Vitamin D stands out from other vitamins. Once processed in the body, it becomes cholecalciferol (vitamin D3), aiding our bones in absorbing necessary calcium. Unfortunately, vitamin D is scarce in foods (found in milk, cereals, tuna, salmon, sardines, eggs, and beef liver), so our bodies mainly generate this vital nutrient when exposed to sunlight.

​

Recommended Dosage

​

The amount of vitamin D our bodies produce, and how much sun exposure we need, depends on various factors, including the time of day, season, latitude, and skin pigmentation. Depending on our lifestyle and location, vitamin D production may be inadequate or even nonexistent during certain seasons. Sunscreen, essential for skin protection, also reduces vitamin D production.

​

According to the American Endocrine Society, the recommended daily dose of vitamin D is:

• 400–1,000 IU for children under 12 months

• 600–1,000 IU for individuals 1–18 years old

• 1,500–2,000 IU for individuals over 18, including seniors

Many older adults don’t get enough sunlight and may struggle with vitamin D absorption. For them, a multivitamin containing vitamin D can help improve bone health.

Most people meet their vitamin D needs through sun exposure. Ultraviolet (UV) B rays with wavelengths of 290–320 nanometers penetrate the skin and convert 7-dehydrocholesterol to previtamin D, which eventually becomes vitamin D3. Seasonal and geographic factors, like cloud cover, smog, melanin levels, and sunscreen use, affect UV exposure and, consequently, vitamin D synthesis.

​

How Much Sun is Enough?

​

As dermatologist Dr. Jasmina Kozarev highlights, we don’t need to expose our whole body to sunlight for vitamin D synthesis. Exposing a portion, like our hands or face, for about 20 minutes a day is sufficient. In our current reality, even standing by an open window or on a balcony can allow our bodies to produce vitamin D, bolstering our immunity.

​

Cloud cover can reduce UV energy by 50%, and shadows (including those from pollution) by 60%. Since UVB radiation doesn’t pass through glass, exposure through closed windows doesn’t promote vitamin D production. Sunscreens with an SPF of 8 or higher typically block vitamin D synthesis, although many people don’t apply enough sunscreen or apply it unevenly, so some vitamin D synthesis still occurs.

​

Various factors impacting UV exposure and the limited research on adequate sun exposure for vitamin D synthesis make general recommendations difficult. However, studies suggest that 5–30 minutes of unprotected sun exposure on the face, hands, legs, or back, at least twice weekly between 10 a.m. and 5 p.m., generally produces sufficient vitamin D.

​

Vitamin D and Cholesterol

​

Interestingly, vitamin D and cholesterol are interconnected. Dr. Jasmina Kozarev points out that international reference cholesterol levels may not suit everyone, as values vary depending on sun exposure. People closer to the equator, who receive more vitamin D, often have naturally lower cholesterol levels. So, while it's essential to maintain healthy cholesterol, “ideal” levels vary by geographic location and season.

​

Deficiency and Excess

​

The effects of vitamin D deficiency are well known: childhood rickets, osteoporosis in older adults, and links to cognitive decline, heart disease, and certain cancers. The International Osteoporosis Foundation reported in 2015 that one in three women and one in five men over 50 globally suffered fractures due to osteoporosis, with fractures occurring every three seconds.

​

Long-term excessive vitamin D intake can also be harmful, with hypercalcemia—a condition where calcium builds up in the blood—posing risks to the heart, blood vessels, and kidneys. Blood levels above 500 nmol/L or daily intake over 30,000 IU over prolonged periods are generally considered toxic. Supplements up to 4,000 IU daily are safe for adolescents and adults, but those with liver or kidney conditions should take lower doses