Updated: Jan 14, 2020
Sunshine. The ultimate, all natural and completely free health supplement. Sunlight is a mood booster, regulates our circadian rhythms and makes the majority of one of the hormones (disguised as a nutrient) in your body. That hormone in nutrient clothing is vitamin D.
Five fast facts about vitamin D
1. Vitamin D is not actually a vitamin. It is really a steroid hormone produced by the kidneys. Back in the early 1920s, pioneering scientists discovered that rickets (a skeletal disease that causes softening of bone, seen in children) could be prevented by taking a novel nutrient, labelled vitamin D. This is the origins of the popular understanding that vitamin D is linked to bone health. In the late 1960s, research started to reveal that this little 'vitamin' does far more than just maintain strong bones. In fact, studies confirmed that vitamin D has a steroid chemical structure. In 1971, researchers suggested that vitamin D should be reclassified as a steroid hormone, instead of a vitamin. By 1997, after mounting evidence indicated that there are vitamin D receptors in nearly every cell in your body, a Nutritional Committee Report for North America and Europe recognised that the scientific community had shifted to seeing vitamin D as a a hormone.
2. Vitamin D turns on and off almost 1000 genes, many in the brain. Vitamin D deficiency is connected to a vast range of symptoms from brain fog, mood changes, anxiety, fatigue, sub optimal muscle function, brittle bones, inflammation, increased susceptibility to infections, blood sugar problems and high blood pressure.
3. Vitamin D is the master controller that regulates the concentration of calcium in your blood stream. It dramatically increases the amount of calcium that you absorb from your diet. Dr Rhonda Patrick explains that if you have sufficient vitamin D (> 75 nmol/L), you'll absorb about 40 % of the dietary calcium you eat. But if you are vitamin D deficient (< 50 nmol/L), you only actually absorb around 10 to 15 % of your dietary calcium.
4. Vitamin D is the only micronutrient that is synthesised from sunlight. Your body makes vitamin D3 from UVB radiation. To do this, sunlight acts on cholesterol in our skin cells to create D3, the prohormone for vitamin D. After that first step, D3 is then metabolised in the liver into the circulating form, 25-hydroxy vitamin D or 25(OH)D. 25(OH)D is what's measured in a blood test because it is the precursor to vitamin D and it reflects both sunlight and dietary sources. 25(OH)D is converted into the biologically active form of vitamin D in the kidneys (1,25-dihydroxy vitamin D, or calcitriol) before acting on multiple sites across the body. This final step and the endocrine action of vitamin D make it quite unique among the vitamins and minerals, earning it its 'hormone' like status.
5. Vitamin D is not found naturally in many foods. It is present in fatty fish and eggs in small quantities and in other (not ideal!) foods due to vitamin D fortification (like some milk, margarine and cereal products). The Society of Endocrinology says that only about 10 % of our vitamin D comes from food.
'The brain is filled with vitamin D receptors and proteins that are turned on by vitamin D.'
- Dr Bruce Ames
Are you deficient?
I can't tell you that, but I can summarise some of the major factors to help you assess your status. Ultimately, a simple blood test is the most reliable method to check your vitamin D level.
You're more likely to be deficient in vitamin D if:
You're indoors a lot, you don't supplement and you don't eat fatty fish, pasture raised egg yolks or cod liver oil.
You're outdoors a lot, but you use sunscreen or wear clothing that covers most or all of your skin.
You're outdoors a lot, but the environment blocks your exposure to UVB rays (such as clouds, pollution, atmospheric ozone, and tall buildings).
You have a darker skin tone. Darker skin contains more melanin, the pigment that gives human skin, hair and eyes their colour. Melanin acts as a natural sunscreen to protect the skin from the sun, but it also blunts the body's ability to make vitamin D from the sun's rays.
You have higher amounts of body fat. Vitamin D is fat soluble. If you hold more body fat, then you'll have more vitamin D stored in adipose tissue and less of it available in your blood stream.
There are other factors like age, latitude and genetic polymorphisms that are all going to play a role in your vitamin D status, too.
Athletes tend to be deficient in vitamin D - but really need it!
Vitamin D and all essential nutrients matter whether you're an elite athlete or a couch potato. That said, if your priority is sports performance and chasing the 1 %, you should be proactively managing your vitamin D status to optimise your bone formation, muscle and immune function, among other things.
A recent meta analysis of 23 athlete studies found that that 56 % of male and female athletes globally had a vitamin D inadequacy (defined at < 80 nmol/L), including 34 % of Australian athletes sampled. The risk increased in winter and early spring because there's little UVB radiation at that time of year. They also detected a higher risk of deficiency among athletes that play indoor sports, even in summer. The authors comment that the high prevalence of vitamin D inadequacy is problematic because it may put athletes at an increased risk for bone and muscle or soft tissue injuries and prolong their recovery.
How much is enough?
The Recommended Daily Intake (RDI) figure for vitamin D has its origin in historical, short term disease (remember the rickets origins?). It's since been established that vitamin D does a lot more than just regulate calcium metabolism. Dr Ames points out that the RDI doesn't reflect the quantities required for optimal long term function across years, decades and a life time. The Endocrine Society has questioned the adequacy of the current RDI in its thorough Clinical Practice Guideline on Vitamin D Deficiency, commenting that 'whether 600 and 800 IU/d of vitamin D are enough to provide all of the potential non-skeletal health benefits associated with vitamin D is not known at this time'.
In Australia, the reference range for 25(OH)D is 50 to 250 nmol/L. If you're at least 50 nmol/L, you'll get a green light. But is that enough?
Luckily, more intelligent minds than I have considered this question to guide us.
As you may have noticed, some of this is in contrast to the reference range that applies in Australia.
Let's try to make sense of this.
It seems like there is a happy medium of at least 40 to 60 ng/ml (100 - 150 nmol/L) and that 100 nmol/L is a borderline level.
It's also clear that too little or too much vitamin D isn't great for you. Like most things, it makes sense that more is not better.
Experts suggest that from 2000 to 4000 IU is a reasonable dosage for most people to supplement, depending on your vitamin D status and ability and opportunities to synthesise D3 from sunlight.
Test and retest
If you don't test, you're not going to be able properly predict your vitamin D status due to the multitude of factors that influence it. If you don't measure it, you can't manage it. Retest at least 1 or 3 months after starting to supplement (and not just for vitamin D, but any nutrient or substance you take).
Keen to learn more? I've also posted an article delving into my experience supplementing vitamin D and K2.
Note! The information presented in this article is to be used for personal education purposes only. If you have any medical issues or queries, you should consult your doctor. Please enjoy my information in the spirit that it's intended - as a practical, user-friendly synthesis of scientific concepts and carefully researched information, for you to ponder and apply as desired to suit you and your body.
Norman, From Vitamin D to Hormone D: Fundamentals of the Vitamin D Endocrine System Essential for Good Health, American Journal of Clinical Nutrition (2008) 88, 491S.
Farrokhyar et al, Prevalence of Vitamin D Inadequacy in Athletes: A Systematic Review and Meta Analysis, Sports Medicine (2015) 45, 365.
Holick et al (Endocrine Society), Evaluation, Treatment, and Prevention of Vitamin D Deficiency: an Endocrine Society Clinical Practice Guideline, The Journal of Clinical Endocrinology and Metabolism (2011) 96, 1911.
Dr Bruce Ames speaking in this excellent episode of Dr Rhonda Patrick's podcast on Triage Theory and micronutrients.
Dr Rhonda Patrick's page, Found My Fitness, has published this fantastic vitamin D infographic for the basic information. Dr Patrick has also put out a detailed Q & A podcast on The most popular questions about vitamin D.
Vitamin D Council, How much vitamin D is required to achieve optimal levels?
Society for Endocrinology, Vitamin D.
Confused about latitude? Osteoporosis Australia has published this great map of the Recommended sun exposure for vitamin D based on location in Australia.