Vitamin D is something of a puzzle. It's both a nutrient that we eat and a hormone that our bodies make. We need it to live, but few foods contain it and we need plenty of sun to synthesize it.

Even if you live in Southern California, wearing sunscreen—a must for preventing skin cancer—means you may not be getting enough vitamin D. To better understand why that matters and what to do about it, we looked into the link between vitamin D and health. It turns out the "Sunshine Vitamin" may hold answers for issues as diverse as bone strength, respiratory infections and autoimmune diseases.

"Vitamin D plays a known role in bone health," says Dr. Lacy Knowles, a primary care physician at Cedars-Sinai. "It's essential for bone growth, and also promotes calcium absorption in the gut."

Low levels of vitamin D may be behind osteoporosis, which causes weak, thinning bones. "If you have weak bones, you're at heightened risk of falling, and osteoporosis makes it more likely you'll suffer a fracture or break," says Dr. Knowles.

Vitamin D may also help increase muscle strength, which can help prevent falls as well. An analysis of a dozen trials that followed 42,000 people aged 65 and older showed that a high intake of vitamin D supplements reduced hip and non-spine fractures by about 20%.

While vitamin D is naturally found in foods such as salmon and egg yolks, and added to others, including fortified milk and breakfast cereals, many people don't get enough vitamin D through diet. As we age, we also tend to spend less time outdoors in the sun, making it hard to synthesize vitamin D. 

"I often recommend that older individuals in particular take supplements," says Dr. Knowles, adding that younger people may get enough vitamin D through diet, but can safely supplement too.

"If you're between 18 to 65, make sure you're getting 800 international units daily to support bone and muscle health," she says. "Adults over 65 need 1,000 to 2,000 IU every day."

But don't take too much. Excessive intake over a long period of time can result in kidney damage and high blood pressure. You should never go over 4,000 IUs a day unless you've been directed to do so by a physician.

In recent years, a growing body of studies has shown that vitamin D may help protect people from respiratory infections, including cold and flu.

The strongest evidence to date comes from a global meta-analysis conducted in 2017. For people with the lowest levels of vitamin D, investigators found that daily or weekly supplementation cut the risk of respiratory infection in half, although all participants who supplemented regularly got some benefits.

How does vitamin D help? Possibly by boosting levels of antimicrobial peptides—natural antibiotic-like substances—in the lungs.

When COVID-19 emerged, researchers wondered if there might be a link between vitamin D and disease severity here too, not least since some people had serious pulmonary symptoms.

In a preliminary report from 2020, a group of Cedars-Sinai researchers noted just that: COVID-19 patients who had a vitamin D deficiency before they got infected had worse symptoms, and were more likely to need acute care, than COVID-19 patients with healthy vitamin D levels. More studies would be needed to figure out if there's a direct causal link between low levels of the vitamin and disease severity.

The implications may be profound for the future treatment or prevention of acute respiratory infections. But what about autoimmune diseases, like multiple sclerosis (MS), where the immune system destroys the fatty substance that coats nerve fibers in the brain and spinal cord?

"There's a definite correlation between vitamin D and MS, and more broadly between vitamin D and immune regulation," says Dr. Ghazal Lashgari, a Cedars-Sinai neurologist specializing in MS. "One study followed a cohort of over 90,000 women over 20 years and found that those with higher levels of total vitamin D intake had a notably lower risk of developing MS."

In certain cases, patients who already have MS should receive vitamin D along with their MS medication. Those with very low vitamin D levels may need to start with large doses prescribed by their physician, followed by a maintenance dose.

Benefits may include a reduction in the progression of brain lesions and therefore disability. Benefits may include a reduction in the severity and frequency of symptoms.

"The benefits could be due to the positive effects vitamin D has on the immune system, although we need more research to confirm this," says Dr. Lashgari, noting that rate of MS is much higher in regions with grey skies, where vitamin D deficiency is also far more common than in sunnier climates.

Vitamin D may be linked to other autoimmune diseases as well. A recent study that followed more than 25,000 participants for five years (women over age 55 and men over 50) showed that those who took vitamin D, or vitamin D and omega-3 fatty acids, had a significantly lower rate of autoimmune diseases—including rheumatoid arthritis, polymyalgia rheumatica, autoimmune thyroid disease, and psoriasis—than people who took a placebo (inactive substance).

While more research is needed, there are good reasons for optimism. And while Vitamin D may not be a miracle, it merits serious consideration as a key component of good health, from prevention to treatment.