By Yang Wei Ling & Nor Aini Jamil
Back in the 1920s, Steenbock and Archie Black discovered that exposing foods to mercury vapour lamps and feeding them to rats with Rickets helped promote their growth and calcium absorption; similar positive effects were observed when rats were directly irradiated. Later, Hess and Weinstock also reported that ultraviolet irradiated cottonseed oil, linseed oil, wheat germ, and lettuce as potent antirachitic agents. Based on these observations, they speculated that it could be considered as a vitamin if the anti-Rickets factor in irradiated foods was comparable to the cod liver oil. This marks the birth of vitamin D, a fat-soluble vitamin. Vitamins A, B and C were already found at that time, so it had to be called D.
Vitamin D is essential for regulating calcium and phosphate metabolism, which play a vital role in maintaining bone health. Inadequate amounts of vitamin D can cause rickets or osteomalacia. Vitamin D has also been associated with other functions such as immune function, cell growth regulation, neuromuscular function and reducing inflammation. However, it is still debatable if a healthy level could help to reduce the risk of non-skeletal disease.
[ihc-hide-content ihc_mb_type=”block” ihc_mb_who=”unreg” ihc_mb_template=”3″ ]A DIY vitamin
Yes, it is a do-it-yourself vitamin as it can be synthesized by our skin upon exposure to ultraviolet-B (UV-B) from the sun. Otherwise, eating fatty fish (salmon, tuna, and mackerel), egg yolks, cod liver oil, mushrooms, and fortified foods such as milk and breakfast cereal could also supply the needed amount. But the majority of vitamin D is “DIY” from skin production in the presence of sunlight and the remaining are obtained from the diet.
How much sunlight exposure do we need?
It is challenging to recommend appropriate sunlight exposure duration due to many factors influencing the cutaneous synthesis of vitamin D. Based on a local study in Malaysia, exposure of the face and hands to sunlight at 11 am for 30 minutes twice a week can help to increase vitamin D levels by 40%. Another study in India indicates that exposure of arms and legs between 10 am and 2 pm for 10 to 30 minutes twice a week is necessary to achieve sufficient vitamin D levels in the Indian skin type. Fair-skinned white individuals may need much less, about 5 to 15 minutes of sunlight twice a week.
Sun exposure is less likely to cause vitamin D toxicity in healthy individuals. However, prolonged exposure to direct sunlight without applying topical sunscreen can cause sunburn and increase skin cancer risk.
Factors affecting the cutaneous synthesis of vitamin D
Latitude, season and time of the day: UV-B radiation between 40° North and 40° South is generally adequate to synthesise vitamin D in the skin throughout the year. However, the UV-B radiation is limited during the winter-time beyond this range of latitudes. Thus, people who reside in this location need additional intakes from food and supplements to maintain sufficient vitamin D level. Besides, the time of the day also plays a crucial role in influencing the cutaneous synthesis of vitamin D. UV-B radiation is generally low in the early morning and late evening. Therefore, vitamin D production in the skin is lesser if exposed during this period.
Clothing style: Clothing acts as a protector between the ultraviolet and the skin. The clothing style related to religious beliefs and cultural practices could be a risk factor for low vitamin D levels. Muslim women wearing religious and cultural dresses such as hijab, niqab and burkha who cover the entire body surface when going outside can lead to limited sunlight exposure. It has been reported that smaller body surface areas exposed to UV-B, for instance, the face and hands, induced lower vitamin D production in the skin compared with larger skin area exposure. Nonetheless, a recent review study indicates that partial skin exposure such as 10% to modest doses of UV may contribute to or maintain a healthy vitamin D status. Yet these findings need careful interpretation as other possible confounding factors such as the skin type could not be considered.
Skin pigmentation: Melanin is a pigment formed by melanocytes in the skin that helps to protect the skin from UV radiation and gives colour to the skin. The electromagnetic radiation in the visible light and UV spectrum was absorbed by melanin. Individuals with dark skin (higher melanin contents) require an extended sun exposure period to synthesise an equivalent amount of vitamin D compared to those with fair skin.
Ageing: Ageing has been reported to be associated with lower vitamin D levels. Ageing can compromise the epidermis’ ability to produce vitamin D. Besides, limited mobility or less time spent in outdoor activities may also contribute to the lower levels of vitamin D in older adults.
Glass window and other factors: As glass absorbs UV-B radiation, sunlight passing through glass, plexiglass, and plastic is less likely to contribute to vitamin D production in the skin. Other factors such as sun-protective behaviour, air pollutants and cloud cover can affect the cutaneous synthesis of vitamin D.
How much dietary vitamin D do we need?
The Malaysian Recommended Nutrient Intake (2017) classifies the recommended dietary vitamin D intakes for healthy individuals into several age groups. Babies from 0 to 12 months need 10µg of vitamin D per day. Children from the age of 1 year and adults, including pregnant and lactating mothers need 15µg of vitamin D per day. Older men and women aged 65 years and older need 20µg of vitamin D per day.
Vitamin D deficiency is prevalent in tropical countries despite the abundance of sunlight received throughout the year. The prevalence of vitamin D deficiency in Southeast Asia varies from 6 to 70%. This could be due to indoor jobs, clothing and sun protection behaviour.
Take home message …
Adequate amounts of vitamin D can be obtained through sensible exposure to sunlight at the right time of a day along with outdoor physical activity and by moderate inclusion of vitamin D rich foods in the diet. Sensible sun exposure can be described as not being over-exposed to sunlight, which could lead to sunburn, as this is the main cause for skin cancer (melanoma and non-melanoma). However, vitamin D supplementation may be needed for individuals at risk of vitamin D deficiency, such as confined indoors and older adults. The intake of supplements should always be based on dietary or medical advice, as excess vitamin D from supplementations may lead to vitamin D toxicity, marked by hypercalcemia, which may persist for a prolonged period of time leading to serious health consequences.
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