VITAMIN D SUPPLEMENTATION DURING COVID-19 LOCKDOWN 
AND AFTER 20 MONTHS: FOLLOW-UP STUDY ON SLOVENIAN 
WOMEN AGED BETWEEN 44 AND 66
Vid VIČIČ
 1*
   , Ruža PANDEL MIKUŠ
 1
 
1 
University of Ljubljana, Faculty of Health Sciences, Chair of Biomedicine in 
Healthcare, Zdravstvena pot 5, 1000 Ljubljana, Slovenia
Received: Sep 06, 2023
Accepted: Sep 11, 2023
Original scientific article
*Correspondence: vidvicic@gmail.com
10.2478/sjph-2023-0026 Zdr Varst. 2023;62(4):182-189
182
DODAJANJE VITAMINA D PRI SLOVENSKIH ŽENSKAH, 
STARIH MED 44 IN 66 LET, MED COVID-19 ZAPRTJEM 
IN SPREMLJANJE PO 20 MESECIH
© National Institute of Public Health, Slovenia. 
Vičič V, Pandel Mikuš R. Vitamin D supplementation during COVID-19 lockdown and after 20 months: Follow-up study on Slovenian women aged between 44 and 66.  
Zdr Varst. 2023;62(4):182-189. doi: 10.2478/Sjph-2023-0026.
ABSTRACT
Keywords: 
Vitamin D
Postmenopausal women
Premenopausal women
COVID-19
Follow-up
Supplementation
IZVLEČEK
Ključne besede: 
vitamin D
pomenopavza
predmenopavza
covid-19
dodajanje vitamina D 
Introduction: The main objective was to assess the impact of the COVID-19 pandemic on vitamin D 
supplementation habits and their changes in the follow-up, 20 months after the study in Slovenian premenopausal 
and postmenopausal women. 
Methods: A cross-sectional study was conducted between March and May 2021. 176 healthy women aged 44 to 
65 from the Central Slovenian region were included in the final analysis. Vitamin D status was determined by 
measuring 25(OH)D concentration. After 20 months an online follow-up questionnaire was sent out, to which 
123 participants responded with complete data. 
Results: Between March and May 2021, 61% of the participants were supplementing vitamin D. 55% of the 
supplementers and 88% of the non-supplementers had insufficient levels (total 25(OH)D <75 nmol/L). After 20 
months in the follow-up, it was found that 62% of participants were taking vitamin D supplements, but only 70% 
of those who had initially reported taking supplements were still doing so. In the follow-up 61% of participants 
stated that they started or increased vitamin D intake due to COVID-19. 
Conclusions: Vitamin D supplementation increased 7-fold compared to pre-pandemic levels and remained at 
a high level after 20 months. However, a significant number of participants discontinued supplementation, 
and only one-fifth were taking vitamin D throughout the entire year. Supplementation is effective for vitamin 
D deficiency prevention only at the individual level, however due to low compliance it should not be the only 
strategy for preventing vitamin D deficiency in the population.  
Uvod: Cilj je bil oceniti vpliv pandemije covida-19 na navade dodajanja vitamina D med pandemijo in po 20 
mesecih pri slovenskih premenopavznih in pomenopavznih ženskah. 
Metode: Med marcem in majem 2021 smo izvedli presečno epidemiološko študijo, ki je zajela 319 žensk, starih 
med 44 in 65 let. V končno analizo smo ob upoštevanju izključitvenih dejavnikov vključili 176 preiskovank. 
Status vitamina D smo določili z meritvijo koncentracije 25(OH) vitamina D. Po 20 mesecih je bil udeleženkam 
poslan spletni vprašalnik za nadaljnje spremljanje. 123 udeleženk je odgovorilo s popolnimi podatki. 
Rezultati: Med marcem in majem 2021 je 61 % preiskovank dodajalo vitamin D. Nezadostnost vitamina D (skupni 
25(OH)D < 75 nmol/L) je bila ugotovljena pri 55 % žensk, ki so dodajale vitamin D, in 88 % žensk, ki niso dodajale 
vitamina D. Po 20 mesecih jih je vitamin D dodajalo 62 %, vendar je vitamin D dodajalo le 70 % od tistih, ki so 
prvotno dodajale vitamin D. V spremljanju je 61 % preiskovank povedalo, da so začele ali povečale dodajanje 
vitamina D zaradi covida-19.  
Zaključki: Dodajanje vitamina D se je v primerjavi z obdobjem pred pandemijo povečalo za 7-krat in po 
20 mesecih ostalo na visoki ravni. Precejšnje število udeleženk je prenehalo z dodajanjem in le petina jih 
je dodajala vitamin D celo leto. Dodajanje vitamina je učinkovita strategija za preprečevanje pomanjkanja 
vitamina D samo na ravni posameznika, vendar ni učinkovito pri preprečevanju pomanjkanja v populaciji.

1 INTRODUCTION
In recent years, the status of vitamin D has gained a lot 
of attention in connection with respiratory infections, 
especially with COVID-19. A recent systematic review 
and meta-analysis of randomized control trials has found 
that vitamin D supplementation reduces the risk of acute 
respiratory infections compared with a placebo (1). Meta-
analyses have found an association between vitamin D 
status, the risk of COVID-19 transmission and clinical 
outcomes in COVID-19 patients. Vitamin D status was 
also identified as a risk factor for severe COVID-19 illness 
(2–4). Similarly, in a systematic review and meta-analysis 
by Chiodini et al. (5), the results showed a connection 
between vitamin D status and COVID-19 severity (defined as 
“Acute Respiratory Distress Syndrome requiring admission 
to intensive care unit”) and mortality. Patients with low 
vitamin D levels on hospital admission had a higher risk 
of respiratory distress and death (5). In severely deficient 
patients in the late phases of COVID-19 pneumonia the 
role of vitamin D supplementation is still unclear (6).
However, the COVID-19 pandemic has also affected daily 
life, including dietary habits and lifestyle factors. The 
effects of the pandemic on these factors varied, depending 
on the type and severity of responses and actions taken 
by different countries. In a large cross-sectional survey 
conducted in Spain between April and June 2020, 35% 
of participants reported an increase in food intake, 
while 41% reported a decrease in snacking compared to 
before the lockdown. The majority of participants said 
their time spent cooking had increased (7). In an online 
survey conducted in the Netherlands between July and 
November 2020, the majority of responders (66%) reported 
no change in eating habits compared to the pre-lockdown 
period, 22% reported eating healthier and 12% less healthy 
than before (8).
An online survey report on the effects of the COVID-19 
pandemic on different lifestyle factors in a sample of 
the Slovenian population (n=1026) found that about 
14% of the participants reported a negative change in 
dietary habits, and 14% reported having better dietary 
habits; others reported no change. 32% of participants 
reported using food supplements during the COVID-19 
pandemic (38% by women and 26% by men), and almost 
half of the participants (49%) who reported better dietary 
habits during the pandemic also reported taking food 
supplements (9). Another study done in Poland, which 
examined the use of food supplements, specifically the 
use of zinc and vitamin D, found that both were more often 
chosen by people with higher education (59%) and with 
medical and related education (55%). Food supplements 
containing only vitamin D were used by 23% of participants 
in the first wave, 38% in the second wave and 33% in the 
third wave of COVID-19 (10).
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In a study by Vičič et al. (11) (March-May 2021), vitamin 
D status in Slovenian premenopausal and postmenopausal 
women was assessed. One of the main findings was that 
premenopausal women had 11.8% lower total 25(OH)D and 
32.2% lower bioavailable 25(OH)D. Additionally, significant 
predictors of vitamin D status were identified, of which 
supplemental vitamin D intake was most important  
(r (175)=0.56, p<0.001), followed by time spent outside 
(r (175)=0.23, p [0.003]). Food vitamin D intake, physical 
activity and BMI were not significant. The odds of having 
vitamin D insufficiency (<75 nmol/L total 25(OH)D) between 
the vitamin D non-supplementers and supplementers  
(>5 µg of vitamin D/day) were: OR = 6.23( p<0.001; 95% CI 
[2.72, 14.274]) (11).
Compared with the results of Hribar et al. (NUTRIHEALTH 
study, February-April 2019) (12), Vičič et al. (11) found that 
the prevalence of vitamin D deficiency (<50 nmol/L) in the 
adult population was much lower: 24.4% compared with 
81.6%. Similarly, the prevalence of insufficient 25(OH)D 
levels (<75 nmol/L) was much lower: 67.6% compared with 
98.0% This can be explained by supplement use, which 
was much higher than in the NUTRIHEALTH study (61.4% 
compared with 8.8%, respectively) (11,12). 
This is a substantial change that was influenced by expert 
recommendations (13) and media coverage of vitamin D 
supplementation effects on COVID-19 disease severity 
and infection risk (14). Žmitek et al. (14) investigated the 
effect of educational intervention – a press release to the 
mass media. They compared the results from the pre-
intervention survey (April 2020, first COVID-19 lockdown) 
and post-intervention survey (December 2020, second 
COVID-19 lockdown). The supplementation rate increased 
from 33% in April to 56% in December 2020. 
To further investigate, Vičič et al. (11) suggested that 
follow up studies should be performed to determine 
if COVID-19-inspired vitamin D supplementation would 
persist after the pandemic.
Therefore, the main objective was to assess the impact 
of the COVID-19 pandemic on vitamin D supplementation 
habits and their changes in the follow-up, 20 months after 
the study in Slovenian premenopausal and postmenopausal 
women. 
2 MATERIALS AND METHODS
2.1 Study’s design and participants
Between March and May 2021, a cross-sectional HIS (Health 
Interview Survey) and HES (Clinical Health Examination 
Survey) (15) study was conducted. Three hundred and 
nineteen (319) healthy women aged 44 to 65 from the 
Central Slovenian region were recruited for the study, of 
whom 176 participants met all the inclusion criteria. The 
flowchart of the study is presented in Figure 1.  

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2.2 Data collection
Participants were initially recruited by healthcare workers 
at two health centres during preventive health visits. A 
telephone survey was performed by trained registered 
nurses (RN) and nutritionists (MNutr). The questionnaire 
was based on a shortened food frequency questionnaire 
(FFQ), and questions on self-reported body weight and 
height, health status, use of food supplements with 
vitamin D, food intake, menstrual status, sun exposure 
status, skin type, socio-economic and socio-demographic 
status were added (supplementary material is available on 
request from the corresponding author). The participants 
named the specific products and dosage. Supplemental 
intake (µg/d) was calculated from the product label.
During the phone interview, a visit to the laboratory was 
arranged. Due to the challenges presented by the third 
COVID-19 lockdown, we employed snowball sampling as a 
method of recruitment (16). We provided all participants 
with values of 25(OH)D, including those who did not meet 
the inclusion criteria, with enclosed explanation. They 
were given a link to an online contact form where new 
participants could apply. This has proven an effective 
tool for recruiting new participants and assured truthful 
answers (supplementary material is available on request 
from the corresponding author).
The collection of blood samples was carried out during 
regular working hours in the selected healthcare centres 
and University Medical Centre Ljubljana between 1 March 
2021 and 31 May 2021. All samples were transported to 
a central laboratory at the University Medical Centre in 
Ljubljana, where laboratory analysis was carried out.
Detailed description of the study’ s design, inclusion 
and exclusion criteria, the content validity of the 
questionnaire, data collection process, laboratory analysis 
and the calculation of bioavailable 25(OH)D are presented 
in a previous article (11). 
2.3 Follow-up design and participants
On October 17, 2022, we sent an online follow-up 
questionnaire about vitamin D supplementation. The short 
questionnaire was made using the web-based platform 
1ka.si (17). It was optimized for maximum response rate 
and composed of 4 to 9 questions, opening depending 
on the participant’s answers. We repeated the same 
questions concerning vitamin D supplementation from the 
study questionnaire and attached additional questions 
on months of supplementation, COVID-19 impact on time 
spent outside and effect of participation in the study on 
vitamin D supplementation. The mean time required for 
completion was 2:04 minutes. In contrast to the study, 
this questionnaire was filled in by participants themselves 
(supplementary material is available on request from the 
corresponding author). 
The last response was received on November 22, 2022. Out 
of 176 participants who met the criteria, 123 participants 
responded with complete data.
2.4 Statistical analysis
Endocrine Society cut-off values were used for assessment 
of total 25(OH)D levels in serum: target concentration for 
optimal vitamin D effects: 75–125 nmol/L, insufficiency: 
50–75 nmol/L and deficiency: <50 nmol/L (18,19). 
Supplementers were defined as participants with 
supplemental intake from foods, food supplements, or 
vitamin D medicines >5 µg per day. This corresponds to 
25% of the Recommended Dietary Allowance (RDA) for 
adults, as set by DGE (the German Nutrition Society) (20).
Participants’ reported highest level of completed education 
was classified into categories as defined by the International 
Standard Classification of Education (ISCED) (21).
Values are presented as a mean±SD or as a percentage (%). 
The data were distributed normally. A two independent 
samples t-test was used to examine differences between 
groups. For proportions the differences between groups 
were examined with a Z-test (22). 
The statistical analysis was conducted using SPSS (IBM, 
version 27) and MS Excel 2019.
Figure 1. Flowchart of the study that included healthy women 
aged 44 to 65 from the Central Slovenian region. 
The study was carried out between March and May 
2021, and the follow-up study between October and 
November 2022.

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185
higher bioavailable 25(OH)D (6.7±4.9 nmol/L) than non-
supplementers (4.3±3.1 nmol/L; t(174)=[3.61], p<0.001). 
Mean vitamin D intake from food and supplementation 
(food supplements, vitamin D medicine) was 2.5±3.3 µg/d 
in non-supplementers and 37.6±25.2 µg/d in supplementers 
(Table 1, Figure 2). 10.2% of the supplementers and 47 .1% of 
non-supplementers were found vitamin D deficient (total 
25(OH)D <50 nmol/L), and 54.6% of the supplementers and 
88.2% of non-supplementers were found with insufficient 
levels (total 25(OH)D <75 nmol/L) (Table 1). 88% of 
supplementers supplemented with dosages of 20 µg/d or 
more. Only 3% of non-supplementers reached food vitamin 
D intake >5 µg/d (Figure 2).
3 RESULTS
Population characteristics, supplementation rates and 
vitamin D intake via food in the participants of the study 
are presented in Table 1.
3.1 Study between March and May 2021
A total of 61.4% of the participants were supplementing 
vitamin D with prescription vitamin D or food 
supplements. Vitamin D supplementers had 33.3% higher 
25(OH)D (76.2±27.1 nmol/L) than non-supplementers 
(50.8±19.6 nmol/L; t(174)=[6.718], p<0.000) and 35.8% 
Age
Menstrual status
BMI (self-reported)
Smoking
Education (ISCED)
Time spent outside 
Moderate physical activity 
Sunscreen use
Suntanning habits
25(OH)D 
DBP
Albumin 
Bioavailable 25(OH)D 
Food intake (µg/d) 
Supplement use (≥5 µg vitamin D/d) 
Supplemental intake (µg/d)
Intake all sources (µg/d)
 
year
Premenopause
Postmenopause
kg/m
2
Current smoker
Primary (L1)
Secondary (L2-3)
Short-cycle tertiary (L5)
Bachelor’s or eq. (L6)
Master’s or eq. (L7)
Doctoral or eq. (L8)
min
h/week
High exposure
Medium exposure
Low exposure
nmol/L
<30
30-50
50-75
>75
mg/L
g/L
nmol/L
54.3±4.8
34.3%
65.7%
25.5±4.2
13.0%
1.9%
27.8%
14.8%
13.0%
31.5%
11.1%
54.3±16.4
3.0±3.3
92.6%
4.6%
58.3%
37.0%
76.2±27.1
3.7%
6.5%
44.4%
45.4%
608±485
47.5±2.2
6.7±4.9
2.2±1.4
100,0%
35.4±25.3
37.6±25.2
53.8±5.0
39.2%
60.8%
25.7±4.4
13.1%
3.4%
29.0%
14.2%
13.6%
29.0%
10.8%
53.3±17.7
3.2±4.2
90.9%
6.8%
64.2%
29.0%
66.4±27.4
8.5%
15.9%
43.2%
32.4%
576±436
47.1±2.2
5.7±4.5
2.2±1.3
61.4%
21.7±26.2
24.1±26.2
53.1±5.2
47.1%
52.9%
26.1±4.6
22.1%
5.9%
30.9%
13.2%
14.7%
25.0%
10.3%
51.8±19.6
3.5±5.4
88.2%
10.3%
73.5%
16.2%
50.8±19.6
16.2%
30.9%
41.2%
11.8%
526±342
46.5±2.1
4.3±3.1
2.1±1.2
0.0%
0±0.08
2.5±3.3
0.120
 t
0.091
 z
0.091
 z
0.375
 t
0.114
 z
0.363
 t
0.447
 t
0.322
 z
0.1443
 z
0.040
 z
0.003
 z
<0.001
 t
0.004
 z
<0.001
 z
0.675
 z
<0.001
 z
0.226
 t
0.003
 t
<0.001
 t
0.627
 t
<0.001
 t
<0.001
 t
Category/
Unit
Supplementers
n=108
Total
n=176
Non-
supplementers
n=68
𝑝 -value Variables
Table 1. Population characteristics, vitamin D status, supplementation and food intake of healthy women aged 44 to 65, from the 
Central Slovenian region. The study was carried out between March and May 2021 (n=176).
BMI=body mass index, DBP=vitamin D binding protein. All values are presented as a mean±SD or %. The 𝑝 – value was determined using 
a two independent samples t-test (
t
) or two sample Z-test for proportions (
Z
). 𝑝<0.05 was considered statistically significant ( 𝑝-values of 
significant variables are in bold print).
Laboratory analysis
Vitamin D intake and supplementation

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186
Supplement use (≥5 µg vitamin D/d) 
Supplemental intake 
Started/increased vitamin D supplementation 
COVID-19 impact on time spent outside
Effect of participation in study on 
vitamin D supplementation
µg/d
No impact
Increased
Decreased
No effect
Positive effect
100,0%
34.9±24.9
63.2%
51.3%
38.2%
10.5%
42.0%
57.8%
61.8%
21.6±25.9
61.0%
51.2%
37.4%
11.4%
51.8%
48.2%
0.0%
0.12±0.7
57.4%
51.1%
36.2%
12.8%
62.8%
37.2%
<0.001 
t
0.522
 z
0.984
 z
0.826
 z
0.697
 z
0.026
 z
0.026
 z
Category/
Unit
Supplementers
n=76
Total
n=123
Non-
supplementers
n=47
𝑝 -value Variables
Table 2.
Figure 2.
Figure 3.
Results from the online follow-up questionnaire of 123 healthy women aged 44 to 65 from the Central Slovenian region, 
included in the follow-up study carried out between October and November 2022 (n=123).
All values are presented as a mean±SD or %. The 𝑝 – value was determined using a two independent samples t-test (t) and two samples 
Z-test for proportions (Z). 𝑝<0.05 was considered statistically significant ( 𝑝-values of significant variables are in bold print).
Total vitamin D intake of non-supplementers (top) 
and supplementers (bottom) (>5 µg/d). Both 
histograms are at the same scale (0-145 µg/d).
Percentage of participants who have reported to 
have started or increased vitamin D supplementation 
due to COVID-19, in study and follow-up.
3.2 Follow-up between October and November 2022
As shown in Table 2, vitamin D supplementation after 20 
months remained at similar levels. 
Compared to the study in March and May 2021, the 
percentage of participants who reported starting or 
increasing vitamin D supplementation in the follow-up 
increased (Figure 3).
Vitamin D supplementation
COVID-19 related changes in lifestyle and vitamin D supplementation

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187
The results of the follow-up also show changes in 
supplementation habits. While the overall percentage 
of supplement use between study and follow-up did not 
change (61.4% vs. 61.8%, respectively), certain changes 
can be noted when comparing supplementers and non-
supplementers. After 20 months, only 69.7% of the original 
supplementers were still supplementing vitamin D, 48.9% 
of non-supplementers started supplementing and 51.1% 
maintained non-supplementation. 
When asked about months of supplementation, 36% of the 
supplementers supplemented vitamin D throughout the 
year, and more than 50% supplemented vitamin D from 
October till March (Figure 4). 
4 DISCUSSION
The results from our study clearly indicate a major positive 
influence of the COVID-19 pandemic on supplement intake 
and serum 25(OH)D levels of the Slovenian population, when 
compared to previous Slovenian studies (23). Other studies 
also found a major increase in the vitamin D supplementation 
rate (10, 14) and increased supplementation with food 
supplements containing vitamin D, vitamin C, selenium 
or zinc with the intention of improving immune system 
function (9). However, there was a lack of data on actual 
vitamin D supplementation dosages.
Interestingly, other studies found a major negative impact of 
the COVID-19 pandemic on 25(OH)D levels in the paediatric 
population. This negative effect was attributed to a decrease 
in sun exposure due to lockdowns (24–26). In premenopausal 
and postmenopausal women, we report the opposite. In our 
follow-up 37% of participants reported an increase and 11% 
reported a decrease in time spent outdoors.
Figure 4. Seasonal rate of supplementation with vitamin D during the year in premenopausal and postmenopausal women vitamin D 
supplementers aged 44-65 from the Central Slovenian region (n=76). Participants marked each month individually or chose 
the whole year. 
High supplement use and lower prevalence of deficiency 
or insufficiency in our study could be attributed to 
Slovenian leading experts rapidly recommending vitamin 
D supplementation in the general population. The 
emphasis was on supplementation in vitamin D deficient, 
vulnerable, high-risk individuals and COVID-19 patients 
(27). Supplementation with 20 to 50 μg (800 – 2000 IU) 
of vitamin D was recommended from October to May as a 
preventive measure in healthy individuals who get enough 
sun exposure during the summer months (13). 
In a meta-analysis of clinical studies on vitamin D 
supplementation in postmenopausal women, 10 studies 
reported a compliance rate with vitamin D supplementation 
over 80% (28). Despite the COVID-19 pandemic, the overall 
rate of vitamin D supplementation did not decrease in our 
study. However, there was a shift in the supplementers, 
as 30% stopped taking supplemental vitamin D, and almost 
half of non-supplementers started supplementation.
The high supplementation rate in the follow-up could be 
due to the effect of our study, as participation in our study 
had a significant impact on vitamin D supplementation 
(Table 2), with nearly half of the participants reporting 
that they initiated or increased their intake after the study. 
Participants were informed about their 25(OH)D level, its 
significance and about recommended supplementation 
(supplementary material is available on request from the 
corresponding author).
The COVID-19 pandemic also caused a lot of misinformation 
concerning vitamin D, especially on the internet (29), which 
can result in cases of supplement induced vitamin D toxicity 
(30,31). In our study, two cases (in a sample of 244 cases) 
were detected with 25(OH)D >250 nmol/L. They were not 
included in the analysis. We talked with the participants 
and in both cases, they were using high-dosage vitamin 

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188
D bought online. In the first case the subject was taking 
1750 µg/day in the form of 7 capsules, each containing 250 
µg of cholecalciferol. The product was imported from the 
USA. In the second case the subject bought legal product 
in the form of highly concentrated drops (25 µg/drop). She 
was taking the product in unmeasured sips, directly from 
the bottle with the intention “to prevent COVID-19”. The 
actual dosage was unknown. 
Supplementation is an effective strategy for vitamin D 
deficiency prevention only at the individual level. Use 
of food supplements is usually associated with healthier 
lifestyle (32), higher education, light skin colour and 
overall health (33). Our results also show that only 45% 
of supplementers reached 25(OH)D levels of >75 nmol/L, 
despite mean supplemental intake of 35.4±25.3 µg/d. This 
may mean that for most of our population higher levels of 
supplementation is needed to reach optimal levels. 
Food fortification could be a viable solution for 
improvement of vitamin D status in the population. In our 
previous publication we provided a fortification model and 
an economic evaluation of biofortification of hen eggs, 
with or without milk (including yoghurt) fortification (34).
4.1 Strengths and limitations of the study
This follow-up study’s unique strength is that it examines 
vitamin D supplementation habits during the COVID-19 
pandemic and 20 months after. We collected qualitative 
data and actual vitamin D dosages. Therefore, it shows 
long-term compliance with vitamin D supplementation. 
Due to COVID-19 restrictions, BMI was calculated from 
self-reported weight and height. Several studies have 
evaluated the relation between self-reported and actual 
BMI. BMI computed from self-reported weight and height 
can differ from actual BMI, especially in people with 
higher BMI (35,36). 
Another limitation of our study was the focus of questions 
on current supplementation and not supplementation in 
past months, which also contributed to 25(OH)D levels. 
5 CONCLUSIONS
The results from our study indicate a major positive 
influence of the COVID-19 pandemic on serum 25(OH)D 
levels and an almost sevenfold increase in the prevalence 
of vitamin D supplementation compared to the Slovenian 
population before the pandemic. However, a significant 
number of the participants discontinued supplementation, 
and only one-fifth were taking vitamin D throughout the 
year. Supplementers had considerably better 25(OH)
D levels, however less than half reached >75 nmol/L. 
Supplementation is an effective strategy for vitamin D 
deficiency prevention at the individual level, however due 
to low compliance it is not a reliable long-term strategy to 
prevent deficiency in the population.
CONFLICTS OF INTEREST
The authors declare that no conflicts of interest exist.
FUNDING
The study was financed by: Department of Public Health, 
Faculty of Medicine, University of Ljubljana research 
programme ARRS grant No. P3-0360, Slovenian research 
programme for comprehensive cancer control SLORApro 
and University Medical Centre Ljubljana, The Division of 
Gynaecology and Obstetrics, research programme ARRS 
grant No. P3-0124, Metabolic and hereditary factors of 
reproductive health, delivery II.
ETHICAL APPROVAL
The study protocol was approved by the Slovenian National 
Medical Ethics Committee (Ministry of Health, Republic 
of Slovenia), identification number KME 0120-68/2019/9 
(approval letter ID 0120-68/2019/9, date of approval: 22 
March 2019).
AVAILABILITY OF DATA AND MATERIALS
The data presented in this study are available on request 
from the corresponding author.
REFERENCES
1. Jolliffe DA, Camargo CAJ, Sluyter JD, Aglipay M, Aloia JF, Ganmaa 
D, et al. Vitamin D supplementation to prevent acute respiratory 
infections: A systematic  review and meta-analysis of aggregate data 
from randomised controlled trials. lancet Diabetes Endocrinol. 2021 
May;9(5):276–292. doi: 10.1016/S2213-8587(21)00051-6.
2. Teshome A, Adane A, Girma B, Mekonnen ZA. The Impact of vitamin D 
level on COVID-19 infection: Systematic review and meta-analysis. Front 
Public Heal. 2021 Mar 5;9:624559. doi: 10.3389/FPUBH.2021.624559.
3. Pereira M, Dantas Damascena A, Galvão Azevedo LM, de Almeida 
Oliveira T, da Mota Santana J. Vitamin D deficiency aggravates 
COVID-19: Systematic review and meta-analysis. Crit Rev Food Sci 
Nutr. 2022;62(5):1308–1316. doi: 10.1080/10408398.2020.1841090.
4. Pal R, Banerjee M, Bhadada SK, Shetty AJ, Singh B, Vyas A. Vitamin 
D supplementation and clinical outcomes in COVID-19: A systematic 
review and meta-analysis. J Endocrinol Invest. 2022 Jan;45(1):53–68. 
doi: 10.1007/s40618-021-01614-4.
5. Chiodini I, Gatti D, Soranna D, Merlotti D, Mingiano C, Fassio A, 
et al. Vitamin D status and SARS-CoV-2 infection and COVID-19 
clinical outcomes. Front public Heal. 2021;9:736665. doi: 10.3389/
fpubh.2021.736665.
6. Jordan T, Siuka D, Rotovnik NK, Pfeifer M. COVID-19 and vitamin D - a 
systematic review. Zdr Varst. 2022 Jun;61(2):124–132. doi: 10.2478/
sjph-2022-0017.

7. Casas R, Raidó-Quintana B, Ruiz-León AM, Castro-Barquero S, 
Bertomeu I, Gonzalez-Juste J, et al. Changes in Spanish lifestyle 
and dietary habits during the COVID-19 lockdown. Eur J Nutr. 2022 
Aug;61(5):2417–2434. doi: 10.1007/s00394-022-02814-1.
8. Dijksterhuis GB, van Bergen G, de Wijk RA, Zandstra EH, Kaneko D, 
Vingerhoeds M. Exploring impact on eating behaviour, exercise and 
well-being during COVID-19 restrictions in the Netherlands. Appetite. 
2022 Jan;168:105720. doi: 10.1016/j.appet.2021.105720.
9. Hočevar Grom A, Belščak Čolaković A, Rehberger M, Lavtar D, Korošec 
A, Gabrijelčič Blenkuš M, et al. Pandemija Covid-19 v Sloveniji: Izsledki 
panelne spletne raziskave o vplivu pandemije na življenje (SI-PANDA), 
16. val. Ljubljana: Nacionalni inštitut za javno zdravje; 2021. 44 p. 
10. Puścion-Jakubik A, Bielecka J, Grabia M, Mielech A, Markiewicz-
Żukowska R, Mielcarek K, et al. Consumption of food supplements 
during the three COVID-19 waves in Poland-focus on zinc and vitamin 
D. Nutrients. 2021 Sep;13(10). doi: 10.3390/nu13103361.
11. Vičič V, Kukec A, Kugler S, Geršak K, Osredkar J, Pandel Mikuš 
R. Assessment of vitamin D status in Slovenian premenopausal 
and postmenopausal women, using total, free, and bioavailable 
25-hydroxyvitamin D (25(OH)D). Nutrients. 2022;14(24):8–11. doi: 
10.3390/nu14245349.
12. Hribar M, Hristov H, Gregorič M, Blaznik U, Zaletel K, Oblak A, et al. 
Nutrihealth study: Seasonal variation in vitamin D status among the 
Slovenian adult and elderly population. Nutrients. 2020 Jun 1;12(6):1–
17. doi: 10.3390/NU12061838.
13. Pfeifer M, Siuka D, Pravst I. Priporočila za nadomeščanje 
holekalciferola (vitamina D3) v obdobjih respiratornih okužb in za 
nadomeščanje holekalciferola pri posameznikih s COVID-19 [Internet]. 
2020. p. 8. Available from: https://www.kclj.si/dokumenti/FINAL_
Okt_2020_PRIPOROCILA_VITAMIN_D_in_covid-19_za_infektologe.pdf
14. Žmitek K, Hribar M, Lavriša Ž, Hristov H, Kušar A, Pravst I. Socio-
demographic and knowledge-related determinants of vitamin D 
supplementation in the context of the COVID-19 pandemic: Assessment 
of an educational intervention. Front Nutr. 2021;8:648450. doi: 
10.3389/fnut.2021.648450.
15. Tolonen H, Koponen P, Al-Kerwi A, Capkova N, Giampaoli S, Mindell 
J, et al. European health examination surveys - a tool for collecting 
objective information about the health of the population. Arch Public 
Health. 2018 Jun 28;76:38. doi: 10.1186/s13690-018-0282-4.
16. Goodman LA. Snowball Sampling. 1961 Mar 1 [cited 2021 Jul 
28];32(1):148–170. 
17. 1ka [Internet]. 2022 [cited 2022 Nov 29]. Available from: https://1ka.
arnes.si
18. Holick MF, Binkley NC, Bischoff-Ferrari HA, Gordon CM, Hanley DA, 
Heaney RP, et al. Guidelines for preventing and treating vitamin D 
deficiency and insufficiency revisited. J Clin Endocrinol Metab. 
2012;97(4):1153–1158. doi: 10.1210/jc.2011-2601.
19. Vieth R, Holick MF. The IOM—Endocrine society controversy on 
recommended vitamin D targets: In support of the Endocrine society 
position. In: Feldman D, editor. Vitamin D. 4th ed. Academic Press; 
2018. p. 1091–1107. doi: 10.1016/B978-0-12-809965-0.00059-8.
20. Debenjak P, Debenjak D, Hlastan-Ribič C, Salobir K, Pokorn D. 
Referenčne vrednosti za vnos hranil. Ljubljana: Ministrstvo za zdravje; 
2004. 
21. UNESCO Institute for Statistics. International Standard Classification 
of Education ISCED 2011. Montreal: UNESCO Institute for Statistics; 
2012. 84 p. 
22. Zou KH, Fielding JR, Silverman SG, Tempany CMC. Hypothesis testing 
I: Proportions. Radiology. 2003 Mar;226(3):609–613. doi: 10.1148/
radiol.2263011500.
23. Hribar M, Benedik E, Gregorič M, Blaznik U, Kukec A, Hristov H, et al. 
A systematic review of vitamin D status and dietary intake in various 
Slovenian populations. Zdr Varst. 2022 Mar;61(1):55–72. doi: 10.2478/
sjph-2022-0009.
24. Yu L, Ke HJ, Che D, Luo SL, Guo Y, Wu JL. Effect of pandemic-related 
confinement on vitamin D status among children aged 0-6 years in 
Guangzhou, China: A cross-sectional study. Risk Manag Healthc Policy. 
2020;13:2669–2675. doi:  10.2147/RMHP.S282495.
25. Rustecka A, Maret J, Drab A, Leszczyńska M, Tomaszewska A, Lipińska-
Opałka A, et al. The impact of COVID-19 pandemic during 2020-2021 
on the vitamin D serum levels in the paediatric population in Warsaw, 
Poland. Nutrients. 2021 Jun;13(6). doi: 10.3390/nu13061990.
26. Beyazgül G, Bağ Ö, Yurtseven İ, Coşkunol F, Başer S, Çiçek D, et al. 
How vitamin D levels of children changed during COVID-19 pandemic: 
a comparison of pre-pandemic and pandemic periods. J Clin Res 
Pediatr Endocrinol. 2022 Jun;14(2):188–195. doi: 10.4274/jcrpe.
galenos.2022.2021-10-6
27. Siuka D, Pfeifer M, Pinter B. Vitamin D supplementation during the 
COVID-19 pandemic. Mayo Clin Proc. 2020 Aug;95(8):1804–1805. doi:  
10.1016/j.mayocp.2020.05.036
28. Hassanein MM, Huri HZ, Baig K, Abduelkarem AR. Determinants and 
effects of vitamin D supplementation in postmenopausal women: 
A systematic review. Nutrients. 2023 Jan;15(3). doi: 10.3390/
nu15030685.
29. Quinn EK, Fenton S, Ford-Sahibzada CA, Harper A, Marcon AR, 
Caulfield T, et al. COVID-19 and vitamin D misinformation on YouTube: 
Content analysis. JMIR Infodemiology. 2022 Mar;2(1):e32452. doi: 
10.2196/32452.
30. Bhat JR, Geelani SA, Khan AA, Roshan R, Rathod SG. Vitamin D 
toxicity due to self-prescription: A case report. J Fam Med Prim Care. 
2022;11(4). 
31. Alkundi A, Momoh R, Musa A, Nwafor N. Vitamin D intoxication and 
severe hypercalcaemia complicating nutritional supplements misuse. 
BMJ Case Reports CP. 2022;15(7). doi: 10.1136/bcr-2022-250553.
32. Iłowiecka K, Maślej M, Czajka M, Pawłowski A, Więckowski P, Styk T, 
et al. Lifestyle, eating habits, and health behaviors among dietary 
supplement users in three European countries. Front Public Health. 
2022;10:892233. doi: 10.3389/fpubh.2022.892233.
33. Forrest KY, Stuhldreher WL. Prevalence and correlates of vitamin D 
deficiency in US adults. Nutr Res. 2011/02/12. 2011;31(1):48–54. doi: 
10.1016/j.nutres.2010.12.001.
34. Vičič V, Mikuš RP, Kugler S, Geršak K, Osredkar J, Kukec A. Vitamin 
D fortification of eggs alone and in combination with milk in women 
aged 44-65 years: Fortification model and economic evaluation. Zdr 
Varst. 2023 Mar;62(1):30–38. doi: 10.2478/sjph-2023-0005.
35. Spencer EA, Appleby PN, Davey GK, Key TJ. Validity of self-reported 
height and weight in 4808 EPIC-Oxford participants. Public Health 
Nutr. 2002 Aug;5(4):561–565. doi: 10.1079/PHN2001322.
36. Hodge JM, Shah R, McCullough ML, Gapstur SM, Patel A V. Validation 
of self-reported height and weight in a large, nationwide cohort of  
U.S. adults. PLoS One. 2020;15(4):e0231229. doi: 10.1371/journal.
pone.0231229.
10.2478/sjph-2023-0026 Zdr Varst. 2023;62(4):182-189
189