The Mediterranean Way of Life, Part 4: Why There's a Genuine "Vitamin D Winter" North of the Alps

Of everything that differs between the Mediterranean and areas north of the Alps, sunlight is the most directly measurable – and research shows just how strongly latitude determines how much vitamin D the body can actually still produce on its own in winter.

This is the final part of our four-part series on the Mediterranean way of life. After diet, stress and social connection, Part 4 closes with the element that most directly distinguishes the Mediterranean region from Central Europe: sunlight and the body's own vitamin D synthesis.

Why sunlight is the body's most important source of vitamin D

Vitamin D holds a special place among vitamins: the body can produce it itself in the skin under UVB exposure. A review of vitamin D status in Europe puts the share of total vitamin D supply coming from the body's own synthesis at an estimated 80 to 100 percent – dietary sources play a comparatively minor role, since only a few foods (oily fish, egg yolk) provide meaningful amounts. Skin synthesis requires sufficiently intense UVB radiation, which depends on solar angle, season, cloud cover, skin type and latitude.

Why a genuine "vitamin D winter" exists north of the Alps

Several European modelling studies have systematically mapped UVB availability across different latitudes. An analysis covering nine European countries/regions between 35° and 69° north latitude found that at around 35° North (e.g. Crete), sufficient UVB for vitamin D synthesis is available essentially year-round, while the so-called "vitamin D winter" – the period with insufficient UVB radiation for meaningful skin synthesis – lasts up to eight months at around 69° North (northern Norway). For Central Europe at roughly 47 to 54° north latitude (Switzerland sits at around 46 to 47° N), several models put the length of this "vitamin D winter" at around four to six months, typically from October/November to March/April. A further study covering 46 European capital cities found up to a five-fold difference in mean UVB dose between Nicosia (Cyprus) and Reykjavik (Iceland). In Germany, according to one cited survey, the proportion of men with low vitamin D levels (below 25 nmol/l) was 9.8 percent in summer versus 21.4 percent in winter – a clear seasonal difference that supports the UVB modelling data.

What the research shows about vitamin D, vitamin K2 and bone density

The evidence on bone density is nuanced. A meta-analysis of randomised controlled trials on vitamin K and bone mineral density in 4,800 middle-aged and older participants found favourable effects of vitamin K2 at the hip and waist, while the data for the lumbar spine remained inconsistent – the authors themselves describe the effect across different skeletal sites as "not yet conclusively established." A randomised, double-blind trial combined D3 and K2 in people with type 2 diabetes over three months and observed synergistic effects on bone-related markers (osteocalcin carboxylation) as well as insulin sensitivity. A one-year randomised controlled trial in postmenopausal women with osteopenia, using D3 and K2 as part of a combination formula, observed an increase in bone density at the lumbar spine (+4.3 percent) and femoral neck (+2.2 percent) compared with placebo. As for the cardiovascular effects of vitamin K2, a recent review describes the evidence as still limited; the authors speak of a "possible complementary role" and call for further research on dosing and target populations.

What role vitamin K2 plays alongside D3

Vitamin D3 promotes calcium absorption from the intestine; vitamin K2 acts as a cofactor in activating proteins that help direct calcium into bone and regulate it in tissue (including osteocalcin and matrix Gla protein). Within the EU, it's authorised to state that vitamin D contributes to the normal absorption and utilisation of calcium and phosphorus, and to the maintenance of normal bones and normal muscle function, and that both vitamin D and vitamin K contribute to the maintenance of normal bones; vitamin K also contributes to normal blood clotting. MK-7, the form of K2 commonly used in supplements, remains in the blood longer than other K2 forms due to a half-life of around two to three days, allowing for more even daily dosing.

Safety, limitations and who should take particular care

High doses of vitamin D taken over long periods can raise blood calcium levels (hypercalcaemia); the daily doses recommended in Switzerland and the EU for supplements sit well below the threshold where this becomes relevant, but very high doses should not be taken without medical consultation. In cases of impaired kidney function or known disorders of calcium metabolism, vitamin D supplementation should be discussed with a doctor beforehand. Particularly important: people taking anticoagulant medication of the vitamin K antagonist type (e.g. phenprocoumon, warfarin) should only use vitamin K2-containing products after consulting their treating physician, since vitamin K directly affects how these medications work. Infants require separate, medically supervised dosing recommendations. Pregnant and breastfeeding women can generally take the commonly recommended daily doses without concern; when in doubt, consulting the treating healthcare professional is advisable.

Conclusion: what's well established – and what remains open

That latitude and season substantially determine the body's own vitamin D synthesis is among the best-established relationships in this entire series – the UVB modelling data and measured seasonal blood levels agree across several independent European studies. As for the role of vitamin K2 alongside D3, the bone density evidence points to a plausible but skeletal-site-dependent benefit, while the cardiovascular evidence, by the authors' own assessment, remains too thin for conclusive statements. That brings our four-part series on the Mediterranean way of life to a close: from olive oil, through siesta and social networks, to sunlight, the same pattern keeps recurring – individual factors are researched to varying degrees, and the full picture only emerges from how they work together.

This article is for general informational purposes only and does not replace individual medical advice. Dietary supplements are not a substitute for a balanced and varied diet and a healthy lifestyle.