INTRODUCTION

The aging process is associated with a loss of muscle mass and strength, as well as a decrease in bone mineral density (BMD), which can lead to reduced mobility, greater risk of falls and the appearance of fractures¹,². In recent years, special emphasis has been placed on maintaining an adequate vitamin D status to optimize muscle strength and BMD in order to reduce falls and fractures³-⁵. Although a recent meta-analysis questions the usefulness of vitamin D supplements to reduce the risk of falls, BMD decrease and fractures6, there are sufficient arguments that demonstrate the importance of vitamin D on muscle and bone health. Vitamin D stimulates the absorption of calcium from the intestine and maintains the serum calcium levels that are required for normal bone mineralization and for the maintenance of muscle function⁷. Several in vivo studies suggest vitamin D’s role in regulating muscle mass and its function. Observational studies show that vitamin D deficiency in the elderly is associated with reduced muscle mass and strength⁸-¹⁰, lower physical performance⁸,¹¹, and increased risk of falls¹². In addition, a meta-analysis of 17 clinical trials showed that supplementation with vitamin D in subjects with basal calcidiol levels below 10 ng/mL had a positive effect on hip muscle strength13. These studies suggest that vitamin D can affect muscle mass and function. However, it is not clear whether vitamin D plays a direct or indirect role. In recent years, the local conversion of calcidiol to calcitriol, the most active vitamin D metabolite, which is synthesized mainly in the kidney through its precursor calcidiol, has been increasingly important⁷. This local synthesis has been reported in several other cell types, such as in osteoblasts¹⁴-¹⁷, prostate cells18 and monocytes19, which reinforces the importance of reaching adequate levels of calcidiol in the body.

Therefore, the aim of our study was to analyze in an unselected population the effect of calcidiol (25-OHD) levels on muscle strength in both hands, activities of daily life related to the functional capacity of the individual and the changes in the BMD.

MATERIAL AND METHODS

The initial study protocol was designed to ascertain the prevalence of vertebral fracture. To do this, 624 men and women over 50 years of age were randomly selected from the municipal registry of Oviedo, Spain. The protocol con- sisted of all subjects completing a questionnaire on risk factors related to osteoporosis. This questionnaire was designed for the EVOS study, translated into several languages, and had an adequate reproducibility index²⁰, ²¹. Similarly, the entire cohort underwent two lateral radiographs (this radiographic study was not completed in only two cases), the collection of anthropometric measurements such as height and weight to determine the body mass index (BMI), and a densitometric study. All subjects had sufficient ambulatory capacity to climb two floors without a lift and 99% lived in their own home.

After the prevalence study, this cohort was followed prospectively for 4 years by means of 2 postal questionnaires to find out the incidence of non-vertebral osteoporotic fracture. In the fourth year of the follow-up period (between the second and the third postal questionnaire), participants who had answered at least one of the two previous questionnaires were invited to repeat the same tests performed in the prevalence study, to which measures of muscular strength of grip in both hands were added to him with a dynamometer that owns a scale that goes from the minimum 0 to the maximum of 300 mm of Hg, a survey with 12 items on the difficulty or not to carry out daily life activities, as well as a biochemical study of general markers and bone and mineral metabolism. In this second cross-sectional study, 404 subjects participated (212 women and 192 men), of which 322 agreed to take part in the biochemical study. A total of 32 subjects (9.9%) were excluded from the analysis as they had undergone osteoporotic treatment, including treatment with vitamin D. From a total of 290 subjects, we had all the data in both cross sections.

RESULTS

Table 1 shows sociodemographic, anthropometric, clinical variables and biochemical markers of the cohort analyzed as a function of serum levels of calcidiol. In those with calcidiol levels of ≥20 ng/mL, there was a predominance of men, younger age, higher BMD values in all the skeletal segments analyzed, lower frequency of previous fractures, higher levels of calcitriol and lower levels of PTH and total alkaline phosphatase.

Table 1. Demographic, anthropometric characteristics, clinical variables and biochemical markers as a function of serum levels of calcidiol 

Calcidiol values =20 ng/mL (28.6% of the cohort) were associated with greater grip strength in both hands compared to levels <20 ng/mL (Figure 1). After adjusting for age, sex, BMI and seasonality, calcidiol levels <20 ng/mL alone were associated independently with decreases in grip strength (OR=2.35; 95% CI: 1.03-5.38). On the other hand, that association was lost in the right hand (OR=1.91; 95% CI: 0.92-3.98).

Figure 1. Grip strength measurements (mm Hg) in hand A) left; B) right according to the serum levels of calcidiol. *p <0.05 for calcidiol <10 ng/mL and calcidiol between 10-20 ng/mL 

The daily life activities as a function of calcidiol levels are reflected in table 2. Of the 12 activities analyzed, the inability or difficulty to "lean to catch a soil object" was significantly associated with lower levels of calcidiol (p=0.009, table 2).

Table 2. Levels of calcidiol (ng/mL) depending on the difficulty or not to perform certain activities of daily life 

Likewise, the difficulty or inability to: "get out of bed" was associated with lower levels of calcidiol; "Pick up a book or object from a high shelf"; "Leaning from a chair to take object from the floor"; "Remove the stockings or socks" and "run 100 meters without stopping" (Table 2). Only "bending down to get an object from the floor" and "getting up from the bed" were significantly associated with calcidiol levels after multivariate adjustment for age, sex, BMI and seasonality. Thus, increments of 10 ng/mL of calcidiol were associated with a decrease of 30% and 58%, respectively, in the difficulty or inability to "bend over to pick up an object from the floor" or to "get up from bed".

The stratification of calcidiol levels showed that, in the multivariate adjustment, the presence of calcidiol deficiency (<10 ng/mL) not only significantly increased the inability or difficulty to "get out of bed: (OR=2.14; 95% CI: 1.21-3.77)" but also to "take a book or object from a high bookshelf: (OR=2.02; 95% CI: 1.09-3.73)", "lean from a chair to take object from the floor: (OR=1.78; 95% CI: 1.03-3.07)" and "remain seated in a hard chair for 1 hour: (OR=1.78; 95% CI: 1.03-3.07)".

The percentage of change in BMD at the level of the lumbar spine, femoral neck and total hip as a function of the serum levels of calcidiol is shown in table 3. The presence of calcidiol levels <20 ng/mL was associated with greater losses of BMD, both at the femoral neck and total hip level, with no significant differences at the lumbar level. After a multivariate analysis, changes in BMD at femoral neck and total hip level were independently associated with calcidiol levels <20 ng/mL (typified beta coefficient=0.130, p=0.041 and typified beta coefficient=0.142, p=0.033, respectively).

Table 3. Percentage of change in BMD at the level of the lumbar spine, femoral neck and total hip between the two cross-sectional studies as a function of serum levels of calcidiol 

DISCUSSION

In this study, low levels of calcidiol (<20 ng/mL) have been found to contribute to a lower muscular strength of grip in the hands, to more difficulties to perform certain activities of daily life, as well as to greater losses of BMD in the hip.

There are both basic and clinical evidences that support the participation of vitamin D in skeletal muscle function²³. In recent work in people with spinal cord injuries requiring rehabilitation, low levels of calcidiol were independent predictors of decreased physical function²⁴. The lower muscle strength of grip in relation to the low levels of calcidiol found in our study has also been reported by other authors25. Thus, in a longitudinal study of Dutch adults, aged between 55 and 85 years, serum levels of calcidiol below 10 ng/mL were associated with a 40% loss in grip strength compared to baseline²⁶.

In our study, some activities of daily life were found to be compromised by the low levels of calcidiol, this effect being more marked in the presence of calcidiol deficiency (<10 ng/mL). The activities that were most affected were those that had more to do with the functional capacity of the organism than those dependent on greater muscular strength such as "transporting an object of 10 kg for 10 meters" or "lifting a box with 6 full bottles and place them on a table." Other recent studies have also associated low levels of vitamin D with the greatest difficulty in performing activities of daily living. Thus, in a recent study by Arbex Borim et al., the reduction of muscle strength or dynapenia combined with low levels of calcidiol were found to be a risk factor that conditioned the development of daily life activities in a sample of 4,630 people over 50 free of disability at the outset of the study, followed for 2 years27. Similarly, Wicherts et al., analyzing a study of men and women between 65 and 88 years old, found that those with calcidiol levels below 20 ng/mL had a worse state and physical performance at both baseline and 3 years of follow-up compared to those with levels higher than 30 ng/mL11. Another Dutch prospective study showed that vitamin D levels were associated with functional limitations in the age stratum between 55 to 65 years and in those over 65 years²⁸. However, others authors have not found any association between calcidiol levels below 10 ng/mL and lower hip flexion, knee extension force, grip strength, gait speed or disability in activities related to mobility of the upper extremities. This last study was carried out in 628 women over 65 years of age followed for 3 years and who presented a moderate to severe disability at the beginning of the study. The existence of very few participants with low calcidiol values may have limited the possibility of obtaining differences²⁹.

The association between levels of calcidiol and BMD revealed in a recent meta-analysis is more contradictory6. Epidemiological evidence indicates that the highest levels of calcidiol are associated with higher BMD in both the young and aging population, maintaining a linear relationship to levels of 30 ng/mL, an association that does not seem so clear and solid in black populations. or Hispanics of North America³⁰. Our data indicate a direct association between calcidiol levels <20 ng/mL and BMD at femoral neck and total hip level. A 2014 meta-analysis concluded that there was very little evidence that vitamin D influenced BMD, since there was no consistent relationship between vitamin D supplementation and BMD in most of the anatomical sites analyzed (lumbar spine, total hip, trochanter, whole body or forearm), although a positive association was observed in the femoral neck, as was observed in our study³¹. Similarly, a recent article shows that patients with hip fractures have lower levels of calcidiol, lower bone mass, decreased bone quality and an increased risk of fracture³². It is important to highlight that in our study all subjects who were receiving treatment for osteoporosis, including supplements with vitamin D, were eliminated, which does not allow us to assess the possible effect of vitamin D supplementation on bone mass.

Our study has limitations, but also strengths. Regar ding the former, the fact of having a single biochemical determination (after 4 years of follow-up) without knowing the values at the beginning of the study limits the associations found. On the other hand, the questionnaire on difficulties to carry out activities of daily life was not self-administered but administered by an interviewer, which could have biased the responses of the participants, especially in those questions that referred to personal hygiene difficulties. As strengths, the analyzed cohort participated in the EVOS-EPOS study, being one of the few groups that completed all the study guidelines. The participation percentages of more than 80% in the four postal follow-ups carried out during 8 years guarantee the representativeness of the sample analyzed. In addition to the articles published with data from the full cohort of the EVOS-EPOS study, the cohort of the city of Oviedo, which has been used for this study, has contributed individually to the publication of several original articles in high-impact journals³³- ³⁹.

To sum up, calcidiol levels above 20 ng/mL are asso ciated with greater muscle grip strength in the hands, better performance in activities of daily life such as "bending over to pick up an object from the ground" and "Get up from the bed" and with a greater BMD in total hip and femoral neck, suggesting that maintaining calcidiol levels above 20 ng/mL would favor an adequate musculoskeletal function.