We are in the middle of winter, which means you're probably hearing a lot about which vitamins and supplements you should take to boost your immune system and keep your body feeling healthy. This includes everything from zinc to vitamin C to vitamin D. Appropriate intake of vitamin D, the "sunshine" vitamin, is linked to a healthy immune system, balanced energy, and cognitive health, among many other positive health benefits. Because sunlight is one of the strongest sources of vitamin D, and because there aren't too many food sources of this vitamin, it's common for people to be deficient during the wintertime, and many Americans are deficient in it in general.

You get sick often.

Both of our dietitians say that Vitamin D is pivotal in helping our immune system, and "it equips our bodies to fight off viruses and bacteria effectively," says Manaker. But because of vitamin D's support in the health of our immune system, a common telltale sign of deficiency is if you're getting sick more often.

"Deficiency may lead to a weakened immune response, making individuals more susceptible to infections and illnesses," says Goodson. Manaker adds, "If you find yourself continually battling one cold after another, it might be due to insufficient levels of this critical nutrient within your system."

You experience mood changes or depression.

Have you ever had someone recommend a vitamin D supplement to you during the wintertime? Because we get a lot of our vitamin D from sunlight, we are more susceptible to deficiency during winter months when the sun isn't as strong or isn't out as long. This not only impacts our immune health, but it can also impact our mood.

How is our mood connected to vitamin D? Manaker notes that "Vitamin D plays an important role in mood regulation," and Goodson says "There is evidence suggesting a link between low levels of vitamin D and mood disorders, including depression." Research has also consistently shown that low levels of vitamin D are linked to greater symptoms of anxiety and depression.

Manaker says "A more overlooked sign of Vitamin D deficiency is mood changes, particularly depression or a feeling of persistent sadness." If you're experiencing some of these symptoms, vitamin D may help.

However, there are many reasons you may have mood swings or symptoms of depression that go beyond a vitamin D deficiency. If this is something you are feeling, it is important to get in touch with a healthcare professional.

You feel fatigued.

Another sign that you're not getting enough vitamin D may be that you're experiencing consistent fatigue and tiredness. This is because "Vitamin D is involved in energy production within the body," says Goodson.

This is separate from tiredness from a busy schedule or not getting good quality sleep. Manaker says that if you have a deficiency in vitamin D, "You might feel consistently weary or struggle to get through your day due to lack of energy despite getting sufficient rest."

Research shows this impact as well. A study published in the North American Journal of Medical Sciences found that in patients with "stable chronic medical conditions" who were experiencing fatigue, vitamin D therapy helped their fatigue levels.

You experience muscle pain.

This is a sign many people may not think of, but if you're experiencing muscle pain that isn't associated with your gym routine or consistent feelings of weakness, it may be connected to a vitamin D deficiency.

"Vitamin D is important for muscle function, and inadequate levels may lead to muscle aches and weakness," says Goodson. "This can affect both skeletal muscles and the smooth muscles of the body."

Research shows that vitamin D is needed for muscle strength and postural stability, and this vitamin has even been found to help decrease the risk of falls associated with muscle weakness in elderly populations. So if something is feeling off with the strength of your muscles or you're regularly feeling muscle aches, it may be time to check your vitamin D levels.

Your bone health is being affected.

"Vitamin D plays a crucial role in the absorption of calcium, which is essential for maintaining strong and healthy bones," says Goodson. Because of this role, vitamin D deficiency can negatively impact the health of our bones, which according to our dietitians can manifest in a couple of different ways.

"A deficiency can lead to a decrease in bone density," says Manaker, "which can make you more susceptible to fractures and injuries." Along with more potential injuries, you may also experience more consistent pain. "Vitamin D deficiency can also result in bone and back pain, and in severe cases, it may even contribute to conditions like osteoporosis," adds Goodson.