The Effects of Sun Exposure on Human Metabolic Health

Written by: Dr. Alexis Jazmyn

UVB light is perhaps the most demonized of all light in the solar spectrum. Simultaneously, it is also among the most important. UV light must be considered as an essential nutrient when it comes to health. The UV content of sunlight is highest around solar noon, and varies based on latitude; with equatorial regions getting consistently high UV index (UVI) light year-round. As you move towards the poles, the UV light content of sunlight begins to vary throughout the year as the Earth tilts towards and away from the Sun, with UVI being the highest in the summer and lowest in the winter. Of note, it is important to understand that the glass used to make the windows in your home, office, and car all block 100% of UV light (and 40% of near-infrared light). Thus, simply by virtue of living an indoor lifestyle, one will be critically deficient in UV light as well as near-infrared (NIR) light.

VITAMIN D

UVB light stimulates the production of pre-vitamin D3 from 7-dehydrocholesterol in the skin. What many don’t know is that about a dozen other vitamin-D-like molecules are also synthesized in the skin in response to UVB light. 

FIGURE 1. The many vitamin D-like molecules made when UVB light strikes
the skin (PMID: 32441029)

Vitamin D does play important roles in bone health and immune function but, most importantly, vitamin D serves as a biomarker for how much time an individual spends outdoors. This concept is made abundantly clear when referencing the medical research around vitamin D. On one hand, insufficient vitamin D status is highly associated with the development of autoimmune diseases, cancer (including melanomas), cardiovascular disease, diabetes, neurodegeneration, depression, and more. On the other hand, the clinical studies using vitamin D supplements for disease remediation and prevention have been totally lackluster, with the results showing little-to-no benefit of vitamin D supplementation on health outcomes. This is because vitamin D is not a metric to be optimized through supplementation. Instead it is providing insights into how much time individuals are spending outdoors in Nature and connected with the Earth and Sun. 

Serum 25(OH)-vitamin D levels should be optimized to between 60-80 ng/mL, which accounts for differences in skin tone. For example, individuals whose ancestors hail from more northern regions require less total sun exposure to optimize vitamin D levels because their skin contains less melanin (which absorbs all light, including UV light). Conversely, individuals with more melanated skin whose ancestors hail from equatorial regions require more total sun exposure to reach optimal vitamin D levels. Mainstream medicine tells us that D levels ~30 ng/mL are sufficient, but this is totally suboptimal. In order to get full suppression of parathyroid hormone (a hormone that is produced when vitamin D levels are insufficient), vitamin D levels must be >40 ng/mL, and in individuals living a more outdoor lifestyle, levels >60 ng/mL are easily achieved. Therefore, we should each strive to reach optimal levels via sun exposure (without sunscreen, which blocks UVB light from interacting with your skin!). Optimized vitamin D status is associated with protection against virtually all chronic disease (the mechanisms of which we will discuss shortly). To gamify your vitamin D optimization journey, consider downloading the Dminder app which allows you to track how much vitamin D you are making per second during a sun session.

Let’s also discuss the flawed logic in the demonization of UV light. All of the research studies showing the harms of UV light have been conducted with UV light in isolation. NONE of them have been conducted in the context of full spectrum sunlight. If you remember nothing else, please understand this limitation in “The Science” which is directly informing “The Policy”. The same literature that demonizes UV light (studied in isolation) also tells us that red and infrared light protect against UV-light induced skin and eye damage. ~50% of midday sunlight is composed of red and infrared light, with less than 10% of this sunlight being within the UV spectrum. In Nature, we NEVER encounter UV light without an abundance of red and infrared light. Period. Thus, studying UV light outside of the context of full-spectrum sunlight is not only moot, but disingenuous, duplicitous, and highly misleading. 

POMC

Outside of vitamin D, there is another molecule produced in response to UVB light that is of incredible importance (and this one I bet you haven’t heard of!). This molecule is a pro-hormone called pro-opiomelanocortin or POMC. POMC synthesis is stimulated when UVB light touches the skin and the eyes. It is cleaved into 10 distinct hormonal products, 6 of which we will briefly focus on here: ɑ-, β-, and γ-MSH, and ɑ-, β-, and γ-endorphins.

FIGURE 2. The cleavage products of the prohormone pro-opiomelanocortin (POMC)

MSH stands for “melanocyte stimulating hormone” and for good reason. These MSH hormones stimulate the production of melanin by melanocytes in the skin. In other words, they help you tan in response to sun exposure. However, in order to fully leverage this effect, both the eyes AND the skin must be exposed to UVB light simultaneously. That means no sunscreen, no sunglasses, no prescription glasses, no contacts, and as much skin exposed as possible (fun fact: the neurons that produce the most POMC are under the skin of the abdomen and the calves, so exposing these two areas is a top priority). One of the primary reasons people burn more easily in the sun today than ever before (outside of a diet enriched in industrial seed oils) is the normalization of wearing sunglasses and prescription lenses when spending time outdoors. Outside of this, it’s crucial for fair-skinned individuals to build what Dr. Jack Kruse calls a “solar callus” to avoid sunburn, aka gradually building up your sun exposure over time and going into the shade or putting on some light clothing if you feel yourself starting to burn (instead of slathering on the sunscreen). You can also leverage red and infrared light to avoid sunburn–red and infrared light exposure on the skin helps to support melanin production and reduce risk of burning. This is as simple as getting out during sunrise and sunset daily, which are highly enriched in red and infrared light, and devoid of UV light or, if you have a red light panel, you can leverage this as well.

In addition to their functions in supporting melanin production, ɑ- and β-MSH also play crucial roles in an area that almost everyone cares about: obesity, diabetes, body composition, inflammation, and reproductive function. Indeed, ɑ- and β-MSH in the brain act to suppress appetite, increase energy expenditure and metabolic function at the whole body level, modulate the immune system to reduce inflammatory burden, and regulate the production of sex hormones and fertility. In other words, if you’re somebody who gains 10 pounds by looking at a donut, struggles with metabolic dysfunction, an unrelenting appetite, inflammatory illnesses, PCOS, dysmenorrhea, low testosterone, obesity, or that stubborn 15-30 pounds that doesn’t seem to budge, then midday sun exposure your best friend when it comes to creating the life, body, virility, and health you desire.

Now, onto the endorphins! With mental illnesses at an all time high, alongside issues of willpower, addictions, cravings, and self-sabotaging behaviors, all the mainstream medical model has done to address the issue is to drug the population into submission, providing short-term gratification at the expense of long-term vitality. ADHD, anxiety, depression, cognitive decline, memory loss, anger, frustration, irritability, rigidity in thought and body, reaching for vices to get by… these are all signs of a burned out, low dopamine brain state. Indeed, the dopamine-depleted state characteristic of individuals leading an indoor lifestyle drives compulsive behaviors including over-consumption of hyperpalatable foods, social media, pornography, stimulants like coffee, nicotine, and adderall, and recreational drugs. Enter the endorphins: endogenously produced opioid compounds (like morphine) that boost mood, increase focus, decrease anxiety, and improve cognition. What is Nature doing by yoking endogenous opioid production to UV light exposure? She is, in effect, addicting us to the Sun because our bodies REQUIRE sunlight for the survival and reproduction of our species. 

The endorphins enhance our baseline dopamine levels which, outside of mood and motivation, have incredibly powerful knock on effects to things like the ability to think critically as well as the quality of our neuromuscular function, relevant both during exercise as well as at a baseline level (e.g. posture and gait). What does this mean? It means that the quality of your movement will be higher if you are exercising outdoors in the Sun. These benefits are further augmented by the presence of the red and IR light in sunlight which boost energy production in exercising muscle.

UVB light has also been shown to significantly improve the health and diversity of the microbiome via the gut-skin axis. In other words, you can leverage UVB light to improve your gut health independent of the food you eat. A healthy microbiome requires full spectrum sunlight on your skin, spending time in natural environments which increases your exposure to a diversity of microbial life, as well as prebiotics and postbiotics coming in through the diet. All three of these pillars are equally important when it comes to supporting the microbiome.

UVA LIGHT

UVA light potently stimulates the production of a molecule called nitric oxide. Nitric oxide (NO) is a critically important factor in the regulation of blood pressure. NO triggers a process known as vasodilation, which expands blood vessels–increasing the reach of the vascular system to facilitate nutrient and oxygen delivery to tissues and waste removal from tissues. NO is known to be crucial for cardiovascular health, while also increasing exercise performance, expediting the healing process for wounds and injuries, boosting sexual function, ameliorating erectile dysfunction, lowering blood pressure, and improving respiration in the lungs. Common issues associated with low NO are fatigue, vision loss, low libido, depression, high blood pressure, poor exercise tolerance, and more.

Bottom line: UVA and UVB light in the context of the Sun is not only NOT toxic, it is actually one of Nature’s most potent forms of medicine, supporting the health and functionality of every tissue in the body either directly or indirectly. 

If you reside somewhere that lacks high quality sunlight (i.e. high UV light) for part of the year, then Nature is instructing you to get cold. In the ancestral environment, you either live somewhere equatorial with high quality sun and warm temperatures year-round, or you live towards the poles where you receive high quality sun part of the year and cold weather the other part of the year. As it turns out, cold exposure possesses many of the same benefits to the body as the Sun. There is evidence that cold exposure actually prompts your mitochondria to make their own UV light in addition to making more infrared light (i.e. heat). Cold exposure also dramatically boosts levels of norepinephrine and dopamine, increasing mood, energy levels, and focus. Cold exposure also stimulates the production of new mitochondria, and increases metabolic rate by stimulating the mitochondria to make more heat (IR light). Thus, both the Sun and cold exposure have immense power to support metabolic health.