You should supplement with at least 400 IU of vitamin D every day. And possibly a lot more. Not because there is strong evidence for vitamin D supplementation reducing COVID-19 risk. There isn’t. But because having sufficient vitamin D is essential for overall health. Which is what really matters. Especially during a pandemic.

There has been a lot of chatter about vitamin D since the pandemic began, and many online health ‘experts’ have been recommending vitamin D supplements for the prevention and/or treatment of COVID-19. But, as The National Institute for Health and Care Excellence (NICE) recently concluded in a review of the evidence, there is insufficient evidence to support this.

More specifically NICE found:

• No evidence for the effectiveness of vitamin D supplementation for the treatment of COVID-19
• No evidence for the effectiveness of vitamin D supplementation for the prevention of SARS-CoV-2 infection
• Some associations (correlations) between vitamin D status and COVID-19 cases and outcomes, which could not be untangled from other variables linked to vitamin D status and COVID-19 risk (and even if they could, they wouldn’t show causation)

That’s pretty clear cut. Future well-designed studies may demonstrate independent associations and causality, but for now there is not much to go on.

However, current evidence does point towards a possibility that sufficient vitamin D may be protective against COVID-19. Certainly, there is no indication that it increases risk. And, given the importance of vitamin D for health in general (and musculoskeletal health in particular), why not supplement with it? This seems to be the view of the NHS, who are currently sending free vitamin D supplements to the ‘clinically extremely vulnerable’, and recommending a daily dose of 400 IU.

In fact, the NHS (informed by NICE) recommends that everyone over the age of 4 should take 400 IU of vitamin D every day during the autumn and winter months. This has nothing to do with COVID-19, it’s an attempt to ensure that most people remain sufficient throughout the year. Between 29% and 40% of the UK population are vitamin D deficient in winter. In the summer this drops to 2-13% (due to increased sun exposure).

The question is, is 400 IU/day during autumn and winter really enough? I’m not convinced it is. 

On the one hand, the Vitamin D and Health report published by the Scientific Advisory Committee on Nutrition (SACN) in 2016 – used as the basis for the NICE/NHS advice – set the Recommended Nutrient Intake (RNI) for vitamin D, for everybody 4 years and older, at 400 IU/day year-round. The reason for this is to “protect the most vulnerable groups in the population and to take account of variable exposure to sunshine and diet”. This RNI, then, seems like a sensible baseline to start from.

(I’m not sure why NICE reduced the recommendation to 400 IU/day during the autumn and winter months only. Perhaps they thought it would be more doable by more people, and something is better than nothing.) 

To be clear, the RNI of 400 IU/day includes the vitamin D from food and supplementation. But, given that current vitamin D intakes from food are very low (just over 100 IU/day for adults, and less than 100 IU/day for children), highly variable, and can’t be known with any accuracy, we should use the 400 IU/day as a supplementation dose.

Is there a case for taking more than 400 IU/day? I think there is, for most people. The RNI from SACN is adequate for 97.5% of the population to achieve a vitamin D concentration in the blood (plasma, to be precise) of 25 nmol/L or more. Why 25 nmol/L? This is the threshold below which the risk of musculoskeletal problems (such as rickets, osteomalacia, muscle weakness, and falls) increases substantially. SACN found insufficient evidence to support causal relationships between vitamin D status and other aspects of health.

So, 400 IU/day is the bare minimum. Just enough to stop us falling to pieces. It’s certainly not going to be the optimum amount for most people.

Well, what is? I can’t answer that because it’s going to be influenced heavily by genetics, health status, diet, and sunlight exposure, among other things. But consider these points:

• The target range for serum vitamin D (25-Hydroxy Vitamin D or 25(OH)D) in an NHS blood test is 60-150 nmol/L. That’s a lot higher than the 25 nmol/L SACN target
• The Institute of Medicine in the United States defined a minimum target of 50 nmol/L serum 25(OH)D, and an RDA (their version of the RNI) of 600 IU/day for ages 1-70 years, and 800 IU/day for ages 71 years and older
• The Endocrine Society Task Force on Vitamin D defined deficiency as being below 50 nmol/L serum 25(OH)D, but ‘to maximise the effect of vitamin D on calcium, bone, and muscle metabolism’, serum 25(OH)D ‘should exceed 75 nmol/L’, requiring at least 1000 IU/day of vitamin D for children (ages 1-19 years) and 1500-2000 IU/day for adults
• Vitamin D intakes of up to 2000 IU/day for children ages 1-10 years and 4000 IU/day for adolescents and adults are completely safe, unless you have a medical disorder that pre-disposes to hypercalcaemia (including: normocalcaemic hyperparathyroidism; granulomatous diseases such as sarcoidosis and tuberculosis; and genetic pre-disposition such as occurs in IIH)

Why is the RNI from SACN so low? In coming to their conclusions, SACN took account of all the data above. The RNI of 400 IU/day is just enough for the vast majority of people, while being completely safe for everybody in the country, even people with the conditions listed above and those taking medication which may interact with higher doses of vitamin D. Public health advice is a difficult balancing game, and I think SACN have done a thorough job here. (If you want to understand vitamin D metabolism and its influence on health, the SACN report is a great place to start.)

So, what should you do? At the very least, take the RNI of 400 IU/day of supplemental vitamin D year-round. This is absolutely essential for the health of your muscles and bones, and probably many other areas of your health. If you want to get your vitamin D levels in the optimum range, and give yourself a comfortable buffer against vitamin D deficiency (and you don’t have a health condition or medication that may negatively interact with higher doses), you may want to consider taking more than 400 IU/day. Either way, you should get your vitamin D status tested (with your GP or a private lab) so you can be more targeted.

Supplementing with Vitamin D may or may not help to protect you against COVID-19, but it is crucial for your health in general.