68.3 ± 26.7	71.7 ± 21.0
Extension peak torque, N·m	267.4 ± 82.6	268.6 ± 74.4	271.5 ± 76.1	285.1 ± 109.2	245.5 ± 67.9
Dietary intake
Energy intake, kcal/d	2071.7 ± 455.4	2023.9 ± 497.6	2008.1 ± 385.3	2128.7 ± 545.5	2117.5 ± 428.8
Carbohydrate intake, g/d	215.3 ± 59.0	217.7 ± 87.3	206.0 ± 40.3	226.0 ± 53.9	212.8 ± 53.9
Fat intake, g/d	87.9 ± 23.6	86.4 ± 18.7	85.3 ± 27.0	87.3 ± 28.9	92.0 ± 20.1
Protein intake, g/d	83.0 ± 25.2	72.2 ± 17.7	84.7 ± 32.0	86.3 ± 26.3	87.3 ± 21.6

HIIT, high-intensity interval training group; HIIT + EMS, high-intensity interval training plus whole-body electromyostimulation group; PAR, physical activity recommendations for adults proposed by the World Health Organization group; VO₂max, maximum oxygen uptake.

^aData are shown as mean ± standard deviation.

Figure 2 shows changes in 1,25(OH)₂D after the intervention study among the 4 groups. Compared with the control group, 1,25(OH)₂D increased in PAR (Δ = 10.99 ± 3.44 pg/mL; P = 0.01), HIIT (Δ = 11.63 ± 3.51 pg/mL; P = 0.009), and HIIT + EMS groups (Δ = 14.01 ± 3.59 pg/mL; P = 0.001) without statistical differences between them (all P values > 0.1, Figure 2). All results persisted after including sex, age, and energy/macronutrients intake as covariates (all P values < 0.02).

Figure 2.

Changes in 1,25-dihydroxyvitamin D after the intervention study in the 4 groups. Parallel bars indicate significant differences between groups. *P < 0.05, **P < 0.01, ***P < 0.001, analysis of covariance adjusting for baseline values, with post hoc Bonferroni-corrected t test. The data are shown as mean ± standard deviation. HIIT, high-intensity interval training group; HIIT + EMS; high-intensity interval training plus whole-body electromyostimulation group; PAR, physical activity recommendations for adults proposed by the World Health Organization group.

Appendix Figure A1 (available online) shows the association between changes in body composition variables and changes in 1,25(OH)₂D after the intervention programs. No association was found between changes in body mass, lean mass, fat mass percentage, and bone mineral density with changes in 1,25(OH)₂D (all P values > 0.1, Appendix Figure A1 [available online]), which remained after adjusting for sex and age (allP values > 0.1, Table 2).

Table 2.

Association between changes in body mass, lean mass, fat mass percentage, and maximum oxygen uptake (VO₂max) in absolute values and in relation to body mass, handgrip strength, and extension peak torque with 1,25-dihydroxyvitamin D changes unadjusted (model 0), adjusted for sex (model 1), and adjusted for age (model 2) ^a

	Δ 1,25-Dihydroxyvitamin D (pg/mL)
	Model 0		Model 1		Model 2
	P	β	P	β	P	β
Δ Body mass, kg	0.91	0.015	0.87	0.022	0.96	0.007
Δ Lean mass, kg	0.18	0.173	0.18	0.173	0.22	0.165
Δ Fat mass, %	0.15	−0.187	0.14	−0.192	0.18	−0.180
Δ VO2max, mL/min	0.008	0.325	0.009	0.327	0.01	0.328
Δ VO2max, mL/kg/min	0.009	0.324	0.01	0.323	0.01	0.322
Δ Handgrip strength, kg	0.002	0.378	0.002	0.384	0.002	0.387
Δ Extension peak torque, N·m	0.008	0.327	0.009	0.331	0.01	0.332

^aP value of multiple regression analysis. β is the standardized regression coefficient. Values in boldface indicate statistically significant differences (P < 0.05).

Appendix Figure A2 (available online) shows the association between changes in physical fitness variables and changes in 1,25(OH)₂D after the intervention programs. A significant positive association was found between the changes in VO₂max (in absolute values and in relation to body mass) and changes in the 1,25(OH)₂D (β = 0.325, R² = 0.105, P = 0.005, and β = 0.324, R² = 0.106, P = 0.008; Appendix Figure A2a and A2b [available online], respectively). Similarly, a significant positive association was found between changes in both extension peak torque and handgrip strength, and changes in the 1,25(OH)₂D (β = 0.327, R² = 0.107, P = 0.008, and β = 0.378, R² = 0.143, P = 0.002; Appendix Figure A2c and A2d [available online], respectively). All associations remained after including sex and age as covariates (all P values ≤ 0.02, Table 2).

Discussion

The current study sought to elucidate (1) the effects of 3 different exercise training interventions on 1,25(OH)₂D in healthy sedentary adults and (2) whether exercise-related changes in body composition and physical fitness were associated with changes in 1,25(OH)₂D in this study cohort. The main findings of the present work were that, compared with the control group, the participants included in the intervention groups benefited from a significant improvement in 1,25(OH)₂D independently of the exercise modality. Interestingly, while no significant association between changes in body composition and changes in 1,25(OH)₂D was observed, a significant positive association was found between changes in physical fitness and changes in 1,25(OH)₂D. Taking together, these results suggest that physical exercise is an effective strategy to increase 1,25(OH)₂D in healthy sedentary adults and that these training effects may partially explain changes in physical fitness.

Three main physiological mechanisms have been postulated for exercise-mediated improvement of vitamin D status. ²⁴ The first is that white adipocytes are considered a reservoir of 25(OH)D because these cells can uptake 25(OH)D from the bloodstream. ²⁹ It seems therefore plausible that a fat-loss exercise intervention produces an increment of 25(OH)D availability. Indeed, Rock et al ²⁹ reported that weight loss (presumably accompanied by a decrease in fat mass) was associated with higher serum 25(OH)D in overweight and obese women. Furthermore, skeletal muscle myocytes—in addition to their traditional physiological functions—also act as a 25(OH)D store. ¹ Given that chronic physical exercise could promote subsequent increments of skeletal muscle mass, a greater pool of 25(OH)D would be available to be released into peripheral circulation. ¹ Last, previous scientific studies demonstrated that physical exercise optimizes not only the synthesis and release of 25(OH)D from the liver (also decreasing its breakdown) but also its hydroxylation in the kidney resulting in greater 1,25(OH)₂D levels in animal models. ⁴² However, there is limited evidence regarding the effects of physical exercise on vitamin D metabolism in humans.

Aerobic exercise is an effective stimulus to increase 25(OH)D levels after an acute bout of exercise ³⁴ and at the end of a chronic exercise intervention.^17,24,35 On one hand, Sun et al ³⁴ observed a direct effect of an acute bout of aerobic exercise on the increase in serum 25(OH)D concentrations, which persisted until 24 hours after exercise. On the other hand, significant increments of 25(OH)D were noted in response to an aerobic exercise intervention (ranging from 5 to 12 weeks) in pregnant women ¹⁷ and elderly adults.^24,35 However, while Vainionpää et al ³⁷ did not find any significant change in 25(OH)D after a 1-year aerobic exercise intervention in women free of disease, Evans et al ¹³ showed a significant decrease of 25(OH)D in response to a 16-week military training intervention in young adults. These discrepancies could be explained by different facts, including (1) the individual’s age, sex, or health status; (2) the analytical procedures to determine 25(OH)D; (3) the different 25(OH)D pretest values; and, in some cases, ¹⁴ (d) the lack of a control group, which makes unclear whether the obtained changes in 25(OH)D are explained by the exercise intervention or by seasonal variation that implies different sunlight exposure and dietary habits. ²² However, there is no study investigating the effects of exercise on 1,25(OH)₂D. This study suggests that a well-designed and structured 12-week exercise intervention induces significant increments of 1,25(OH)₂D in healthy sedentary adults independently of sex, age, and exercise training modality, which partially supports the aforementioned findings.

A poor vitamin D status has been consistently associated with an increased incidence of chronic metabolic and cardiovascular diseases. ²¹ Low physical fitness levels and an altered body composition have been also related to a higher incidence of age-related chronic pathological conditions and all-cause mortality.^18,38 On one hand, physical fitness depends on different physiological parameters including arterial oxygen availability, muscle contractibility, cardiac output, shunting of blood to myocytes, and oxygen extraction performed by these myocytes among others, all of them improved by physical exercise and modulated by vitamin D metabolites. ⁶ On the other hand, it is well known that individuals with sarcopenic obesity (ie, high fat mass and low lean mass) present reduced 25(OH)D levels not only because a large fat body mass surface provides superior distribution space for fat-soluble compounds but also because these individuals usually have important alterations of lifestyle factors, including unhealthy dietary habits or physical activity behavior. ⁴³ The positive effects on physical fitness and body composition induced by physical exercise could be partially modulated by changes in vitamin D metabolism. We have recently shown that the PAR, HIIT, and HIT + EMS are effective to improve physical fitness ³ and body composition ⁵ in this study cohort. Remarkably, the current findings support the notion that exercise-induced changes in physical fitness are closely related to changes in 1,25(OH)₂D, which is in line with the previous rationale, while no association was obtained between changes in 1,25(OH)₂D and changes in body composition. Further studies are needed to understand these associations.

The present study had several limitations. We only included healthy sedentary adults (45-65 years old), and hence, we do not know whether these results can be extended to younger, older, and/or physically active individuals. The sample size of this study was relatively small, so this study is likely underpowered to detect statistical differences in 1,25(OH)₂D between the different training modalities, although we observed a robust increment of the 1,25(OH)₂D in the PAR, HIIT, and HIIT + EMS groups compared with the control group. Further trials involving a greater number of participants are needed to accurately determine training-induced changes when comparing these 3 exercise methodologies. Finally, we did not measure 25(OH)D levels, which would have allowed us to better understand the role of exercise on vitamin D metabolism.

Conclusion

These results show that a 12-week exercise intervention produced an increment of 1,25(OH)₂D independently of age, sex, and exercise modality in healthy sedentary adults. Furthermore, we also found a significant positive association between changes in physical fitness and changes in 1,25(OH)₂D in this study cohort. Therefore, we suggest that the link between an exercise intervention and the increase of physical fitness could be mediate changes in vitamin D metabolism.

The present findings suggest that physical exercise, independently of its modality, can be considered as a strategy to increase 1,25(OH)₂D in middle-aged adults. This conclusion supports the implementation of physical exercise as a strategy not only aiming to reverse the seasonal decrease of 1,25(OH)₂D in winter explained by low sunlight exposure (ie, we performed the baseline assessment in September and the postintervention test in December) but also for obtaining subsequent increases of this hormone even in these a priori adverse conditions. Finally, a longer intervention (>12 weeks) is needed to well understand the study’s clinical implications.