Vitamin D helps your body absorb calcium and supports bone, muscle, and immune function. Your skin makes it from sunlight, and it's found in fatty fish, egg yolks, and fortified foods. People at higher risk of deficiency include those with little sun exposure, darker skin, older adults, and people who are overweight — for them, a supplement can genuinely help.
But the large VITAL trial found that routine high-dose vitamin D supplements did not reduce heart disease or cancer in the general, mostly-sufficient population. The lesson: assess your risk rather than mega-dosing by default. Vitamin D is essential, but more is not automatically better, and very high doses can be harmful.