We all know the importance of getting our daily vitamins. They keep our bodies healthy, boosting our physical fitness and immune system. Many vitamins can be absorbed through diet. But, some are environmental.

During the winter, it can be tough to get enough vitamin D. We synthesize vitamin D from ultraviolet-B (UVB) light from the sun hitting our skin. Our ability to synthesize vitamin D declines as we age, and things like pollution, clouds or a winter coat can stop us from being able to make enough of it on our own.

Vitamin D controls the functions of over 200 genes and is necessary for growth, development and health. That’s why it’s so important to maintain levels of vitamin D: it strengthens the immune system. We’ll take the best immune system we can get during the winter!

“Because vitamin D is involved in supporting essential functions like immunity and cancer prevention, as well as neurological, cardiovascular, and bone health, it’s easy to see just how dangerous falling short can be,” said Dr. Frank Lipman.

There is some good news: vitamin D doesn’t immediately drop once we start wrapping up for chilly weather. “Our bodies can store vitamin D in the liver and fat tissues, so there are ample opportunities to make vitamin D in spring, summer and fall,” said Carol Haggans, a registered dietitian from the National Institutes of Health Office of Dietary Supplements.

Indeed, you aren’t in immediate peril. However, studies shower that our vitamin D levels start dropping in September right through March. So, it’s vital to shore up your levels throughout the winter.

Vitamin D can be found in two forms in foods — vitamin D3 comes from animal proteins, vitamin D2 comes from vegetables. Fatty fish, eggs and mushrooms are excellent natural sources of vitamin D. Additionally, many products are fortified with vitamin D, like milk. All milks — nut, soy and dairy — are fortified. Some sources are better than others, 10 ounces of salmon a week will supply your whole supply of vitamin D. But, reading labels means you can mix and match your sources and be covered! Vitamin D is fat-soluble; eating vitamin D-rich foods with healthy fats, like avocado or egg yolks, helps your body absorb it.

However, you can bolster your vitamin levels in other ways too. There are vitamin D lamps that provide UVB inside your house in the winter. But they can be pricey. Getting any amount of sun is helpful. As little as 20 minutes a couple of times a week can be very beneficial. And, speak to your doctor about whether a supplement might be useful for you. Too much vitamin D can cause kidney stones and, surprisingly, weaken your bones. Getting the amount “just right” is essential.