So many, and every year they discover more. The most well known is Rickets, which is actually a massive calcium deficiency caused by having no vit D to facilitate calcium uptake. However, low Vit D has been found to impact sleep-wake schedule, memory, multiple neurological functions, digestion, cholesterol levels (vitamin D synthesis uses cholesterol, meaning exposure to sunlight causes lower cholesterol levels). Vit D deficiency causes fatigue, memory deficits, increased pain sensation, insomnia, and many other issues.

If you live in any northern climate you are almost guaranteed to be D deficient, you get less sunlight, cover more of your skin when you are in the sun, and if you use sunscreen then it is likely you get almost no Vit D from natural synthesis.

The most commonly recommended dose for daily D intake is around 400-500 IU, which is what is needed to prevent rockets. FDA sets the maximum D intake at 4000 IU daily. However, current research has found tthat. 30 minutes in direct sunlight with shorts and T-shirt on can produce 50-100K IU. Now consider that humans used to spend 8+ hours a day in direct sunlight while farming and you can recognize that the current suggestions are vastly outdated. While it used to be considered that 50,000 IU Was an overdose, most researchers now believe that 50,000 IU should be the minimum daily intake.

Its also important to note that the synthetic Vit D we make supplements with is vastly inferior to naturally produced Vit D. It contains only 1 of the 3 main forms of the vitamin, and it is the least bioavailable form to boot. The best way to treat vitamin D deficiency is to do so naturally, through exposure to sunlight. In northern climates where this isn't always possible, especially in the winter, newer tanning beds now have lights that produce UV-B in similar amounts to natural sunlight (UV-B is the catalyst for vit D synthesis in your skin, older tanning beds only produced UV-A).