As the Days Get Shorter: Let’s Talk About Vitamin D

November in New England means cooler mornings, changing leaves, and noticeably shorter days. It also marks the beginning of a seasonal change we may not think much about: our vitamin D levels.

Vitamin D levels tend to be at their highest around August and September. As we move through fall and into winter, they begin to decline, typically reaching their lowest levels toward the end of winter.

Here in Massachusetts, that seasonal change matters. During the winter months, the sunlight simply isn’t strong enough for our skin to make meaningful amounts of vitamin D. We’re also spending more time indoors and, when we are outside, we’re bundled up with much less skin exposed to the sun.

How Does the Sun Give Us Vitamin D?

We often hear vitamin D called the “sunshine vitamin,” but the sun doesn’t actually give us vitamin D. UVB rays from sunlight trigger a process in our skin that allows our bodies to make vitamin D. From there, the liver and kidneys process it into a form our bodies can use.

Interestingly, when we are outside matters too. UVB rays are strongest around the middle of the day. Early morning and late-afternoon sunlight are much less effective for vitamin D production. And sitting in a sunny window doesn’t count: glass blocks the UVB rays needed for our skin to make vitamin D.

What about sunscreen? Sunscreen does block UVB, so in theory it can reduce vitamin D production. But studies of everyday sunscreen use have shown mixed results. More recent research suggests that consistent use of very high-SPF sunscreen may have a modest effect on vitamin D levels. That doesn’t mean skipping sun protection; it simply adds another interesting piece to the vitamin D puzzle.

Why Does Vitamin D Matter?

Vitamin D helps our bodies absorb calcium and phosphorus, making it important for healthy bones. It also supports normal muscle function.

One tricky thing about low vitamin D is that you may not know you have it. Many people with low levels have no obvious symptoms. More significant deficiency can cause muscle weakness and bone pain.

Can We Get Vitamin D From Food?

Yes, although relatively few foods naturally contain it. Fatty fish such as salmon, mackerel, tuna, and sardines are among the best natural sources. Egg yolks and certain mushrooms contain smaller amounts. Vitamin D is also added to foods including milk, some orange juices, yogurt, and breakfast cereals.

As we settle into fall and prepare for another New England winter, vitamin D is a great example of how our bodies respond to the changing seasons. Understanding where it comes from, why we need it, and how our levels naturally change throughout the year can help us be a little more thoughtful about our health as the days get shorter.