Survivalist Tip: Vitamin D is actually a rarity in the vitamin world, found in only a few foods; mainly fish, eggs, milk and cod liver oil. Five forms of Vitamin D have been identified: 1 – 5. (Scientists were too excited to give them individual names.) Vitamin D’s basic function is to maintain normal blood levels of calcium and phosphorus. As you know, calcium is a critical element in the health of bones and teeth. Phosphorus is an element represented by the letter “P” (as if that wasn’t obvious); it is essential to all life forms because it helps to extract energy from food, particularly fats and starches. It speeds up the healing process and works with calcium to strengthen bones. Strong bones will be of the utmost importance as you struggle to survive in the immediate aftermath of the apocalypse. Strong bones will help you search for food, build fires in the outdoors to cook it and beat the crap out of anybody that tries to steal that food. The ancient Mayans – as short and stocky as they were – had incredibly strong bones. How else do you think they were able to build those massive pyramids without draft animals? Therefore, it’s important that you a ready supply of Vitamin D in your cache of goods. You may not build any pyramids, but strong bones will come in handy when you remodel your house after the axes switch.
Chief Writing Wolf 
My son (madly into martial arts) takes vitamin D – I fully agree with you.