Tuesday, October 3, 2023

What is ‘adequate’ sun exposure for vitamin D?

It is theoretically possible but practically impossible for most people to get enough vitamin D from sun exposure. The article discusses various factors that hamper such an effort.

Executive Summary

Experts advise 10 minutes of daily sun exposure to get enough vitamin D. However, that number is only for white-skinned people, wearing shorts at noon in certain parts of the world!

The requirement shoots up for darker skin types. It changes based on the time of the day, latitude and season, body exposure and position, etc. If you add the issues such as skin tanning and photoageing, it is practically impossible to get adequate sun exposure for most people.

A simple alternative is to take a daily vitamin D supplement of 2,000 IU or more.

Vitamin D is called the sunshine vitamin. When our skin is exposed to sunlight, it manufactures vitamin D. So health researchers advise one to get adequate sun exposure on a daily basis. Unfortunately, the word ‘adequate’ differs for each person, location, time of the day, and of the year. So it has created a controversy. In this article, I will discuss what is ‘adequate’ for you.


Vitamin D is an important vitamin that helps in many body functions. It regulates calcium levels in the blood and is needed for our immunity. It also plays a protective role in conditions such as cancer, heart disease, type 2 diabetes, depression, and dementia.

How Sunlight Produces Vitamin D

The sunlight is made up of rays of various frequencies. In childhood, we have learned about the seven colours of the rainbow—VIBGYOR or violet, indigo, blue, green, yellow, orange, red—coming from the sunlight. The sun rays also contain some invisible rays called ultraviolet–B or UV-B. These rays interact with a protein called 7-DHC in our skin, converting it into vitamin D.

Here is something for your general knowledge: The ‘C’ in 7-DHC stands for cholesterol. The compound 7-DHC is made from the cholesterol in your blood. So some researchers argue that if you are deficient in vitamin D, your body thinks that it is because you don’t have enough cholesterol in your blood and stimulates its production, leading to a high cholesterol problem. A corollary to that is to expect a lowering of blood cholesterol by reducing vitamin D deficiency. Read this article to learn what scientists are finding there.

One more tidbit: Only 20% of the cholesterol in your body comes from food; the rest is manufactured by your body. So what is the fuss about reducing dietary cholesterol? But let’s keep that discussion for some other article.

Vitamins D, D2 and D3

If you browse medical literature, or more likely the internet, you will read phrases such as vitamin D, D2 and D3. What are these different vitamins?

The vitamin D that our body uses is called calcitriol. It is manufactured by our kidneys as and when required from a compound called calcifediol. This calcifediol circulates in our blood and is measured by laboratory tests. So when a lab reports your vitamin D levels, it is the levels of calcifediol in your blood, not the real thing (calcitriol). Incidentally, that calcifediol is neither vitamin D, D2 nor D3. Such is life!

Calcifediol is made by your liver from a compound called cholecalciferol or Vitamin D3. It can also be made from another slightly different compound called ergocalciferol or vitamin D2. To add to our confusion, vitamins D2 and D3 make slightly different forms of calcifediol. But luckily, both of them are called calcifediol and both are measured together in the blood tests. So that distinction is only for puritanical biochemists. (

Vitamin D Production Sequence

  • The compound 7-DHC in the skin makes vitamin D3 in the presence of UV-B rays from the sunlight.
  • The liver converts vitamin D3 to calcifediol. Calcifediol is converted to vitamin D by kidneys for final use.
  • Our bodies cannot make vitamin D2. But if the body somehow gets vitamin D2, the liver converts it to calcifediol. That calcifediol also is converted to vitamin D by kidneys for end use.
  • Just for the sake of knowledge, the calcifediols made from vitamin D2 and D3 are slightly different. But both of them are called calcifediol and both are converted to real vitamin D–calcitriol– by the kidneys.

The laboratory tests measure calcifediol and report it as vitamin D or vitamin D3. But it is neither vitamin D, D2 nor D3.

Just when you thought you are getting a hang of it, it might help to know that calcifediol is also called calcidiol.

Vitamin D2 or ergocalciferol comes only from plant sources such as yeasts and mushrooms. Vitamin D3, on the other hand, is found in animal sources such as egg yolk, fatty fish, and cheese. The compound that our skin makes is also vitamin D3.

If you love arcane technical stuff, here is the production diagram of all those vitamin D brethren.

Exhibit 1. Vitamin D production in the body. Alexander Pico, Chris Evelo, Lucia Riberio, Martijn van Iersel, Kristina Hanspers, CC BY-SA 3.0, via Wikimedia Commons

Lab Tests and Units

To add to our misery, labs in different countries use various units for reporting vitamin D numbers. The labs in the USA and India report the values in ng/mL. Many other countries use nmol/L. You should multiply ng/mL values by 2.5 to convert them to nmol/L.

The recommended normal range for vitamin D in the blood is 30 to 100 ng/mL or 75 to 250 nmol/L. This is the level needed in your blood, not the daily requirement.

The body uses vitamin D on a continuous basis. So its levels in the blood will drop down, if not replenished on a regular basis.

Daily Requirement of Vitamin D

Since our body makes vitamin D from vitamins D2 or D3, when we talk about daily intake of vitamin D, we are actually referring to vitamins D2 or D3.

Many government bodies recommend a minimum daily intake of 15-20 µg of vitamin D. That number is also quoted in IU or International Units. 1 µg = 40 IU. So the minimum recommendation is for 600–800 IU a day.

However, many researchers and independent entities such as the Vitamin D Council recommend 2000 IU per day.

Considering that varying amounts of vitamin D are needed for different body tasks, it is quite possible that the optimal intake number is 4000–6000 IU a day, as some experts claim. Keep an open mind about these developments; the last word on this is not heard yet.

Amount of Sun Exposure

The good thing about vitamin D3 production by sun exposure is you will never overdose on it. The body shuts down further production of vitamin D3 once it manufactures enough.

The conventional wisdom is that 10 minutes of sun exposure is adequate. Unfortunately, there are too many ifs and buts to this number. Let us discuss them one by one.

Skin Colour

For starters, the 10 minute time is for a white-skinned person.

Our skin has a layer of melanocytes, the cells that absorb sunlight and also give colour to the skin. The darker the skin, the more is the absorption of sunlight in melanocytes and the less is the amount that reaches the skin layer underneath, which produces vitamin D3.

Here is Exhibit 2, a ready reckoner chart for sun exposure calculation. Match your skin tone and use the multiplier to calculate the time needed.

Exhibit 2. Sun exposure chart for various skin tones. Copyright 2022 Health Sachet.

My skin colour is roughly 4th from the left. So I need to multiply the 10 minute time by 4. I will need a sun exposure of 40 min daily.

Time of the Day

The 10-minute rule is for sun exposure at noon. Typically, the sun is high up in the sky at noon and very little of its rays are absorbed in the atmosphere.

Towards late afternoon, the sun approaches the horizon and its rays have to traverse through a thicker slice of the atmosphere to reach us. So more of its rays are absorbed in the atmosphere, which is why the sun feels milder in the late afternoon.

Obviously, we would need a higher exposure time of that late-afternoon sun. Of course, that would depend on the time of the afternoon. But roughly, at 4 pm, you would need 50% more time (which varies based on where you are on earth!) than at noon. So I would need 40 minutes x 1.5 = 60 minutes or 1 hour of the afternoon sun if I want to avoid the scorching heat at noon.

Body Exposure

The 10-minute rule requires exposing your face, arms, legs, and back. If you cannot stand in the sun in your shorts, you can safely assume that you are reducing the sun exposure by a big percentage. Here is a reckoner about how much of your skin surface is exposed to sunlight:

  • 12% if you are wearing smart casuals: a full-sleeved shirt and trousers (hands and face are exposed);
  • 26% if you are wearing a half-sleeved shirt instead of a full-sleeved one (hands, arms, and face are exposed);
  • 46% if you are wearing shorts and a t-shirt (hands, arms, face, and legs are exposed);
  • 72% if you are wearing shorts and no t-shirt (hands, arms, face, legs, and back exposed);

The 10-minute rule assumes a 66% skin exposure. In other words, if I were to stand in the sun in my office dress, I will need to multiply the number by further 5.5 times (12% versus 66%). So I would need to stand in the sun for 1 hour x 5.5 = 5.5 hours. Of course, this presumes that the sun will stay at the same place in the afternoon.

There is also a problem of social stigma. In many conservative cultures, it may not be proper for people to be seen outdoors without proper clothing. And in almost all cultures, women may not wish to be seen outdoors daily without ‘adequate’ clothing. But wait, there are further issues.

Body Position

The engineer in me is itching to tell you that the sun exposure will change depending on how you are facing the sun. If you are standing, the sun rays will have far less exposure than if you were to lie down like you would on a beach.

The rule is to check the size of your shadow. That is the amount of sunlight that falls on your body.

Now, it is quite unlikely that I will lie down on the floor in my office dress. And it is equally unlikely that I would be standing in my shorts without a t-shirt during my office hours. That should also be true with you unless you are lucky enough to have a job as a lifeguard on a beach.


Latitude means the (angular) distance north or south of the equator. The more the latitude of your place, the sun’s rays will come at a larger angle, reducing your sun exposure.

Of course, you (and I) don’t want to worry about the 3-D geometry to calculate how complex that can get based on the latitude of your current location. Suffice to say that the farther north or south you are from the equator, the lesser sun exposure you should pencil in.

Time of the Year

The 10-minute rule is for the summer. In many parts of the world away from the equator, seasons are extreme. During winter, the sunlight is much milder. So the 10-minute rule is actually:

  • 10 min in summer;
  • 20 min in spring/autumn; and
  • 30 min in winter.

In winter, I will need to stand in the sun for 5.5 hours x 3 = 16.5 hours, if I am residing in the North Americas, northern Europe, Russia, Australia, New Zealand, etc. Luckily, I live in Mumbai, India, which is fairly close to the equator.

This is already getting insane. Looks like anything short of me lying down naked for an hour daily on my building rooftop is not going to cut it for me.

Summer Times

In many countries near the equator (like mine), the summers are scorching. Do you really want to stand in the afternoon sun sweating profusely or in dry climates, dehydrating? Every day?

Winter Times

In some countries (unlike mine), the air outside during winter is freezing cold. Do you really want to stand in your shorts to catch the sunlight? Every day?

Glass Cabins

Some people think they can find a smart solution to the above dilemma. They would sit in their air-conditioned offices, in a cabin with glass windows. So they would get the lovely sunshine on their shoulders making them happy and producing vitamin D while the room air is quite pleasant. Alas, no such luck.

The window glass allows light rays to pass through it. But it blocks the UV-B rays in the sunlight. Since these ultraviolet rays are invisible to your eyes, you don’t notice any difference. But your skin does. Those are exactly the rays that help your skin produce vitamin D. Without them, your glass cabin will give you a sun-tan but no vitamin D. Read on this website: Can your skin make vitamin D if you sit in a glass cabin?

Now, about this suntan thing.

Sun Tanning

In many cultures, fairness is prided upon, rightly or wrongly. Many men and most women would like to retain fairer skin for whatever reason. But exposing your skin to sunlight will darken it over a period of time.

It seems tanning is a social signalling mechanism to show that the person is wealthy enough to have enough time to go to a beach and acquire suntan. But that is true in developed countries.

In developing countries such as mine, there is no special status attached to darkened skin. In fact, in such countries, tanned skin indicates you may have a physical labour job, which is less desirable socially.

Add a complexity that suntan will be visible mainly on fair skin, which is more common in developed countries. So, you can expect that in developing, brown- or dark-skinned countries such as mine, most people will not want to look darker by standing in the sun.

Skin Damage

The sunlight has harsh ultraviolet rays. Frequent exposure to them can lead to the breakdown of skin proteins such as collagen and elastin, leading to premature skin ageing.

You can check the skins of farmers who work outdoors the whole day. They rarely get vitamin D deficiency as their skins make enough vitamin D. But their skins, especially the facial skin which is thinner, is often photo-damaged, saggy, wrinkled, and listless.

For most people, such skin damage is probably not worth it if there is another way to get vitamin D.

My Views

  1. I hope I have conveyed to you that except for a select population of white-skinned people living in some parts of the world, most people will not be able to get ‘adequate’ sun exposure without hampering their lifestyle in a significant way.
  2. Why do all these if you can simply pop in a vitamin D supplement and get on with life? I am being practical—health is not the only thing that I desire in life.
  3. Vitamin D supplements come in two formats: a mega-dose of 50,000–60,000 IU once-a-week and a regular dose of 1,000–2,000 IU once-a-day.
  4. The mega-dose should be used only on a doctor’s prescription and if there is a severe vitamin D deficiency detected in a blood test.
  5. Everyone else is advised to take the regular daily dose.
  6. For using the daily dose, one does not need to test for blood vitamin D levels. Such tests are often very expensive, compared to the cost of the daily dose itself. In my country, the blood test would cost 100 times the cost of a daily tablet of 2,000 IU. How about your country?
  7. Since our body uses vitamin D on a daily basis, it is almost impossible to end up with too high blood levels of vitamin D by taking just 2,000 IU a day. However, it is quite likely to get that problem if one takes the mega-dose without a doctor’s supervision. So to me, it is a no-brainer that everyone should take a daily dose of 2,000 IU vitamin D.
  8. For technical reasons, a vitamin D supplement should also contain vitamin K2 in it. While most people won’t have a vitamin K2 deficiency, those who do could end up with serious problems such as heart attacks and kidney stones, on intake of just vitamin D supplements. It is like insurance—most people don’t need it; but those who do, are glad that they have that insurance.

To Read More

First published on: 19th March 2022
Image Credit: Mohamed Hassan from Pixabay


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