It's common knowledge that 15 to 20 minutes in the sun can give you a sufficient amount of vitamin D for the day. But what if you're not able to be in the sun? How do you know if you're experiencing a vitamin D deficiency?Vitamin D not only helps with your metabolism and immunity, but also with your overall mood and body function. However, because signs of a vitamin D deficiency can be subtle, it can be hard to determine if you're experiencing a lack of it. In order to determine the different signs of a vitamin D deficiency, we spoke with Nicole Avena, PhD, Assistant Professor of Neuroscience at Mount Sinai School of Medicine, and Visiting Professor of Health Psychology at Princeton University."Vitamin D is produced by skin cells when they are exposed to light, this makes it unique among other vitamins which usually need to be ingested," says Avena. "While the sun is typically the best source for vitamin D, it isn't always easy to be in the sun due to UV exposures and also due to limited daylight in certain parts of the world, like in the northern climates. The body's ability to produce and absorb vitamin D decreases significantly in older people compared to younger ones. Especially for older people with restricted mobility or health conditions that require them to remain inside, it is nearly impossible for them to get sufficient vitamin D from the sun."Here are a few of the signs that you'll need to step into the sun—or eat more foods rich in vitamin D. And speaking of healthy eating, here's What Happens to Your Body When You Drink a Smoothie Every Day.1 You'll experience bone pain."Vitamin D regulates the metabolism of calcium and phosphate and is important in maintaining strong, hard bones," says Avena. This means that when there's a vitamin D deficiency, there can be issues with overall bone health—and can result in bone or even joint pain."In severe cases of vitamin D deficiency, the bones become demineralized and soft," says Avena. "In children, this presents itself as rickets. In adults, this can become osteomalacia. There is still research going on to see if vitamin D deficiency is involved in diabetes, high blood pressure, and autoimmune disorders."Here are 5 Health Benefits of Vitamin D, and Why It's Important.2 You'll feel weak.Yes, physically weak. One study by the Western Journal of Medicine that muscle weakness is certainly a sign that someone isn't getting enough vitamin D—but it can also be reversible.3 You'll feel tired and lethargic.Avena points out feeling tired and even lethargic can be a sign of a vitamin D deficiency. One study published by the North American Journal of Medical Sciences took a closer look at this claim and did find that 77.2% of patients with low vitamin D deficiency were presented with fatigue. When their vitamin D numbers were normalized, their symptoms of fatigue were significantly improved.For even more healthy eating tips, be sure to sign up for our newsletter.4 You're feeling moody.Studies do show there is an interesting tie between vitamin D deficiency and depression. One study published by the Journal of Issues in Mental Health Nursing concludes groups that are at risk for vitamin D deficiency (the elderly, adolescents, obese individuals, and those with chronic illness) are also the same groups that reported risks for depression. A similar study was also published by the British Journal of Psychiatry. If you are feeling moody and don't have an explanation for it, evaluating your vitamin D intake might be a helpful next step. Remember, it's also best to talk to your doctor about any ongoing depression that you are experiencing.5 You're having muscle cramps.Along with feeling weakness in your muscles, it is likely that a vitamin D deficiency will cause you to experience extra cramping as well. According to Cleveland Clinic, while it is a subtle sign, muscle cramps can be one of the signs. How to add more Vitamin D in your diet"You can get vitamin D from some foods," says Avena. "Many oily fish like mackerel and herring contain vitamin D, as does egg yolk. However in most foods, vitamin D is relatively low, and most people's diets do not contain sufficient amounts of the vitamin."Avena says you can find some dairy products that are fortified with vitamin D, or even take a daily supplement like Vitafusion to ensure you don't have to experience the signs of vitamin D deficiency again. Also, incorporating these The 11 Best Vitamin D-Rich Foods can help.