Vitamin D deficiency in Korea: test, then top up wisely

Why vitamin D deficiency in Korea is so common

National health surveys have repeatedly found that a large share of Korean adults have low vitamin D levels. Vitamin D deficiency in Korea comes down to everyday life: long hours in offices, cafes and subway stations, commuting before and after the sunniest part of the day, careful daily sunscreen and sun-shielding habits, and winters when the sun is too low for much vitamin D production.

Smoggy or fine-dust days keep people indoors. Diets in Korea are often low in oily fish, egg yolks and fortified foods compared with some Western countries. Expats who moved from sunnier climates, night-shift workers, people with darker skin and anyone who covers up outdoors are particularly likely to run low.

The 25-OH-D test and what results mean

The test measures 25-hydroxyvitamin D, the main circulating form. Fasting is not required. Cutoffs vary slightly between guidelines, but commonly used ranges are:

Results can vary by season, so a level measured in late summer is often higher than the same person's level in February. We take timing into account when interpreting your number.

• Below 10–12 ng/mL: severe deficiency
• Below 20 ng/mL: deficiency
• 20–30 ng/mL: insufficiency in many guidelines
• 30 ng/mL and above: generally considered sufficient
• Very high levels, often above 100–150 ng/mL, raise concern for toxicity, usually from over-supplementation

Symptoms, and who should be tested

Mild deficiency often causes no symptoms. Lower levels can be associated with tiredness, bone or muscle aches, muscle weakness and, over time, reduced bone density. Studies linking vitamin D to immunity, mood and many other conditions are mixed, so we avoid overpromising.

Testing is most useful if you have osteoporosis or a fracture history, take steroids or certain anti-seizure drugs, have kidney or liver disease, have malabsorption such as celiac disease or prior bariatric surgery, are pregnant or planning pregnancy, are older, or have symptoms plus limited sun exposure. Routine screening of everyone without risk factors is not universally recommended.

Oral vitamin D vs a vitamin D shot

Oral vitamin D3 is effective, inexpensive and lets you adjust the dose easily. It is the first choice for most people. Taking it with a meal containing fat improves absorption.

A vitamin D injection gives a large dose at once, raising levels over the following weeks. It can suit people who struggle to take daily pills, have poor absorption, or want a single top-up before winter. It is not necessarily better than tablets, and it cannot be adjusted once given.

Dosing depends on your level, weight and health conditions. More is not better: excess vitamin D can raise blood calcium. People with kidney stones, high calcium, sarcoidosis, certain lymphomas or advanced kidney disease need individual advice before high-dose treatment. We usually recheck levels after about three months.

Food, sunlight and everyday prevention

Food alone rarely corrects a deficiency, but it helps maintain levels. Good sources include oily fish such as mackerel (godeungeo), salmon and sardines, egg yolks, and some mushrooms exposed to UV light. Fortified milk and cereals are less widespread in Korea than in North America, so check labels.

Sun exposure is a balance. Dermatologists rightly recommend sunscreen to reduce skin cancer and photoaging, and we do not advise abandoning it. Short, sensible exposure of arms and legs in late spring and summer can contribute, while supplements cover the winter gap. For many people living in Seoul, a modest daily maintenance dose after correcting a deficiency is the simplest long-term approach, with occasional rechecks rather than frequent testing.

Red flags and limits

Vitamin D deficiency is rarely an emergency. However, signs of high calcium, often from excessive supplementation, need prompt care: persistent nausea or vomiting, extreme thirst and urination, confusion, constipation and weakness. Severely low calcium can cause muscle spasms or tingling around the mouth.

Go to an ER or call 119 in Korea for confusion, seizures, severe muscle cramps or spasms, an irregular heartbeat, or a suspected fracture after a minor fall.