Vitamin D is one of the most common nutritional gaps there is. Research published in the American Journal of Clinical Nutrition estimates that 30 to 50% of people run low, and most have no idea, because a shortfall rarely announces itself. There's usually nothing to feel day to day, so it goes unnoticed for years.
It helps your body absorb calcium and keep your bones and muscles working properly. When levels stay low over time, that's associated with ongoing tiredness and achy bones or muscles, which isn't dramatic, and that's part of why it's so easy to miss.
Most people top up through a mix of sunlight and food, with salmon, mackerel, sardines, and eggs the strongest sources. Adults usually need around 600 IU (15 mcg) a day, and it's best not to exceed 4,000 IU (100 mcg), since vitamin D stores up in the body rather than washing out.
It matters most for anyone who gets little sun: indoor workers, people who cover up or have darker skin, and older adults.