According to google trends, “social distancing” is one of the most frequently searched terms on the net as people seek clarity regarding global recommendations facing the COVID-19 pandemic. In fact, data shows that the term has reached peak popularity mid-March in a matter of 2 weeks (1). Social distancing is applied at different levels across the world, from strong recommendations to keep a social distance of 2 meters, to imposing strict home confinement or only allowing an hour of exercise outside. That being said, the general message is “stay home”, stay home to flatten the curve and allow health professionals to do their job and save lives.
While staying home is our best weapon against Covid-19 today, we have to be conscious of the nutritional and health impacts of prolonged confinement (should this be the case) a reality that defies every good habit that we have instilled in our minds and in those of our children. With limited time outside, one concern that is less obvious than weight gain or mental health is Vitamin D insufficiency due to reduced sun exposure. Compared to overweight and obesity which affect around 45% (2) of the global population, about 50% has Vitamin D insufficiency (3); however, far from the awareness and popularity of the obesity issue, the importance of Vitamin D and the risks associated with its insufficiency are less known.
So what is Vitamin D and why is it important? can Vitamin D go beyond maintaining good health and actually play a protective role when it comes to COVID-19? And finally, how can we make sure we are meeting our Vitamin D requirements during these peculiar times?
Vitamin D in a nutshell
Vitamin D is referred to as the sunshine vitamin for the simple reason that it is primarily synthesized by the skin through sun exposure. Vitamin D can also be obtained through diet however dietary sources of this vitamin are generally not sufficient to cover daily requirements. For this reason, Vitamin D supplementation is often prescribed, especially in winter months. To know more about Vitamin D metabolism, see the summary diagram below
Why is Vitamin D important?
The primary role of Vitamin D is the maintenance of bone health by regulating the absorption of calcium and phosphorous, both essential for bone formation and maintenance. Therefore, consequences of vitamin D insufficiency (serum levels between 20 ng/ml and 29ng/ml) include bone loss, osteoporosis and a high risk of fractures. Vitamin D deficiency (serum levels lower than 20 ng/ml) leads to bone disease such as rickets and osteomalacia.
Beyond bone metabolism, Vitamin D has been investigated for its role in immunity. If we go back in time to the beginning of the 20th Century, we can see that the link of Vitamin D with immunity is not a new discovery. Indeed, concentrated light irradiation—later explained by the action of Vitamin D—was used to cure lupus, and oral vitamin D supplementation to cure leprosy (4). Today, further work is being conducted to describe the role of Vitamin D in immunity based on the fact that VitD receptors were found to be expressed on most proliferating immune cells (5). Epidemiological studies do show positive associations of vitamin D deficiency with the severity of diseases such as tuberculosis, sepsis, influenza as well as with overall mortality. This link to immunity is further supported by a recent review (6) showing a strong inverse relationship between Vitamin D levels and the development of autoimmune diseases. Some clinical trials have shown an improvement in disease outcomes with megadoses of Vitamin D and although further supportive clinical research is needed, there is no doubt that data available today is promising (5,6)
Can Vitamin D play a protective role against Covid-19?
If the effect of Vitamin D on the immune response is increasingly supported by the scientific community, the big question is, could it play a protective role in the fight against COVID-19?
The most recent pooled analyses examining data from different prospective studies on Vitamin D and respiratory tract infections conclude that supplementation is associated with a reduced risk of respiratory tract infections (7,8). Furthermore, vitamin D is more protective in participants with lower Vitamin D status and when supplementation was given at lower doses daily or weekly (equivalent to 800-2000 IU per day) rather than less frequent bolus doses (8).
There is no data suggesting that Vitamin D is protective against covid-19 infection specifically given the novelty of this virus and how little we know about it today. Nevertheless, symptoms of covid-19 infection are primarily manifested in the respiratory tract and reports show that people with compromised immunity are at greater risk of contracting and dying from the virus. Given the growing body of evidence pointing towards a potential role of Vitamin D in strengthening the immune system, Vitamin D seems to be one of these nutrients we should not ignore in the COVID-19 era we are witnessing.
How can we maintain good levels of Vitamin D while staying home?
If the world was following its usual course, we would be preparing for the summer: long walks in the sun, days at the beach or the pool, gathering on terraces of restaurants and bars, and our only concern would be NOT to forget to wear sunscreen. However, this is not today’s situation and so maybe we need to include our vitamin D discipline into our routine and that of our children. The question is how?
Whether you can go out for a short walk, spend some time in your backyard or on your balcony, or even watch the empty streets from your apartment window, a regular exposure of bare skin (without sunscreen) to sunlight for 5-15 minutes a day, around midday is believed to suffice for the production of Vitamin D, without exposing the skin to the harmful effect of UVA and UVB (9). However, it is important to understand the factors modulating Vitamin D production from sunlight in order to stay safe and adapt sun exposure accordingly. Some of these factors are:
– Location: The further we are from the equator the less likely we are to receive UVB light in the winter and consequently to produce Vitamin D from the sun.
– Season and time of day: In general, we are more likely to get exposure to UVB light in the late spring and summer months. The most favorable times are around midday. Again, these change according to location.
– Age: Older individuals have less vitamin D precursor in the skin and might be less efficient at producing vitamin D as well. This results in Vitamin D levels in older adults (over 65) being one fourth of those in their 20s according to the Harvard Medical School (10).
– Skin colour: melanin, the natural pigment present in the skin, absorbs UVB, making it less available for the Vitamin D precursor found in the skin. This competition is the reason why dark skin requires longer sun exposure than light skin to produce the same amount of Vitamin D.
– Use of sunscreen: an important part of our summer activities, sunscreen is there to protect our skin from burns by blocking UVA and UVB rays. This of course should reduce the production of Vitamin D by the skin as was shown in clinical trials, although some research shows that wearing moderate SPF sunscreen does not completely prevent this process from happening (11). As with everything, it is important to find a balance between allowing the body to synthesize vitamin D through sun exposure and keeping our skin safe by protecting it against skin cancer.
The recommended intake from diet or supplements (RDA) of Vitamin D is 600 IU (International Units) for children (> 1 year) and adults, 800 IU for adults over 70 years; and the maximal recommended intake (Upper limit) to avoid toxicity is 2500-3000 IU in children and 4000 IU in adults (12).
Although diet alone is generally not sufficient to cover daily requirements, some sources can contribute greatly to complement the vitamin D we produce from sunlight and are part of a healthy diet. They include oily fish, salmon being the richest source followed by mackerel and herring; cod liver oil; egg yolk; and fortified foods. Fruits and vegetables are not good sources of vitamin D except for some mushrooms (portabello mushrooms, chanterelles and other UVB treated mushrooms). Fortified food products are a good way of increasing our Vitamin D intake and depending on local regulations, milk, orange juice, and breakfast cereals are commonly fortified foods.
Changes in lifestyle involving desk jobs, sedentary behavior, screen time and an overall reduction of time spent outside, are thought to drive the high rates of Vitamin D insufficiency observed globally. As we are getting less and less Vitamin D from the sun and knowing the benefits of vitamin D on health, practitioners are prescribing supplementation of up to 1000 IU of vitamin D per day. If we add our current lockdown context to the equation, supplementation can play an essential role in preventing hypovitaminosis D (low blood vitamin D levels) in a situation of prolonged home confinement. On the other end of the spectrum, and although vitamin D toxicity is rare, overusing Vitamin D supplements is the only plausible way (in healthy individuals) of reaching blood levels of Vitamin D considered toxic (150 ng/ml). While research shows that acute toxicity can only be reached with doses higher than 10,000 IU per day, a prolonged intake of 4000 IU per day can lead to chronic toxicity (13). Hypervitaminosis D (vitamin D toxicity) results in a buildup of calcium in the blood, symptoms including vomiting, weakness, and can lead to bone pain and kidney stones.
To take home…
Vitamin D, like the sun, is one of these things we take for granted until we no longer have access to it. When we consider the high global rates of insufficiency, the data showing its protective role in immunity, and the sudden reduction of our time outdoors imposed by the covid-19 pandemic, it becomes obvious that getting sufficient Vitamin D should be an important part of the recommended strategy to maintain a healthy lifestyle and strengthen our immunity. Although there is no data to date that shows any effect of Vitamin D —or any dietary intervention for that matter— on the outcomes of covid-19 infection rates, severity or mortality, reports have shown that the most vulnerable individuals are those with underlying conditions and a compromised immune system. With this in mind, responsibly exposing our skin to sunlight when possible and consuming vitamin D rich foods or supplements can ensure that we contribute to the maintenance of a healthy immune system at a time when it is needed most.
Amira Kassis, PhD