Written by Ben Bunting: BA(Hons), PGCert. Sport & Exercise Nutrition. L2 Strength & Conditioning Coach.
We get vitamin D from the sun, so is it really necessary to take a supplement that contains vitamin D?
This article will cover vitamin D; it's role, benefits, effects, and whether we really need to start supplementing.
Here are the key sections:
- What is vitamin D?
- How is vitamin D supplements made?
- Vegan options
- Supplementation Benefits
- Supplements vs the Sun
- Side Effects
- Military Studies
Let's start at the beginning so you, the reader, can get a better grasp and understanding of vitamin D.
What is Vitamin D?
Vitamin D is a fat-soluble vitamin that was discovered in the early 1900s by a man called Sir Edward Mellanby from Great Britain primarily in the quest to cure rickets which was affecting large numbers of the population across the United Kingdom.
It was concluded from experiments and testing that it wasn't the vitamin A found in cod liver oil that was previously considered to cure the disease of rickets, but a new vitamin which was named vitamin D. 
This vitamin is found in foods, but it's not actually very abundant, so in many cases, it has to be added. In which case, these are referred to as fortified foods.
Because there are so few foods available with vitamin D, arguably one of the most common ways we get vitamin D is from the ultraviolet rays of the sunshine.
When our skin is exposed to these rays it triggers vitamin D synthesis. 
All vitamin D is inert when the body receives it from either food or sun exposure. This means, that it is chemically inactive, or has no effect until it goes through two hydroxylations in the body.
It must go through the liver and then the kidneys for conversion before it becomes activated and useful. 
This process is what makes vitamin D unique...
When is a vitamin not a vitamin?
When it is vitamin D. How so?
Vitamin D was discovered in the early 20th Century along with 12 other vitamins when doctors and scientists were trying to understand nutritional deficiencies and how to overcome them.
Technically speaking, 'D' isn't a vitamin.
While it is essential to health like other vitamins, the difference being that it is actually produced and converted in the body to be used.
The encyclopedia Britannica actually states that it is categorized as a pro-hormone as its an inactive precursor of a potent metabolite. 
This means that when our body either ingests vitamin D or is exposed to ultraviolet rays it needs to be transformed by the body to be utilized. No other vitamin requires this action. 
Furthermore, vitamin D is many chemicals rather than just one and is a steroid hormone that's produced from cholesterol when the sun's rays hit the skin.
In addition, no other vitamin is produced from skin exposure to sunlight, making this truly unique. 
Unlike many other vitamins, 'D' is fat-soluble, not water-soluble.
This means it can be stored within the body for long periods of time rather than being excreted.
Being fat-soluble also means that when the few foods that contain vitamin D are cooked, the vitamins aren't lost. 
Sources of Vitamin D
There are not many foods that actually contain vitamin D naturally.
The few exceptions are egg yolk and oily fish such as:
Cod liver oil is also key, it was this that helped chemists initially understand and name the vitamin.
However, many people also think that milk and other dairy products are sources of vitamin D.
Theoretically, they can be, but these are fortified foods.
That means vitamin D is added to them to help prevent deficiencies in a population.
Yet, this fortification is not a global standard. Some foods are fortified in some countries and not in others.
For instance, the US and Canada differ slightly.
The US fortifies mainly milk and breakfast cereals whereas Canada prefers to fortify margarine. 
In the United Kingdom, fortification of wheat flour is considered the viable option to reach those most at risk from deficiency. 
One thing for certain is that foods that contain vitamin D naturally need to be eaten every day, that would mean oily fish in abundance. 
The only other option would be a single tablespoon measure (15ml) of fish oil every other day.
Another option that is widely recommended by numerous health authorities is a vitamin D supplement that provides no more than 100mcgs daily. 
Vitamin D Deficiency
Having learned that there aren't that many options to get enough vitamin D in your diet other than oily fish or fish oils, and with no global standard amongst food fortification, supplements may seem like the safe 'go-to' option.
In addition, with this in mind, you may not be too surprised to learn that there are many people around the world that suffer from vitamin D deficiency.
According to the American Journal of Clinical Nutrition from a published article back in 2008, approximately 1 billion people worldwide were deficient, and it is now classed as a pandemic. 
Bear in mind that those statistics are over 12 years old when this article was written and accounted for approximately one-sixth of the global population.
Even now, with a global population of 7.8 billion according to data from Yale University, 1 billion is a sizeable chunk, and the deficiency levels may be even higher now. 
Who is affected the most?
Clearly, there are three ways to get vitamin D. These are:
Therefore, by not getting enough from these three sources, you are likely to be suffering from a deficiency.
Other reasons could be medical.
Your body may not be able to absorb or convert vitamin D, or you could be taking medications that affect your body's ability to absorb or convert the vitamin.
Yet, even so, some groups of people are more at risk from a deficiency than others.
Obviously, with very few food sources (and arguably palatable) available to boost D levels, a good option is sunshine.
However, with the increased risk and awareness of skin cancer, plus the aging effect of too much time spent in the sun many people avoid long exposure to it or cover-up to protect themselves.
Furthermore, there are just groups of people that are just more at risk from a deficiency than others according to information from the US Library of Medicine  which is corroborated by advice from the United Kingdom's National Health Service. 
Those who live in climates without much sunshine or who spend much of their time indoors or work night shifts are at risk from a deficiency.
Older people tend to spend more time inside, their skin and kidneys are less able to synthesize and convert the sun's rays.
People with darker skin also have less ability to produce vitamin D from the sun and need longer exposure to ultraviolet rays than those with lighter skin, this can reduce the opportunity to get as much of the vitamin.
Infants that are breastfed need supplementation or fortified food sources as breast milk lacks the required amounts of vitamin D.
Then there are medical reasons, such as disease which can hinder the processes of vitamin D absorption.
Obese people suffer from a deficiency due to their body fat binding to some of the vitamin which then prevents it from getting into the blood.
This can the same for people with a type of cancer called lymphomas, chronic kidney and liver disease plus Crohn's or celiac disease.
Those people suffering from osteoporosis, hyperparathyroidism, and diseases caused by chronic inflammation such as tuberculosis are also at risk.
Additionally, those who are treating ailments with medications can also find themselves at a risk from a deficiency as their medications can affect the way their body metabolizes and processes vitamin D.
Those people who are on medicines to treat HIV/AIDS or anti-seizure and antifungal drugs can pose a risk.
In light of the COVID-19 (Coronavirus) pandemic, there are even further calls to make people aware of vitamin D and the risk of a deficiency as people spend more time indoors.
There are also speculations that those who are at risk of a vitamin D deficiency are more likely to be admitted to an Intensive Care Unit upon contracting COVID-19. 
Although there appears to be a “statistically significant” association between death rates and vitamin D there lacks conclusive proof that vitamin D can be beneficial towards COVID-19 outcomes according to researchers from Trinity College Dublin and the University of Liverpool. 
As of September 2020, a peer reviewed study was published that concluded that there is a significant association between lower COVID-19 rates and higher levels of circulating vitamin D levels.
Low Vitamin D: The Risks
So far we have learned that vitamin D isn't actually a vitamin, but, like vitamins, it is essential nutrient.
There are also precious few foods that naturally contain vitamin D, and those that do, need to be eaten often and maybe an acquired taste – not everyone is a fan of strong tasting fish.
That leaves us with the sun.
However, even this free source may not be adequate in some parts of the world, your skin may not be able to absorb it properly, your work patterns may not favor daylight hours and then there is the risk of skin cancer.
Getting enough of this vitamin is more difficult than it seems.
Yet, the implications of a deficiency aren't trivial.
Here we shall run through the risks associated with a deficiency.
A deficiency that is categorized as a pandemic and was the cause behind one of the most devastating health consequences of the Industrial Revolution throughout Northern Europe and parts of the USA. 
This is a disease which perfectly encapsulates the physical effects of vitamin D deficiency and the importance vitamin D has for bone health.
It was rickets or the search to cure rickets, which led to the discovery of vitamin D from fish oil.
Rickets is a condition that causes soft and weak bones, mainly affecting bone development in children.
This is painful and can lead to deformities.
However, adults can also be at risk. This is then called osteomalacia.
There is an increased risk of fractures from osteoporosis of the hips, forearms, and vertebrae. 
During the early twentieth century, the effects of rickets were two-fold.
It caused pain and issues for children developing, and it also made it difficult for women to give birth as their pelvis structure wasn't optimal.
The cause of rickets is brought upon by the correlation of vitamin D deficiency with a lack of calcium and phosphorus absorption within the intestine.
This is a condition that was almost consigned to the past, however, in the United Kingdom, cases have risen again, this is due to the low levels of vitamin D in the blood amongst a significant number of the population. 
As far back as the early 1940s, there have been observations of those people with low exposure of sunlight developing serious life-threatening cancers compared to people who lived in regions that experience more sunshine. 
It has since been much more widely documented that those who live in the higher latitudes of the globe are more at risk of a variety of lethal cancers due to vitamin D deficiency from less exposure to sunlight. 
What's the mechanism?
Exposure to sunlight stimulates the skin to produce vitamin D. This is also a mechanism of prostate cells.
In short, the higher the exposure to vitamin D, the more prostate cells are used and less likely to become malignant. 
Yet, far from being unique to prostate cells, this mechanism has also been observed amongst other cells in the body such as the brain, breast, lung, colon, and others.
Studies suggest, with strong support from research, that if we have enough vitamin D in our blood for our cells and tissues to synthesize it can prevent malignancy and control cell growth. 
Qualified data tells us that people who live in regions above 37-degree latitude are more likely to develop autoimmune diseases compared to people who live south of 37 degrees latitude. 
This affects most people within Europe and North America whereby there is an increased prevalence of diabetes mellitus, multiple sclerosis, and rheumatoid arthritis.
Studies into these conditions and vitamin D have noted positive outcomes when subjects were administered vitamin D supplementation. 
It is now recognized that vitamin D is a potent immunomodulator; enhancing the immune system.
The relationship between latitude, vitamin D deficiencies, and high blood pressure was reported as far back as 1979 by Stephen G Rostand of the University of Alabama at Birmingham. 
To confirm these reports, two groups of patients suffering from hypertension were exposed to tanning beds exposing them to light that would either simulate summer sunlight or winter sunlight over three sessions per week for a duration of three months.
The group that was exposed to summer sunlight saw a reduction of blood pressure that placed them within the normal range.
Whereas the group exposed to winter sunlight saw no change in their blood pressure. 
Further study into heart disease has discovered that a patient is more likely to develop heart failure if they are vitamin D deficient according to a study published in the Journal of the American College of Cardiology. 
Whilst it is not entirely understood how vitamin D status relates to cardiovascular health complications, it is understood that vitamin D is cardio-protective. 
The Journal of Military and Veterans' Health of Australia notes that there is a link between low levels of vitamin D and reduced testosterone.
A study published in Volume 24 of 2016 found that those with the lowest vitamin D concentrations also had the lowest levels of testosterone and higher Body Mass Index (BMI) compared to those with higher vitamin D. 
Moreover, a study published in 2010 by the Hormone and Metabolic Research Journal declared that they had found a connection between vitamin D supplementation and increased testosterone levels. 
These results are supported further by a study into the seasonal differences between hypogonadism (extremely low testosterone) and vitamin D amongst Korean men.
Scientists from the Pusan National University School of Medicine, Busan, and the College of Medicine of Kosin University concluded that there is a seasonal link between low testosterone levels and reduced vitamin D exposure. 
Furthermore, as men age, and after a peak seen in their late teens, testosterone secretion declines.
The problem is, testosterone is great for the physiology of our body, so a decline 'slows' the body down and makes it harder to recover, build muscle or even lowers our mood. We've written an article about the benefits of testosterone, here.
There is evidence that supports vitamin D deficiency is related to an increased risk of neurodegenerative diseases such as Alzheimer's, Multiple sclerosis, and Parkinson's. 
This can affect multiple brain functions such as:
- Cognitive impairment
- Memory loss
- Reduce motor functioning
For instance, it is recognized that there is a higher incidence of multiple sclerosis sufferers in northern areas of France compared to the southern regions which receive more sunlight. 
Not only is there a higher prevalence, but the onset is sooner too as reported by research published in Nature Reviews Neurology. 
Furthermore, to cement the relationship between D and its protective action against neurodegenerative disease one only has to look at Norway.
A country nestled in the higher latitude but has a lower prevalence of multiple sclerosis due to their diet of oily fish. 
Like multiple sclerosis, Parkinson's is the degeneration of the Central Nervous System.
Recent data demonstrates that Parkinson's sufferers tend to have sub-optimal levels of vitamin D. 
This is a relationship that has been identified in patients throughout America and China. 
Approximately 24 million people across the globe suffer from Alzheimer's which affects cognitive function and results in dementia.
It is a degenerative disease that is on the rise, with a prediction of 48 million sufferers within 20 years (at the time of press). 
Scientists are not clear exactly how vitamin D has a positive effect on Alzheimer's, but it is known that it does reduce oxidative stress. 
Numerous studies have identified a clear positive correlation with higher levels of vitamin D and higher cognitive function, whereby the vitamin D mitigates cognitive decline. 
This trend was replicated in a study from France involving women aged 75 years or above over a seven-year period of study. 
Likewise, there is an association with a higher risk of all-cause dementia and Alzheimer's to those who have a severe deficiency of vitamin D as was concluded by a study published in the American Academy of Neurology. 
Mental health is a key topic that has gained less stigma more recently. Conditions such as depression can affect anyone, from any walk of life, and can contribute to the loss of life. 
Current statistics from the World Health Organization show that only a quarter of those with depression actually receives treatment, and treatment which usually consists of medications and or psychotherapy has a success rate as low as 60% which may rise to 80%. 
However, it could be suggested that there is a fear, economic pressures, and lack of understanding surrounding medication due to 52.1% of patients reportedly discontinuing medication treatment after just 60 days. 
Therefore we have a situation whereby only 25% of people suffering from mental illness are being treated, and the drop out rate is significantly high.
This presents itself as a major health concern as depression is considered chronic and reoccurring.
And, it is reported that those who do not adhere to the recommended treatment are at significant risk of relapse. 
The deficiency of some vitamins has been identified in the past as being associated with mental disorders .
These have included:
It has been recognized that vitamin D plays a role in women's mood and mental well being. 
This also supports accepted findings that vitamin D has an impact on seasonal affective disorder (SAD).
Studies have registered a reduction of depressive symptoms for those administered a dose of vitamin D who are affected by SAD compared to a control group. 
Further research has noted that there is a clear trend amongst those suffering from depressive symptoms, anxiety, schizophrenia and other mental disorders to have abnormally low levels of vitamin D. 
That said, the connection between mental disorders and low serum vitamin D is not fully understood.
Can supplementing with Vitamin D be beneficial?
With a correlation of low vitamin D status and mental disorders, the question is whether increasing a person's level of vitamin D will improve their condition.
It seems that from the studies available, there is no significant effect on mental health by taking vitamin D if there isn't already a deficiency.
However, a study conducted over a period of three months resulted in the participants seeing a significant improvement of their well being after taking up to 100mcg daily. 
This was also demonstrated in a study conducted over a twelve-month period with depressed and overweight patients.
They found that there was a significant improvement in depression which was more evident in those with higher depression levels. 
It must be noted that there are no clear outcomes.
Various studies have included different parameters and no set dose of vitamin D has been used.
The World Health Organization recognizes that a deficiency of vitamin D is linked to infections such as:
The WHO also states that pneumonia is the leading cause of death in children globally and understands that studies point towards vitamin D supplementation being a possible treatment. 
This is supported by a review (meta-analysis) of various studies published by the British Medical Journal that concluded vitamin D supplementation is a safe method of protecting against acute respiratory tract infection.
Those who were already deficient in serum vitamin D gained the most benefit. 
The European Journal of Medical Research outlines the benefit of vitamin D supplementation for not just respiratory protection but for immunodeficiency virus and hepatitis C.
As they report that 37% of the general population within Europe are suffering from a deficiency, they recommend a daily dose of 1000iu as being a safe and cost-effective preventative treatment. 
A reduction of vitamin D is a common finding in patients who suffer from hepatitis B and C which causes chronic liver diseases.
While not fully understood, it is theorized that the anti-inflammatory, anti-tumor, and anti-fibrotic of vitamin D may have a role in possible prevention. 
We have covered sleep and its importance for sound physical and mental recovery here.
To summarise, sleep is extremely important for the processes of the body. It helps with learning, judgment, awareness, alertness, and physical performance.
In periods of extreme stress and sleep deprivation, life-threatening accidents such as 'friendly fire' during conflicts are well documented. 
With sleep being such an important factor in good health, there has been a number of studies to understand the relationship between vitamin D values and sleep duration.
Initial studies found that low vitamin D levels are associated with excessive day time sleepiness and shorter sleep intervals.  
These results were backed up by a more recent study using polysomnography equipment that measured the lower Rapid Eye Movement (REM) for those with low vitamin D values. 
Further research found that those who were given a diet of oily fish were able to fall asleep quicker than those who didn't eat fish.
The study published in the Journal of Clinical Sleep Medicine reporting findings of a positive correlation between vitamin D and sleep efficiency. 
A number of studies indicate that supplementation of vitamin D is able to reduce musculoskeletal pain, even after a short duration of six weeks. 
Additional studies have found that the benefits of vitamin D are not limited to musculoskeletal pain and can have a positive effect on other conditions such as arthritis and myalgia as observed in hospital-bound patients with pain-related conditions. 
It was also observed in Pakistan that women who traditionally wear the burqa are less likely to get the required exposure of vitamin D.
Those who suffered from fibromyalgia and were administered vitamin D to increase their serum levels saw their pain score reduce over the period of supplementation. 
As we have learned, there are many negative effects of vitamin D deficiency.
There are probably many more, however, let's look at the most important factor. The association between a deficiency and mortality; death if you will.
Fortunately, there has been a great deal of research into this.
The analysis of 25 studies has highlighted the connection between low vitamin D concentration and all-cause mortality.
The evidence presented illustrates that those with the lowest vitamin D levels had almost twice as likely a chance of dying compared to those with the highest levels of serum vitamin D.
Evidence suggests that those who take a supplement containing 2000 to 4000iu daily would place people in the higher levels of vitamin D. 
The result of this meta-analysis demonstrated that those who have extremely low serum concentrations of vitamin D are at risk. This amounts to 20 ng/mL.
It is even said that those with 30 ng/mL are still at risk.
The consensus is that if people take a supplement containing just 1000iu it can place them at a safer level. 
How is Vitamin D Supplements Made?
So, the evidence available is highlighting the importance of vitamin D supplementation.
In which case, how are they made? And, what are they made from?
We have read about fish oil (a very strong taste which repeats on you) being beneficial and obviously sunshine.
In which case how do you 'package' or 'bottle' sunshine?
There are two types of vitamin D
These are D2 and D3.
They come from different sources; D2 from plants and D3 from animals.
There is much debate between which version is 'better' or more effective at raising vitamin D serum status in humans.
Generally speaking, the evidence seems to favor D3 as being the most effective, this is because it is absorbed better by the body.
However, this may be only when taken as a large 'bolus' single dose, usually by injection.
When taken daily as an oral supplement, the additional benefit of D3 over D2 is reported as 'lost'.
Therefore, there is little reason to use one more than the other.
Vitamin D Supplement Sources
For supplements, vitamin D3 is made from lanolin which is extracted from sheep wool that has been subject to Ultraviolet
Whereas D2 is extracted from yeast.
Both forms are considered to be equally effective as a benefit to health and preventing conditions such as rickets. 
Vegan Vitamin D3
So you may be thinking if vitamin D3 supplements are processed from sheep wool, this is clearly an animal source and not suitable for vegans or vegetarians.
You would be correct. So what is the answer?
As mentioned, there is a general favoring of D3 over D2.
Vitamin D2 is generally from yeast, so that is not an animal source and a vegan-friendly source.
However, if you want vitamin D3, the vegan option is lichen. 
It is lichen that is used in Military Muscle.
Lichen – the vegan option
Lichens are a fungus or algae and D3 is now extracted from these sources. 
They're a plant source of vitamin D3 and provides a suitable meat-free alternative for those who do not eat meat for religious or dietary reasons. 
This may also be due to performance reasons since a plant-based diet is attributed to improved cardiovascular health which is critical for athletes. 
Furthermore, it has been identified that a plant-based diet can contribute to an improved global environment. 
One thing to consider if you are opting for a vegan source of vitamin D3 in your supplement is the capsule.
Often overlooked, but many capsules consist of gelatin.
This is an animal source and will negate any vegan claim to a product, even if vegan D3 is included.
The alternative to gelatin is hydroxypropyl methylcellulose (HPMC), this is a plant-based capsule made from vegetable cellulose which has very similar behavior to gelatin capsules.
You can read more about HPMC vegan capsules here.
Vitamin D supplementation Benefits
The benefits of vitamin D supplementation are clear.
It has a magnitude of health benefits, not less reducing all-cause mortality.
The evidence dictates that a dose of 4000iu will place the user in the higher serum status bracket, for a very cheap cost.
What are the benefits of supplementation over alternative sources?
Not all regions of the world are exposed to long periods of sunshine. Those areas that lay in the higher latitude regions see a higher prevalence of vitamin D deficiency. 
2. Skin Tone
People with darker skin tones need more time in the sun than those with lighter skin. This is not always possible due to the variation of location and lifestyle. 
Your job may mean that you are based indoors for much of the day, or your could be working on night shifts. This means that exposure to sunlight is limited. 
It has been identified that those people who wear the burqa are likely to be vitamin D deficient, this is also passed on to the unborn child if the person is pregnant. 
As already broached. There are very few food sources that can provide natural vitamin D.
Those that are available tend to be oily fish and egg yolks. Some fungi such as certain mushrooms are also a source of vitamin D. 
The issue is that these foods need to be eaten daily just to reach the required vitamin D requirements.
Fish oil is the only source with substantial vitamin D content that can be ingested every other day.
However, people may not like these foods or even have regular access to them.
6. Elderly Persons Hypertrophy
Some elderly people suffer from muscle mass loss. Supplementation of vitamin D along with fish oil and protein in conjunction with resistance and aerobic training resulted in improved strength and muscle gain. 
7. Sun Safety
On the other end of the spectrum, some climates have long seasons of strong sunshine, but even in hot climates, populations suffer from deficiencies due to lifestyle behaviors and the use of sun protection measures like sunscreen. 
Skin cancers are preventable, and there is a lot of information about sun safety which advises against getting too much sun exposure by staying under the shade, wearing hats with sun protective clothing, and consistent sunscreen application. 
With all of these limitations in mind, the benefits of a supplement can be a much effective, safe, and low-cost option to ensure people, worldwide, can get the recommended amounts of vitamin D.
Furthermore, as we have already discussed, dietary requirements needn't be an issue as a vegan option is available in capsule format.
Vitamin D Supplement Vs. The Sun
The sun is a free and abundant energy source.
In many respects is having the sun available could be seen as nature's way of not needing to provide many food sources with high vitamin D levels.
We must also remember as vitamin D is fat-soluble, we can also store it, unlike some other vitamins that do require constant replenishment.
So, how does a supplement stack up against the sun? Because nature seems to have us covered.
We have already mentioned a supplement can come as either meat or plant-based, so all dietary requirements are covered.
A supplement can also come in different strengths, as such you can tailor the amount you may need.
This may be particularly necessary for those with darker skin tones or the elderly that are unable to synthesize vitamin D as effectively as younger people.
In essence, a supplement can be a reliable source, whereas the sun is not.
As we have covered, there are limitations, be they regional (latitude and climate) or imposed by humans.
This could include religious practices such as the wearing of veils or full body clothing to concerns of skin cancers, as such the culture of covering up to protect from the harmful sun's rays.
The United Kingdom presents a variable case for sun safety, low exposure, and vitamin D deficiency.
It is recommended that 10 to 15 minutes of exposure for lighter-skinned people and up to 40 minutes of sun exposure for darker skin tones between the months of April and September is sufficient. That is to minimize the risk of skin cancer and burn. 
This may seem like a low amount of time, even for a country where the scorching sun isn't associated, particularly for a country above 37 degrees latitude and has an adult population with 40% having a deficiency. 
It is then recommended by Public Health England that a supplement is taken throughout the rest of the year. 
Alternatively, working practices have led people inside buildings for long periods of daylight or night working.
Again, they are not able to receive sufficient sunlight.
The point being, it can be hard to judge the safety of the sun, the time required, and then whether our working or lifestyle practices can make the most of this free source.
On the other hand, a supplement is a low cost and effective all year round, for all people.
Supplementation Side Effects
Taking a supplement seems like a foolproof way to prevent and treat a number of conditions, safely.
There's no need to eat strong-flavored foods daily, or risk skin cancers.
We can also adjust the amounts required. It all seems very simple.
So, are there any unwanted side effects from taking a vitamin D supplement?
Fortunately, vitamin D intoxication is considered one of the rarest medical conditions presentable.
It is considered that 10,000iu or 250mcg is tolerable for a daily dose of vitamin D without adverse effects, although a very high dose appeared to be no more beneficial than a lower dose. 
There is a report of toxicity, whereby a person accidentally took over 1,000,000iu (25mg) daily for over a six month period believing his dose to be just 2,000iu. 
Should there ever be a rare case of toxicity from vitamin D, the following side effects have been reported:
- Hypercalcemia (Excessive blood calcium levels)
- Frequent urination
- Excessive thirst
- Kidney dysfunction (Extreme cases)
- Altered mental state (Extreme cases)
There is an extremely low risk of suffering from vitamin D intoxication.
Due to the high levels of stress and lack of regular sleep for military personnel, there have been studies in the serum vitamin D levels of operational soldiers.
One such study saw that over half of operational special forces soldiers had insufficient vitamin D status along with suppressed testosterone levels, even in the southern latitudes. 
This presents a risk of limiting human performance.
Submariners are particularly at risk due to extended periods without being exposed to sunlight.
Research has found that supplementation of 2000iu (50mcg) daily increased vitamin D serum status and reduced the risk of stress fractures. 
A study looked into the role that vitamin D supplementation may have for recruits entering the US Marine Corps.
Training is stressful both physiologically and psychologically.
The results found that vitamin D holds an important relevance towards improving immune response for recruits under stressful conditions. 
Depression amongst Service Personnel
A study has linked soldiers being based at higher latitudes, having lower vitamin D serum status, and a higher risk of depression. 
The Deployment Health Clinical Center recognized that low levels of vitamin D in the blood are associated with fractures, injury, diseases, cancer, pain, immunodeficiency as well as post-traumatic stress disorder as well as mild brain injury symptoms.
They also noted that those deployed to the Middle East were suffering from low vitamin D due to long sleeves, sunscreen, night-time operations, and general avoidance of sun.
These practices combined with heavy loads placed them more at risk of injury. The study concluded that the ability to increase vitamin D blood levels through supplementation is cheap, quick, and available. 
There is a well-defined need for vitamin D supplementation for people across the globe.
Vitamin D deficiency is free of prejudice towards social class, status, religion, or race.
Even those who live in regions of high sunshine exposure are at risk of low serum status.
It has been identified by numerous health authorities, scientists, doctors, and researchers, both in a civilian and military context that low vitamin D status is at pandemic levels and presents a risk towards multiple health parameters and even mortality.
Food sources containing sufficient levels of vitamin D are few and far between, and of a strong taste.
This coupled with the potential dangers of extended sun exposure place supplementation as the low risk, effective option.
Case studies have shown that the chances of toxicity or ill-health from mega-doses of vitamin D to be extremely rare, yet it is cost-effective and widely available.
Research has demonstrated that it can be a preventative and treatment to many health conditions, as such it is recommended by health authorities that citizens around the globe should use a supplement.