Vitamin D on the sunny side

How much vitamin D can the skin produce from the sun? If you take a closer at this interesting question, you will first come across the Holick rule. As you might remember, Dr. Michael Holick was the first to research and describe the ubiquitous functions of vitamin D. This prompted researchers to re-evaluate the substance previously only known as the vitamin for bones.

The Holick rule is based on a series of measurements that investigated the extent to which UV irradiation affects measurable vitamin D in the laboratory. The researchers used the minimal erythema dose (MED) as a reference value; it is the lowest radiation dose that causes skin redness. The rule derived from these results states that the vitamin D synthesis resulting from a sun exposure of ¼ MED to ¼ of the body surface corresponds to approx. 1,000 IU of orally ingested vitamin D.  But, to use the rule sensibly, the individual MED would have to be known in advance. Summary:  The Holick rule is of little use in practice.

But there’s even more to it. Researchers use the following formula to calculate the epidermal production rate of vitamin D in studies:

Q_D = q_e x S_type x S_der x t_ex x UV_VitD

S_type stands for skin type, S_der for the exposed dermal surface, t_ex for the exposure time in the sun, and UV_VitD for the energy unit of the radiation.

In practice, however, the conclusion is just as unsatisfactory as that of the Holick rule: The skin of a person with skin type III and 600 cm² surface area produces 10 µg (400 IU) of vitamin D at ⅓ MED.

In addition to the difficult determination of ¼ or ⅓ MED, these formulas lack important parameters such as age, physical condition, or medication. By the age of 70, the skin’s capacity to synthesize vitamin D is reduced by about 75%. On one hand, the skin condition changes. On the other, there is no longer sufficient 7-dehydrocholesterol available to synthesize vitamin D.

More practical, but riskier, are instructions that are based directly on the MED. We thus find a further calculation of the Holick rule and the statement that “the exposure of the body in swimwear with a minimum MED of solar radiation – the UV dose that starts to cause visible reddening of the skin – corresponds approximately to the oral intake of 10,000 to 25,000 IU of vitamin D”. The type of swimwear should not be forgotten. Those who sunbathe in a modest swimsuit will produce 3,000 to 5,000 IU in the time up to sunburn; those who sunbathe in something more revealing (e.g. a bikini or Speedo^®) will produce 15,000 to 20,000 IU. And if you stand upright instead of lying down and/or use sunscreen, you will produce less. Exactly how much less has yet to be determined.

The authorities have already gone further. The Swiss Federal Office of Public Health provides its citizens with a table with the length of time a person must spend in the sun in order to produce 600 IU of vitamin D. Under the heading “risk-free vitamin D formation in the sun is possible”, the Swiss Federal Office is trying to make the public more aware of the body’s endogenous vitamin D production. With the help of this table, you can calculate the approximate time you need to stay in the sun – if it’s even shining.

A laudable effort. Unfortunately, the Federal Office of Public Health assumes that 600 IU/day is sufficient for the Swiss population and concludes: “On sunny days from mid-March to mid-October, it is possible to produce sufficient vitamin D through the unprotected skin of the face, arms, and hands without the risk of sunburn. In the midday sun, the required vitamin D is produced in less than ten minutes.”

However, this puts the Federal Office at the same scientific level as 40 years ago. We now know that about 2,000 IU of vitamin D/day are necessary to maintain a good level of vitamin D throughout the year. This has been shown by recent publications. A study with African tribesmen proved that their daily sun exposure corresponded to a vitamin D intake of 2,000 IU. Individual hunters and gatherers had an average vitamin D status of 110 nmol/l (44 ng/ml). Nowadays, a value of 100–150 nmol/l (40–60 ng/ml) is considered optimal.

The balance between maximum vitamin D synthesis and minimum skin exposure to UV is another challenge that is not without pitfalls. Thus, the provocative thesis of one researcher – that sunscreen blocks vitamin D synthesis much more effectively than it protects against sunburn – seems to be confirmed.

However, sunscreens are rarely applied as recommended by the manufacturer. As a rule, protection is effective only at a layer thickness of 2 mg/cm². Vitamin D production is assumed to be blocked in this case. However, if only 1.5 mg/cm² is applied, which corresponds to the average application, the body’s endogenous vitamin D synthesis can take place. The MED is also reached more quickly. An in vivo study underlines these findings. A one-week holiday in a sunny destination, which led to sunburn among the young male participants (despite applying sunscreen), significantly increased their vitamin D levels by 28.0 ± 16.5 nmol/l.

The type of sunscreen also plays a role. A sunscreen with a high UVA filter has a lower proportion of the UVB filter. Because only the UVB rays synthesize the precursor of vitamin D, sunscreens with high UVA protection will still enable vitamin D formation.

The issue of vitamin D and summer is highly topical and very complicated. The question of when vitamin D should be supplemented is or is not necessary in summer is not easy to answer. Rough guides provides by bureaucrats are neither meaningful nor purposeful. And most people do not reach the required 2,000 IU/day until they are already in the damaging erythema range. The skillful combination of short unprotected exposures to the sun and longer sunbathing with sunscreens containing a high amount of UVA and a low amount of UVB filters allows younger people to produce sufficient levels of vitamin D. A holiday on the beach is usually accompanied by sunburn, but at least vitamin D formation is guaranteed.

So what should you do? If you have an office job and don’t go outside much on weekends (if the sun shines at all), we think you should continue to supplement with 2,000 IU/day. The same applies to older people, who can only produce a small amount of vitamin D – even when exposed to the sun – because of the altered skin structure. If you go out in the sun one or two days a week protected with a lot of sunscreen, you are probably on the safe side with a supplement of 1000 IU/day.

However, because vitamin D synthesis is associated with individual fluctuations, only a laboratory diagnosis can provide complete certainty. A vitamin D test at the end of the summer will show whether the amount supplemented and the amount formed were actually sufficient for optimal vitamin D levels.