Featured photo courtesy of Life Mental Health.
The sun’s radiation that hits Earth’s surface was covered over the last post, where I discussed how the depletion of the ozone layer at most high-population latitudes is misunderstood and overblown, and that global dimming has actually reduced the amount of sunlight the surface is receiving on average.
Continuing the Sunlight Series, this post will finally discuss the effect of sunlight on your body. The main two beneficial effects of sunlight on the human body are:
- Regulation of your circadian rhythm (biological clock)
- Production of vitamin D in the skin (UV-B radiation does this)
To review, sunlight is made up of a continuum of wavelengths that is mostly infrared, visible, and ultraviolet radiation. Ultraviolet radiation can be broken up into UV-A, UV-B, and UV-C, with UV-C being the highest in energy, and UV-A being lowest in energy. UV-C is entirely screened out by the atmosphere, while the atmosphere lets about 1-3% of UV-B through to the surface, and essentially all of the UV-A radiation.
UV-B is responsible for both vitamin D production in the skin, and sunburns. Ironically, sunburns are linked to skin cancers, while sufficient vitamin D levels are linked to lower cancer rates. Vitamin D is an extremely important and underemphasized hormone that performs a myriad of essential functions in the body. The fear of skin cancer is, as is all too common with public health initiatives, misguided and misunderstood. This will be covered in a later post.
In this post, let’s take a look at vitamin D and its role in health and disease prevention.
Vitamin D was famously discovered by Elmer McCollum in 1922, and it was shown in his experiments to cure rickets, a debilitating condition in which bones are soft and weak. After American biochemist Harry Steenbock discovered in 1923 that the irradiation of certain foods could increase their vitamin D concentrations it became common to add vitamin D to foods (such as milk). By the time his patent expired in 1945, rickets was essentially eliminated in the US.
Vitamin D is considered to be a collection of compounds, as opposed to one single molecule. The most common types considered for supplementation are vitamin D2 (ergocalciferol) and vitamin D3 (cholecalciferol). Vitamin D is not classified as an essential dietary nutrient since UV-B radiation causes synthesis of vitamin D in human skin (specifically D3), although it can also be ingested.
The synthesis of D2 arises from UV-B rays striking a molecule called ergosterol that exists in fungus (yeast, mushrooms). D3 results from UV-B irradiation of the cholesterol precursor (what could turn into cholesterol) 7-dehydrocholesterol, which resides in mammals at different locations. In humans, the highest 7-dehydrocholesterol concentrations are in the outermost layer of skin (the epidermis). The body is doing this on purpose. It knows that UV-B radiation will hit your skin (if you don’t block it out).
Either D2 or D3 or both then go to the liver where it is converted to calcidiol. There has been some controversy in the past over whether D2 can be considered equal to D3 in terms of efficiently converting to calcidiol. A review of the present literature (here and here) has shown that D3 is much more effective in humans than D2, although D2 still does result in calcidiol production.
Calcidiol then travels to the kidney where it is converted to the actual biologically active form of vitamin D: calcitriol. This is the hormone that plays a significant role in the human body and is at least partly responsible for:
- Calcium absorption from the gut (food), and reducing calcium loss from bones (hence why it prevents rickets)
- Insulin regulation
- Neuromuscular function
- Immune function
- Reduction of inflammation
- Modulation of cell growth
- Prevention of multiple sclerosis
- Prevention of several cancers
Basically, it boils down to this:
UV-B Radiation ==> D2 or D3 (skin) ==> Calcidiol (Liver)
==> Calcitriol (Kidney) ==> Happiness
Humans cannot be healthy without enough vitamin D, and although vitamin D can be absorbed through food and supplements, the best source is unfiltered sunlight of sufficient intensity. However, vitamin D is known to be toxic in various ways at extremely high concentrations. High doses of and reliance on supplementation can cause these high concentrations, but absorbing it through skin slows down the production of vitamin D. This is a natural feedback mechanism that has evolved to optimize human health. The recommendation to avoid unfiltered sunlight entirely is thus misguided.
Determining how much vitamin D is in the human body is difficult. There are two tests for vitamin D levels: one measures blood calcidiol levels (what gets made in the liver from D2 or D3), while the other measures the biologically active calcitriol (produced in the kidneys) levels in the bloodstream. While there is disagreement over which test is superior, the calcidiol test is the most common.
Experts disagree over exactly how much vitamin D we should carry in our bloodstreams and optimal levels likely vary by individual. According to a 2005 study, estimates for the lower bound of calcidiol levels range from 50 nmol/L to 80 nmol/L, (nanomoles per liter, just an amount per liter of blood) with the authors suggesting that older individuals aim for levels closer to 80 nmol/L to lower the risk of bone fracture. Levels higher than 150 nmol/L can be considered toxic. Since the potentially toxic level is almost double what is now considered the bare minimum of 80 nmol/L, it is best to err on getting more vitamin D than less. There is a large buffer zone of healthy levels between 80-150 nmol/L.
Vitamin D (specifically, D2 and D3) intake is measured in international units (IU). The current official recommended daily intake, from the American Office of Dietary Supplments, is along the lines of 400-800 IU per day. In the absence of unfiltered sunlight, this is insufficient to get people to the 80 nmol/L threshold, and is only enough to prevent rickets, but not enough to obtain many of the benefits of vitamin D. Production from UV-B radiation on human skin varies by individual, as lighter-skinned people produce vitamin D much more efficiently than darker-skinned people. For a lighter-skinned person at midday in the middle of summer, 10-15 minutes of whole-body sun exposure will generate and release up to 20,000 IU of D3. This would be considerably less for a darker-skinned individual who should seek more sun exposure.
Other than variable D3 production by individual, other factors impacting D3 production include amount of skin exposed, geographic latitude, time of day, cloud cover, and air pollution levels. At latitudes far enough from the equator, during winter the sun is low enough in the sky that essentially all UV-B is screened out by the atmosphere, and vitamin D production is halted. The further from the equator, the longer this period is. During times of the year where vitamin D production is possible, the sun’s intensity is only high enough to produce significant vitamin D during midday when the sun is high in the sky, generally from 10-4. If it’s cloudy or the air is heavily polluted (cities), UV-B levels are drastically reduced even during summer.
The recommendation of this author after reviewing the recent studies is to seek out high-intensity sunlight when possible as a first-line of defense against vitamin-D deficiency. If time of year, weather, or scheduling conflicts are so severe that this is unfeasible, supplementation of up to 10,000 IU of D3 per day has been shown to be safe and effective. However, supplementation is not the preferred method as there is no biofeedback to prevent toxically high levels of vitamin D. If an individual is dark-skinned, then more time outside with more skin exposed during peak UV-B times or even heavier supplementation is necessary. This is especially necessary at latitudes far from the equator. Soak it up.
Readers may now be thinking “what about sunburns and skin cancer?”. These are valid concerns that I will be addressing in the next post.
Until next time,