Introduction

Globally, snoring is a common sleeping problem across all age groups, including adults1,2,3,4,5. About 1 billion of the world’s 7.3 billion adults aged 30–69 years are estimated to have the most common type of sleep-disordered breathing, obstructive sleep apnea (OSA)5, of which snoring is a key clinical manifestation6,7. OSA can lead to adverse health consequences, including hypertension and arrhythmia6. The burden of snoring is much higher in resource-limited settings, such as Sub-Saharan Africa, where healthcare systems are ill-equipped to deal with its consequences3,8,9. For instance, African OSA patients showed a higher prevalence of comorbidities, such as diabetes mellitus10. Several risk factors have been reported to be associated with snoring in adults, including increasing age4, male sex4,11, cigarette smoking4,11, alcohol consumption11, and obesity3,4,11. Moreover, snoring is associated with increased odds of developing hypertension and type 2 diabetes mellitus (T2DM)3, dyslipidemia, and metabolic syndrome12.

Vitamin D is a fat-soluble vitamin that plays a key role in health maintenance13. Recently, greater attention has been paid to the role of vitamin D in the development of several health problems, including snoring/OSA14, sleep duration14,15,16,17, reduction in quality of life, and social relationships16.

To combat these health problems (vitamin D deficiency and snoring), researchers have called for raising awareness and appropriate management on a global scale18. In addition, other negative consequences of these conditions, such as poor academic performance19,20,21, must be considered. However, clinical studies have reported contradictory results regarding the association between vitamin D and OSA18. While some studies have shown a significant association22,23,24, others have shown no such association25,26,27,28. Such contradictory data necessitate further studies to understand the association between vitamin D and OSA.

The practical steps for tackling these health problems and their complications on a global scale require a thorough understanding of the local context. First, the status of vitamin D in the community and its association with snoring need to be assessed, and appropriate healthcare measures must be applied accordingly. There is a high prevalence of vitamin D deficiency in various population groups, including children, adolescents, adults, and pregnant women, in different regions of Sudan29,30,31. Moreover, snoring and its complications in adults have been reported in different regions of Sudan, including the studied region (Northern Sudan)3,19. However, the association between vitamin D and snoring has not yet been assessed in Sudan. Therefore, this relationship must be explored to address the health problems associated with snoring and its complications. Thus, the current study aimed to investigate the association between serum 25(OH)D concentrations and snoring among adults in Almatamah, River Nile State, Northern Sudan.

Materials and methods

Study area

River Nile State is one of the 18 states in Sudan. According to the 2008 Sudan census, the total population of this state was 1,120,44132. The River Nile State has seven localities (the lowest administrative units in Sudan).

Definition of case and control

This study followed the Strengthening the Reporting of Observational Studies in Epidemiology (STROBE) guidelines33. The participants were considered snorers if they self-reported snoring that had occurred in the past 30 days. Diagnosing snoring in adults through self-reporting is commonly used in different countries3,34. Any participant who did not report snoring was considered a control.

Study population and design

This community-based matched case-control study was conducted in four villages in the Wad Hamid district, Almatamah locality, River Nile State, Northern Sudan. The Wad Hamid district is adjacent to Khartoum State, about 100 km from Khartoum, the capital of Sudan. The study was matched for age and sex.

Among the seven localities, Almatamah was selected based on our previous study, which showed a higher prevalence of snoring in adults (one-third of the adult participants) that was positively associated with increased odds for hypertension and T2DM in this region of Sudan3. One of the three districts in the Almatamah locality was selected randomly (i.e., Wad Hamid). A systematic sampling method selected four villages from the randomly selected districts. Then, based on population density, 20 to 30 households from each village were chosen to obtain the desired sample (snoring adults and non-snoring controls). The first member of each household who agreed to participate and met the study inclusion criteria was selected. If the selected house was uninhabited or the inhabitants refused to participate, the next house was chosen to reach the target number for the study. The investigators trained four medical officers in data collection methods to standardize the data collection procedure and, thus, ensure data quality.

After signing an informed consent form, all adult (aged ≥ 18 years) Sudanese residents (including both men and women) who had no symptoms were enrolled from the chosen households using a lottery method. Participants aged less than 18 years, pregnant women, patients with poor cognitive functions, and severely ill patients were excluded from the study.

Data collection

For the data collection, the World Health Organization’s (WHO) three-level stepwise approach questionnaire was used35. The questionnaire aimed to gather data on the participants’ sociodemographic characteristics, including age in years, gender, employment status (employed/unemployed), marital status (married/unmarried), education level (< secondary and ≥ secondary), and cigarette smoking (never and former/current). Anthropometric and serum 25(OH)D measurements were conducted using the standard methods described below.

Procedures

The participants’ weights were measured in kilograms (kg) using the standard procedure (well-calibrated scales adjusted to zero before each measurement). The participants removed their shoes and excess clothing and stood on the scale with minimal movement, with their hands by their sides. Next, their heights were measured in centimeters (cm) while they stood straight, with their backs against the wall and feet together. Their body mass index (BMI) was computed as the weight in kg divided by the square of the height in meters (kg/m2). The BMI was categorized according to the WHO classification as underweight (< 18.5 kg/m2), normal weight (18.5–24.9 kg/m2), overweight (25.0–29.9 kg/m2), and obese (≥ 30.0 kg/m2)36.

Processing of blood samples

In this study, 3–5 milliliters (ml) of blood were collected from the cubital vein in a plain tube and allowed to clot at room temperature under aseptic conditions. The blood was then centrifuged and stored at − 20° C until the assay of 25(OH)D was performed using the enzyme-linked immunosorbent assay (fully automatable 450 nm, reference wavelength between 620 and 650 nm) and following the manufacturer’s instructions (Euroimmun, Lubeck, Germany). The manufacturer’s quality control measures and six standard solutions (calibrators) set between 0 and 120 ng/mL were applied for each assay. The sample was considered vitamin D deficient if the serum 25(OH)D level was ≤ 20 ng/ml37.

Sample size calculation

A total of 238 participants were included (140 non-snoring controls vs. 98 snoring adults) with a ratio of 1.5:1, based on a previous study in Sudan3. The sample size was calculated assuming that participants with vitamin D deficiency are twice as risk to snore, based on a previous study24. This sample size was calculated to detect a difference of 5% at α = 0.05, with a power of 80%.

Statistical analysis

The data were entered into the computer using the IBM Statistical Package for the Social Sciences® (SPSS®) for Windows, version 22.0 (SPSS Inc., New York, United States). The proportions were expressed as frequencies (%). The continuous data were evaluated for normality using the Shapiro–Wilk test and were non-normally distributed. The non-normally distributed data were expressed as the median [interquartile range (IQR)]. Adjusted regression analysis (binary) was performed with snoring as the dependent variable, sociodemographic factors (age, gender, educational level, occupation, marital status, cigarette smoking, and BMI), and 25(OH)D as the independent variable. All variables were entered to construct multivariable models that considered crude associations between snoring and the variables (the model will predict better with more covariates). Backward elimination (conditional) was performed to adjust the model for covariates. Serum 25(OH)D and vitamin D deficiency levels were entered into the model one by one. Adjusted odds ratios (AORs), 95% confidence intervals (CIs), coefficients, and standard errors were calculated as they were applied. A two-sided P-value of < 0.05 was considered statistically significant.

Results

This study enrolled 238 adults (98 snoring adults and 140 controls). Of the total, the median (IQR) of the age and BMI were 50.0 (40.0‒58.3) years and 27.0 (23.3‒31.2) kg/m2, respectively. Of the 238 adults, 49.2%, 17.6%, 67.2%, 47.8%, and 20.2% were male, married, educated (≥ secondary level), employed, and cigarette smoking, respectively; 29.8%, 7.6%, 31.1%, and 31.5% were normal weight, underweight, overweight, and obese, respectively.

Compared with the controls, snoring adults had lower 25(OH)D levels (14.0 [10.4‒20.9] vs. 15.6 [11.8‒22.9], P = 0.016) ng/ml. Thus, more snoring adults had vitamin D deficiency compared to the controls (73 [74.5%] vs. 85 [60.7%], P = 0.026), as shown in Table 1.

Table 1 Characteristics of the studied adults and univariate analysis of snoring in Northern Sudan (n = 238).
Full size table

The results of the multivariable logistic regression revealed that serum 25(OH)D levels were inversely associated with snoring in the adults (AOR = 0.96, 95% CI 0.93–0.99), and vitamin D deficiency increased the odds of snoring among them (AOR = 1.89, 95% CI 1.07–3.33). Age, sex, education, occupational status, marital status, cigarette smoking, and BMI did not differ significantly between the two groups, as shown in Table 2.

Table 2 Multivariable logistic regression analysis of factors associated with snoring among adults in Northern Sudan (n = 238).
Full size table

Discussion

The main finding of this study was that serum 25(OH)D levels were inversely associated with snoring in adults. In addition, adults with vitamin D deficiency had almost twice the risk of becoming snorers. As mentioned above, several previous studies have revealed a significant association between vitamin D deficiency and snoring22,23,24. For example, Zhang et al.’s prospective cohort study included 444,975 participants from the UK Biobank without prior OSA. It revealed an inverse association between serum 25(OH)D levels and the risk of new-onset OSA in overweight and obese participants22. Shendi et al.’s study in the United Arab Emirates, which included 148 OSA patients, showed an increasing severity of OSA with vitamin D deficiency23. Kerley et al., in their case-control study, showed that 25(OH)D was significantly lower in OSAS cases than in matched, non-OSAS controls24. This study supports previous studies, including systematic reviews and meta-analysis studies that have recommended using vitamin D as a biomarker of health in snoring among adults and children18,38,39. For example, in their recent updated meta-analysis, Li et al. revealed that the serum 25(OH)D levels in all OSA subgroups decreased compared to the control group39.

On the other hand, other studies have shown no association between snoring and vitamin D status26,27,28. A study conducted by Yassa et al. among 121 OSAS patients showed a significant relationship between OSAS severity and vitamin D status (vitamin D status does not alter the severity of OSAS). There is no significant relationship between vitamin D and the apnea-hypopnea index (AHI). vitamin-D deficiency might be the result of lifestyle changes due to OSAS rather than the cause26. In Turkey, Akyıldız et al. analyzed 262 cases of obstructive sleep apnea syndrome (OSAS) and reported that neither the presence nor the severity of OSAS had a significant association with serum 25(OH)D levels27. Salepc et al. revealed that vitamin D levels did not differ by OSA diagnosis status or severity28.

The exact mechanism by which vitamin D could influence snoring/OSA or vice versa is poorly understood. However, researchers have attributed the relationship between OSA and low levels of vitamin D to inflammatory and non-inflammatory pathways, genetic polymorphisms of the vitamin D receptor (VDR) and the vitamin D binding protein (VDBP), and the autonomic nervous system18. Moreover, it has been reported that vitamin D deficiency has a role in promoting several inflammatory processes, oxidative stress, hypoxia, as well as impairing immune and muscle functions, all of which could contribute to the pathogenesis of OSA40.

This study was conducted mainly to assess the association between vitamin D and snoring in adults. However, the other variables that were evaluated (sex, smoking, and BMI) were no different between the two groups. Previous studies have reported other factors associated with snoring, such as increasing age4,14, male sex4,11, cigarette smoking4,11, and obesity3,4,11. Moreover, previous studies have shown a positive association between increasing BMI/obesity, vitamin D, and snoring/OSA22,24,27. The lack of association of snoring and BMI in this study could be attributed to the study design (i.e., matched for age and sex). Matching for age and sex could explain this, as both factors are closely linked to increasing BMI/obesity; that is, matching for age and sex indirectly matches the BMI. For instance, our studies from Sudan showed that increasing age and sex were associated with obesity3,41. In addition, our recent study revealed that increasing age and increasing BMI/obesity were positively associated with snoring in Sudanese adults3.

The results of this study have implications for improving adults’ health since the studied factor (snoring) and the identified factor (vitamin D) are manageable. It is worth emphasizing that the early identification of snoring and vitamin D deficiency might lead to early treatment of these health problems and their associated comorbid diseases, such as cardiovascular conditions6. In line with this evidence, the medical officers advised the identified snoring adults to seek further consultation with the nearest healthcare facilities. Moreover, the results of this study will be communicated to policymakers to promote further studies investigating the impact of snoring on adults’ health.

Strengths and limitations of the study

The present study has several strengths. To the best of the authors’ knowledge, this is the first study of its kind to address the association between vitamin D and snoring in Sudan, where there is a high prevalence of vitamin D deficiency. Thus, it adds to the sparse data on snoring in that country3,19. It is an age- and sex-matched case-control design, and this minimizes confounding. Moreover, the employment of a validated WHO questionnaire, standardized anthropometry, and ELISA for the measurement of serum 25(OH)D. Likewise, conditional logistic regression controls for matching variables, and the distinction between continuous vitamin D and deficiency status in models is clear. Another point of strength reports a substantial inverse correlation between serum 25(OH)D and snoring (AOR = 0.96, 95% CI: 0.93–0.99) and elevated snoring odds with deficiency (AOR = 1.89, 95% CI: 1.07–3.33).

However, the present study has some limitations that must be acknowledged and overcome in future studies. This study was a case-control study, which cannot determine temporal/causal relationships (does deficiency lead to snoring, or does snoring/OSA lower vitamin D?). Conducting a longitudinal study will clarify the association and direction between snoring and vitamin D status among adults. This study was conducted among adults in Northern Sudan only, thus limiting the generalization of the results to adults in the whole of Sudan. Moreover, no data were collected in the present study on characteristics such as ethnicity and physical activity, which could influence snoring42,43 and vitamin D status44. In this study, snoring was self-reported. Reliance on self-reported snoring (past 30 days) without objective measures (e.g., polysomnography) or snoring could be recorded in some way while the patient sleeps, given that many people who snore are unaware they do so until someone points it out. This was a small sample size (n = 238) and borderline P-values (e.g., P = 0.045 for 25[OH]D), indicating that the results may require replication and a lack of power.

Conclusion

Our study revealed a significant positive association between vitamin D deficiency and snoring in adults in Northern Sudan. Practical guidelines for screening/treatment of vitamin D in snorers are suitably conservative in light of design limitations.