5 Factors That May Disrupt Your Body’s Vitamin D Absorption, and What To Do About It

Photo: Stocksy/Elizabeth Rajchart
You've likely heard it before that when the sun is shining and you don't have to be inside, getting out and catching some rays can offer you a valuable nutrient: vitamin D. First, it's imperative to underscore the fact that Cancer.org recommends that you always wear sunscreen even indoors—however, some very minimal, highly-protected exposure to sunlight (as well as consuming vitamin D-rich foods) can offer this key nutrient, which benefits your bones, brain, and immune system.

As someone that loves numbers and specifics (not to mention sun protection), sometimes I find myself wondering just how much time I should spend outdoors each day to get my daily value of vitamin D. Unfortunately, it’s clearly a bit of catch-22: Although sunlight offers vitamin D, it’s also harmful to your health without proper protection like sunscreen, clothing, time in the shade, and overall caution about sunburns, according to the Skin Cancer Foundation. Their experts—as well as the Centers for Disease Control and Prevention—share that some exposure to sunshine can be beneficial to your wellbeing so long as you take the necessary protective precautions for your age, skin type, and health.

Experts In This Article
  • Shengyi Mao, MD, My professional focus is practice innovation and providing high-quality, low-cost preventative healthcare. A major way I work to innovate and improve care is through quality improvement (QI). I lead a variety of primary care QI initiatives such improving hypertension, cancer...

So, how are we supposed to get vitamin D safely? "Our bodies are very efficient at making vitamin D in the skin, but how much varies a lot from person to person due to several factors," says Shengyi Mao, MD, Clinical Assistant Professor of Internal Medicine, The Ohio State University Wexner Medical Center. "This makes it difficult to provide specific recommendations for how much time to spend in the sun. Some folks may be primed to get exactly what they need without extra effort, while others may need to get more in their diet from outside or talk to their doctors about supplementation." Here, we asked Dr. Mao to break down exactly what kinds of factors may determine your body's vitamin D absorption.

What is vitamin D, and why do you need it?

First things first. "Vitamin D is a vitamin that all humans need. It helps our bodies absorb calcium and phosphorus, which are critical for bone health," Dr. Mao says. In kids, she explains that it helps grow healthy bones; in adults, it helps maintain strong bones and prevent fractures or breaks. "But this nutrient is truly a powerhouse of important benefits that reach a lot farther beyond strong bones." In fact, according to Dr. Mao, vitamin D possesses anti-inflammatory, antioxidant, and neuroprotective qualities that support brain health, immune system function, and even prevent things like joint pain, low energy, and depleted moods. In turn, low vitamin D levels in the winter are part of the reason some folks may experience seasonal affective disorder.

How your body gets vitamin D

Vitamin D, unlike a lot of other important vitamins, does not occur naturally in a lot of foods, according to Dr. Mao. In fact, the United States government started to fortify milk with vitamin D in the 1930s as a public health measure against Rickets, a musculoskeletal illness caused by vitamin D deficiency. Since then, more foods have become fortified with the nutrient, including alternative milks, cheeses, cereals, flours, orange juice, snack foods, and more. (For those wondering, a major natural source of vitamin D is fish, including tuna, salmon, flounder, herring, sardines, and tilapia—huge win for sushi fans everywhere.)

Is there a specific amount of time to spend in the sun in order to get your vitamin D requirements?

As mentioned, aside for your diet, it's possible for your body to get its needed vitamin D from the sun. However, this is a bit trickier than you might think: The sun doesn't just shoot out vitamin D to land on your skin and do its thing. Instead, according to Dr. Mao, the UVB rays in sunlight cause a molecular reaction within your skin and fat cells to create the nutrient we know as vitamin D. This nutrient is fat soluble, which means that it is created and/or absorbed and housed in fat cells until it is sent to the liver to be used. Unfortunately, UVB rays are also the very rays from the sun that cause your skin to burn, so it is imperative you wear sunscreen to protect yourself from cancer-causing rays.

All of the above are the reasons that Dr. Mao stresses that the more you know about what can disrupt your vitamin D absorption, the more informed your diet and supplementation decisions can become. Here's how reaping the amount of vitamin D your body needs can naturally get foiled.

5 key factors that can impact your vitamin D absorption

1. Sun protection

Sun protection is important and healthy, even though it also prevents optimal absorption of vitamin D. According to Dr. Mao, things like clothing coverage, sunscreen, and glass prevent you from getting the nutrient from the sun. Even though your drive home from work may be sunny or your desk is placed near a window, UVB rays can't penetrate glass (though UVA rays certainly can) and therefore don't offer the same kind of vitamin D-rich sunlight.

2. The amount of melanin you have in your skin

Melanin is a molecule in the body that determines a person's skin pigmentation. "Vitamin D absorption is reduced as the amount of melanin a person has increases," Dr. Mao says. Black and brown individuals, according to a 2021 study published in the peer-reviewed clinical journal Nutrients, often face a 20- to 25-fold prevalence of vitamin D deficiency.

3. Your age, body composition, where you live, and ability to be outside

Adding to the list of things that can disrupt vitamin D absorption are factors like location, age, outside time, body composition, and location. There are a lot of reasons that someone may not be able to spend a lot of time outside, whether that's risky for dehydration, a time-consuming job, or living in a climate that doesn't offer a lot of sunlight. Because vitamin D you get from the sun is acquired via the skin and housed in fat cells, fat stores can sometimes hoard the nutrient meaning that fat and plus size people can sometimes utilize less sunlight based vitamin D

"Location, weather conditions, seasons, and time of day also make a huge difference on how much ultraviolet light is getting through to the person's skin and can impact vitamin D levels," says Dr. Mao. "For example, vitamin D levels often decrease in the winter due to decreased sun exposure."

4. Certain illnesses and conditions

"Certain illnesses can prevent the metabolism of vitamin D at numerous stops along its journey to getting where it's needed in the body, like kidney and liver disease," Dr. Mao says. Those with type 2 diabetes may have reduced vitamin D absorption, and so do gastric bypass patients. Bariatric patients sometimes risk becoming vitamin D deficient, too, as the portion of their stomach that is removed is the upper small intestine, which is where the majority of vitamin D is typically absorbed in the body.

5. Having a specific diet, like veganism or gluten-free

"One common misconception is that vitamin D can easily be found in foods. It is not naturally occurring in many foods, so it is very difficult to get enough vitamin D in our diets," says Dr. Mao. This is why some folks who follow a gluten-free diet can often miss out on fortified foods like cereals, whole grains, wheat flour, and bread. The same goes for vegan individuals who don't get fortified vitamins from things like dairy or naturally vitamin D-rich foods like fish or eggs.

Another misconception is that more is better, according to Dr. Mao. "In general, it is very safe to supplement vitamin D, but extremely high levels can lead to kidney stones, bone pain, confusion, weakness, vomiting, and more," she says. That's why talking to a provider about your options is a great path forward.

At the end of the day, there isn't one factor or guarantee that you are going to be vitamin deficient forever. In fact, most people likely experience some barrier to getting the vitamin D that they need at some point in their lives. The best thing you can do is stay informed about what your specific roadblocks are. This can help inform your decision to ask for a vitamin D blood test at your yearly physical or to make the decision to eat more vitamin D-rich foods or take vitamin D supplements upon approval from a healthcare provider.

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