This study conducted in a sample of 150 Brazilian community-dwelling older people individuals showed that the prevalence of hypovitaminosis D was high (60.7%); in contrast, there was no association of hypovitaminosis D with muscle strength and functional capacity. This finding may be attributed to the fact that the sample included older people individuals with lower active age groups and few comorbidities, and the majority was practicing physical activity.
The high prevalence of hypovitaminosis D in this sample corroborates previously reported findings. Recently, a systematic review and meta-analysis of 72 studies on the prevalence of hypovitaminosis D in Brazil reported a rate of insufficiency of 45.85% and a rate of deficiency of 41.53%. The study mentioned included all age groups; however, it highlighted a greater presence of hypovitaminosis D among the older people group .
In recent decades, research has confirmed the important role of vitamin D in several outcomes, especially after the discovery of its activity on various organs and tissues . Its relationship with the muscular system was more noticeable after the discovery of the VDRs in muscle tissue in the 1980s . However, to date, the evidence on the influence of vitamin D deficiency on the muscle strength and functional capacity of older people individuals is still inconclusive. Aspell et al. (2019) evaluated 4,157 individuals with a mean age of 69.8 ± 6.9 years and reported that vitamin D deficiency was associated with decreased muscle strength among English community-dwelling older people individuals . The value adopted to verify the vitamin deficiency used in this study was 30 nmol/L, which corresponds to 12 ng/mL. In our study, the recommendations of the SBPC/ML established levels below 30 ng/mL to indicate insufficiency and below 20 ng/mL to indicate deficiency. The absence of an international standardization for the classification of vitamin D deficiency hampers a comparative analysis between the two studies .
In 2018, Mendes et al. evaluated 1,425 older people individuals (mean age: 75.4 ± 7.5 years in women and 74.2 ± 6.7 years in men) and found a significant association between vitamin D deficiency and decreased gait speed and HGS, especially in men (41.5% of the total sample) . Of the participants of our study, only 28% were men, with a much smaller age group (67.18 ± 5.33 years), which could explain, at least in part, the conflicting results. It is already established that men have greater muscle strength than women in all age groups and that the higher the age group, the lower the muscle strength and functional capacity . Verlaan et al. (2018) conducted a multicenter study of 380 community-dwelling older people individuals aged over 65 years from 6 European countries (Belgium, Germany, Ireland, Italy, Sweden, and the United Kingdom) . They found that older people individuals with vitamin D deficiency also presented decreased muscle mass (on DXA), strength (HGS), and functional performance in the SPPB (4–9 points). In our study, despite low schooling and living in a developing country, such as Brazil, 82.7% of the older people participants displayed good functional performance; further, 50.6% practiced regular physical activity, with few comorbidities, and were often exposed to sunlight. This differentiated profile of the Brazilian older people individuals may have contributed to the results found.
Notably, some studies have demonstrated similar results: not finding an association between hypovitaminosis D and decreased muscle strength and functional capacity. Mathei et al. (2013) assessed the relationship between the levels of 25(OH)D, HGS, and gait speed and balance test findings in 367 octogenarians. In this sample, only 12.8% presented vitamin D levels of 30 ng/mL or more . Interestingly, there was no association found between hypovitaminosis D and the variables studied, although it was expected that the older age group would present worse results, which did not occur. Verreault et al. (2002) followed a cohort of 628 women (age: ≤65 years) for 3 years, performing evaluations every 6 months. The sample was divided into three groups according to the levels of vitamin D (≤ 10 ng/mL, 10–20 ng/mL, and ≥ 21 ng/mL) at baseline. Their analysis showed that the decline in strength (HGS) and muscle function (SPPB score) was similar in the three groups, regardless of the initial level of vitamin D . Annweiler et al. (2009) evaluated the HGS, IQS, renal function, and parathyroid hormone (PTH) and calcium levels in 440 older people women with a mean age of 80.1 ± 3.5 years . No association was found between vitamin D deficiency and decreased muscle strength. Advanced age and a higher number of chronic degenerative diseases were associated with greater decreases in muscle strength. The association of muscle weakness with increasing age is already well established in the literature and was one of the findings.
A possible physiological explanation for the lack of association of vitamin D deficiency with muscle strength and functional capacity in the older people population found in this study could be the decrease in the number of VDRs in the muscle tissue. Currently, the exact mechanism of the relationship between receptors and vitamin D deficiency is not yet established, that is, whether this reduction is caused by vitamin D deficiency itself (negative regulation of the receptor) or if by another factor inherent to aging .
The complex interaction between muscle strength and vitamin D, especially in older people individuals, is still contradictory. The existence of possible confounding factors, such as the level of PTH, presence of kidney diseases, comorbidities, use of medications or supplements, and practice of physical activity, is not always controlled in studies, which may have contributed to the reporting of divergent results. Further, the large methodological variability, especially between the forms of measurement and classification of 25(OH)D, and diversity of assessments for muscle strength and functional capacity limit the comparison of data and may explain, at least in part, the large disparity of the findings. Conversely, the heterogeneity of the aging process, senility, clinical and socio-cultural factors, and life habits could also interfere in muscle and functional performance and should also then be considered.
In this study, sex (female), increased age, presence of pain, and use of supplements were more related to decreased HGS than the serum level of 25(OH)D. Both sex and age are factors that explain the decrease in muscle strength, that is, men have higher HGS than women, and muscle strength decreases with age .
Our study had an exploratory design; however, it is important to highlight that we analyzed not only the serum levels of vitamin D and the functionalities but also the use of vitamin supplements. Herein, we found that the older people participants who were using some type of supplementation had lower HGS, which may suggest that they were experiencing functional decline and thus had the need for supplementation.
There was no association between hypovitaminosis D and the IQS; increased age, sex, presence of pain, and physical activity were most related to this variable. The participants of this study were healthy community-dwelling older people individuals, of whom 50.6% practiced regular physical activity, and 82.7% had a good functional status. The performance of physical activity is considered a premise to reduce the physiological losses inherent to aging; it is widely studied and has been proven to be efficient in maintaining functional capacity and muscle strength . The presence of pain could be a source of bias for the performance of muscle contraction required in the IQS assessment. In our sample, the older people participants who complained of pain in some joint presented lower quadriceps strength. In a systematic review with meta-analysis, Latey et al. (2017) reported that the intensity, frequency, and duration of pain are associated with muscle weakness .
Our study used the SPPB for the evaluation of functional capacity. Most of the older people participants presented good performance in the SPPB, which resulted in a low variability in this variable. Previous studies have reported that life habits, such as the practice of physical activity and good diet, can influence the functional performance of older people individuals; however, the influence of the vitamin D levels on these outcomes needs further investigation in future studies [24, 26].
This study has several limitations. First, as a cross-sectional study, which has an inherent limitation of not allowing the establishment of a cause-effect relationship. Therefore, the findings must be analyzed with caution. Second, the medications and supplements taken by the participants were not qualitatively assessed. Last, we did not evaluate the diet or time of sun exposure, which are factors that may influence the level of vitamin D.