Imagine someone born and raised under the tropical sun of Indonesia, exposed to abundant sunlight their entire life. Then they fly to Japan to work. Suddenly they live in a country with long winters, indoor working hours, and often overcast skies.
Without realizing it, their body begins to lack something they previously got for free: vitamin D.
More worryingly, this deficiency is not just about brittle bones. A recent scientific review published in the Journal Nutrients (2025) found that low vitamin D levels are directly linked to eight of the ten leading causes of death worldwide: heart disease, cancer, and even Alzheimer's.
Why Foreign Workers in Japan Are Vulnerable to Vitamin D Deficiency
There are three factors that put foreign workers from Indonesia and other Southeast Asian countries at higher risk compared to their counterparts from Western countries.
Darker skin requires more sunlight
Melanin, the pigment that gives skin its color, functions like natural sunscreen. The darker someone's skin, the more UVB rays are blocked before they can be converted into vitamin D beneath the skin.
Data from the United States illustrates how serious this problem is: 53.1% of the Black population experiences vitamin D deficiency, compared to only 12.2% among the white population. Similar conditions are very likely experienced by workers from Southeast Asia living in Japan.
Japan is a temperate country with limited UVB rays
Indonesia sits on the equator; sunlight arrives almost perpendicular year-round. Japan is located at a much higher latitude, especially in winter. The low angle of sunlight means UVB rays (the only wavelength of light that can trigger vitamin D production in the skin) barely reach the earth's surface from October to March.
Research shows vitamin D deficiency rates reach 57% in regions at latitudes 60–80° N during winter.
Predominantly indoor work patterns
The majority of foreign workers in Japan work in factories, warehouses, or other enclosed facilities. Limited time outdoors means minimal sun exposure, exacerbating the two factors above.
Vitamin D Is Not Just a "Bone Vitamin"
For decades, vitamin D was only known as a nutrient for maintaining bone health. That view is now outdated.
At the cellular level, vitamin D acts like a genetic foreman: its active hormone penetrates cell walls, enters the cell nucleus, and directly regulates the expression of hundreds to thousands of genes. One study found that supplementation for 6 months was able to up-regulate or down-regulate up to 1,289 genes in white blood cells.
This explains why adequate vitamin D levels impact nearly every organ system in the body:
Disease | Impact of Optimal Vitamin D Levels |
|---|---|
Heart disease & hypertension | Blood pressure decreases; CVD mortality risk significantly reduced |
Breast cancer | 80% lower risk at levels ≥60 ng/mL |
Infections & COVID-19 | Production of cathelicidin, a compound that destroys viruses and bacteria |
COPD (chronic lung disease) | Risk reduction up to 57% |
Alzheimer's & dementia | Significant risk reduction at high levels |
Type 2 diabetes | Risk decreased 52–71% at levels 40–50 ng/mL and above |
Numbers You Need to Know
Old guidelines set safe vitamin D levels at 20 ng/mL. However, recent scientific reviews conclude this number is only sufficient to prevent bone problems, not for protection against chronic diseases.
Researchers now recommend serum blood levels between 40–70 ng/mL for optimal protection.
To achieve these levels, the recommended daily supplementation is:
- 2,000 IU/day: minimum to maintain levels above 30 ng/mL
- 4,000–6,000 IU/day: to achieve protective levels of 40–70 ng/mL
- Patients with excess weight (obesity) may need more, because vitamin D is fat-soluble and tends to be "stored" in adipose tissue
The good news: vitamin D is a supplement that is cheap, safe, and widely available at Japanese pharmacies (ドラッグストア). Toxicity only occurs at extreme levels above 150 ng/mL, which is nearly impossible to achieve from standard supplementation.
First Step: Check Your Blood Levels
Because each person's body response to supplements varies by up to 20%, the only way to know if you are deficient is through a blood test.
In Japan, vitamin D level testing (ビタミンD検査) can be done at general clinics or during annual health checkups (健康診断). Ask your doctor to check your 25(OH)D blood levels.
First Steps You Can Take Today
Moving from a tropical climate to Japan is a major change, not only culturally and socially, but also biologically. A body that for years relied on tropical sunlight suddenly must adapt to an environment with far less UVB.
Vitamin D deficiency is a real health risk experienced by many foreign workers in Japan, yet it often goes unnoticed because it does not cause clear symptoms early on. The impact is only felt years later in the form of serious chronic diseases.
Simple steps like checking blood levels, starting supplementation if needed, and taking time to sunbathe even briefly, can make a big difference to your long-term health.
Nuance Union is here to support foreign workers in Japan: not only in documentation and employment matters, but also in ensuring you receive relevant and accurate health information. If you have questions about health as a foreign worker in Japan, don't hesitate to contact us.