Upper respiratory tract infections (URTIs) are among the most common acute illnesses worldwide, contributing significantly to morbidity and loss of productivity, especially among young adults in densely populated environments such as universities and colleges [1,2]. While most URTIs are self-limiting, frequent episodes can negatively affect academic performance and quality of life [3]. Several factors influence susceptibility to URTIs, including nutritional status, lifestyle habits, and immune function [4].

Vitamin D, traditionally recognized for its role in calcium-phosphate metabolism and bone health, has emerged as a crucial modulator of both innate and adaptive immunity [5,6]. It exerts immunomodulatory effects by enhancing the pathogen-fighting capacity of monocytes and macrophages, and by modulating inflammatory cytokine production [7]. Vitamin D receptors are expressed in various immune cells, including B cells, T cells, and antigen-presenting cells, highlighting its role in immune defense mechanisms [8].

Undergraduate students, often subject to irregular routines, poor dietary practices, and prolonged indoor activities, may be particularly vulnerable to both vitamin D deficiency and recurrent URTIs [5]. Despite this, there is limited research exploring the correlation between vitamin D levels and URTI incidence in this demographic.

Therefore, the present study aimed to evaluate the relationship between serum vitamin D levels and the frequency of URTIs among undergraduate students, with the hypothesis that lower vitamin D concentrations would be associated with a higher incidence of infections.

A sample size of 180 participants was determined using a prevalence estimate of vitamin D deficiency of 40% from previous literature, with a 95% confidence interval and 5% margin of error. Students were selected using simple random sampling from the university’s enrollment list.

Inclusion and Exclusion Criteria

Students aged 18–25 years who had been residing in the university campus or nearby for at least six months were eligible. Participants with chronic illnesses, autoimmune disorders, recent vitamin D supplementation (within the last three months), or on long-term medications affecting vitamin D metabolism were excluded.

Data Collection

Participants completed a structured questionnaire that captured demographic data, lifestyle habits (dietary patterns, physical activity, and sun exposure), and the number of upper respiratory tract infection (URTI) episodes experienced in the past six months. URTI was defined as an acute illness involving symptoms such as sore throat, nasal congestion, rhinorrhea, and/or cough lasting less than two weeks, with or without fever, and not requiring hospitalization.

Biochemical Assessment

Venous blood samples (5 mL) were collected from each participant under aseptic conditions. Serum 25-hydroxyvitamin D [25(OH)D] concentrations were measured using a chemiluminescent immunoassay (CLIA) on an automated analyzer (e.g., Abbott Architect i2000SR). Based on Endocrine Society criteria, vitamin D status was classified as deficient (<20 ng/mL), insufficient (20–29 ng/mL), or sufficient (≥30 ng/mL).

Statistical Analysis

Data were analyzed using SPSS software version 26.0 (IBM Corp., Armonk, NY, USA). Continuous variables were expressed as mean ± standard deviation (SD), while categorical variables were presented as frequencies and percentages. Group differences were assessed using one-way analysis of variance (ANOVA) for continuous variables and chi-square tests for categorical variables. Pearson’s correlation coefficient was applied to assess the relationship between serum vitamin D levels and the frequency of URTIs. Logistic regression analysis was performed to estimate odds ratios (OR) with 95% confidence intervals (CI) for the risk of recurrent URTIs (≥3 episodes) according to vitamin D status. A p-value of less than 0.05 was considered statistically significant.

Baseline Characteristics

A total of 180 undergraduate students participated in the study, with a mean age of 20.9 ± 1.8 years. The sample comprised 98 females (54.4%) and 82 males (45.6%). The majority of participants reported limited sun exposure (<30 minutes/day) (62.8%) and irregular dietary intake of vitamin D–rich foods (71.7%). The mean body mass index (BMI) was 22.1 ± 2.8 kg/m². Serum vitamin D levels ranged from 9.4 to 42.6 ng/mL, with a mean of 21.8 ± 7.5 ng/mL.

The distribution of vitamin D status was as follows: 76 students (42.2%) were deficient, 66 (36.7%) insufficient, and 38 (21.1%) sufficient (Table 1).

Table 1. Distribution of vitamin D status among participants (n = 180)

Association Between Vitamin D Status and URTI Frequency

The mean number of URTI episodes in the preceding six months was significantly higher in the vitamin D–deficient group (3.1 ± 1.2) compared to the insufficient (2.0 ± 0.9) and sufficient groups (1.4 ± 0.6) (p < 0.001) (Table 2).

Table 2. Comparison of mean URTI episodes across vitamin D status groups

*One-way ANOVA with post-hoc Tukey’s test.

Correlation and Risk Analysis

Pearson’s correlation analysis revealed a significant inverse relationship between serum vitamin D levels and URTI frequency (r = –0.46, p < 0.001) (Table 3). Logistic regression demonstrated that participants with vitamin D deficiency had 2.8 times higher odds of experiencing ≥3 URTI episodes compared to those with sufficient levels (95% CI: 1.5–5.1, p = 0.002).

Table 3. Correlation between serum vitamin D level and URTI frequency

The present study demonstrated a significant inverse relationship between serum vitamin D levels and the incidence of upper respiratory tract infections (URTIs) among undergraduate students. Participants with deficient vitamin D status experienced a higher number of URTI episodes compared to those with sufficient levels, aligning with the proposed immunomodulatory role of vitamin D in respiratory health (6,7).

Our findings are consistent with earlier epidemiological data showing increased susceptibility to respiratory infections in individuals with low serum 25-hydroxyvitamin D concentrations (8,9). Vitamin D enhances innate immunity by stimulating the production of antimicrobial peptides such as cathelicidin and defensins, which have direct antiviral and antibacterial effects on respiratory pathogens (1). Moreover, it modulates adaptive immunity by shifting the balance from a pro-inflammatory Th1 response toward a more regulated Th2 and T-regulatory profile, potentially reducing tissue damage during infections (2).

Several randomized controlled trials have demonstrated that vitamin D supplementation can reduce the risk of acute respiratory infections, particularly in those with baseline deficiency (3,4). In our cohort, vitamin D–deficient students had almost threefold higher odds of recurrent URTIs, suggesting that suboptimal levels may represent a modifiable risk factor in this age group. This association may be particularly relevant in university settings, where close interpersonal contact and crowded environments facilitate viral transmission (5).

The high prevalence of vitamin D deficiency (42.2%) observed in this study is comparable to previous reports in young adult populations, especially in regions with limited sun exposure or lifestyles characterized by prolonged indoor activities (6,7). Inadequate dietary intake of vitamin D–rich foods and the use of sunscreen further exacerbate this deficiency (8). Given these lifestyle factors, targeted interventions such as dietary counseling, safe sun exposure practices, and supplementation may be beneficial.

Interestingly, while previous studies have often focused on children, the elderly, or high-risk clinical groups (9,10), fewer have examined healthy young adults. Our results add to the growing body of evidence indicating that vitamin D deficiency is not limited to vulnerable populations and may have clinical relevance even in otherwise healthy students.

The cross-sectional design of the study limits causal inference. Reverse causality cannot be excluded, as frequent infections could potentially influence outdoor activity and sun exposure, leading to lower vitamin D levels. However, the biological plausibility supported by mechanistic and interventional studies strengthens the likelihood of a causal link (11-15). Furthermore, self-reported URTI frequency may be subject to recall bias, though the structured questionnaire and symptom-based definition helped minimize misclassification.

Future research should include longitudinal designs to establish temporal relationships and randomized controlled trials to assess the efficacy of supplementation in reducing URTI burden in young adults. Such studies could also evaluate optimal dosing strategies and seasonal variations in vitamin D status.

In conclusion, this study highlights a significant association between low serum vitamin D levels and increased URTI incidence among undergraduate students. Addressing vitamin D deficiency in this group may represent a practical approach to reducing infection rates and improving overall well-being.

1. Saeed BQ, Jairoun AA, Ashraf Khamis A, Hatim Abdelrahim L, Abobakr Aljomhi A, Adrees AO, et al. Vitamin D deficiency and insufficiency among university students: prevalence, risk factors, and the association between vitamin D deficiency and episodes of respiratory tract infections. Risk Manag Healthc Policy. 2021 Jun 28;14:2733-41. doi:10.2147/RMHP.S308754. PMID:34234589.
2. Liu X, Baylin A, Levy PD. Vitamin D deficiency and insufficiency among US adults: prevalence, predictors and clinical implications. Br J Nutr. 2018 Apr;119(8):928-36. doi:10.1017/S0007114518000491. PMID:29644951.
3. Chen CM, Mu SC, Chen YL, Tsai LY, Kuo YT, Cheong IM, et al. Infants' vitamin D nutritional status in the first year of life in Northern Taiwan. Nutrients. 2020 Feb 4;12(2):404. doi:10.3390/nu12020404. PMID:32033065.
4. Jayatissa R, Lekamwasam S, Ranbanda JM, Ranasingha S, Perera AG, De Silva KH. Vitamin D deficiency among children aged 10–18 years in Sri Lanka. Ceylon Med J. 2019 Dec 31;64(4):146-54. doi:10.4038/cmj.v64i4.8991. PMID:32121673.
5. El-Adawy EH, Zahran FE, Shaker GA, Seleem A. Vitamin D status in Egyptian adolescent females with iron deficiency anemia and its correlation with serum iron indices. Endocr Metab Immune Disord Drug Targets. 2019;19(4):519-25. doi:10.2174/1871530318666181029160242. PMID:30370867.
6. Contreras-Manzano A, Villalpando S, Robledo-Pérez R. Vitamin D status by sociodemographic factors and body mass index in Mexican women at reproductive age. Salud Publica Mex. 2017 Sep-Oct;59(5):518-25. doi:10.21149/8080. PMID:29267648.
7. Al Shaikh AM, Abaalkhail B, Soliman A, Kaddam I, Aseri K, Al Saleh Y, et al. Prevalence of vitamin D deficiency and calcium homeostasis in Saudi children. J Clin Res Pediatr Endocrinol. 2016 Dec 1;8(4):461-7. doi:10.4274/jcrpe.3301. PMID:27476528.
8. Esbenshade AJ, Sopfe J, Zhao Z, Li Z, Campbell K, Simmons JH, et al. Screening for vitamin D insufficiency in pediatric cancer survivors. Pediatr Blood Cancer. 2014 Apr;61(4):723-8. doi:10.1002/pbc.24844. PMID:24194420.
9. Parian-de Los Angeles E, Retoriano K, Arnaldo H, Ronquillo-Nolasco ME, Urtula R. Vitamin D status of breastfed Filipino infants aged less than 6 months in an urban community. Pediatr Gastroenterol Hepatol Nutr. 2021 Jul;24(4):403-12. doi:10.5223/pghn.2021.24.4.403. PMID:34316475.
10. Al-Zubeidi H, Leon-Chi L, Newfield RS. Low vitamin D level in pediatric patients with new onset type 1 diabetes is common, especially if in ketoacidosis. Pediatr Diabetes. 2016 Dec;17(8):592-8. doi:10.1111/pedi.12342. PMID:26694737.
11. Visuthranukul J, Phansuea P, Buranakityanon P, Lerdrungroj P, Yamasmith E. Prevalence and risk factors of vitamin D deficiency among living with HIV adults receiving antiretroviral treatment in tropical area: cross-sectional study. Heliyon. 2023 Sep;9(9):e19537. doi:10.1016/j.heliyon.2023.e19537. PMID:37809448.
12. Garg S, Dasgupta A, Paul B, Maharana SP. Vitamin D insufficiency risk score for screening for vitamin D insufficiency. Indian J Endocrinol Metab. 2019 Sep-Oct;23(5):552-6. doi:10.4103/ijem.IJEM_539_19. PMID:31803596.
13. Cui A, Xiao P, Ma Y, Fan Z, Zhou F, Zheng J, et al. Prevalence, trend, and predictor analyses of vitamin D deficiency in the US population, 2001–2018. Front Nutr. 2022 Oct 3;9:965376. doi:10.3389/fnut.2022.965376. PMID:36263304.
14. Marwaha RK, Tandon N, Garg MK, Kanwar R, Narang A, Sastry A, et al. Vitamin D status in healthy Indians aged 50 years and above. J Assoc Physicians India. 2011 Nov;59:706-9. PMID:22616336.
15. Jeon YD, Cho SD, Youm YS, Song JY, Lee KJ, Park KB. The prevalence of vitamin D deficiency in patients undergoing total knee arthroplasty: a propensity score matching analysis. Arch Osteoporos. 2022 Mar 23;17(1):53. doi:10.1007/s11657-022-01097-7. PMID:35320426.