The Must-Know Facts On Supplementing & Testing Vitamin D

Mirthe Precision Health
8 min readOct 17, 2023


Why you may not be getting enough even if you are supplementing, why it matters, and how to fix it

You may have felt it over the weekend. Autumn has all of the sudden arrived, with temperatures plunging down to 10 degrees during the day. After an extended warm and sunny autumn thus far with plenty of occasions to load up on sunshine and vitamin D, we now have to wrap up and likely many will cosy up inside, rather than spending time outside.

And with our bodies needing sunrays to make vitamin D, now is the time to get your vitamin D supplement out of the drawer, and make sure to get back onto a daily routine of taking it. You are already on it? That’s wonderful! However, chances are you are still not getting enough to reap all of its benefits.

How come?

Listen to it on apple podcast, or watch on youtube.

The National Guidelines recommend 10mg of vitamin D supplementation per day for adults and children over 4, and 8.5–10mg for children from 1–4 years of age. They also recommend not getting more than 4000 IU/ day for adults and children from age 11, and children aged 1 to 10 years should not have more than 50 micrograms (2,000 IU) a day. Infants under 12 months should not have more than 25 micrograms (1,000 IU) a day (1).

However, this amount will likely only get you into the low end of the normal vitamin D range on blood testing, technically getting you out of ‘deficiency mode’, but far from ‘optimal’, still missing out on many of its wonderful health benefits.

I see this often in my patients that come in for blood tests for health optimisation. They already take a supplement they picked up somewhere, mostly including anywhere between 500–2000 IU/ day. During blood work with their GP, their vitamin D levels show as ‘fine’, and off they go from their GP’s office.

Sadly, ‘normal lab ranges’ for vitamin D are usually anywhere from 25–200 nmol/l, some labs in the UK even calling any vitamin D above 50 nmol/l too high and resulting in your GP telling you to stop or reduce your intake.

Yet research suggests that to get all the best protection from breast and colon cancers, autoimmune disease like Multiple Sclerosis, and many other chronic issues,, we actually want to get to a range of 100–150 nmol/l (2).

Your blood levels of vitamin D for optimal health should be 100–150 nmol/l!

To get up to that level, most individuals need around 4000–5000 IU per day throughout the winter months.

And even during summer, if one mostly stays out of the sun, slops on sunscreen at any given moment one might encounter any sunrays to prevent skin ageing, the same dose may be needed to get and keep one at those optimal levels even over summer. In particular in individuals with darker skin, more of the UVB rays needed for vitamin D production are absorbed by melanin in the skin, and don’t make it through, leaving darker skinned individuals at even a more increased risk of vitamin D deficiency.

I generally recommend testing once in a while to not over or under supplement, as everyone responds slightly different to supplementation (3).

Testing vitamin D in blood is a good way to make sure you are not over- or under- supplementing, but are getting optimal amounts.

However, make sure to add K2!!

One note of caution, in particular at that higher level of supplementation, is to make sure to take a combination of vitamin D with K2.

While vitamin D helps to increase calcium absorption (good for our bones!), it is the vitamin K2 that will tell the body where to safely deposit this calcium — ie our bones and teeth, rather than unwanted and potentially dangerous calcifications in joints, arteries and more (4).

But what’s all the fuss, and why is vitamin D so important?

Research suggests that vitamin D deficiency is linked to increased susceptibility towards a whole range of ill health effects, including:

  • Low bone density and brittle bones, such as osteopenia and even osteoporosis
  • Brittle teeth
  • Cardiovascular disease, including high blood pressure, strokes and heart attacks
  • Autoimmune disease, including Multiple Sclerosis, Hashimoto’s and others
  • Cancers
  • Infections such as acute and chronic viral and bacterial disease, including influenza, but also the often overlooked HPV risk in women, lingering Epstein Barr virus, long covid, and more
  • Depression and other mood disorders
  • Infertility
  • Sex hormone imbalances

As such, adequate vitamin D has been shown to both

  • Lower estrogen when it is too high, which is often found in young women suffering from PMS, endometriosis, cysts, fibroids, painful periods and more.
  • Increase estrogen when it is too low, such as in post menopausal women (and some women when still in their reproductive years, in particular very skinny women).
  • It even has been linked to improved fertility, much like progesterone.

Some of the most common symptoms of vitamin D deficiency

Some of the most common and well known symptoms of vitamin D deficiency are fatigue, bone loss (osteopenia or even osteoporosis) and a weak immune system, catching any cold that goes around. However, there are some more symptoms that have been linked to low vitamin D levels in research, but are often overlooked, including:

  • Insomnia and difficulties sleeping (5).
  • Muscle twitches
  • Depression and low mood
  • Hormone issues such as PMS, perimenopausal symptoms including hot flashes, via vitamin D’s balancing effect on the sex hormones estrogen and progesterone (6, R).
  • An increased risk of heart disease, high blood pressure and obesity (7).
  • An increased risk for cancers, such as breast and colon cancer, and autoimmune diseases, such as Multiple Sclerosis (8).

How quickly can you become deficient in vitamin D?

You have topped up on sunshine this summer, and are now wondering if that may be enough to get you through the winter months?

The the body is indeed able to store some vitamin D in our fat tissue. Research suggests that after supplementation of a few years, once stopped there will still be some vitamin D released from that storage the following year (9).

Another study shows that loading up on sunshine and vitamin D stores over summer can prevent vitamin D levels from dipping to extremely low levels over winter (10).

However in order to keep vitamin D levels in blood optimal for cancer and autoimmune prevention, bone health and overall best health outcomes, supplementation over the winter months is crucial, unless you are jetting off to spend the winter somewhere in the sun.

Can’t you top up your vitamin D levels through diet?

Mushrooms that are exposed to sunlight can produce some vitamin D2, which can help us increase vitamin D levels. However, us humans need a different version, D3, which we produce upon sun exposure, and we first need to covert the D2 from mushrooms once eaten, making this less efficient.

Foods that contain some vitamin D3 are:

  • Eggs
  • Liver, such as cod liver oil
  • Meat and fish

However, despite these foods containing some vitamin D, foods alone are usually insufficient to keep our vitamin D levels at an optimal level for best health outcomes and supplementation in winter is imperative for optimal health.

Is a supplement the easiest way to keep vitamin D levels topped up?

Yes. Ideally one also gets some sunshine for its many other health benefits beyond just vitamin D production (more on this in this blog post from last year), but in order to maintain vitamin D levels at an optimal level, most people in the UK would benefit from regular supplementation.

Should you take vitamin D every day?

In order to get up to that level, most individuals benefit from supplementing with around 4000–5000 IU/ day throughout the winter months (or equivalent weekly doses, ie 28.000–35.000 IU if preferring to take it once per week).

And even during summer, if one mostly stays out of the sun, slops on sunscreen at any given moment one might encounter sunshine, and in particular with darker skin, the same dose may be needed to get and keep one at those optimal levels. I generally recommend testing once in a while to not over or under supplement, as everyone responds slightly different to supplementation (11).

If you have darker skin, you have more melanin, so less UV light gets absorbed to create vitamin D3. You will need more sun exposure to produce vitamin D3 than those individuals with lighter skin.

If you spend time outside during autumn and winter do you still need to take a vitamin D supplement?

In the UK, and at similar latitudes, summer midday sunlight contains enough UVB for vitamin D synthesis (if one goes outside and gets exposed to sunshine without wearing sunscreen, which blocks the needed UVB rays!) — while the weaker sunlight of winter provides only a negligible amount of vitamin D synthesis.

Research suggest that in order to remain sufficient year-round levels, a relatively high circulating level of vitamin D should be achieved by the end of the summer through sun exposure to take us through to spring. which often is not the case.

The same research also shows that, white Caucasians across the UK need nine minutes of daily sunlight at lunchtime from March to September for blood vitamin D levels to remain ≥25 nmol/L throughout the winter. This assumes forearms and lower legs are exposed June-August, while in the remaining, cooler months only hands and face need be exposed. Exposing only the hands and face throughout the summer does not meet requirements (12).

However, as specified above, the 25 nmol/l in most cases are not enough to promote optimal health and prevention. I therefor usually recommend to my patients to supplement, even if they are getting some UK winter sunshine, to keep their immune system and bones strong throughout the winter months, while testing their levels in blood at least 1x/ year.

What’s the best time a day to take a vitamin D supplement?

While most research suggest that any time of the day, as long as it easy for you to remember, is fine, there are some recent scientific papers hinting that earlier in the day may be better, and ideally with a meal.

Our bodies have a natural circadian rhythm, that dictates when during the day specific hormones get released, and vitamin D levels appear to be highest around noon (13), which overlaps with when we naturally would be exposed to the most sunshine and UV light, which is how we get the most vitamin D. Getting vitamin D during the day may also act as a primer for better melatonin secretion at night, which is crucial for getting a good night’s sleep (14, 15, 16).

However, there currently is no clear research on whether taking vitamin D at night time can work just as well.

Research also suggests that absorption of vitamin D appears to be the best when taken with the main meal of the day (17), which ideally, for a healthy metabolism, would correlate with lunchtime also.

Did you find this helpful? Let me know in the comment below or send me an email at, I would love to hear your thoughts!

Are you looking for a bespoke health strategy to take the guesswork out of your health? Functional Medicine is a lab & science based approach to help to achieve & maintain optimal health.

I currently see patients both

📍 In person in Kensington, London (UK),

👩🏼‍💻 And worldwide online via zoom.

Email me at or check out my website for more details on how I work if you areinterested in booking in an initial consultation ❤️.




















Mirthe Precision Health

Functional Medicine Practitioner. Hormones, epigenetics, health optimisation. IG/ TikTok/ Youtube/ Fb: @mirthe_precisionhealth,,