The question

Vitamin D is absolutely integral to human health! This vital ‘nutrient’ provides many benefits to the body. Even though we still refer to it as a vitamin or a nutrient, the vast majority (approximately 90%) of active vitamin D₃ in the body is produced when our skin is exposed to sunlight. So that’s it then, we should get our vitamin D from sunlight, debate over! Not so fast! This topic is a little more nuanced than that. Sufficient, effective sun exposure on a regular basis can be tricky to achieve. It is certainly possible to consume vitamin D from the diet, but there are only a very limited number of food sources that contain sufficient amounts to meet daily requirements. As a result of these 2 key issues, it is now estimated that 40-75% of the world’s population is vitamin D deficient! (1) That´s right – you could be one of the 7 out of 10 people with low vitamin D, so read on to learn how to resolve this. 

Functions of vitamin D

Firstly why do we need vitamin D? Scientific studies have shown that vitamin D is needed for and supports a wide range of health-related functions: (2)

• Necessary for bone strength by aiding the absorption of calcium, phosphorus and magnesium to lay down new bone tissue
• Helps to regulate vascular health and may positively influence blood pressure
• Stimulates insulin secretion from the pancreas and may reduce diabetes
• Supports both infectious and inflammatory immune system response
• Potent antioxidant properties with some research suggesting potential for anticarcinogenic properties and reduced cancer mortality
• Supports oestrogen production, may help to regulate the menstrual cycle and reduce symptoms of PMT
• Has beneficial effects on the brain and may reduce the risk of cognitive decline and dementia

Vitamin D from sunlight

If sunlight exposure is the primary method of receiving up to 90% of this incredibly important nutrient, then we really should understand more about the complex relationship between sunlight and human exposure.

Standing outside on a bright sunny day, feeling the warmth of the sun on your skin, is a prized experience in the colder climates both north and south of the tropical and sub-tropical regions of the world, especially after a long winter season. Whereas for those living in the relentless heat of the tropical zones, it is an ongoing, daily battle to keep cool and avoid sunburn with much less fluctuation across the seasons!

Exposure to the rays of the sun (UVB wavelength) is known to help generate vitamin D through the conversion of a naturally occurring precursor under the skin called 7-dehydrocholesterol (7-DHC). This is converted into pre-vitamin D₃, which is then rearranged into the active form of vitamin D₃.

The standard guidelines indicate that the body needs a minimum of 20-25 mcg or 800-1000 IU of vitamin D per day. Although, it must be noted that reaching this minimal nutrient status is a different matter to preventing all adverse effects from insufficent vitamin D. A group of scientists in the vitamin D research community are advocating a daily requirement 4 times higher at 4000 IU per day to ensure optimal health effects (1). However, reaching this level of vitamin D from sunlight alone on a hot summer’s day would likely lead to sunburn for lighter skinned people.

How much vitamin D is actually produced when the skin is exposed to sunlight depends upon a range of factors, including:

• Time of day
• Season of the year
• Geographical latitude
• Altitude
• Length of sunlight exposure
• How much skin is directly exposed to the sun
• The colour or pigmentation of the skin

Time and season

Both the time of day and the season of the year make a difference to our ability to get vitamin D from sunlight exposure. This is related to the angle of the sun’s UV rays passing through the earth’s atmosphere and also the proximity of our geographical location on the globe to the sun itself.

In the early morning and late evening, the suns rays shine upon us at a sharper angle, causing them to pass through a greater distance of the earth’s atmosphere. The atmosphere helps filter the suns rays reducing Ultraviolet intensity (UVI). Lower UVI at these times of day means less conversion of 7-DHC to active vitamin D₃ as well as less chance of sunburn. Conversely, the higher the sun is in the sky, the more direct the sun’s rays are in relation to our location. This means less distance to pass through the atmosphere, higher UVI and more rapid conversion of 7-DHC to vitamin D₃, but also a much faster sunburn time too.

The earth spins around a central axis point, however, that axis is not vertical, it sits at a titled angle of 23.5 degrees in relation to its orbit around the sun. It is this tilted axis that gives earth its annual seasons. The northern and southern hemispheres gradually change in their position to the sun, being closer in the summertime and further away in the wintertime. This change in distance between the earth’s surface and the sun alters the length of day and the peak UVI. The UVI at midday in the summer will be much higher than UVI during winter midday. As already stated a reduced UVI will affect the rate of conversion to vitamin D₃. It has been well documented that vitamin D deficiency rates are higher in winter months. (3) In the temperate and frigid zones of the earth, above and below 42 degrees latitude, there will be periods during winter (approximately November to February) when it will be very difficult to convert any 7-DHC to vitamin D₃ at all, due to limited UVB radiation that can reach the earth’s surface.

Latitude and altitude

In addition to the time of day and season of the year, the latitude on the earth’s surface will also play a part in the angle of the sun’s rays through the atmosphere. The further north or south an individual is positioned on the planet, the more atmosphere sunlight will need to pass through and the lower the UVI will be relative to equatorial zones at the same time of year.

The ozone layer is the portion of the earth’s atmosphere that largely protects us from the strong UVB rays that cause sunburn and stimulate vitamin D conversion. Some UVB does still makes it through the ozone into the atmosphere below (troposphere). The higher the altitude above sea level, the greater the UVI will be due to the thinner atmosphere present at that location to help filter UVB rays. UVB radiation increases approximately 7% every 1000m in elevation above sea level (4). As a general rule, higher altitudes result in cooler temperatures, but conversely higher altitudes also mean a higher risk of sunburn due to increased UVB exposure. The higher UVI at altitude also speeds up the conversion process so that more vitamin D is formed in less sunlight exposure time.

Length of time and amount of skin exposed

Regardless of the time of year or the geographical position on planet earth, the length of exposure time to the sun’s UVB rays is directly correlated to a greater opportunity to covert 7-DHC to vitamin D₃. But this time of exposure for vitamin D production must be balanced carefully against the risk of burning the skin and causing damage as a result. The skin coverage of the clothing we choose to wear and the colour of an individual’s skin will also play an important part in determining the time exposed to UVB necessary to reach our daily vitamin D from sunlight requirement. The image below will provide some guidance regarding the percentage of skin exposed to sunlight based on which body parts are clothed or not.

The general guidance offered is to expose 18% of the body (face, arms and ahands) to the summer sun around mid-morning or mid-afternoon for 6-10 minutes in order to stimulate the conversion of up to 1000 IU of vitamin D. In the winter times this exposure may need to be increased up to 45 minutes. Well, at least these general guidelines apply to sub-tropical and temperate zones and lighter-skinned people. These rules will need to be adapted for hotter tropical zones and darker-skinned people.

Skin pigmentation

The natural pigment in human skin, called melanin, is present in varying amounts and gives rise to the different shades and colours of skin across the human population. Melanin absorbs and prevents UVB from passing through the skin layers. This helps to reduce the risk of sunburn and skin damage, but it also means that less vitamin D from sunlight is produced under the skin of those with higher melanin levels. There are 6 types according to the Fitzpatrick skin type categories. Type 1 is the lightest skin through to type 6 the darkest pigmentation. It is estimated that darker-skinned people may require 3-6 times longer sun exposure to produce the same relative vitamin D as lighter-skinned individuals.

Dietary vitamin D

So where does dietary vitamin D fit in after this extensive focus on sunlight exposure and all its compounding variables? Firstly, it should be quite obvious that getting enough vitamin D from the sun during the warmer summer months should be relatively easy to achieve with daily, fairly short outdoor sun exposure, even for those with darker skin. The summer is the least likely time to experience vitamin D deficiency.

It is the winter season when dietary sources of vitamin D become a very important contributor to maintaining our levels of this beneficial nutrient. The increased risk of vitamin D winter deficiency can be offset by carefully planning some simple inclusions in your diet. This is especially important for those living in the temperate and frigid zones, above and below 42 degrees latitude north or south.

Vitamin D is a fat-soluble nutrient, meaning it is only available within naturally occurring foods that have sufficient amounts of fat contained within them. The richest sources being primarily from fish and shellfish. Some food sources may be fortified with vitamin D (synthetic form added), but keep an eye out for the addition of the less effective, cheaper vitamin D₂ form, instead of the more beneficial active vitamin D₃ form.

Cod liver oil has been used as a nutritional supplement for hundreds of years, especially in the colder, temperate countries in the northern hemisphere. Whilst science now understands the benefits of omega 3 fats and vitamin D, perhaps traditional wisdom had worked out there was something good in this particular oil that helped them weather the physically challenging winter months a little better. Cod liver oil is a food ‘supplement’ and whilst it is the richest source per 100g, it will usually only be consumed 1-2 tablespoons (15-30ml) per day. This small amount will still deliver 1500-3000 IU so is a useful additional source of vitamin D.

A few commonly consumed foods, not sourced from the oceans, that provide smaller amounts of vitamin D include egg yolks (2.6 mcg/100g), butter (1.4 mcg/100g), and beef liver (1.1 mcg/100g). These fatty, naturally occurring foods will help contribute small, but beneficial amounts to our personal vitamin D reservoir during the short days and longer nights of winter.

If the above foods are difficult to source or just not to your taste preferences, then the use of supplementation may be warranted during winter months. In 2011 researchers at Bastyr University in California ran a study comparing vitamin D₃ supplementation in 3 different forms; oil drops, capsules or chewable tablets. They showed that when taking high doses of 10,000 IU (250 mcg) of vitamin D₃ daily for 12 weeks, all 3 forms proved to be both safe and effective, and significantly increased levels in the blood (6). The recommended daily intake varies depending upon the organisation you choose to rely upon from 1000 IU (25 mcg) to 4000 IU (100 mcg) per day.

Conclusion: Vitamin D from sunlight or diet

So as it turns out, it is not a matter of sunlight versus diet in the vitamin D stakes. Both have their place throughout the seasons of the year. When the sun does makes an appearance during the spring, summer, and autumn seasons we should seek to enjoy some regular exposure on our skin to allow for natural vitamin D formation. Not only it is good for our biology, it is also good for the mind and emotions to get outside and bathe in sunlight. If we get sufficient sunlight during the warmer months, supplementation will not be necessary at that time of year.

• Between 6-20 minutes of summer sunlight for skin types 1 – 3, between 20-45 minutes for skin types 4 – 6 of summer sunlight – the variation will depend on the time of day, season, temperature, latitude, and altitude.
• 18% (face, hands and arms) to 36% (face, hands, arms & legs) of the skin’s surface should be exposed to sunlight.
• Sunlight for vitamin D should not be hindered by sunscreen, which potentially blocks the UVB rays we need to facilitate vitamin D conversion.
• If your shadow is longer than you are tall, then the UVI is lower and slightly longer time in the sun will be required, if your shadow is shorter than you are tall then UVI is higher and shorter times in the sun are advised.
• Sun exposure to get sufficient vitamin D will not require any reddening or burning of the skin.
• If exposure to the sun will go beyond the individualised time limit for optimal vitamin D, then it would be appropriate to cover the body with clothing or to utilise a thorough covering of sunscreen to prevent sunburn.

During the winter months when sunlight is rare, we must then become more dependent upon natural dietary sources of vitamin D from cod liver oil, fish and seafood. If this is not practical and you want to be certain, then also including a good quality vitamin D₃ supplement to ensure our physiological needs are met may be an important strategy during the colder months of the year.

References

1. https://www.grassrootshealth.net/project/our-scientists/

2. https://ods.od.nih.gov/factsheets/VitaminD-HealthProfessional/

3. https://www.grassrootshealth.net/document/vitamin-d-deficiency-by-season/

4. https://www.mdpi.com/2072-6643/2/5/482/pdf

5. https://www.nature.com/articles/ejcn2011105

6. https://pubmed.ncbi.nlm.nih.gov/24684456/

If you enjoyed reading this blog or have your own thoughts or opinions on the subject, then please comment below. Please visit our blog archive for more great reads. You may also want to watch some of the expert interviews via the Fit to Succeed podcast.