The essential supplement we should all be taking between October and March

What is Vitamin D and why do we need it?

Vitamin D is actually a hormone—not a vitamin—that our bodies produce when our skin is exposed to sunlight. We've long known that vitamin D helps regulate how much calcium and phosphate we absorb into our bones, making it crucial for musculoskeletal health. But in recent years, studies have suggested that vitamin D may also play a role in preventing certain cancers, type 1 diabetes, heart disease, rheumatoid arthritis, and inflammatory bowel disease. It’s safe to say this is one nutrient (or hormone!) we really don’t want to be deficient in.

Why the change in guidelines?

Previously, the advice was that if we got enough sun exposure during the spring and summer months (April through September) and ate a balanced diet, we’d make and store enough vitamin D to get us through the colder, darker months. Since vitamin D is a fat-soluble vitamin, our bodies can store it. Some groups—like older adults and young children—were encouraged to take supplements, but most people were thought to be fine without them. After a recent review, though, the new recommendation is that everyone should consider taking a supplement during the fall and winter (October through March).

So, what’s changed? With growing evidence about vitamin D’s broader health benefits—and the increasing emphasis on protecting our skin by staying covered up and using high-SPF sunscreen—experts are concerned that we may not be making enough vitamin D naturally. While sunscreen is essential for protecting our skin from harmful UV rays, it also inhibits vitamin D production. The result? Many of us may not be producing enough to last through the year.

Why can't I just eat more vitamin D-rich foods?

So why are public health authorities recommending a supplement instead of just adjusting our diets? It’s because sunlight has always been our primary source of vitamin D. While some foods do contain vitamin D, they’re not significant sources on their own.

Vitamin D is only found in a few foods, and even then, the amounts are pretty small. There’s also uncertainty about how efficiently our bodies absorb vitamin D from food. So it would be difficult to meet daily needs through diet alone.

Here are some of the best dietary sources of vitamin D and their approximate content:

• 1 medium egg – 1.5 micrograms
• 100g raw salmon fillet – 8 micrograms
• 100g cooked liver – 1 microgram
• 10g fortified margarine – 0.6 micrograms

So why not just fortify more foods?

You might be wondering why we don’t just add vitamin D to more foods. The challenge is that vitamin D is fat-soluble, which makes it hard to incorporate into many products. Plus, many fortification methods rely on fish oil, which has a strong smell and taste—not ideal for most foods! While there’s potential to expand fortification, taste, consumer acceptability, and strict regulatory limits on how much vitamin D can be added all pose barriers.

How can I get more vitamin D from sunlight?

Now that we know sunlight is our best source—how can you make sure you're getting enough? Surprisingly, you don’t need to be outside for long. Just 10 minutes of sun exposure (without sunscreen) between 11 a.m. and 3 p.m., on your bare forearms, each day during spring and summer should be enough to produce sufficient vitamin D. Hanging laundry or watering the garden might be just enough—but always be mindful of sun strength and your skin’s sensitivity to avoid burns or damage.