Vitamin D Insufficiency in a Tropical Country: A Nutrition Paradox

by Jirayu Tanprasertsuk

As winter approaches, days are getting shorter and nights are getting longer in New England. Around this time, I, as a person who was born and raised in Thailand, can’t help missing the warm climate of my home country where the sun shines all year long. Last summer I was very fortunate to have a chance to talk to Dr. Boonsong Ongphiphadhanakul, MD, one of the authors of a study on vitamin D status among the Thai population and a professor in the Faculty of Medicine at Ramathibodi Hospital in Thailand.

To be honest, I thought this interview would be a boring conversation about happy Thai people who get vitamin D from an infinite amount of sun exposure—yet it would turn out to be 30 minutes filled with fascinating information. Through this article, I want to share my personal learning experience on the paradox of how people in a tropical country may not get enough of this “sunshine” vitamin.

Sunlight as a source of vitamin D

Before delving further into this issue, let’s look at some basic science. Vitamin D is either obtained from food or produced when our skin is exposed to UVB in sunlight. The UVB converts 7-dehydrocholesterol located beneath our skin to vitamin D3. Vitamin D3 remains inactive until it is activated twice by the liver and kidney. The first activation in the liver converts vitamin D3 to 25-hydroxycholecalciferol or 25(OH)D, and the second activation in the kidney converts 25(OH)D to 1,25-dihydroxycholecalciferol or 1,25(OH)2D, which is a biologically active form of vitamin D.

Vitamin D insufficiency is more common than we thought

Clinically, one’s vitamin D status is determined by serum 25(OH)D levels. Clinicians use this value because it reflects the total amount of vitamin D coming from diet, supplements, and sun exposure. The Institute of Medicine considers a person with a serum 25(OH)D level less than 12 ng/mL (or 30 nmol/L) to be “vitamin D deficient”, while a person who has a value less than 20 ng/mL (or 50 nmol/L) is “vitamin D insufficient.” The Endocrine Society uses higher cutoff values for people who are at risk of diseases associated with low vitamin D status. Our body requires vitamin D to maintain normal blood levels of calcium and phosphorus, and to build strong bones. While vitamin D deficiency causes rickets in children and osteomalacia (softening of the bones) in adults, optimal vitamin D level has been shown to be associated with many potential health benefits, including increased bone density, and prevention of fractures and falls.

Global vitamin D status is indeed worse than many people would expect. If we use 20 ng/mL as a cutoff value, prevalence of vitamin D insufficiency in the US is approximately 40 percent. A study in South Korea shows prevalence of vitamin D insufficiency in summer can be as high as 50 percent among teenagers. Dr. Ongphiphadhanakul’s study fortunately shows the prevalence of vitamin D insufficiency for Thailand to be only 6 percent, but the prevalence is unexpectedly high at 15 percent among Bangkok residents. The result also shows that living in a big city, being young, and being female are three important predictors that increase the risk of vitamin D insufficiency among Thai people.

Living in a hot city

As a Bangkokian, I have my own hypothesis of why people who live in big city might not get enough of the “sunshine” vitamin. Given the choice of being outside (and sweating like a waterfall) or staying cool and dry inside with the A/C on, it’s a no brainer to stay away from sunlight and scorching heat. Surprisingly, my hypothesis does not explain everything.

“City lifestyle might just be one of many factors,” Dr. Ongphiphadhanakul said. “If we had a group of healthy people exposed directly to sunlight in Bangkok at noon, we would only see a subtle rise in their serum 25(OH)D. However, when we used a UVB lamp to shine UVB on their skin, their serum 25(OH)D level increased dramatically.”

So what does this little experiment imply? Well, it seems like there is nothing wrong with the ability of skin to convert 7-dehydrocholesterol to 25(OH)D, but there is something wrong with the sunlight in Bangkok. I asked him what the reason behind this was.

“We are not sure just yet,” he admitted. “Right now, our hypothesis is that the air pollution may absorb or scatter UVB in the atmosphere, resulting in less UVB reaching the earth surface. We still need further research to confirm this hypothesis.”

One study in Iran seems to find similar results. If being exposed to sunlight in the city is not going to raise my vitamin D level, why bother staying outside? I guess I had made the right call staying indoor and sticking to my bestie (aka A/C).

Getting high vitamin D, like a grandma

As we grow older, we expect our skin to be less efficient at vitamin D production. Because of this reason, many professionals believe older adults are at increased risk of developing vitamin D insufficiency. This was shown not be the case in Thailand (as well as in South Korea, Malaysia, and Iran). Teenagers and younger adults have lower serum 25(OH)D levels than older adults. Another study also shows the prevalence of vitamin D insufficiency to be at a whopping 45 percent among kids in urban areas of Thailand. The reason remains mysterious.

“The abundant sunlight may overcome the disadvantage of decreased vitamin D production in older adult populations,” Dr. Ongphiphadhanakul explained. “After retirement, they may have more leisure time to spend outdoors, while teenagers and young adults may stay busy with school and work and spend more time indoors.”

Come to think of it, I can totally relate this to my own experience. I went to grade school and middle school in Bangkok, and I can remember spending the entire day, including lunch, indoors. Gym class was only once a week and we often played soccer in the shade of tall buildings. I guess 45 percent is about right then.

Tanning is not a thing around here

Light skin color is a paradigm of beauty in Thailand, and there is an ongoing skin-whitening craze among Thai women. Skin-whitening agents are in sunscreen, skin lotion, facial wash, deodorant, and even vaginal wash. I personally know people who take glutathione just to make their skin color look lighter.

Dermatology clinics can be found on every corner in Bangkok, promising to introduce people to the miracle of “whiter, lighter, spotless” skin. Thai women do everything they can to avoid sunlight, which tans the skin. I asked my female Thai friend what kind of sunscreen she usually uses.

“I use SPF 50++,” she replied. “People rarely use SPF 30 anymore. It is too low for the strong sunlight in this country.”

I admit I did not possess much knowledge on sunscreen, so I did a little research myself. It turns out SPF value refers to the efficacy of UVB blockage. While dermatologists say to avoid UV rays to lower the risk of getting skin cancer, vitamin D researchers promote the benefits of sunlight exposure.

Even though it is hard to determine general guidelines for everyone, some researchers recommend “5 – 30 minutes between 10 am and 3 pm at least twice a week to the face, arms, legs or back without sunscreen.” For me, I believe it is a matter of balancing skin health and bone health.

Sun exposure may not be the best strategy

Dr. Ongphiphadhanakul agreed with the right timing and right location, sun exposure could surely increase vitamin D status. I believe one example would be spending an afternoon lying on the beach on Phi Phi islands. However, for people who are susceptible to vitamin D insufficiency or having severe vitamin D deficiency, sun exposure may not be the best strategy to raise their 25(OH)D level.

“This is where nutrition plays an important role,” he suggested. “Supplementation is inexpensive and effective for treating or lowering the risk of certain diseases. We also don’t quite understand how different forms (epimers) of vitamin D behave in our body. The next step in vitamin D research is how to match the right dose and form of vitamin D to the right population.”

Vitamin D supplementation is shown to be effective for treating osteomalacia, rickets, familial hypophosphatemia, and renal osteodystrophy. However, for healthy general population, Dr. Ongphiphadhanakul insisted a good quality of sun exposure is still the best way to maintain their vitamin D status.

Can fortification protect everyone?

A large portion of the U.S. population relies on fortified foods to meet their Vitamin D needs. Milk, orange juice, and breakfast cereal are commonly fortified with vitamin D in the US. I asked Dr. Ongphiphadhanakul if vitamin D fortification in Thailand is a possible solution to decrease the prevalence of vitamin D insufficiency.

“We currently have no policy of vitamin D fortification because of a popular belief that Thai people still get sufficient vitamin D from sun exposure. And even now that we know this may not be true, we still need to consider two important things before implementing vitamin D fortification policy: the degree of fortification and the food medium.”

I asked him what food he would fortify if he could choose freely. “Some scientists suggest rice because we eat a lot of rice,” he said.

This is very true and it would be my choice as well, since rice is widely consumed in developing countries of Asia. We usually eat rice with entrées that are cooked in vegetable oil, which would be a good mediator for vitamin D absorption in our small intestine, since vitamin D is fat-soluble. There have also been suggestions to fortify rice with iron, zinc, and vitamin A, aka ”triple-fortified rice,” to decrease the prevalence of deficiency of those nutrients. However, the idea of vitamin D fortification is still not widely considered in Thailand.

Vitamin D vs. Vitamin A

“The prevalence of vitamin D insufficiency has now exceeded the prevalence of vitamin A insufficiency in Thailand,” Dr. Ongphiphadhanakul said. But how worried should we be about this situation?

“Compared to vitamin A, we don’t know whether the national burden from vitamin D insufficiency is higher or lower. And this is because we still don’t have enough data on the non-skeletal functions of vitamin D.”

To clarify his opinion, we are now clear about vitamin D‘s classical roles in regulation of calcium homeostasis, maintaining bone health and neuromuscular function. However, recent studies have found associations between vitamin D and its potential roles in regulating blood glucose homeostasis, the autoimmune system, and protection from depression, cardiovascular disease, and certain cancers. But we still do not have sufficient evidence to establish a causal relationship between vitamin D and its non-skeletal actions.

“This is also a reason why vitamin D fortification is unlikely to happen soon,” Dr. Ongphiphadhanakul said. “A high prevalence of vitamin D insufficiency does not mean that it’s a national problem. We want to be cautious about the right amount of fortification before we do anything.” It is hard to get vitamin D intoxication from natural food, but if foods are heavily fortified or overly consumed, there may be harmful consequences.

I should end my vitamin D journey by pointing out the irony of this story. While people who live in the high latitude areas are longing for more summer, some people who live closer to the equator are avoiding the sunlight to the extent that they become vitamin D insufficient. It seems like people are never satisfied, but I digress. City lifestyle, shades of tall buildings and air pollution from urbanization may also contribute to the high prevalence of vitamin D insufficiency, and this may be a sign for Bangkokians to finally take good care of the city’s environment.

For now, I’ll suggest to anyone who is planning a trip to Thailand: do not get allured by the tasty local foods and get stuck in Bangkok. Explore the northern part of the country where you can go whitewater rafting in a rainforest while making friends with elephants, or the southern part where undisturbed beaches are as white as snow. And just keep smiling in the Land of Smiles, because you know you are finally boosting your vitamin D level!

Jirayu Tanprasertsuk, aka Boo Boo, is a graduate student in Biochemical and Molecular Nutrition program. His palms and feet are always yellow because of overconsumption of papayas.

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