Why people with obesity may be vitamin D deficient—and what you can do

Before we get into the weight care bit, let’s talk about vitamin D for a sec. Based on the name, you’d think that it’s, um, a vitamin. But it’s actually a hormone—and it’s an essential one, too. Vitamin D helps your body absorb calcium for strong bones and muscles and plays a role in your nervous and immune systems. Years ago, the U.S. started fortifying grocery items like milk, OJ, and some ready-to-eat cereals with vitamin D to help increase our intake. But even today, deficiencies are common. In fact, nearly 42% of all adults in the United States aren’t getting enough. 

The sun is one of our primary sources of vitamin D. When your skin is exposed to the sun's UV rays, your body naturally produces the vitamin. You can also get it from your diet. (Sidenote here: Vitamin D is fat-soluble— you need fat in your diet to absorb it.) Lack of sun and outdoor activity, along with a less nutritious diet, can increase the risk of being deficient, regardless of weight. However, people with obesity are more prone to vitamin D deficiencies. And here’s why:

The connection between obesity and vitamin D deficiency

  There’s evidence that the more someone weighs, the more vitamin D they need because of what’s called “volumetric dilution” in the body, according to research published in the journal Obesity. That’s a science-y term that means the more someone weighs, the higher their blood volume. Because of this, vitamin D can get spread out and diluted throughout the body. So extra vitamin D may be necessary. 

Another factor: Since vitamin D is a fat-soluble vitamin, those with higher amounts of fat in their body will “sequester” or store this hormone in their fat cells—making it difficult for their body to access it. When this happens, vitamin D doesn’t reach the bloodstream like it’s supposed to—and can’t do its important functions, says Rekha Kumar, MD, Found’s Chief Medical Officer. 

Low vitamin D levels are also a concern for people who have had bariatric surgery. After the procedure, it can be harder to absorb essential nutrients, vitamins, and minerals—including vitamin D. That’s because parts of the intestines where many nutrients are absorbed are bypassed. 

[H2] What happens when you’re deficient? 

One of the main things vitamin D does is help with calcium absorption. If you don’t have enough, it may put you at risk for osteoporosis or osteomalacia—conditions that can lead to body pain, fragile bones, and weak muscles. 

Other issues like low-grade inflammation, depression, high cholesterol, insulin resistance, diabetes, heart disease, and metabolic syndrome have also been linked to vitamin D deficiencies. More research is being done to understand the exact connection. Still, it’s believed that low vitamin D  results from having excess weight, as opposed to low levels of the vitamin causing obesity or weight gain. 

The good news is that studies show that when people lose weight, vitamin D levels tend to increase—though it’s not totally clear how much you need to lose for this to happen.

How much vitamin D do you need?  

Adults 19 to 70 years old need around 600 international units (IU) of vitamin D. However, people with a known deficiency or those at risk may need more. Dr. Kumar says she recommends 2,000-4,000 IU daily to people with obesity to give them a chance to achieve healthy levels yet still be safe (too much vitamin D can be toxic). She might recommend upwards of 5,000-10,000 IU per day for bariatric patients.

Sources of vitamin D 

The three main ways to get your vitamin D are:  

1. Sunlight
   

As we mentioned, the primary way your body gets vitamin D is through sunlight. That’s why it’s called the sunshine vitamin. The amount of sunlight needed varies from person to person and can also depend on where you live (the sunnier your climate is year-round, the more your body will likely make). Skin color, age, and clothing are also factors. Even short bouts of time spent outside can do your body some good - but don’t forget the sunscreen! Research shows that regular sunscreen use may not have as much of an impact on our ability to produce vitamin D as we once thought - as we are more than likely not covering our entire body. According to a study published in the British Journal of Dermatology, “concerns about vitamin D should not negate skin cancer prevention advice.”

 2. Diet

Foods like egg yolks, liver, oily fish like salmon, mackerel, tuna, sardines, and fish liver oils are great sources of vitamin D. And, cereals, orange juice, milk, non-dairy milks, and other dairy products like yogurt are often fortified with vitamin D—just take a peek at nutrition labels to see if a product has it. 

3. Supplements

Because you still might not get enough vitamin D through diet or sunlight alone, a supplement may be your best bet—especially during the winter. There are over-the-counter options, multivitamins, and prescription vitamin D supplements. But don’t head to the store before talking to your doctor because it’s possible to get too much vitamin D. Your primary care physician can do a blood test to see where your levels are and recommend a supplement if needed. 

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