You might think that it is not a big deal if you don't manufacture vitamin D in the skin -- you can just pop vitamin D supplements to compensate. But biology is never that simple. Vitamin D taken orally is a very different thing from vitamin D manufactured under the influence of sunlight. An important consideration is that the vitamin D manufactured in the skin is immediately put to use in situ to ward off skin cancer. In a sense, people who regularly apply sunscreen are simulating the same condition brought on by low LDL. An SPF level of 8 or more completely disables the reaction that converts cholesterol to vitamin D, while at the same time not protecting you from the most harmful high frequency rays. This, I suspect, is why the incidence of melanoma cancer in children has been increasing by 3% per year since the 1970's
, as more and more people buy into the sunscreen scare (Melanoma in Children) .
Secondly, disabling the natural production mechanism and replacing it with oral supplements is a lot like destroying the pancreas' ability to make insulin and taking insulin shots to compensate. Your body's natural mechanisms have exquisite control over production levels, turning the spigot on and off based on carefully monitored biological needs:
"The body has two mechanisms to prevent an excess of vitamin D from developing: first, further irradiation converts excess vitamin D in the skin to a variety of inactive metabolites; second, the pigment melanin begins to accumulate in skin tissues after the first exposure of the season, which decreases the production of vitamin D." (Vitamin D Regulation) .
Thus, you can never suffer from a vitamin D overdose if you get your vitamin D exclusively from sun exposure. Even if you get it in part from foods that are natural sources, such as fatty fish and eggs, it is never present in the potentially toxic concentrations that can be found in pills, and it is always accompanied by fats that are an important factor in aiding absorption.
A series of thoughtful and provocative articles by Marshall et al. ,, have urgently argued that the current trend towards recommending vitamin D supplements to just about everybody is misguided and potentially very dangerous (Oral Vitamin D Dangerous) . Like much of what is wrong with modern medical practice, the arguments in favor of vitamin D supplements are deceptively simple and appealing. It is becoming increasingly evident that vitamin D plays a critical role in protecting from cancer and infection, in addition to its critical role in calcium metabolism and bone development. There has been a growing awareness that our nation faces a vitamin D deficiency epidemic. Instead of simply recommending that people spend more time out in the sun, the solution that is recommended is to give everybody a life-time "prescription" of oral vitamin D supplements.
However, Marshall argues that this growing practice may be problematic because it is exposing gut bacteria to concentrated doses of vitamin D, which is acting essentially like an antibiotic. Many of the original bacteria in the gut succumb, and a new mix of biota emerges, with adverse consequences, possibly even contributing to the obesity epidemic:
"Is it possible that the chronic addition of immunomodulatory "Vitamin D" to the diet of Homo sapiens has disturbed the historic composition of gut microbiota, and thus is as at least partly responsible for the current epidemic of obesity? Physicians know that chronic administration of corticosteroids encourages obesity. More research is needed to better define the immunomodulatory activities of this secosteroid, before encouraging even more of it to be added to the food chain." 
But perhaps even more alarmingly, he hypothesizes that the bacteria may be evolving to produce factors that inhibit a critical step that converts the ingested vitamin D into the active form (from 25-D to 1,25-D). The bacteria would be inspired to do this in self defense: the process of natural selection would select for the ones who had acquired this defense mechanism. This is directly analogous to the mechanism that is causing bacteria to become resistant to just about every antibiotic we throw at them. It is unclear what would be the long-term consequence, but it seems really alarming to imagine a situation where the step that activates the vitamin is foiled. One outcome he suggests is an increased incidence of autoimmune disease, something that is definitely happening at this time. But surely there would also be an increased susceptibility to infection and cancer, since the oral vitamin D is being neutralized by the bacteria and therefore wasted.