If calcium is the star of bone health, then vitamin D is its essential co-star — the one that helps calcium actually work.
Many people diligently take calcium supplements, only to forget vitamin D. As a result, the calcium is consumed but not effectively absorbed.
Why Is Vitamin D Important?
One of vitamin D's most critical functions is helping the intestines absorb calcium and maintain bone and dental health. It also supports immune function, muscle performance, and many physiological regulatory processes. Recent research suggests vitamin D's impact may be even broader than previously understood.
Taiwan's Dietary Reference Intakes recommend adults take about 10 mcg (400 IU) per day, with those over 50 advised to increase to 15 mcg (600 IU).
Vitamin D deficiency often has no immediate obvious symptoms. Some people may feel tired, have weak muscles, low mood, frequent colds, or reduced physical capacity. Long-term deficiency increases risk of osteoporosis, fractures, and sarcopenia.
Why Do People Still Get Deficient If the Body Can Make Its Own?
The human body can manufacture vitamin D — when UV rays from sunlight hit the skin, the body begins synthesizing it. This is why it's called the "sunshine vitamin."
However, modern life makes this increasingly difficult. We spend more and more time indoors — in offices, on the subway, in cars. When we do go outside, we use umbrellas, sunscreen, hats, and long sleeves, blocking almost all sunlight. Even in sun-drenched Taiwan, the rate of vitamin D deficiency remains quite high.
How Can Plant-Based Eaters Boost Vitamin D?
Certain mushrooms — such as button mushrooms, shiitake, maitake, and black wood ear — contain ergosterol, which converts to vitamin D2 through photochemical reaction when exposed to sunlight. Since most commercially grown mushrooms are cultivated indoors, try slicing fresh mushrooms and placing them in direct sunlight for 15–30 minutes before cooking to significantly boost their vitamin D2 content.
Some fortified plant milks and breakfast cereals also contain added vitamin D.
For supplements, look for D2 or D3 derived from lichen (algae/fungi sources).*
That said, sunlight remains the best vitamin D source. UVB rays at approximately 280 nm are most effective, and peak between 10 am and 3 pm. Expose your face, neck, and arms to direct sunlight without hats or sunscreen for 15 minutes, 3–4 times per week.
*Note: D2 and D3 have the same functions — both aid calcium absorption, support bone and muscle health, and boost the immune system. D3 generally has better absorption and stays in the bloodstream longer. Be aware that most commercial D3 is derived from wool (lanolin) — the wool industry has significant animal welfare concerns. Choose lichen (algae/fungi)-derived D3 instead.