Do You Have a Vitamin D Deficiency?

Lilli Link, MD
Medically Reviewed
March 5, 2019

Vitamin D deficiency is extremely common and research is showing that it may be associated with a variety of conditions. Here’s how low vitamin D affects you and what you can do about it.

Decades ago, no one was really concerned about vitamin D. It was expected that we were getting sufficient amounts and generally unknown what would happen if we didn’t. Most of our ancestors spent their days outdoors, exposed to sunlight with little protection, which is essential for vitamin D production. Today, we spend most of our days crammed into dark offices, bundled up when it’s cold, or slathering on sunscreen when we’re exposed. This coupled with the fact that there aren’t very many foods high in vitamin D could be the reason an estimated one billion people have low vitamin D .

Over the past few years, studies exploring low vitamin D levels have uncovered the adverse effects vitamin D deficiency can have on our bodies—even attributing to some non-skeletal conditions doctors see everyday, making it more important to test and monitor these levels.

At Parsley Heath, over 90 percent of our patients have a vitamin D deficiency. But what does this even mean? How does vitamin D affect our bodies and how can we make sure we’re getting enough? Luckily, if you are deficient, it is possible to naturally increase your levels with foods high in vitamin D and vitamin D supplements .

What is vitamin D?

Vitamin D, aka the sunshine vitamin, is a fat-soluble vitamin, meaning it’s absorbed by the body through fatty tissue and then stored in these tissues or the liver. It initially comes from exposure to natural sunlight, as your body relies on the sun’s UV light to produce the vitamin in our skin. Unlike most vitamins, vitamin D functions more like a hormone, where every cell in your body has a receptor that allows it to absorb the mineral. It’s also less commonly found in some foods.

What does vitamin D do?

One of the most important functions of vitamin D is its role in maintaining and building bone strength. It allows your body to absorb calcium from your gut and kidneys, helping to strengthen your bones. This process can only happen when vitamin D is present, so while vitamin D levels typically vary due to things like the season, time of day, or where you live, it’s crucial that you have enough of it in your body. Vitamin D is also known to support your immune system , reduce inflammation , and improve some chronic conditions like heart disease or depression.

Vitamin D deficiency symptoms

If you spend most of your time indoors, eat very little fish or dairy , or live at a latitude above 37 degrees north or below 37 degrees south (i.e. further from the equator) you are more likely to have a vitamin D deficiency. You may have one without realizing it, as low vitamin D symptoms are often subtle or go unnoticed.

Common low vitamin D Symptoms

  • Low immunity and getting sick often
  • Depression
  • Excessive fatigue
  • Hormonal imbalance
  • Low bone mineral density

The link between vitamin D and overall health

So how much do we really need to get the maximal benefit? Evidence to date points to optimal levels resting above what most doctors recommend. While there aren’t results from randomized studies (yet) to establish that level, we can turn to observational studies to look at the relationships between vitamin D levels and our overall health. Some of the studies on vitamin D levels is contradictory, but highlighting the connections in these studies is important to understanding its potential effects.


One study that pooled together the results of 11 observational studies  showed that levels of vitamin D greater than 47 ng/mL were associated with a 50 percent reduction in risk for breast cancer. Four different observational studies of colorectal cancer showed that the people with the highest vitamin D levels (27-33 ng/mL, depending on the study) were less likely to die compared with those who had the lowest levels. (13-25 ng/mL). However, more comprehensive studies are needed to draw larger conclusions on this impact, as another study found that higher circulating vitamin D levels were not associated with cancer incidence or mortality.

Autoimmune Disease

Many, though not all, studies show an increased risk of various autoimmune diseases in those with lower levels of vitamin D. One particularly striking study found that for every 4 ng/mL decrease in the level of vitamin D, there was a 19 percent increased risk of developing multiple sclerosis. The highest levels in these studies are not actually that high compared with people who get consistent sun exposure, but even so, are higher than what some doctors think is adequate for good health.

Type 2 Diabetes

Studies have linked vitamin D deficiency with an increased risk of developing type 2 diabetes. Of a study of health adults, higher levels of vitamin D were found to greatly reduce the risk of developing diabetes or prediabetes, as participants with levels above 30 ng/mL had one-third the risk and participants with levels above 50 ng/mL had one-fifth the risk. Though more research is needed on this relationship, it is a promising foundation for vitamin D benefits.


The connection between vitamin D deficiency and depression is an interesting one considering the impact of sunlight on vitamin D levels. Since your body needs sunlight to produce vitamin D, and levels of depression are often increased in lower-light months, there is some evidence that depression could be linked to low sun exposure and therefore low levels of vitamin D. Studies like this one have found that people with lower vitamin D levels were more likely to be depressed.

Heart Health

An Ohio University study found that in individuals with disorders that strain the cardiovascular system, like hypertension and diabetes, vitamin D-3 can help rebuild the strength of endothelial cells—cells that form the lining of your blood vessels and are vital to cardiovascular function. While more research is needed, there is no known system that restores endothelial cells after cardiovascular stress, so the chance that something as accessible as vitamin D could be effective is an exciting discovery.

Dementia and Alzheimer’s

While there is still limited research on the connection between dementia and vitamin D levels, some studies have shown an increased risk of developing different types of dementia, including Alzheimer’s. One study found that those with moderately low vitamin D levels (between 25 and 50 ng/mL) had a 53 percent higher chance of developing Alzheimer’s while those with severe deficiency (less than 25 ng/mL) were shockingly 125 percent more likely.

What to look for in a vitamin D supplement

While some researchers disagree that vitamin D supplementation is an effective way to trigger these benefits, it is widely agreed upon that vitamin D serves as marker of overall health, meaning maintaining adequate levels is essential.

There are two types of vitamin D that are important, including:

  • Vitamin D2 (ergocalciferol): Found in plant foods, namely some mushrooms.

  • Vitamin D3 (cholecalciferol): Found in animal food sources like fish and egg yolks.

If you shop for a vitamin D supplement, you will most likely will find D3 rather than D2. That’s because D3 is the one that does a better job at keeping your blood level up and has been associated with decreased mortality.

Final thoughts on vitamin D deficiency

Vitamin D is clearly a major player in health and disease, so it is no surprise that scientists are finding receptors for vitamin D (where the vitamin attaches to cells and produces an effect) not only in bone cells but also in immune cells and cancer cells. This helps explain why the benefits of higher levels of D are seen in bone health, autoimmune disease and cancer.

At Parsley Health, we screen all of our members’ vitamin D levels and then decide if they need additional supplementation with a high-quality vitamin D supplement  or more time spend in the sun.

Ready to take control of your nutrition? Schedule a call  to learn more about Parsley’s root-cause approach, saving on Parsley , and how we can personalize your health journey.

Lilli Link, MD

Dr. Lilli Link is a board-certified Internist and Functional Medicine Practitioner who graduated from medical school at the University of Chicago, and completed her residency at Columbia Presbyterian Medical Center in New York.

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