FREQUENTLY ASKED QUESTIONS
WHAT IS VITAMIN D DEFICIENCY?
SCIENTIFIC EXPLANATION
Vitamin D deficiency is defined as a low blood concentration of 25-hydroxyvitamin D (25(OH)D), the marker used to assess vitamin D status.
Clinically, deficiency is typically described when levels fall below around 25 nmol/L.
Vitamin D contributes to normal biological processes, including the normal function of the immune system, normal muscle function, maintenance of normal bones and teeth, normal absorption and utilisation of calcium and phosphorus, normal blood calcium levels, and has a role in the process of cell division.
HOW COMMON IS VITAMIN D DEFICIENCY?
Vitamin D deficiency (serum 25(OH)D < 25 nmol/L) and low vitamin D status (< 50 nmol/L) are common worldwide.
More than half of the global population is estimated to have vitamin D levels below what is considered adequate, particularly during winter at higher latitudes.
Read more about the winter gap
WHO IS MORE LIKELY TO HAVE LOW VITAMIN D?
Lower levels are more commonly seen in people who spend little time outdoors, live at higher latitudes, have darker skin, are older adults, regularly cover most of their skin or use sun protection, or have limited dietary intake of Vitamin D.
WHY IS LOW VITAMIN D COMMON TODAY?
Vitamin D is mainly produced when skin is exposed to ultraviolet B (UVB) sunlight. Modern lifestyles, including indoor work, limited daylight exposure, seasonal changes, clothing, and sun protection, can reduce the amount of UVB reaching the skin, even in sunny regions.
Read more about this in The Sun Within
HOW DO I KNOW IF I AM VITAMIN D DEFICIENT?
There are no reliable symptoms that can confirm Vitamin D deficiency. Many people with low levels feel completely well. The only reliable way to know is through a blood test that measures serum 25-hydroxyvitamin D.
HOW CAN I GET VITAMIN D IN THE WINTER?
At higher latitudes, there is insufficient UVB radiation during winter months to support vitamin D production in the skin. As a result, vitamin D intake relies more on dietary sources (such as oily fish), fortified foods, and supplementation to maintain adequate 25(OH)D levels.
CAN YOU GET VITAMIN D THROUGH GLASS?
No. You cannot produce vitamin D through glass.
Vitamin D is made in the skin when it is exposed to ultraviolet B (UVB) light from the sun. Standard window glass blocks UVB rays, even though it still allows visible light and warmth to pass through.
This means that sitting by a sunny window or in a car does not lead to meaningful vitamin D production.
To produce vitamin D from sunlight, direct exposure to outdoor UVB is required.
CAN YOU GET VITAMIN D ON A CLOUDY DAY?
Vitamin D is produced in the skin through exposure to ultraviolet B (UVB) radiation. Cloud cover can reduce the amount of UVB reaching the skin, but it does not block it completely. As a result, some vitamin D synthesis can still occur, although at a reduced rate compared to clear-sky conditions.
CAN YOU GET VITAMIN D FROM A TANNING BED?
Some tanning beds emit ultraviolet (UV) light that can stimulate vitamin D production in the skin. However, this depends on the type of tanning bed.
Vitamin D is produced when the skin is exposed to UVB radiation. While certain tanning beds emit UVB, many are designed primarily to deliver UVA, which does not effectively produce vitamin D.
Because of the associated risks—particularly increased risk of skin damage and skin cancer—tanning beds are not recommended as a way to improve vitamin D levels.
Safer and more reliable approaches include:
- Sensible sunlight exposure outdoors
- Dietary sources and fortified foods
- Supplementation where appropriate
In clinical and public health guidance, tanning beds are not considered a suitable or safe method for maintaining vitamin D status.
WHAT IS THE BLOOD TEST FOR VITAMIN D?
Vitamin D status is assessed by measuring the concentration of 25-hydroxyvitamin D (25(OH)D) in the blood.
This is the primary circulating form of vitamin D and the standard marker used to evaluate vitamin D status.
DOES VITAMIN D BLOOD TEST REQUIRE FASTING?
No, you do not usually need to fast before a vitamin D blood test.
Vitamin D status is assessed by measuring circulating 25-hydroxyvitamin D (25(OH)D) in the blood. This marker is not significantly affected by short-term food intake, so fasting is generally not required prior to testing.
Practical note
Always follow any specific instructions provided by your healthcare provider, as fasting may be required if other blood tests are being done at the same time.
HOW LONG DOES IT TAKE TO RECOVER FROM A VITAMIN D DEFICIENCY?
The time required to correct vitamin D deficiency varies depending on baseline 25(OH)D concentration, intake (sun exposure, diet, or supplementation), and individual physiology.
Vitamin D status reflects circulating 25(OH)D, which changes gradually over time rather than immediately, so restoration of adequate levels typically occurs over weeks to months rather than days.
HOW MANY IU IS 25mcg OF VITAMIN D?
25 micrograms (mcg) of vitamin D equals 1,000 IU.
Conversion rule:
1 mcg vitamin D = 40 IU
So:
25 mcg × 40 = 1,000 IU
FOODS HIGH IN VITAMIN D?
Vitamin D is found in relatively few foods. Most dietary intake comes from a small number of natural sources, alongside fortified products.
NATURAL FOOD SOURCES OF VITAMIN D
- Oily fish — salmon, mackerel, sardines, herring, anchovies
- Cod liver oil
- Egg yolks
- Liver (especially beef liver)
- Some mushrooms (particularly those exposed to UV light)
FORTIFIED FOODS (vary by country):
- Fat spreads and margarine
- Milk and dairy products
- Plant-based milks (e.g. oat, almond, soy)
- Breakfast cereals
- Yoghurt and dairy alternatives
- Some orange juices
Oily fish provides the highest natural amounts. However, even with a balanced diet, it can be difficult to obtain sufficient vitamin D from food alone—particularly in northern latitudes—so fortified foods and supplementation are often used to help maintain adequate levels.
HOW QUICKLY DOES VITAMIN D WORK FOR FATIQUE?
Vitamin D status is reflected by blood levels of 25-hydroxyvitamin D (25(OH)D), which change gradually over time.
While low vitamin D status has been associated with a range of health conditions, there is no defined timeframe in which vitamin D intake leads to improvements in symptoms such as fatigue. Changes in 25(OH)D concentrations typically occur over weeks to months, depending on baseline levels and intake.
CAN I TAKE VITAMIN D WITH LEVOTHYROXINE?
Levothyroxine absorption is quite sensitive. It is best taken on an empty stomach, with water, and separated from other supplements or food because many substances can reduce how much is absorbed.
Vitamin D itself does not directly interfere with levothyroxine in the way calcium or iron can. However, most vitamin D supplements are taken with food (to aid absorption), and some formulations may include calcium or be taken alongside other supplements that do interfere.
Practical guidance:
- Take levothyroxine first thing in the morning, on an empty stomach
- Wait at least 30–60 minutes before eating
- Take vitamin D later, ideally with a meal that contains fat
- If your vitamin D supplement also contains calcium, leave a 4-hour gap
This approach protects levothyroxine absorption while still allowing effective vitamin D uptake.
If your dosing schedule is unusual (e.g. nighttime levothyroxine), the same principle applies—keep them separated.
CAN I TAKE VITAMIN D WITH IRON?
Yes, you can take vitamin D and iron together.
There is no meaningful direct interaction between the two that would make them unsafe or significantly reduce each other’s absorption. Vitamin D is fat-soluble and absorbed via lipid pathways in the intestine, while iron (particularly non-heme iron) is absorbed through separate transport mechanisms. These systems operate independently in the gut.
That said, there are a few practical considerations:
> Iron absorption is more sensitive to timing and context. It is best absorbed on an empty stomach or with vitamin C.
> Iron absorption can be reduced by calcium, tea, coffee, and high-fiber foods.
>Vitamin D absorption is improved when taken with a meal containing fat.
So while taking them together is fine, the “optimal” way depends on your priority:
> If iron deficiency is the main issue → take iron separately (empty stomach or with vitamin C).
> If convenience matters → taking both together with a meal is acceptable, just not maximised for iron absorption.
SHOULD YOU TAKE MAGNESIUM WITH VITAMIN D?
Magnesium is involved in enzymatic processes that relate to vitamin D metabolism, including the conversion of vitamin D into its active forms. However, routine co-supplementation is not universally required. Adequate magnesium status can usually be maintained through diet, and supplementation decisions depend on individual intake and nutritional status.
HOW MUCH MAGNESIUM IS NEEDED FOR VITAMIN D FUNCTION?
Magnesium is involved in enzymatic processes related to vitamin D metabolism, including its activation. However, there is no established magnesium intake level defined specifically for vitamin D function. Instead, recommended intakes are based on general nutritional requirements, typically around 300 mg/day for women and 350 mg/day for men.
HOW CAN YOU GET MAGNESIUM FROM FOOD?
Magnesium is widely available in everyday foods, particularly nuts and seeds (e.g. almonds, pumpkin seeds), whole grains (e.g. oats, brown rice), green leafy vegetables (e.g. spinach, kale, broccoli), legumes (e.g. beans, lentils), fish and seafood, and cocoa and dark chocolate. These foods form the primary dietary sources of magnesium in European populations.
HOW MUCH MAGNESIUM DO PEOPLE GET DAILY FROM THEIR DIET IN EUROPE?
Average intake is around 250–300 mg/day for women and 320–440 mg/day for men. Some people may not consistently meet recommended intake levels.
HOW CAN YOU INCREASE YOUR INTAKE EASILY?
Adding a small handful of nuts or seeds (around 30 g) each day is one of the simplest ways to increase magnesium intake. Wholegrain foods like sourdough bread, along with oats and vegetables, can contribute over the course of the day.
HOW TO RAISE VITAMIN D LEVELS QUICKLY?
Vitamin D levels don’t increase overnight. Because vitamin D is stored and processed gradually in the body, it typically takes several weeks to raise blood levels in a meaningful way.
The most effective way to increase vitamin D levels is through a combination of:
- Consistent daily intake — from supplements or fortified foods
- Taking vitamin D with a meal containing fat — to support absorption
- Safe sunlight exposure — when available, as UVB light triggers vitamin D production in the skin
For many people, especially in northern latitudes or during winter, sunlight alone is not enough, so supplementation can help maintain and improve levels.
Higher intakes may be used in some cases to correct low levels, but this should be guided by a healthcare professional.
In practice, improving vitamin D status is a gradual process, with levels typically rising over weeks rather than days.
ARE THERE FOODS THAT INHIBIT VITAMIN D ABSORPTION?
Vitamin D is absorbed in the small intestine alongside dietary fats. While certain dietary components (such as very high fibre or fat-binding compounds) can influence fat absorption in general, there is no consistent evidence that typical foods significantly impair vitamin D absorption under normal conditions.
Absorption is more dependent on overall digestive function and the presence of dietary fat than on specific inhibitory foods.
WHY DOES VITAMIN D CAUSE TINNITUS?
There is no strong evidence that vitamin D causes tinnitus.
Tinnitus (ringing in the ears) is relatively common and can fluctuate over time, so it may appear to coincide with starting a supplement without being directly caused by it.
In rare cases, very high intakes of vitamin D can raise calcium levels in the blood. When this happens, a range of symptoms can occur, and changes in hearing have occasionally been reported. This is typically associated with excessive doses rather than standard supplementation.
Some research has explored the opposite relationship—whether low vitamin D levels are linked to hearing issues—but findings are still evolving.
If you notice tinnitus after starting vitamin D:
- Check the dose you are taking
- Review any other supplements taken at the same time
- Speak to a healthcare professional if symptoms persist