European Journal of Cardiovascular Medicine

Vitamin D, a fat-soluble secosteroid, plays a crucial role in maintaining calcium homeostasis and bone metabolism, while also exerting influence on muscle function and immune modulation [1]. It is primarily synthesized in the skin through sunlight exposure and to a lesser extent obtained from dietary sources. Despite its significance, vitamin D deficiency is a widespread global health concern, affecting individuals across various age groups and geographic regions [2]. Sedentary lifestyle, limited sun exposure, and inadequate dietary intake have been recognized as major contributors to suboptimal vitamin D levels, even in sun-rich countries [3].

Emerging evidence has linked vitamin D deficiency not only with skeletal disorders such as osteomalacia and osteoporosis but also with non-skeletal symptoms including chronic fatigue, myopathy, and general muscle weakness [4,5]. Fatigue, a non-specific but frequently reported symptom, significantly impairs daily functioning and quality of life. Similarly, muscle weakness, particularly of the proximal muscles, has been associated with lower levels of serum 25-hydroxyvitamin D [25(OH)D] [6].

Although several clinical studies have examined the association between vitamin D levels and musculoskeletal symptoms, there remains a gap in the literature concerning its impact within the general population. The relationship between hypovitaminosis D and non-specific symptoms such as fatigue and muscle weakness is often overlooked in clinical practice, leading to underdiagnosis and undertreatment [7]. This study aims to evaluate the prevalence of vitamin D deficiency and to investigate its correlation with fatigue and muscle strength in a general adult population sample.

A total of 300 individuals aged between 18 and 60 years were included using a convenience sampling method. Participants were recruited from outpatient departments, health camps, and wellness centers. Individuals with known chronic illnesses such as renal disease, endocrine disorders, neuromuscular diseases, or those taking vitamin D supplementation in the past three months were excluded.

Data Collection and Clinical Assessment:

Demographic details including age, gender, occupation, and lifestyle habits (sun exposure, physical activity) were recorded. All participants underwent a clinical evaluation including assessment of fatigue and muscle strength.

Fatigue was measured using the Fatigue Severity Scale (FSS), a validated tool consisting of nine items rated on a 7-point Likert scale. A higher score indicated greater severity of fatigue. Muscle strength was evaluated by handgrip dynamometry using a standardized digital handgrip strength meter. Three readings were taken for each hand, and the highest value was considered for analysis.

Biochemical Analysis:

Venous blood samples were collected under aseptic conditions. Serum 25-hydroxyvitamin D [25(OH)D] levels were measured using chemiluminescence immunoassay (CLIA). Based on Endocrine Society guidelines, participants were categorized as follows:

• Deficient: <20 ng/mL
• Insufficient: 20–30 ng/mL
• Sufficient: >30 ng/mL

Statistical Analysis:

All data were entered into Microsoft Excel and analyzed using SPSS version 25.0. Quantitative variables were expressed as mean ± standard deviation, while categorical variables were expressed as percentages. Comparisons between vitamin D groups were made using ANOVA for continuous variables and Chi-square test for categorical variables. Pearson correlation was used to assess the relationship between vitamin D levels, fatigue scores, and muscle strength. A p-value less than 0.05 was considered statistically significant.

A total of 300 participants were included in the study, comprising 172 females (57.3%) and 128 males (42.7%), with a mean age of 38.6 ± 10.4 years. Based on serum 25(OH)D levels, 162 individuals (54%) were classified as vitamin D deficient (<20 ng/mL), 84 (28%) as insufficient (20–30 ng/mL), and 54 (18%) as sufficient (>30 ng/mL).

The mean Fatigue Severity Scale (FSS) scores and handgrip strength values showed significant differences across the three groups. Participants in the vitamin D deficient group had a notably higher FSS score and lower handgrip strength compared to those in the sufficient group (Table 1).

Table 1: Comparison of Fatigue and Muscle Strength across Vitamin D Status Groups

In terms of gender distribution within the vitamin D categories, females were more likely to be deficient compared to males (p=0.014), as shown in Table 2.

Table 2: Gender-wise Distribution of Vitamin D Status

Pearson correlation analysis revealed a significant negative correlation between vitamin D levels and FSS scores (r = -0.61, p < 0.001), and a positive correlation with handgrip strength (r = 0.58, p < 0.001), confirming a strong association between serum vitamin D status and both fatigue and muscle performance (Table 3).

Table 3: Correlation between Vitamin D Levels, Fatigue, and Muscle Strength

As seen in Tables 1–3, individuals with lower vitamin D levels consistently reported higher fatigue and exhibited reduced muscle strength, reinforcing the clinical importance of evaluating vitamin D status in patients with non-specific musculoskeletal complaints.

This study demonstrates a high prevalence of vitamin D deficiency in the general population and establishes a significant association between low serum 25(OH)D levels with increased fatigue and reduced muscle strength. The results align with previous research that emphasizes the extra-skeletal roles of vitamin D, especially in musculoskeletal and neuromuscular health [1,2].

Vitamin D deficiency has been increasingly identified as a global epidemic, affecting over one billion people worldwide, regardless of age, gender, or geographical region [3]. In this study, more than half of the participants exhibited deficient levels of vitamin D, consistent with findings reported by Mithal et al., who documented high deficiency rates even in sun-rich regions due to lifestyle and dietary factors [4]. Notably, female participants had a higher prevalence of deficiency, which could be attributed to less outdoor activity, cultural clothing practices, and nutritional gaps [5,6].

Fatigue is a non-specific but frequently debilitating symptom that significantly impacts daily functioning and quality of life. Our findings indicated that individuals with deficient vitamin D levels had markedly higher Fatigue Severity Scale (FSS) scores. Plotnikoff and Quigley also reported similar observations in a study where patients with unexplained musculoskeletal pain and fatigue showed significant improvement in symptoms upon correction of hypovitaminosis D [7]. Vitamin D receptors are expressed in muscle and nerve tissues, supporting its role in cellular energy metabolism and neuromuscular transmission, which may help explain the fatigue associated with its deficiency [8,9].

Muscle weakness, particularly in the proximal muscle groups, is another established manifestation of vitamin D deficiency. In our cohort, participants with lower 25(OH)D levels had significantly reduced handgrip strength. This finding corroborates with studies by Bischoff-Ferrari et al. and Ceglia et al., who demonstrated improvements in muscle strength and balance following vitamin D supplementation in older adults [10,11]. Mechanistically, vitamin D plays a role in muscle protein synthesis and calcium handling, which are crucial for muscle contraction and function [12]. A deficiency impairs type II muscle fiber development, resulting in decreased muscle performance and increased fall risk, especially in the elderly [13].

The strong inverse correlation between vitamin D levels and fatigue severity, along with the positive association with grip strength, underscores the clinical relevance of screening for vitamin D in patients presenting with such non-specific symptoms. Despite being widely overlooked, subtle neuromuscular symptoms like chronic fatigue and muscle weakness may be early indicators of vitamin D deficiency, warranting routine assessment [14].

This study is limited by its cross-sectional design, which restricts causal inference. Seasonal variations in vitamin D synthesis and dietary intake were not controlled, and other confounders such as serum calcium, parathyroid hormone levels, or thyroid status were not evaluated. Nevertheless, the findings provide valuable insight into the potential benefits of early detection and correction of vitamin D deficiency in the general adult population [15].

This study highlights a significant association between vitamin D deficiency and increased fatigue and muscle weakness in the general adult population. Routine screening for vitamin D levels may aid in early identification and management of non-specific musculoskeletal symptoms, ultimately improving patient well-being and quality of life.

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