Evaluation of Vitamin D Levels in Karystos Residents: An Investigation of the Mediterranean Paradox

ACHAIKI IATRIKI | 2025; 44(1):16–20

Original Research Article

Iliana Leontari, George Kalapodas


Laboratory of Biopathology, General Hospital of Karystos, Karystos, Greece

Received: 01 Aug 2024; Accepted: 25 Oct 2024

Corresponding author: Iliana Leontari, General Hospital of Karystos, Karystos, Greece, Tel.: +30 2224 350123, E-mail: leontari111@gmail.com

Key words: Vitamin D, mediterranean paradox, Karystos, vitamin levels

 


Abstract

Background: According to recent studies, a considerable portion of the population in Greece demonstrates vitamin D insufficiency [25(OH)D < 30 ng/ml] and severe deficiency [25(OH)D < 12 ng/ml] [1]. The current study was designed based on the “Mediterranean Paradox,” which highlights that residents of sunny regions, such as the Mediterranean, often exhibit vitamin D deficiency despite adequate solar exposure. The purpose of the study was to evaluate the serum vitamin D levels of the residents of Karystos and investigate vitamin D levels in a population where sun exposure is considered adequate throughout the year.

Materials and Methods: The research was conducted from September 2022 to September 2023 and included the analysis of serum vitamin D levels in Karystos residents undergoing routine examinations at the Biopathology Laboratory of the General Hospital of Karystos.

Results: The study results revealed that despite adequate sunlight throughout the year, a considerable percentage of Karystos residents exhibited vitamin D deficiency.

Conclusion: These findings support the “Mediterranean Paradox” phenomenon and indicate the need for further investigation and possible intervention to address vitamin D deficiency in populations with sufficient sunlight exposure.

INTRODUCTION

Vitamin D is one of the most important vitamins for human health, playing a multifaceted role in the body’s physiology. Although primarily known for its contribution to calcium and phosphorus metabolism regulation, recent scientific studies highlight its broader significance in immune system function, cardiovascular health, and the prevention of various chronic diseases. Additionally, vitamin D deficiency has been linked to numerous pathological conditions, making it a considerable target for public health prevention and intervention [2].

Vitamin D is unique because it is produced endogenously in the body through the effect of sunlight on the skin. Some researchers have proposed that sun exposure of 15-20 minutes a day is sufficient to produce the necessary amount of vitamin D for the body [3]. Vitamin D consists of several compounds, with the main representatives being ergocalciferol (D2), derived from plants and commonly added to foods, and cholecalciferol (D3), synthesized from 7-dehydrocholesterol in the skin.

During sun exposure, UVB radiation is absorbed by 7-dehydrocholesterol and converted into previtamin D3 (precalciferol). Over two-three days, previtamin D3 undergoes thermal isomerization leading to vitamin D3 (cholecalciferol). Once in circulation, any form of vitamin D is hydroxylated to 25(OH)D3 in the liver and then in the kidneys, where it is hydroxylated to 1,25-dihydroxycholecalciferol (1,25(ΟΗ)2D3), the active metabolite of vitamin D (calcitriol) [4]. Calcitriol, once released into circulation, binds to a specific carrier protein and is transported to target organs to exert its effects. However, the best indicator for studying vitamin D is the circulating concentration of 25(OH)D, representing the vitamin obtained from both sun synthesis and diet. It has a half-life of two weeks, compared to 1,25-dihydroxycholecalciferol, which has a half-life of just four-six hours [4,5].

Additionally, vitamin D is provided through diet in the form of provitamin. Natural dietary sources include fatty fish (salmon, mackerel, tuna), cod liver oil, eggs, beef liver, and others. In the USA and some EU countries, certain foods such as milk, cereals, margarine, juices, and bread are fortified with vitamin D [6, 7].

Guidelines for vitamin D deficiency and insufficiency vary among organizations, reflecting different research methods and clinical experiences. According to some international clinical guidelines, serum 25(OH)D levels below 10 ng/mL (25 nmol/L) are considered deficient. However, the Institute of Medicine states that serum 25(OH)D levels should not fall below 20 ng/mL (50 nmol/L), while the Endocrine Society suggests that optimal skeletal health and muscle strength require serum 25(OH)D levels of at least 30 ng/mL (75 nmol/L) [8, 9]. The Endocrine Society defines vitamin D deficiency as 25(OH)D < 20 ng/mL and insufficiency as 21-29 ng/mL, with some authors recently advocating for values between 40 ng/ml and 60 ng/ml for better health [10].

The World Health Organization (WHO) has not issued specific guidelines for vitamin D but recommends ensuring adequate sun exposure and consuming vitamin D-rich foods to prevent deficiency. For most people, 90% of their vitamin D requirements are met through sun exposure and only 10% through their diet [10]. Therefore, one would expect that residents of sunny countries like those in the Mediterranean would have adequate 25(OH)D levels in their blood. However, studies in recent years highlight vitamin D deficiency in these populations, referring to the “Mediterranean Paradox” [11, 12].

This study was designed to evaluate the serum vitamin D levels of people living in the Karystos area, where sun exposure is considered adequate for most of the year. The study took place from September 2022 to September 2023 during routine examinations at the Biopathology Laboratory of the General Hospital of Karystos.

MATERIALS AND METHODS

Study Population

The study population included 1,199 Greeks, categorized by gender and age into three groups: 20-44, 45-64, and ≥65 years. No information was obtained on Body Mass Index (BMI), medical history, or current medication.

Sample Collection and Measurement

Vitamin D levels were measured using the Vit D Roche Elecsys reagent from Roche. Measurements were conducted using the electrochemiluminescence immunoassay method on the cobas e411 analyzer from Roche. Blood samples (3.5 – 5 ml) were taken from each volunteer in a special tube without anticoagulant, as serum is required for measuring 25(OH)D. The tubes were left to stand for a few minutes before centrifugation.

Definition of Levels

Vitamin D levels were defined as follows:

  • Sufficient: 25(OH)D > 30 ng/mL
  • Insufficient: 25(OH)D < 30 ng/mL
  • Deficient: 25(OH)D < 20 ng/mL

A value of 3 ng/mL was set as the lower detection limit of 25(OH)D.

Statistical Analysis

Statistical analysis of the data was conducted using p-value and chi-square analysis to compare vitamin D levels based on age and gender, utilizing Python, the SciPy library and the statistical tool R. A significant level was set at P<0.05.

RESULTS

Demographic Characteristics

Participants included men and women aged 20 to 88 years. The sample distribution by age group was: 20-44 years (21.55%), 45-64 years (33.17%), and ≥65 years (45.27%). Women accounted for 64.49% and men for 35.51% of the participants (Figure1, figure 2).

Figure 1. Distribution of the Sample by Age (%).

Figure 2. Distribution of the Sample by Gender (%).

Vitamin D Levels

The mean serum 25(OH)D concentration in the entire sample was below the laboratory’s reference normal limit (30 ng/mL). Specifically, the mean vitamin D value for the 2022-2023 period was 24.33±5.46 ng/mL, indicating insufficiency. The prevalence of vitamin D insufficiency (25(OH)D < 30 ng/mL) was 61.38%, while 23.32% of the population had vitamin D deficiency (25(OH)D < 20 ng/mL). Only 15.3% of the population had sufficient vitamin D levels (Figure 3).

Figure 3. Percentage (%) of the Population with sufficient, insufficient, and deficient Vitamin D levels from 01/09/2022 to 31/09/2023.

Correlation with Age and Gender

Age was not considerably associated with the prevalence of vitamin D insufficiency, although a linear trend with increasing age was observed (t-statistic: -1.432, p-value: 0.37). The chi-square statistic based on the p-value and the degrees of freedom was calculated to 4.36. There was also a differentiation in vitamin D levels between women and men. Women had lower average vitamin D levels compared to men, but this difference was not statistically considerable (t-statistic:-1.016, p-value:0.16). The chi-square statistic based on the p-value and the degrees of freedom was calculated to 1.88. Overall, the effect of age and gender did not show a statistically significant impact on the results.

Monthly Variation

During the first half of 2022-2023, the mean serum 25(OH)D concentrations were 23.58 (8.21; 66.46) ng/mL and 25.05 (3.00; 50.45) ng/mL for the second half, respectively. Significant seasonality effects were observed in vitamin D concentrations. The highest insufficiency and deficiency rates were recorded in February, with a mean vitamin D concentration of 19.40 ng/mL, while the lowest insufficiency rates and highest mean vitamin D concentrations were observed in September, with a mean value of 35.13 ng/mL. Below are the charts illustrating the levels of vitamin D by age group (Figure 4).

Figure 4. Distribution of vitamin level (ng/mL) in the total sample (1) and separately for each age group (2, 3, 4).

The variation in vitamin D levels seems to be explained by seasonality. February follows the month where sun exposure is limited, reducing vitamin D production in the skin. Conversely, September follows the summer months, where sun exposure is increased, leading to higher vitamin D levels.

DISCUSSION

This study aimed to evaluate vitamin D levels among adults living in a sunny region in Greece for most of the year. The Karystos area has a mild Mediterranean climate with approximately 270 sunny days per year.

 The main finding was that a high percentage of participants (over 80%) had serum 25(OH)D concentrations below the reference limit of 30 ng/mL, while only 16% had 25(OH)D levels above 30 ng/mL. This result contradicts the expected relationship between sunlight and adequate vitamin D levels.

Despite Greece’s known sunshine, even during winter, previous research has shown that most of the population suffers from vitamin D deficiency [1]. Our study confirms this phenomenon, as more than two-thirds of the population did not have sufficient vitamin D levels. Significant study limitations include the absence of information on BMI, medical history, current medication, and lack of data on factors affecting vitamin D absorption, such as dietary habits and lifestyle.

It is known that there is a strong correlation between high body weight and vitamin D deficiency. Specifically, obese men and women (BMI > 30) are 75% more likely to present with vitamin D deficiency compared to those of normal weight, due to the deposition of vitamin D in subcutaneous fat tissue [1]. Additionally, reduced physical activity and a sedentary lifestyle limit sun exposure and may contribute to obesity. Dietary habits, such as insufficient consumption of fatty fish, eggs, and vitamin D-fortified products, may also contribute to inadequate vitamin D intake.

The lack of adequate sunlight exposure, especially during the winter months, appears to be the main reason for the low vitamin D levels in the Karystos region. Increased sun exposure is recommended, particularly during the hours when sunlight is most intense (morning and afternoon) throughout the year, to address this deficiency.

Furthermore, this study indicates that perhaps the cut-off levels of vitamin D should be re-evaluated and revised for a more accurate assessment. Revising the reference thresholds may provide a more accurate assessment of vitamin D status in the population and contribute to improving the prevention and management of deficiencies through increased sun exposure, appropriate supplementation, and dietary interventions.

In conclusion, this study demonstrates the need for regular monitoring of vitamin D levels and further research on the factors influencing these levels, even in populations with high sun exposure. The study highlights the importance of individualized health approaches, considering demographic, geographic, and seasonal factors to improve overall health.

Conflicts of interest: None to declare

Declaration of funding sources: None to declare

Author contributions: Conceptualization, I.L. and G.K.; methodology, I.L.; formal analysis I.L. and G.K.; writing-original draft preparation, I.L.; writing-review and editing I.L. All authors have read and agreed to the published version of the manuscript.

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