The Truth Behind Sunlight, Vitamin D and Its Various Forms

Portrait of happy girl looking at camera outdoors

Vitamin D is unusual among the vitamins in that the human body can actually produce vitamin D from the sun. More specifically, we produce vitamin D from a chemical reaction that can only occur in sunlight.

Strictly speaking, this synthesis means that vitamin D is not technically a vitamin but is actually a hormone.

Even though vitamin D is synthesized, it is still a critical vitamin, because most people do not get enough of it.

In Western society particularly, a large proportion of the population suffers from vitamin D deficiency, with some estimates suggesting this may be as high as 70% or 80%.

This isn’t really that much of a surprise, as people have a tendency to spend the majority of their time inside, whether this is at work or at home. After all, modern culture is filled with indoor entertainment, like computers, game systems and television.

Furthermore, when people do go outside, they often cover up or wear sunscreen. While sunscreen reduces the risk of skin cancer, it also reduces the ability to synthesize vitamin D from sunlight (1).

Our exposure to sunlight has dramatically decreased due to modern culture and technology

Forms of Vitamin D

Vitamin D occurs in five different forms, which have slightly different structures and different sources. These are numbered D1, D2, D3, D4 and D5. Of these five types,  the form of vitamin D from the sun is known as vitamin D3, and this is also the form that has the most biological significance for humans.

In contrast, vitamin D2 is produced by invertebrates, plants, and fungi in a similar reaction to the one that produces vitamin D3 in humans and other vertebrates.

The exact function of this vitamin within its host species is not known, but vitamin D2 does act in a similar way to vitamin D3 in humans.

There is little concrete research concerning the difference between these two forms of vitamin D, however, it appears that both forms have similar effects at nutritional doses, while vitamin D3 is more effective at high doses (2).

Woman with vitamin D bottle

Vitamin D3 also tends to be more available biologically, resulting in potentially more impact on vitamin D levels in the blood.

Vitamin D within foods tends to be in the D3 form, but fortified food or drink that contains vitamin D may be either the D2 or D3 form, depending on the specific manufacturer.

Likewise, supplements are sometimes in the vitamin D2 form rather than D3.

This makes it important for people to choose their supplements carefully as the vitamin D3 form means that your body gets more of the vitamin – which is critical to the health benefits that it offers.

This means that vitamin D benefits for health are likely to be stronger for vitamin D3 compared to other variants.

Vitamin D3 is the most desirable form of vitamin D

Sources of Vitamin D

The key natural source of vitamin D is sunlight, as this results in a chemical reaction that leads to the production of the vitamin. This would suggest that people naturally get enough vitamin D, but that isn’t actually the case.

Instead, many people are deficient in vitamin D, to the extent that it harms their health (3).

A secondary source of vitamin D comes from food, especially as some milk, cereal, yogurt and orange juice brands are fortified with vitamin D. Other foods also contain vitamin D naturally, such as eggs, poultry, and fish.

Some of the highest sources of vitamin D in food can be seen in the following graph:

Vitamin D in food

Average vitamin D in selected foods. Data from the National Institutes of Health.

Of these two sources, the amount of vitamin D obtained from food is relatively small and this makes sunlight the primary source of vitamin D.

Additionally, most foods supplemented with vitamin D uses the D2 form of the vitamin, which is less biologically active (4).

This makes the practice of food fortification largely ineffective for preventing vitamin D deficiency.

It also means that you won’t really get vitamin D benefits from food alone.

While vitamin D is found in food, the most common and prevalent source is synthesis that occurs in the presence of sunlight

Measuring Vitamin D

There are two different ways of looking at vitamin D within a person.

The first is to look at how much vitamin D they take in and the second is to work out how much vitamin D is in the blood. The amount of vitamin D that a person takes in is typically given in IUs, which stands for International Units.

IUs are international standards for different vitamins and drugs.

They are extremely useful, as they provide a way of comparing the amount of vitamin D across multiple areas.

Out in the sun

However, there are many factors that influence how vitamin D is used within the body.

Because of this, two people might take 1,000 IU of vitamin D, but the impacts on vitamin D in their blood may be dramatically different.

Tests for vitamin D level typically look at the levels of a prehormone known as calcifediol, often abbreviated to 25(OH)D.

This compound is a precursor to vitamin D in the blood and is a good indicator of vitamin D levels.

It is particularly important as we cannot test for vitamin D levels directly.

In general, the levels of 25(OH)D are given using one of two units.

These are:

  • nmol/L
  • ng/ml

Either unit is acceptable, and you will find that some people talk about 25(OH)D levels in ng/mg (which stands for nanograms per milligram) while others will talk about the levels in terms of nmol/L (which refers to the number of mols in a liter – where a mole is a scientific unit of measurement).

The process of converting between nmol/L and ng/ml is a bit tricky as it involves knowing some things about the chemical itself (the conversion is different for each individual compound).

However, a number of websites have provided the required information to make it easier.

Essentially, the equation to go from one to the other is as follows:

vitamin D conversion

Alternatively, the Vitamin D Service website offers a calculator for conversion. Another site offering a similar service is EndMemo.

In this discussion, I’m going to be mostly using the measurement ng/ml as this is more common within the United States. However, in some cases provide the nmol/L, particularly when that was the measurement that the actual study used.

Research into Vitamin D

Interest in vitamin D benefits has been growing within the public sphere and among researchers. As a result, there have been a growing number of research studies into vitamin D levels, the effect of vitamin D and optimal levels of vitamin D.

Doing scientific research

While there is now a wide amount of information available on the subject, there are still many questions that haven’t been answers and many challenges in the discussion of vitamin D in general.

One ongoing issue is accurate assessment of vitamin D in the blood. As I mentioned before, this is done by measuring the level of 25(OH)D.

However, there are significant differences in the processes used to test for levels of this compound.

Because of this, the outcomes of such tests can vary significantly and potentially overestimate or underestimate the level of vitamin D in the blood. For some tests, estimates suggest that they are incorrect more than 40% of the time

A second key challenge is the type of research undertaken.

A large number of studies in the field of health and nutrition tend to be observational. This approach involves collecting data from a large number of people and then using this data to look for trends.

Observational studies do have considerable power because they allow researchers to look for trends across a lot of people.

The main limitation of this approach is that trends don’t necessarily imply cause and effect.

For example, there tends to be a positive trend between height and performance on math problems – but obviously, neither of those variables affects the other. Instead, the real relationship is mediated by age, as people get taller and better at math (in general) as they age.

So, the outcomes of any observational study have to be treated with care.

The results might indicate the possibility of a cause-and-effect relationship, but they do not prove one. Instead, they suggest the need for further research, particularly through experimental studies.

This issue is very significant for the case of vitamin D and health.

Sad thinking girl

Conducting an observational study on the topic can be relatively easy as large data sets from other research often exist – and there are a lot of observational studies that compare various health outcomes with vitamin D levels.

Typically, these studies will look at 25(OH) concentration in the blood serum, but in some rare cases, they will consider supplementation with vitamin D instead. These studies can be interesting, and many of the studies that I will discuss in this article fall in this area.

As I mentioned before, the issue is that this type of study focuses on correlation.

For example, a person who is physically sick may go outside less often and is likely to have lower vitamin D levels as a consequence. Now, an observational study could find that, on average, ​people who have a certain illness (like chronic fatigue, for example) are more likely to be vitamin D deficient. The same study might then argue that vitamin D may play a role in that disease. 

Now, it's true, vitamin D could be significant in the disease. But, the observed trend can also simply be explained by the fact that sick and tired people get outdoors less often.

This issue doesn't make observation studies invalid, far from it.

They are still important because they are relatively easy to conduct and can look at a large sample.

However, the more powerful studies are experimental in nature. These are the studies that tell us the most about vitamin and the role that it has on health. 

For example, this might involve having participants supplement with vitamin D and comparing their outcomes with the outcomes of a group that didn’t supplement.

Some studies also use this structure with multiple different doses of vitamin D. That approach can help researchers pinpoint the optimal levels of vitamin D.

So, some of the research on vitamin D is certainly much more powerful than other pieces of research.

Nevertheless, they all contribute to the overall picture that vitamin D does play a role in promoting health – and that vitamin D deficiency can have many negative impacts on health.

Vitamin D has been a strong focus of research, although there is much still to learn

Vitamin D in Different Populations

Vitamin D is a growing concern in modern times, but research suggests that our ancestors did not have as many issues with getting enough of the vitamin.

Similar outcomes have been found by looking at vitamin D levels in dark-skinned hunter-gatherer tribes from around the equator (actually getting the blood to do so must have been challenging).

One study looked at two groups of people who live traditionally and wear little clothing when they are outside (the Maasai and the Hadzabe). The authors of the study found that the levels of vitamin D in this group ranged from around 58 to 171 nmol/L (5).


Another study looked at the levels of vitamin D in five indigenous populations within East Africa. That study found a mean level of 106.8 nmol/L (6).

Those numbers convert to around 23.2 to 68.4 ng/ml for the first study and 42.8 ng/ml for the second study. It is not clear what these concentrations mean for healthy levels of vitamin D within Western lifestyles.

Nevertheless, the information offers important indications of vitamin D levels in these populations and this information may help to guide recommendations and research.

Certainly, the information is an important indication that levels of vitamin D that naturally occur in dark-skinned populations may be much higher than previously assumed. It also suggests that numbers around 40 to 60 ng/ml may be healthy and may even be levels that our ancestors experienced.

Vitamin D levels in hunter-gatherer tribes tend to be relatively high

How Much is Enough?

For most people, getting enough sunlight to produce the vitamin D levels that they need isn’t a viable option.

There are many reasons for this. For example, people may be too busy, may not have many opportunities to get outside every day, may live in high smog areas or may be housebound.

Additionally, getting vitamin D from sunlight is a bit of a tradeoff, because sunlight exposure increases the risk of skin cancer (7).

This may be one reason why the Institute of Medicine in the United States hasn’t made any formal recommendations about how much sun exposure people should get to obtain the vitamin D that they need. Instead, they have simply focused on ideal levels of vitamin D, although these have been contested.

Any guideline for how much sunlight you should be getting in a day to get the optimal amount of vitamin D is going to depend on how much vitamin D a person needs for their health.

Yet, there is no consensus about what this number actually is.

It's also important to note that the human body is a complex system, with many different interacting components. This means that people may vary considerably in the amount of vitamin D that they need and even in how their body uses the vitamin D that they have.

For example, some research suggests that vitamin D, magnesium and calcium can all interact with one another, which could result in different impacts depending on your levels of each of these three nutrients. Likewise, vitamin K may also interact with vitamin D. ​

The various interactions throughout your body mean that even if researchers could come to a consensus about ideal vitamin D levels, it's likely that those levels wouldn't be suited for everybody. 

Getting Vitamin D From the Sun

The creation of vitamin D occurs as the result of a specific reaction, which occurs in ultraviolet (UV) light. This reaction only occurs at wavelengths of light between 270 and 300 nanometers. This only occurs when the UV index is higher than three.

Girl in the sun in a field

​Now, the wavelengths of available light can vary from one season to the next. Often, in winter, the wavelengths are not sufficient to promote much vitamin D production.

In these seasons, even the people who spend considerable amounts of time outdoors will not be getting enough vitamin D from sunlight.

Sunscreen also plays a role.

When people do get exposed to the sun, they often wear sunscreen to lower their risk of skin cancer.

Unfortunately, sunscreen also blocks out the wavelengths of light involved in the production of vitamin D.

There is also no clear picture of just how much sunlight exposure people need to synthesize sufficient vitamin D.

This is an area that is subject to a large amount of debate, particularly as there are so many different factors that can influence how much vitamin D you produce.

This means that there is no simple answer to the question 'how much sunlight do I need?'.​

For example, some researchers argue that having 5 to 30 minutes of exposure to the sun (without sunscreen) between 10am and 3pm is enough to get sufficient vitamin D (8) and that this includes exposure on the arms and legs.

Another example is the claim that 10 to 15 minutes exposure three times a week is sufficient for vitamin D production (9).

However, other research suggests that this is not the case.

One study on the topic instructed women to get 20 minutes of sun exposure at midday each day for four weeks on their face, forearms and hands. The authors found that this had no significant net effect on the level of vitamin D in blood serum (10).

This may indicate that exposure to the sun needs to be longer to have a significant impact – although the study may also have been influenced by some participants wearing sunscreen.

It's also important to note that individual factors can influence vitamin D production too, such as skin color, age and weight. So, the idea amount of sunlight for one person may not be enough for another.​

Personally, I feel that the idea of needing half an hour of exposure (or less) is inaccurate.

There are many people in the United States (and other developed countries) that do get this level of exposure. If this truly was enough, the prevalence of vitamin D deficiency would be much lower than it actually is.

The site Sunsafe RX offers additional insight into the relationship between the sun and vitamin D production. 

This suggests that a decent amount of sunlight exposure is needed to truly see vitamin D benefits and this may not be practical or possible for everyone. This may mean that getting your vitamin D from the sun may not always be a viable option, especially in the winter.

There is no consensus on how much vitamin D or how much sunlight people need for their health


Gel liquid caps

Vitamin D is a critical nutrient for our overall health and we’re only just starting to realize how significant the vitamin truly is.

For one thing, vitamin D deficiency is associated with a number of health risks and many of the symptoms of this deficiency resemble fibromyalgia (11). In fact, this deficiency is so widespread and such an issue that it is sometimes referred to as a hidden epidemic.

Getting sufficient vitamin D from the sun is a major challenge, especially as we don’t truly know how much vitamin D people even need. Because of this, many people turn to vitamin D supplementation as an alternative and they normally choose the vitamin D3 form.

In fact, recognition of vitamin D’s significance is growing to the point that many doctors are now testing for vitamin D deficiency and prescribing supplements.

For many people, supplementing with vitamin D offers a valuable approach for fighting the risks of vitamin D deficiency. This type of supplementation also has the potential to contribute to health benefits.

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What is your experience with vitamin D? Do you supplement the vitamin or do you think you get enough from the sun and simply being outdoors?​

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