Vitamin D, often called the sunshine vitamin, is a fat-soluble nutrient essential for bone health, immune function, and mental well-being. Yet nearly half of all people don’t get enough. “When your vitamin D levels are normal, you feel fine,” says Rachel O’Connor, a clinical dietitian at NewYork-Presbyterian/Columbia University Irving Medical Center. “Even if you don’t notice a deficiency, preventing one is crucial. Maintaining proper levels helps protect bone density and supports your immune system.”

Here’s what you need to know about vitamin D and how to keep your levels where they should be.

### Why is vitamin D important?
“Vitamin D helps your body absorb calcium and phosphorus, directing these minerals to your bones and teeth,” O’Connor explains. “Along with calcium, it guards against bone loss. Beyond strengthening bones, it also plays a key role in immune health and reducing inflammation.”

Vitamin D may also affect mental health. Research suggests low levels are linked to a higher risk of depression. “Supplements might help with depression, but mental health is complex—vitamin D alone isn’t a cure-all,” O’Connor notes.

### Getting vitamin D from food
Food is the simplest way to boost your intake. Fatty fish like salmon, trout, and sardines are great sources, as are dairy products like milk, yogurt, and cheese. Many foods, such as orange juice and breakfast cereals, are fortified with extra vitamin D. “Since vitamin D is fat-soluble, pairing it with a meal—especially one with healthy fats—helps your body absorb it better,” O’Connor advises.

### Sunlight and vitamin D
Your skin produces vitamin D when exposed to sunlight, but factors like location and sunscreen use can limit this. “In places like the Northeast, consistent sun exposure isn’t always possible,” O’Connor says. “Sunscreen also blocks some vitamin D production, so getting your levels checked can help determine if you need a supplement.”

### Choosing a supplement
The two main forms are D2 (from food) and D3 (from sunlight and animal sources). “D3 is more effective at raising and maintaining vitamin D levels,” O’Connor recommends. “Most people don’t need more than 600–800 IU daily, but those with a deficiency can safely take up to 4,000 IU.”

### Signs of deficiency
“Vitamin D deficiency doesn’t always have obvious symptoms,” O’Connor says. Fatigue, muscle pain, and weakness may be clues, but only a blood test can confirm low levels. “A deficiency raises the risk of fractures, especially in older adults or those with osteoporosis.”