For their study published in Nature Communications, the researchers analyzed anonymized routine data from 292,187 patients from all over Bavaria, from both outpatient and inpatient settings. The data came from laboratory information systems as part of a data protection-compliant research platform.

The scientists compared laboratory values two years before the pandemic (March 2018 to February 2020) and two years during the pandemic (March 2020 to February 2022). Their results show that the mean serum level of 25-hydroxyvitamin D fell from 26.7 µg/l to 26.0 µg/l during this period. At the same time, the proportion of all people with vitamin D deficiency (below 20 µg/l) increased from 31.2 to 35.2 percent.

These differences were statistically highly significant (p < 0.001) and remained even after age, gender and season were taken into account.

"One possible explanation is that lockdowns, working from home and changes in leisure habits have reduced the sun exposure of many people," explains Prof. Eva Grill.

Three different statistical methods were used to validate the results - classic descriptive analyses, propensity score matching and machine learning. All methods independently confirmed the decrease in mean vitamin D levels and the increase in deficiency rates.

The analysis also showed clear differences between population groups. Older women over the age of 60 were particularly affected by the decline in vitamin D levels during the pandemic. However, there was also a sharp increase in vitamin D deficiency among younger adults aged 18 to 39 - their deficiency rate rose from 34.6 to 37.8 percent. Vitamin D is not only crucial for bone health, but also has an immunomodulatory effect. A deficiency can increase the risk of infections, autoimmune diseases and bone fractures.

There is also an urban-rural divide: Deficiency rates were consistently higher in urban regions than in rural areas. "Urban living conditions - less sunlight due to building development, limited green spaces, higher air pollution - make vitamin D synthesis even more difficult," says Grill.

The researchers also interpret their data against the background of gender-specific behavioral differences. Overall, women take vitamin D supplements more frequently than men, but at the same time suffered greater losses during the pandemic. According to Grill, this could be "an indication that social and psychological factors such as increased workload, stress and limited access to preventive services played a role for women."

The study not only provides surprising insights. "Our work shows that routine data from medical care can be an early warning system for public health," emphasizes Grill. "They make it possible to quickly identify trends such as the decline in certain nutrients or the increase in risk factors - without having to wait for time-consuming surveys."

Grill also sees the design of the study as a perspective for future health reporting. By integrating laboratory, billing and care data, developments in micronutrients, metabolic parameters or vaccination rates, for example, can be tracked in real time - an important step towards a data-driven prevention policy.

Lea Skapetze, Daniela Koller, Andreas Zwergal, Stefan Feuerriegel, Anna Rubinski & Eva Grill: Monitoring changes in vitamin D levels during the COVID-19 pandemic with routinely-collected laboratory data. Nat Commun 16, 8772 (2025).

https://doi.org/10.1038/s41467-025-64192-6