What dose to take?

Many experts recommend that we carefully monitor the calcidiol levels in our blood and adjust the dose. However, we don't really know what the ideal level should be. If my hypothesis is correct, then we should simply make sure the total dose we get from the Sun and from supplements is at a level consistent with being outside during summer all day long. This amounts to a dose of the order of 10,000 IU per day.

Note that dynamical equilibrium between production and destruction of vitamin D in the skin is reached on time scales of roughly a half an hour to an hour. This means that we would reliable get this maximum dose, if we would be outside with most of the skin exposed to the Sun for an hour or longer. Even if the sky is overcast, we would still get this dose, if we are outside for several hours. The dose at which dynamic equilibrium is reached doesn't change if the flux of UV radiation becomes less, what changes is the time needed to reach equilibrium.

This implies that the vitamin D dose has a natural constant value which is set at the maximum dose we can theoretically get. What is "theoretical" for us today, was quite normal for our ancestors. The calcidiol level cannot be considered to be a constant, as this depends on how fast your body uses vitamin D. If your body is fighting an infection, the immune system will use vitamin D at a faster rate than normal. The natural dose would only start to become less than the maximum dose, if the weather were so bad that your Stone Age ancestors would not get outside for long periods of time or if they would have to wear thick clothes covering most of their bodies.

My recommendation is therefore to take 10,000 IU per day during those parts of the year that you don't get any vitamin D from the Sun. Note that the UV index has to be 3 or higher for significant amounts of vitamin D to be produced in the skin in a period of an hour or less. 

When we do get large amounts of vitamin D from the Sun, a safe dose to take is 5,000 IU per day. Most people don't get the theoretical maximum amount of vitamin D every single day during the summer, the 5,000 IU per day then makes up for that. If you spend a lot of time indoors during noon during the summer, then you should take a higher dose. E.g. if you only get out in the midday Sun in the Weekend, you can safely use 10,000 IU per day for 5 days per week.

Of course, if your vitamin D levels were too low to begin with , you should follow the recommended treatment, which does involve monitoring of calcidiol levels. But once your levels are in the normal range, you should not use a low dose of 1,000 or 2,000 IU per day but instead make sure you get on average 10,000 IU per day from the Sun plus supplements.

My Answer

Reading the recent news on vitamin D, one can´t help but ask why the body has chosen to use vitamin D. Yes, it is a deliberate choice, as vitamin D is not like other vitamins that directly influence important processes in the body. Vitamin D acts as a hormone, it switches on genes that produce enzymes that in turn regulate important processes like the immune system. Calcium is absorbed from our food, not by vitamin D, rather by proteins that are produced by genes which are expressed in a response to vitamin D (actually calcitriol, vitamin D is converted to calcitriol).

Then, given that recent research indicates that for optimal health, we should take an amount of vitamin D that one cannot get from food alone, one should ask why animals have evolved to make some of their important processes artificially dependent on a hormone that isn't going to be produced when the Sun isn't high in the sky.

My answer to this question is that the body uses vitamin D as an indicator of the availability of food from the environment in the near term. So, when vitamin D levels go down in Fall, the body uses this to let the immune system function in a more energy efficient way, allowing the body to gain more fat reserves before the start of Winter. This then compromises the immune system somewhat. Now, the less body fat an animal has, the sooner vitamin D levels will fall (because vitamin D is stored in body fat), so it will start to save energy sooner in Fall, which makes sense from the point of view of my hypothesis.

Then during Winter, vitamin D levels will go down further, and more energy saving measures will be taken by the body. Eventually the bones won't be maintained anymore. Getting calcium from food costs energy. It is cheaper to get the calcium we need to maintain proper concentrations in our blood, from our bones than from our food (because the concentration of calcium in bones is very high, it is easy to get it from there). Obviously this comes at the expense of maintaining our bones, but this won't do a lot of damage if this situation only lasts for a few weeks.

Of course, instead of energy, one can also compare using proteins as enzymes in one way or in another way and consider the change in optimal use when food may get in short supply due to the arrival of Winter.