Grow Youthful: How to Slow Your Aging and Enjoy Extraordinary Health
Grow Youthful: How to Slow Your Aging and Enjoy Extraordinary Health

Vitamin D, sunlight, and more benefits

Why we are deficient in vitamin D

Symptoms of vitamin D and sunlight deficiencies

Sunlight, ultraviolet and vitamin D

Sunlight, time-of-day, resilience

How much vitamin D do you need?

Food sources of vitamin D

Vitamin D supplementation

Vitamin D resistance

Vitamin D overdose


Why we are deficient in vitamin D

Getting sufficient vitamin D is one of the keys to staying healthy and long-lived. Hundreds of studies show that most people are deficient in vitamin D, especially those who live modern lifestyles or who live in cities. Even the most conventional journals now acknowledge that a lack of vitamin D is responsible for a wide variety of ailments (1, 2, 44, 72), listed below.

Humans evolved in equatorial Africa, where there is constant and regular sun throughout the year. With all that sunlight our African ancestors synthesised much higher levels of vitamin D than most humans get today. Remember that they were out all day hunting and gathering, and mostly unclothed as it wasn't too cold. About 50,000 years ago a genetic mutation occurred that was responsible for the appearance of white skin in humans. White skin, with less melanin, synthesizes vitamin D in sunlight six times faster than dark skin (3, 14, 15). These white-skinned humans were able to successfully migrate to higher latitudes around the world because they could synthesise sufficient vitamin D to survive in the long winters and lower levels of sunlight.

People with black or brown skins who live far from the equator are at a particularly high risk of vitamin D deficiency. The early 2020 data on covid in Sweden found that 50% of all covid deaths were in the Swedish Somali refugee population, even though this population was only 1% of the total population. Further investigation found that their black skins prevented them from synthesising sufficient vitamin D at that high latitude, and they were not supplementing or eating a high-vitamin D diet.

We evolved to live outdoors, yet most people spend most of their time inside buildings (or vehicles). The majority of the world's population now lives above latitude 35 degrees. This means that most people are sun-deficient. The majority of these people are also unable to synthesise vitamin D from sunlight during the winter months, depending on the angle of the sun, their altitude, and the amount of skin exposure they get. For example, in Seattle (47 N) and London (52 N), there is insufficient sun for skin to make vitamin D from October to April. People living at high latitudes have an increased risk of diseases caused by a lack of vitamin D (8, 9, 14, 15, 72).

It is possible that during winter we can synthesise sufficient vitamin D in the skin provided we are at sufficiently high altitude. Measurement of UV-B shows that it increases significantly with altitude. There is anecdotal evidence that many Indians living at high altitude in the Himalayan mountains have good levels of vitamin D at the end of winter.

Vitamin D is not really a vitamin. It is more like a hormone and is also used to make other hormones. It should probably be called a pre-hormone and a good name is viterol (derived from the words vitamin and steroid).

One of Vitamin D's characteristics is it behaves rather like the stress hormone cortisol. There are a range of pharmaceutical drugs that act somewhat like cortical steroids, such as prednisone, which are often prescribed for chronic muscular and joint pain. Do not take these synthetic cortical steroid look-alikes - they have terrible side effects, especially in the long term. They also create dependence on them. Instead, you can get the same relief, and gain many other healthy advantages, by ensuring vitamin D sufficiency in your body.

Vitamin D is stored in both fat and muscle.

The half-life of vitamin D circulating in the body is approximately one month. By the end of winter in the high latitudes most people are deficient in vitamin D. If you have plenty of exposure to the sun during summer months you buildup a reservoir of vitamin D which can see you through the winter months.

Ageing. The precursor for vitamin D is 7-dehydrocholesterol (7-DHC), an oil in the epidermis of your skin. Sunlight on your skin converts 7-DHC to vitamin D. Young people have plenty of 7-DHC in their skin, but as people age the skin tends to dry and the level of 7-DHC in an old person is a small fraction of that when they were younger.

Obesity more than halves vitamin D production because 7-DHC is trapped in fat cells rather than being in the epidermis.

Sugar prevents the production of vitamin D in your skin. This is why diabetics are nearly always severely deficient in vitamin D. Sugar also prevents the absorption of vitamin D from the skin and from food. Sugar interferes with the activation of vitamin D before it can be used. Sugar turns into liver fat, which also interferes with the proper use of vitamin D. With so much sugar being added to foods today, and with most people having such sweet tastes, it is no wonder that most people have dangerously low levels of vitamin D. (82)

Symptoms of vitamin D and sunlight deficiencies

Insufficient vitamin D is associated with a wide range of modern degenerative diseases. (22, 44, 45, 72)

Sunlight, ultraviolet and vitamin D

When you sunbathe, vitamin D is created by the action of ultraviolet-B light on cholesterol (7-DHC) close to the skin surface (epidermis). For good health you need sufficient cholesterol in your diet. It is one of the reasons why coconut oil, a saturated fat, is used as a sunbathing oil. A better alternative with a content closer to 7-DHC is lanolin. After being in the sun, the newly created vitamin D on the oily skin surface needs 48 hours to be absorbed. Do not wash off the oil for 48 hours after sunbathing. If you have a shower after sunbathing, do not wash your skin with soap or you will wash away the newly synthesised vitamin D. I suggest that you do not ever use soap for washing your skin unless it is the only way to get off grime or a smell.

A light-skinned adult human will synthesise 5,000 IU of vitamin D in 5 minutes of naked sunbathing at midday on the equator. Five minutes per day is all this person needs to get his/her optimal daily supply of vitamin D. In a little over half an hour, they'll get their entire week's optimal health allowance. A healthy young adult can synthesise over 20,000 IU of vitamin D per day, provided they are not deficient in magnesium that is required for the process.

As you move further from the equator, further from mid-summer, further from midday, to low altitudes close to sea level or in cloudy or polluted air, less vitamin D will be synthesised by your skin. However, at a temperate latitude or at a high altitude in mountains, with a few more minutes per day it is still easy to get your full vitamin D requirement just from the sun.

The 45 degree rule. The rule of thumb is that the sun must be clearly visible and at least 45 degrees above the horizon before you can make sufficient vitamin D. If you stand on a flat surface, your shadow must be shorter than you and clearly visible. If your shadow is longer than you are, it is cloudy, or the air is very dirty or polluted, the sun may produce little or no vitamin D in your skin.

A black-skinned person will only synthesise one sixth as much as a white-skinned person. Dark-skinned people are much more at risk of vitamin D deficiency at temperate and cooler latitudes (3, 15).

Older people. In addition to having much lower levels of 7-DHC in their skin (dry skin), the elderly synthesise vitamin D slower than young people. A 70 year old person synthesises about a quarter as much vitamin D as a 20 year old. Few people over 70 years of age can make sufficient vitamin D in the sun, so they all need to supplement.

Ultraviolet radiation. Sunlight has three main types of ultraviolet radiation: A, B and C.

Ultraviolet-A radiation is of a lower frequency, in the range of 400-315 nm. UV-A is able to penetrate the atmosphere, including clouds and polluted air. It can even pass through glass windows.

UV-A penetrates through the outer human skin deep down to the elanocytes. UV-A is the dangerous ultraviolet radiation, which does its damage even on cloudy days, or if you are behind a glass window such as when you are driving. It is easy to get severely burned through over-exposure to the sun in cloudy weather. Most sunscreens do not block UV-A. (34)

Ultraviolet-B radiation is of a mid frequency, in the range of 315-280 nm. UV-B cannot penetrate glass, and is easily blocked by sunscreens. It is partially blocked by cloud, dust, air pollution and the atmosphere in general. It gets through best in the middle of a clear day, when the sun is immediately overhead. UV-B does not penetrate the skin deeply.

When the sun is low on the horizon, or when the sun's rays are weak, blocked by cloud or atmospheric pollution, UV-B rays are filtered out and relatively more of the UV-A rays get through.

Ultraviolet-C radiation is of such a high frequency that it is mostly unable to penetrate through the atmosphere. You will get practically none from sunlight.

Sunburn. UV-B only acts on the surface of the skin, causing sunburn if you get too much of it. Sunburn is a valuable and important signal - your body telling you that you have had too much sun. One of the (many) reasons I don't use sunscreens is that they hide this essential signal. (30) Even a little redness or burning causes skin damage, wrinkles, skin aging, and keratoses.

Sunglasses are a fashion statement invented in the early 1900's when people tried to mimic early aviators. People who wear them every time they go outside (and even when they are indoors) are harming themselves. We need sunlight on the retinas of our eyes. However, sunglasses may be necessary if you are not resilient and the light seems glaring. If there is additional reflection of sunlight such as in the snow on in a sandy desert on a sunny day, sunglasses can reduce the extreme level of light entering the eyes.

The best time of day to make vitamin D is in the middle of the day when the sun is directly overhead and UV-B exposure is maximised. Expose as much skin as possible for just a few minutes (longer if you are dark-skinned). Take care not to burn or go even slightly red. You have made most of your vitamin D for the day by the time your skin is halfway to getting red.

Vitamin D from sunlight is more effective in health maintenance, and stays in the body about twice as long as vitamin D from food and supplements.

Sunlight, time-of-day, resilience

In Grow Youthful, I strongly recommend getting sufficient sunlight at all times of the day as regularly as you can. Sunlight regulates sleep cycles and helps create melatonin. The infrared in sunlight warms the body and is essential for healing, immunity, energy production and other body systems. Low frequency ultra violet rays (UV-A) release the nitric oxide stored in compounds in your skin, dilating blood vessels, promoting cardiovascular health, enhancing the immune system and boosting energy production. Having sufficient nitric oxide protects your skin from sunburn. This is one reason why keeping out of the sun and slathering on sunscreen actually makes you less resilient and more susceptible to sunburn. (16)

Build up resilience. According to Weller (16), the highest risk of damage from the sun comes from intermittent exposure to excessive sun instead of regular exposure to sufficient sun. You need to build up your resilience with regular exposure to the sun every day if possible, including midday sun. The worst sun damage occurs when someone who is not sun-resilient binges in the sun and gets burned. Think of someone who lives in a winter city who travels to a holiday in a sunny place and gets sunburned on the first few days. Excessive and intermittent exposure that leads to sunburn is the most damaging, especially for young people.

Stay in the sun about half the time needed to turn your skin very slightly pink. If your skin turns the slightest pink then get out of the sun immediately - you have had too much. This faint pink should disappear within minutes and if it does not you have had too much sun for your level of resilience. If you get some sun every day, you will find that it takes longer for your skin to get slightly pink as you adapt.

Early morning sunlight is beneficial. Try to get outside between sunrise and 8 or 9 am. Early morning light on your eye's retina assists with the production of melatonin and other hormones. It also promotes production of a protein named filaggrin. Filaggrin protects the skin from radiation and other damage. In other words, getting early morning sun makes you more resilient and protects you from stronger sun in the middle of the day. Filaggrin also assists the immune system and helps prevent allergies and immune reactions such as rheumatoid arthritis and asthma.

People who avoid the sun are twice as likely to die as those who sunbathe every day. An epidemiological study (32) that followed 30,000 women for over 20 years showed that all-cause mortality was about double in those who avoided sun exposure compared to the highest sun exposure group. Outdoor workers have half the rates of skin cancers as indoor workers. Tanned people beat untanned people on virtually every health measure. Long term sun exposure is associated with less melanoma.

The researchers concluded that those who avoid the sun at all costs and slather on sunscreen to minimise sun exposure are doing themselves more harm than good. This is the exact opposite to the message we have been getting from the sunscreen industry and captured government regulatory agencies.

How much vitamin D do you need?

The level of vitamin D needed for good health is a lot higher than the US Food and Nutrition Board set as the Recommended Dietary Allowance (RDA). The RDA recommends 200 international units (IU) per day for children and adults less than 50 years old, 400 IU for adults age 50-70, and 800 IU for adults over the age of 70. The RDA was set at a level that prevents rickets but does not prevent numerous other diseases.

An optimal level of vitamin D can prevent and heal cancers (4, 42, 45, 46, 47, 48, 49, 50, 51, 52, 75, 76, 77, 78, 79, 80), multiple sclerosis (5), cardiovascular disease, autoimmune diseases, (66, 70) and infections such as influenza (6, 7).

A maintenance dose of vitamin D for a healthy adult is 4,000 IU per day. For someone who is deficient, ill, or facing physical trauma like surgery the daily dose can be ten or more times higher. If taking a high dose, I urge you to have your blood vitamin D level tested so that you can monitor and adjust the supplementation level.

Here is the most recent thinking on blood vitamin D levels:



Severely deficient
Optimal *

* Optimal for good health, healing and disease prevention.

To convert ng/ml to nmol/L multiply by 2.5

Food sources of vitamin D

Without sufficient sun exposure the optimal blood level of vitamin D of 50-99 ng/ml for an adult requires 5,000-7,000 IU per day from supplementation and / or from food.

Getting sufficient vitamin D from food is difficult. Getting sufficient vitamin D from sunlight will vary, usually being easy near the equator but limited by higher latitudes, seasons especially winter in high latitudes, being older than 50 years, and other lifestyle factors. Supplementation is the only option for people with these limitations.

The best food sources of vitamin D are wild (not farmed) oily fish like cod, tuna, salmon, mackerel and sardines. Other food sources include free-range eggs, free-range meat, and fresh or sun-dried mushrooms. However, the non-fish foods contain so little vitamin D that you would have to eat ridiculous quantities to get sufficient vitamin D (for example, forty free-range eggs per day). In summary, little vitamin D is naturally present in our food, especially for those who consume a modern, processed food diet or for people who avoid animal-based foods.

Cod liver oil is by far the best food source of vitamin D. One tablespoon of cod liver oil provides about 1,400 IU of vitamin D. This seems to be sufficient for a maintenance dose of vitamin D for a healthy adult. Somehow cod liver oil seems to increase the efficiency and absorption of vitamin D compared to taking vitamin D in a supplement form.

Do NOT take four tablespoons of cod liver oil per day to get a maintenance dose! Cod liver oil is rich in other nutrients like vitamin A, and taking a high dose of cod liver oil risks building up harmful levels of vitamin A and other nutrients.

Vitamin D supplementation

There are three kinds of vitamin D supplements. Almost all of them are synthesised in laboratories rather than being made from animals or plants.

  1. Vitamin D2 (ergocalciferol), the form of vitamin D found in plankton, and mushrooms exposed to UV-B light.
  2. Vitamin D3 (cholecalciferol), the form of vitamin D our skin makes, and that is generally found in animals.
  3. Calcifediol (also known as calcidiol, 25-hydroxycholecalciferol, or 25-hydroxyvitamin D3), the active form of vitamin D that circulates in the blood. When taken as a supplement calcifediol is fast-acting, raising the level of vitamin D in the blood within hours. It is often sold in tablet form and called rapid vitamin D. It takes many days or even several weeks before D2 or D3 are converted to calcifediol, the active form of vitamin D that circulates in the blood. Studies have found that calcifediol is more potent, is better absorbed, has better bioavailability and is more quickly cleared from the body. (57)
  4. However, I do not necessarily recommend calcifediol as the form in which to normally supplement vitamin D. Calcifediol is probably best used in an emergency, when a sudden illness, injury, trauma, surgery or vaccination require that you get your vitamin D levels boosted as soon as possible. Remember that D2 and D3 are closer to the form in which you get vitamin D from food and sunlight, and so are likely to be less intrusive to your body than supplementing in a form that you would never get from nature.

    Although it takes days or even a week or two to fully convert vitamin D2 or D3 to calcifediol, I have heard numerous people describe how a large dose of vitamin D2 or D3 had a potent effect by the next day.

    Vitamin K2 assists with the absorption and use of vitamins D2 and D3. (71) Vitamin K2 is often sold in a combined capsule or liquid spray with D3. Vitamin K2 is produced by bacteria in a healthy gut biome, and is found in animal fats. The best food sources of vitamin K2 are high quality unpasteurised artisan hard or strong cheeses which contain live bacterial culture and are made from raw milk, butter from grass-fed cows, meat from grass-fed animals, egg yolks from free-ranging hens and home made raw sauerkraut.

    Also ensure that you are magnesium sufficient and zinc sufficient. (Nearly everyone is not.) Magnesium is essential for the proper absorption and use of vitamin D. Vitamin D and zinc support each other in their activities. If you are deficient in one of them, it will make you appear deficient in the other. A deficiency in one of them inhibits the production, absorption and thousands of different ways in which they are both used.

    If supplementing with a high dose of vitamin D3 (such as 50,000 IU or more per day), it is important to ensure sufficiency of vitamin K2, vitamin B6, magnesium, zinc and boron to prevent the accumulation of calcium outside of the bones.

    Take vitamin D supplements irregularly. Rather than taking the same dose every day, take the week's dose over one, two or three days instead.

    Vitamin D resistance

    Several bacteria, especially spirochete bacteria like those that cause Lyme disease, also cause what I call vitamin D resistance. These bacteria cause a decrease in the efficiency of vitamin D receptors by 60 to 80 times. This means that someone diagnosed with Lyme disease can have a blood test that shows a normal level of vitamin D, but at a cellular level is suffering from a severe vitamin D deficiency. (58)

    Several other bacteria like Chlamydia and Mycoplasma, some viruses, and some moulds like Aspergillus, also cause this breakdown in the efficiency of vitamin D receptors.

    Vitamin D efficiency. Supplementing vitamin D will not remedy vitamin D resistance. (70) Instead, what is required is to increase the efficiency of vitamin D receptors. The following will help improve vitamin D response (efficiency): (58)
    Zinc, magnesium and boron sufficiency.
    Eliminating seed oils from the diet.
    Quercetin, which is found in onions, especially red onions. The richest food source is capers.
    Omega-3 oil sufficiency.
    Intense exercise bouts (if it is possible to do this in a healthy manner without risking injury).
    TUDCA bile salts.

    Vitamin D overdose

    It is almost impossible to build up an excess of the natural vitamin D that your body makes from sunlight, because production slows down as its level approaches optimal sufficiency.

    Daily consumption of 50,000 IU of a vitamin D supplement for several months may cause severe hypercalcemia (elevated calcium level in the blood), which is the primary symptom of vitamin D toxicity and in the worst cases can result in death. (In contrast, getting 5,000 IU of vitamin D per day prevents the buildup of calcium in blood vessels.) (12)

    The risk of taking a vitamin D overdose is significantly higher if you are deficient in magnesium or vitamin K2. (71) In addition, the risk of hypercalcemia is raised if you are taking any kind of calcium supplement.

    If you take a toxic excess of a vitamin D supplement, your body will try to pee it out. You may be dehydrated and excessively thirsty (polydipsia) at the same time, as your body produces excess urine (polyuria). Other symptoms may include vomiting, abdominal pain, apathy and confusion.


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