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.