“Is it true that your body can make vitamin D from sunlight?” “Which foods are rich in vitamin D?” “What is the difference between vitamin D2 and D3?” “And how much is 1,000 IU in micrograms?”
Once you start looking into vitamin D, it quickly becomes clear that it behaves a little differently from many other vitamins.
Vitamin D is a fat-soluble vitamin involved in the use of calcium and phosphorus and in normal bone formation.
One of its most distinctive features is that vitamin D does not come only from food. Your skin can also produce vitamin D when it is exposed to ultraviolet B radiation, or UVB.
This means vitamin D status is influenced not only by how many micrograms you eat, but also by factors such as season, sunlight exposure, age and lifestyle.
Supplement labels can add another layer of confusion. You may see “Vitamin D3 1,000 IU” on one product and “25 mcg” on another, while some supplements contain D2 and others D3.
In this guide, we explain what vitamin D is, what it does in the body, its relationship with sunlight, the difference between D2 and D3, vitamin D-rich foods, daily intake, factors associated with low vitamin D status, 25(OH)D blood testing, age-related considerations, IU and mcg conversion, supplement selection and safety.
Contents
What is vitamin D?
Vitamin D belongs to the same group of fat-soluble vitamins as vitamins A, E and K.
Unlike water-soluble vitamins such as vitamin C and the B vitamins, vitamin D is absorbed along with dietary fat.
The term “vitamin D” actually refers to a group of related compounds. The two forms most relevant to nutrition are:
- Vitamin D2 (ergocalciferol)
- Vitamin D3 (cholecalciferol)
Vitamin D is also unusual because it does not have to come entirely from food. When UVB radiation reaches the skin, the body can produce vitamin D3.
Did you know? Vitamin D can be made in the body
Vitamins are generally known as nutrients that must be obtained from the diet because the body cannot make enough of them on its own.
Vitamin D is a little different because the skin can produce it with the help of sunlight.
That makes vitamin D one of the few nutrients where both diet and environmental exposure need to be considered.
What does vitamin D do in the body?
One of the best-known roles of vitamin D is its relationship with calcium and phosphorus.
In Japan, the authorised nutrition function statement for vitamin D explains that it:
helps promote calcium absorption in the intestinal tract and supports bone formation.
Vitamin D participates in the absorption of calcium and phosphorus in the intestine and in mechanisms that help regulate their levels in the body.
Bone is not formed simply by eating more calcium.
Calcium intake is one part of the process, while vitamin D and other nutrients and hormones are involved in how calcium is absorbed and used.
A useful way to think about vitamin D is therefore as a nutrient that helps support the body’s normal use of calcium.
What is the difference between vitamin D2 and D3?
Several forms of vitamin D exist, but D2 and D3 are the two most important from a nutritional perspective.
| Form |
Full name |
Typical sources |
Used in supplements? |
| Vitamin D2 |
Ergocalciferol |
Mushrooms and other fungal sources |
Yes |
| Vitamin D3 |
Cholecalciferol |
Fish, other animal-derived foods and skin synthesis |
Widely used |
D2 and D3 are both vitamin D
Both D2 and D3 contribute to vitamin D status in the body.
However, human studies have generally found that vitamin D3 tends to raise and maintain blood 25(OH)D concentrations more effectively than vitamin D2.
For this reason, when comparing supplements it can be useful to check not only whether the label says “Vitamin D”, but also whether the product provides D2 or D3.
Did you know? Mushrooms and fish usually provide different forms
Vitamin D2 can be formed when ergosterol in mushrooms and fungi is exposed to ultraviolet light.
Vitamin D3, on the other hand, is found in foods such as fish and can also be produced in human skin from 7-dehydrocholesterol.
So two foods described as “sources of vitamin D” may actually contain different forms of the vitamin.
Vitamin D and sunlight
Sunlight is one of the features that makes vitamin D particularly unusual.
When ultraviolet B radiation reaches the skin, 7-dehydrocholesterol in the skin begins a process that ultimately produces vitamin D3.
The amount produced is not the same every time you go outside.
Factors that influence vitamin D production from sunlight include:
- season
- latitude
- time of day
- cloud cover and weather
- how much skin is exposed
- skin melanin levels
- age
- use of sunscreen
How many minutes of sunlight are needed for vitamin D?
A common question is, “How long do I need to be in the sun to make enough vitamin D?”
There is no single number that applies to everyone.
The amount of time required can vary substantially depending on location, season, time of day, weather, clothing, exposed skin area and skin pigmentation.
For that reason, it is not possible to give one universal recommendation such as “10 minutes a day” for every person.
UVB exposure in summer can be very different from winter exposure, and conditions can vary considerably between northern and southern regions.
Rather than deliberately aiming for long periods of sun exposure, it is more practical to consider normal outdoor activity together with diet while also keeping sun protection in mind.
Did you know? Sunlight through a window is different
Most ordinary window glass blocks a large proportion of UVB radiation.
So sitting beside a bright window is not the same as having sunlight reach exposed skin outdoors when it comes to vitamin D production.
At the same time, this does not mean longer sun exposure is always better. Ultraviolet radiation also affects the skin, so vitamin D should be considered together with appropriate sun protection.
One of the most interesting things about vitamin D is that the form produced in the skin or absorbed from food is not immediately the final active form used by the body.
Vitamin D goes through several metabolic steps.
1. Vitamin D3 is produced in the skin
UVB exposure initiates the conversion of 7-dehydrocholesterol in the skin, eventually producing vitamin D3.
2. The liver converts vitamin D to 25(OH)D
Vitamin D is transported to the liver, where it is converted into 25-hydroxyvitamin D, or 25(OH)D.
3. Further conversion occurs mainly in the kidneys
25(OH)D is then further metabolised into active forms of vitamin D that participate in normal physiological processes.
In simple terms:
sunlight → skin → liver → kidneys and other tissues → biologically active forms
Did you know? Vitamin D is not “finished” the moment sunlight reaches your skin
It is common to hear that “sunlight makes vitamin D”, which is true, but the process does not end in the skin.
The vitamin still needs to go through metabolic steps in the liver and kidneys before it is used in its active forms.
Regular vitamin D supplements and prescription active vitamin D are different
The vitamin D2 and D3 commonly found in dietary supplements should not be confused with active vitamin D medicines used under medical supervision.
Ordinary vitamin D supplements are nutritional products, while active vitamin D preparations are prescription medicines used for specific medical purposes.
If you are taking an active vitamin D medicine, speak with your doctor or pharmacist before adding an over-the-counter vitamin D supplement.
What is 25(OH)D?
When reading about vitamin D, you will often see:
25(OH)D
25-hydroxyvitamin D
This is a vitamin D metabolite produced in the liver and is commonly used as a blood marker for assessing vitamin D status.
25(OH)D reflects vitamin D coming from:
- food
- supplements
- vitamin D produced in the skin
This makes it more informative than simply asking how much vitamin D you ate yesterday.
Blood test results should be interpreted with the reference range
25(OH)D may be reported in either ng/mL or nmol/L.
Definitions of deficiency, insufficiency and adequate status are not always identical across organisations and clinical guidelines.
For this reason, it is better to interpret your result together with the laboratory reference range and advice from your healthcare professional rather than relying on one number found online.
How much vitamin D do we need each day?
According to the Dietary Reference Intakes for Japanese (2025 edition), the adequate intake for vitamin D is 9.0 mcg per day for adults aged 18 and over.
The adult tolerable upper intake level is 100 mcg per day.
| Group |
Adequate Intake |
Upper Intake Level |
| Men aged 18+ |
9.0 mcg/day |
100 mcg/day |
| Women aged 18+ |
9.0 mcg/day |
100 mcg/day |
| Pregnancy |
9.0 mcg/day |
– |
| Breastfeeding |
9.0 mcg/day |
– |
The dash in the pregnancy and breastfeeding rows means that a separate upper intake value is not listed in that table. It does not mean that high-dose vitamin D can be used freely during pregnancy or breastfeeding.
Because vitamin D can also be produced in the skin, Japanese dietary guidance notes that sunlight exposure should be taken into account when considering vitamin D.
In other words, vitamin D cannot be understood simply by comparing dietary intake with a single daily number.
How much vitamin D do adults in Japan get from food?
According to Japan’s 2024 National Health and Nutrition Survey, average dietary vitamin D intake among adults aged 20 and over was 6.9 mcg per day across men and women combined.
This figure represents average intake from food.
Because vitamin D can also be produced in the skin, this number alone cannot tell us whether an individual has adequate vitamin D status.
Diet, outdoor activity, season and age all need to be considered together.
Foods rich in vitamin D
Vitamin D is found naturally in food, but it is not evenly distributed across the food supply.
Some of the richest natural sources are fish and certain mushrooms.
| Food |
Example serving |
Vitamin D |
| Grilled chum salmon |
80 g |
31.2 mcg |
| Grilled sockeye salmon |
80 g |
30.4 mcg |
| Grilled eel |
80 g |
15.2 mcg |
| Grilled Pacific saury |
100 g |
13.0 mcg |
| Canned mackerel in water |
90 g |
9.9 mcg |
| Stir-fried wood ear mushrooms |
30 g |
11.4 mcg |
| Stir-fried maitake mushrooms |
30 g |
2.3 mcg |
Vitamin D content can vary depending on species, production method, exposure to light and cooking method.
Fish is an important dietary source of vitamin D
Fish provides vitamin D3, and salmon, eel, Pacific saury, sardines and mackerel are among the better-known dietary sources.
Among plant foods, mushrooms are particularly notable because they can provide vitamin D2.
Because vitamin D is not abundant in all common foods, knowing which foods contribute meaningful amounts can make it easier to review your usual diet.
Vitamin D absorption and fat
Vitamin D is fat-soluble.
Vitamin D from food is absorbed along with dietary fat in the digestive system.
For supplements, studies suggest that taking vitamin D with a meal or snack containing some fat may support absorption.
This is one way vitamin D differs from water-soluble nutrients such as vitamin C and many B vitamins.
Its fat-soluble nature also means that vitamin D can accumulate in the body.
That is why supplement dosage matters and why more is not automatically better.
Who may be more likely to have low vitamin D status?
Vitamin D status depends on more than dietary intake alone.
Sunlight exposure, absorption and metabolism all play a role.
Factors that may affect vitamin D status include:
- limited sunlight exposure
- spending most of the day indoors
- low intake of vitamin D-rich foods
- older age, when the skin’s ability to produce vitamin D declines
- higher levels of melanin in the skin
- medical conditions that affect fat absorption
- liver or kidney conditions that affect vitamin D metabolism
This is why it is not always helpful to assume that someone is getting enough vitamin D simply because they eat fish occasionally or spend some time outdoors.
A more complete view considers diet, sunlight, age and overall health together.
What happens with significant vitamin D deficiency?
More pronounced vitamin D deficiency can affect calcium utilisation and bone metabolism. In children, severe deficiency can contribute to rickets, while in adults it can contribute to osteomalacia.
These are medical consequences of vitamin D deficiency and should be distinguished from everyday nutritional supplementation.
If deficiency is suspected, medical assessment including measurement of 25(OH)D may help clarify current vitamin D status.
Does vitamin D become more important at different ages?
Japan’s adequate intake for adults remains 9.0 mcg per day across adult age groups, but lifestyle and physiology change with age.
Younger adults
People who spend most of the day indoors for work or study may have relatively little daylight exposure.
A diet that includes little fish may also provide fewer natural dietary sources of vitamin D.
Women around menopause
Around menopause, changes in hormone levels often lead women to think more about bone health.
Vitamin D is one part of that picture, together with calcium, protein, physical activity and overall nutrition.
Many studies have examined vitamin D supplementation and bone-related outcomes in postmenopausal women, but results can differ depending on baseline vitamin D status, dose, study population and other factors.
For this reason, it is more useful to consider diet, exercise, sunlight exposure and medical assessment when appropriate rather than focusing on a single nutrient alone.
Older adults
The skin becomes less efficient at producing vitamin D with age.
Older adults may also spend less time outdoors, further reducing opportunities for skin production.
Changes in appetite and food intake can also affect dietary vitamin D intake, making both diet and lifestyle worth reviewing.
Vitamin D, calcium and bone
Vitamin D is often discussed in relation to bone, but bone health depends on many factors.
These include:
- calcium
- phosphorus
- protein
- vitamin D
- vitamin K and other nutrients
- physical activity
- hormonal factors
Within this larger system, vitamin D plays an important role in supporting calcium absorption and utilisation.
For a broader look at bone health, see our related article on osteoporosis below.
IU vs mcg: how much is 1,000 IU?
International vitamin D supplements often use labels such as:
Vitamin D3 1,000 IU
while Japanese dietary reference values are expressed in micrograms, or mcg.
The two units can be converted:
1 mcg = 40 IU
1 IU = 0.025 mcg
| IU |
Equivalent in mcg |
| 400 IU |
10 mcg |
| 1,000 IU |
25 mcg |
| 2,000 IU |
50 mcg |
| 4,000 IU |
100 mcg |
Did you know? 1,000 IU sounds large, but it equals 25 mcg
The number “1,000” can look very large on a supplement label, but for vitamin D, 1,000 IU equals 25 mcg.
Likewise, Japan’s adult upper intake level of 100 mcg is equivalent to 4,000 IU.
When comparing supplements, converting IU and mcg into the same unit makes the numbers much easier to understand.
Reading a real product label
One product available from Brilliant Life Products NZ is Health Life Coenzyme Q10 320 mg + Vitamin D3 1,000 IU, which provides 1,000 IU of vitamin D3 per capsule.
Using the conversion:
1,000 IU × 0.025 = 25 mcg
So the vitamin D3 amount is 25 mcg per capsule.
Understanding this conversion makes it easier to compare international supplement labels with Japanese dietary reference values.
It is still important to look at the full daily intake from food and other supplements rather than judging a product only by the number printed on the front label.
How to choose a vitamin D supplement
1. Start with both diet and sunlight exposure
Unlike many nutrients, vitamin D comes from both food and skin production.
Consider how often you eat fish or mushrooms and how much time you typically spend outdoors during daylight hours.
2. Check whether the product contains D2 or D3
Supplements may contain either vitamin D2 or vitamin D3.
Both are forms of vitamin D, but they differ in origin and in how strongly they may affect blood 25(OH)D concentrations.
3. Convert IU and mcg before comparing products
Remember that 1,000 IU and 25 mcg represent the same amount of vitamin D.
Converting labels to the same unit helps avoid misleading comparisons based only on the size of the printed number.
4. Check the total daily serving
Look at the amount per capsule or tablet as well as the recommended number of servings per day.
If you use more than one supplement, check whether vitamin D is also present in a multivitamin or another product.
5. Choose a format that fits your routine
Vitamin D products are available as capsules, tablets, liquids and other formats.
Along with dosage, consider serving size, format and other ingredients when comparing products.
Safety, excess intake and medication considerations
The adult upper intake level in Japan is 100 mcg per day
Japan’s Dietary Reference Intakes (2025 edition) set an adult tolerable upper intake level of 100 mcg per day for vitamin D.
That is equivalent to 4,000 IU per day.
The upper intake level is not a target.
For everyday supplementation, it is better to follow the product directions and consider vitamin D from all supplements you use.
Vitamin D is fat-soluble, so dosage matters
Because vitamin D is fat-soluble, it can accumulate in the body.
Very high intakes over a prolonged period can raise blood calcium levels and lead to health problems.
This is why vitamin D supplementation should be based on an appropriate dose rather than the idea that more is always better.
Can too much sunlight cause vitamin D toxicity?
Vitamin D toxicity is not generally caused by ordinary sunlight exposure.
Excess vitamin D is more commonly associated with prolonged use of very high-dose supplements.
This does not mean unlimited sun exposure is safe. Ultraviolet radiation has its own effects on the skin, so sun exposure should always be considered together with appropriate sun protection.
If you take medication or are receiving medical treatment
Vitamin D can be relevant to the use of certain medicines and to conditions involving calcium metabolism, kidney function or the parathyroid glands.
If you are receiving treatment, have kidney-related concerns, have been advised about abnormal calcium metabolism, or take prescription medication, speak with a doctor or pharmacist before adding a vitamin D supplement.
Summary
Vitamin D is a fat-soluble vitamin involved in calcium and phosphorus utilisation and normal bone formation.
Key points include:
- vitamin D is a fat-soluble vitamin
- it can come from both food and skin production
- the two main nutritional forms are D2 and D3
- D2 is commonly associated with mushrooms, while D3 is found in fish and produced in the skin
- vitamin D is metabolised through the liver and kidneys
- ordinary D2/D3 supplements are different from prescription active vitamin D medicines
- 25(OH)D is commonly used as a blood marker of vitamin D status
- the adult adequate intake in Japan is 9.0 mcg per day
- average dietary intake among Japanese adults aged 20 and over is 6.9 mcg per day
- the adult upper intake level in Japan is 100 mcg per day
- fish and certain mushrooms are important dietary sources
- season, sunlight exposure and age can influence vitamin D status
- there is no single universal number of minutes of sun exposure suitable for everyone
- the skin’s ability to produce vitamin D declines with age
- 1 mcg = 40 IU
- 1,000 IU = 25 mcg
- when comparing supplements, check the form, dose and total daily intake
Vitamin D is sometimes called the “sunshine vitamin” or associated mainly with bone, but its biology is more complex than either description suggests.
It can come from food or be produced in the skin, and it then goes through several metabolic steps before being used by the body.
For this reason, vitamin D is best considered by looking at diet, sunlight exposure and lifestyle together.
If you choose a supplement, checking D2 vs D3, IU vs mcg, the total daily amount and overlap with other supplements can make product labels much easier to understand.
Related Articles
- What Is Osteoporosis? Causes, Testing, Diet, Exercise and Prevention
Learn more about bone health, including bone density, nutrition and physical activity.
- What Is Magnesium? Roles, Foods, Daily Intake, Deficiency & Supplement Guide
A detailed guide to another essential nutrient involved in bone and mineral metabolism.
- What Is Zinc? Roles, Foods, Daily Intake, Deficiency & Supplement Guide
Learn about zinc-rich foods, absorption, elemental zinc and mineral balance.
- When Is the Best Time to Take Supplements?
A practical guide to supplement timing, including considerations for fat-soluble nutrients.
- How to Combine Supplements: What to Know About Ingredients and Timing
Useful information for checking overlapping ingredients when using more than one supplement.
- What Is Coenzyme Q10? Ubiquinone vs Ubiquinol and How to Choose a Supplement
Learn more about CoQ10, which is included alongside vitamin D3 in one of our current products.
Main References
- Dietary Reference Intakes for Japanese (2025 edition), Ministry of Health, Labour and Welfare
- National Health and Nutrition Survey 2024, Ministry of Health, Labour and Welfare
- National Institute of Health and Nutrition, Japan: Vitamin D
- eJIM, Ministry of Health, Labour and Welfare: Vitamin D
- Standard Tables of Food Composition in Japan
- Japan Ministry of the Environment: UV Radiation and Health Guidance
- NIH Office of Dietary Supplements: Vitamin D – Fact Sheet for Health Professionals
- Systematic reviews of vitamin D supplementation and bone-related outcomes in postmenopausal women
- Systematic reviews and meta-analyses of vitamin D, calcium, fractures and falls in adults
Important Information
This article is intended to provide general nutrition and health information about vitamin D.
It is not intended to diagnose, treat, cure or prevent disease, and it does not claim that any particular food or supplement has therapeutic effects.
If you have a medical condition, are pregnant or breastfeeding, have kidney or calcium-metabolism concerns, or take medication, speak with a doctor, pharmacist or other qualified healthcare professional before adding a vitamin D supplement to your routine.
