Most of us know that vitamin D is an essential vitamin we get from sunshine, but what exactly does it do for us and how do we know if we’re getting enough?

Vitamin D is made in the skin after sun exposure, but we can also get it from dietary sources such as oily fish (mackerel, tuna, and sardines), mushrooms and fortified dairy and nondairy substitutes. This vitamin is critical for immune health, bone health, and microbiome diversity, and, as you may have guessed, most people don’t get enough of it during the darker winter months.

Margherita T. Cantorna, a microbiologist and immunologist, explains that sunscreen, darker skin pigmentation, clothing, and reduced daylight all inhibit the body’s vitamin D absorption. Humans need at least 600 IU per day of vitamin D, so how can you make sure you’re getting enough? If you avoid sunlight or live in a northern region, it is recommended that you take a vitamin D supplement and eat extra vitamin D rich foods like salmon, eggs, yogurt, olive oil, and cheese. 

It is possible to have too much vitamin D, though. More than 4,000 IU per day is not recommended, but colder seasons and indoor lifestyles mean most of us are far from at risk for hitting this upper limit. So if you want to feel healthier and stronger this winter, consider looking at your vitamin D intake and making some tuna salad for lunch!