About half the world’s population suffers from a deficiency of Vitamin D, including some Canadians, whose reserves fall short during a long, grey winter.

“Most of the vitamin D we get, especially during Canadian summers, comes from sun exposure and is more than adequate,” says Philip Britz-McKibbin, a professor in the department of Chemistry & Chemical Biology.

His latest research explores the need for more personalized interventions to improve health outcomes for critically ill Vitamin D-deficient children who are prone to respiratory infections, sepsis and poor clinical outcomes during hospitalization.

“The challenge is that we burn through those summer stores during the long winter months when there is inadequate UV-B exposure from sunlight. Canadians may also be Vitamin D-deficient even in the summer if working indoors, wearing sunscreen and avoiding sun exposure.”

Here, he offers insights on why Vitamin D matters and how to make sure you’re getting enough.

What is Vitamin D and what role does it play in the body?

Vitamin D refers to a group of related fat-soluble vitamins that are key to maintaining healthy bones. The two main forms are Vitamin D2 and Vitamin D3; the latter is produced naturally in the skin when it is exposed to adequate sunlight and is also more commonly found in animal-based foods and supplements.

Vitamin D is best known for preventing nutritional rickets, a condition historically associated with the curvature of the spine and bowed legs in affected children.

We saw this particularly during the peak of industrialization in late 19th-century England when child labour was prevalent in dirty, sunless cities.

Severe deficiency can lead to similar malformations in adults, known as osteomalacia, which also increases the risk of bone and joint pain and muscle weakness.

In recent decades, the roles of Vitamin D3 beyond musculoskeletal health have become more apparent. Vitamin D is sometimes referred to as a pleiotropic prohormone, meaning it has many functions beyond just bone mineralization.

One of the most important roles, especially during winter, is its impact on immune function and inflammation. There is a well-documented inverse correlation between respiratory illnesses and mortality risk in winter in northern latitudes, which mirrors Vitamin D status.

Studies have shown that adequate Vitamin D levels are associated with a lower risk of acute respiratory infections that can be prevented by optimal Vitamin D supplementation. Poor Vitamin D status is also associated with higher risk for other chronic health conditions ranging from autoimmune disorders, cancer and depression.

How can people tell if they are deficient?

There aren’t always obvious symptoms for mild deficiency, but one possible clue is getting sick more often — particularly with respiratory infections.

Chronic inflammation may also be a sign, though it can result from many causes and is often associated with various chronic diseases and abnormalities. It’s unclear whether the disease itself causes lower Vitamin D levels or if the deficiency precedes the disease.

Weak bones, muscle pain, poor wound healing and mood changes may also coincide with Vitamin D deficiency; however, these various symptoms are not reliable nor specific. For these reasons, a more objective assessment, like a blood test, is needed to measure circulating levels of Vitamin D, especially for high-risk groups including pregnant women, children and individuals with prediabetes.

What does excessive vitamin D look like?

Excessive intake of Vitamin D3 supplements may lead to symptoms such as nausea and vomiting. It can also cause elevated calcium levels, increasing the risk for kidney injury if left untreated. This typically only happens with extremely high levels are consumed over prolonged periods, often due to adulterated Vitamin D products containing concentrations much higher than labeled.

Overdosing from diet alone is also unlikely, since fortified foods contain low levels by design. That’s why many physicians take a cautious approach to supplementation. While this helps prevent toxicity, it may fall short of supporting optimal immune function and long-term health outcomes on an individual level.

Interestingly, sun-derived Vitamin D3 generation does not carry the risk of toxicity and provides many other health benefits. The body has a natural mechanism to prevent the over-accumulation of sun-derived Vitamin D3, making sun exposure the best source when possible.

How can people ensure they are getting enough vitamin D – especially in winter?

Fortunately, Vitamin D deficiency is a modifiable risk factor that can be addressed through sensible sun exposure, dietary habits and supplements. The body produces Vitamin D3 naturally when skin is exposed to UV-B radiation from the sunlight, but in colder months, northern climates, overcast days or due to lifestyle changes, that’s not always possible.

Dietary sources include oily fish, like salmon or mackerel, and fortified foods in Canada, such as milk and other dairy products. Natural food sources are limited, which is why supplements are often necessary, especially in the winter.

Current recommendations for Vitamin D intake are still quite conservative. They typically recommend as low as 400 international units (IUs) per day for adults under 65 years old, which might be adequate to prevent osteomalacia but not enough to promote immune health, which often requires higher levels.

Some studies suggest that significantly higher daily intakes, up to 4,000 IU, may be more effective. But recommendations vary and should be tailored to the individual.

What is next in your research into Vitamin D nutritional interventions?

New advances in precision nutrition highlights that “a one-size-fits all” approach to dietary interventions is rarely effective for everyone.

Our recent paper, in collaboration with the Children’s Hospital of Eastern Ontario, tackled this issue by examining the impact of a large single dose of Vitamin D3 to rapidly correct Vitamin D deficiency in pre-screened critically ill children.

Despite providing a similar weight-normalized dose of Vitamin D3, we found large differences in treatment responses due to variations in how the body absorbs and  metabolizes Vitamin D3 after ingestion.

Nevertheless, most children’s Vitamin D levels were restored within three days using a large single dose, compared to daily low dose maintenance therapy of Vitamin D3 as a standard care of practice.

Although this strategy was needed in a critical care setting, it is not optimal for most individuals who benefit more from daily or weekly dosing regimens.

Work is underway in a multicentred to examine if rapid normalization of Vitamin D status improves clinical outcomes following pediatric critical illness since it represents a simple, cost-effective and safe intervention in vulnerable Vitamin D-deficient children.