Vitamin D is more like a hormone than a vitamin. Unlike other vitamins, the main source is not food but a series of chemical reactions in the skin that require ultraviolet light (UVB) as a catalyst. Natural food sources are mainly animal products in which the vitamin D stores of other animals are consumed. However, most omnivorous diets only supply a small proportion of daily needs and the natural source has always been sunshine. The need for sun-derived vitamin D explains why populations that lived at higher latitudes evolved paler skin as an adaption to reduced UVB exposure.
The vitamin D3 from sun exposure or supplements (most are the D3 rather than the D2 form) is then converted into 25-hydroxy vitamin D3 which is the form of vitamin D stored in the body and the one that is measured by blood tests. The final product is the hormone calcitriol which has many sites of action other than its primary role in regulating blood calcium level. It regulates calcium absorption from the gut, calcium excretion by the kidneys and bone formation/breakdown.
It has long been observed that low vitamin D levels were associated with chronic diseases including heart disease, stroke, diabetes, cancer and multiple sclerosis. Vitamin D was therefore suspected of having a protective effect against these diseases and many clinical trials of vitamin D supplementation have been conducted. Two recent reviews published in The Lancet found that vitamin D supplements did not provide protection against diseases such as heart disease, diabetes and colorectal cancer (see Resources section). It is now thought that the low vitamin D level associated with these diseases is a consequence of these diseases rather than the cause of them. The increase in whole body inflammation associated with obesity and chronic diseases reduces blood levels of 25-hydroxy D3.
A whole foods plant based diet may boost vitamin D function. It prevents, slows or reverses most of the chronic diseases associated with low vitamin D levels. And while eating meat and other animal products increases measurable indicators of inflammation (which lowers vit. D levels), whole plant foods such as grains and vegetables reduce metabolic inflammation.
Excessive calcium intakes from dairy foods and supplements suppress the production of the active form of vitamin D, calcitriol, and may have long term adverse effects on the production of this hormone (see The China Study, p.208). Animal protein may also affect this system by driving a higher excretion of calcium.
Vitamin D stores (25OH Vit D3) can be measured with a blood test. For most people, this test is unnecessary and the Medicare benefits schedule was modified in early 2015 to restrict funding of this test to those at high risk of deficiency. Commonly used laboratory tests may be inaccurate so the result should be taken as an approximation only (see: Two new vitamin D blood tests are often highly inaccurate, researchers say). There is some uncertainty over what constitutes an adequate blood vitamin D level. Most laboratories use 75-150 nmol/ml as the recommended range. However, possible adverse health effects have only been observed at levels below 50nmol/ml, therefore this level should be considered adequate.
Vitamin D is an essential “nutrient” and just as other nutrients are best obtained from natural sources, food, vitamin D is best obtained from natural sources, sunshine. Many nutrients can be harmful, not just useless, when taken as isolated supplements. You can overdose on vitamin D supplements with long term (months) consumption of only 5-10x the recommended supplementation dose of 1000iu – 2000iu (25mcg – 50mcg) per day. You cannot overdose on vitamin D produced from sun exposed skin even though a single episode of sun exposure may give 10 000iu of vitamin D. Of course, fair skin can be easily damaged by excessive sun exposure and we are all aware of the role of sun exposure in skin cancer. There is less awareness of the role of diet in the development of skin cancer.
It is likely that sun light has effects on our physiology other than vitamin D synthesis. Exposure to UVB light alters the function of skin cells and immune cells in the skin. Bright light also helps to synchronize and regulate our circadian rhythm which is thought to be important for health as well sleep, wakefulness and mood.
Vitamin D stores can see us through 30-60 days without sun exposure or supplements and even longer periods of partially adequate sun exposure so it is not necessary to obtain adequate vitamin D every day or every week. In tropical Australia and during the summer months in the Southern parts of the country, the majority of people can maintain adequate vitamin D levels with short periods of sun exposure to the face, neck and arms during the middle parts of the day when the sun is high in the sky. During the winter months of May to August, people in Southern Australia may need, on average, 20 minutes per day (or more if less skin is exposed) to maintain adequate vitamin D levels. These recommendations are based on fair skinned people and those with darker skin may require as much as 3 to 10 times as much sun exposure to produce adequate vitamin D. There is a SunSmart App designed to help you balance adequate vitamin D with sun protection. One of the most useful resources is the Realtime UV Levels website which provides both predicted and real-time UV levels for several Australian cities. This information will give you a better understanding of how much UVB there is at different times of the day in the current season and assist you to take the middle path between vitamin D deficiency and sunburn. Even if you work indoors most days, a short period of intense UVB exposure to a large area of skin on a weekly basis will provide adequate vitamin D.
In summary. A source of vitamin D is essential and the best source is UVB light from the sun. This need not be on a daily basis as vitamin D is stored in the body. Vitamin D supplements do not have a role in the prevention or treatment of disease. Those who live in Southern Australia may require a supplement during the winter months if their lifestyle provides minimum sun exposure during the middle parts of the day. Supplementation is more likely to be necessary for those with darker skin and for those whose culture requires minimal skin exposure.
Resources
- Ultraviolet radiation index – ARPANSA (Australian). You can check the real time UV levels in your region.
- How Important is Vitamin D? Facts You Need to Know – Conor Kerley, PhD. 2018
- Vitamin D Supplements Are Harmful—Sunshine and Food Determine Health – Dr John McDougall, 2015
- Shining a Light on Vitamin D – Thomas Campbell, MD, 2013
- Vitamin D: Values for Normal Are Exaggerated – Dr John McDougall, 2011
- Vitamin D Pills Are of Little or No Benefit and Some Harm – Dr John McDougall, 2010
- What is the optimal level of vitamin D? – Australian Family Physician article (2014)
- Two new vitamin D blood tests are often highly inaccurate, researchers say
- Free SunSmart Global UV app – SunSmart is an Australian website
Peer-reviewed articles:
- Autier, P., Boniol, M., Pizot, C., & Mullie, P. (2014). Vitamin D status and ill health: a systematic review. The Lancet Diabetes & Endocrinology, 2(1), 76-89.
- Bolland, M. J., Grey, A., Gamble, G. D., & Reid, I. R. (2014). The effect of vitamin D supplementation on skeletal, vascular, or cancer outcomes: a trial sequential meta-analysis. The Lancet Diabetes & Endocrinology, 2(4), 307-320.
- Editorial: Vitamin D: chasing a myth? (2014). The Lancet Diabetes & Endocrinology, 2(1), 1. doi: 10.1016/s2213-8587(13)70164-5
Page last updated 6 April 2015