Supplementation is one option among a few — here's the fuller picture.
A short stretch of bare arms and legs in direct sun, a few times a week, is how your body makes vitamin D naturally. It has to be direct skin, not through glass.
A caution worth repeating: more sun is not automatically better. Overexposure still burns skin and raises long-term skin damage risk, regardless of how much vitamin D it makes. A brief, unburned exposure — a matter of minutes, not hours — is the goal. If your skin starts to redden, you've had enough; cover up or move to shade rather than "getting more D" for the day.
Fatty fish, eggs and fortified foods carry some vitamin D — but diet alone rarely closes a real gap, since very few everyday foods contain much of it. A nutritionist can help you get the most out of what's actually on your plate.
When sun exposure and diet aren't enough — common for people spending most daylight hours indoors — a supplement offers a reliable, controlled amount. It's worth raising with your doctor or pharmacist, who may point you toward something like Core D3.