Vitamin D insufficiency affects almost 50 percent of the population worldwide. An estimated 1 billion people across all ethnicities and age groups lack enough of this vitamin, which is crucial for your overall well-being according to research.

Among people in the U.S., that number is 41.6 percent of adults — including 69.2 percent of Hispanics and 82.1 percent of African-Americans — data show. The trend is only increasing. 

Vitamin D deficiency occurs as a result of insufficient sunlight exposure and dietary limitations.

Vitamin D is unique because the body synthesizes it itself through exposure to sunshine, and it plays a huge role in calcium balance, Dr. Jennifer Franceschelli-Hosterman, DO, Geisigner Health System, says. It helps bones mineralize, and it promotes growth and maintenance of strong bones, she adds. Studies have found “widespread and alarming” rates of Vitamin D deficiency in patients with metastatic bone disease.

Obesity and being overweight increases the body’s need for the vitamin because of the higher amount of fat tissue, Dr. Franceschelli-Hosterman says. Vitamin D is a fat-soluble vitamin extracted from the blood by fat cells, altering its release into circulation. “[Obese people] have more trouble converting Vitamin D to a more usable form and often need 2 to 3 times the usual daily dose.”

Research has pointed to Vitamin D deficiency as a risk factor for heart attacks, congestive heart failure, peripheral arterial disease (PAD), and the conditions associated with cardiovascular disease, according to Johns Hopkins Medicine. People with low levels of Vitamin D (versus the optimal level) were 64 percent more likely to have a heart attack and had an 81 percent higher risk of dying from heart disease, research suggests.

Vitamin D directly interacts with the cells that are responsible for fighting infection. The immune system can become overactive if it doesn’t have enough vitamin D. If unbalanced, it can also attack the body’s organs.

A significant correlation was found between serum vitamin D deficiency and poor sleep quality. The study also showed that the mean scores for subjective sleep quality, sleep latency, and sleep duration were significantly higher in the vitamin D-deficient participants, indicating that the vitamin D-deficient participants had poorer sleep quality as well.

This is an odd symptom, but excess sweating in the head as opposed to the rest of the body has been associated with low Vitamin D levels. Excessive sweating in newborns due to neuromuscular irritability is still described as a common early symptom of Vitamin D deficiency, according to Mayo Clinic.

Vitamin D and calcium go hand in hand when it comes to bone strength. The important nutrient helps your body better absorb calcium, which results in enhanced bone health and strong, healthy bones.

There is rapidly increasing epidemiological and strong experimental evidence suggesting a role for Vitamin D in inflammatory bowel disease, which is a chronic illness that causes inflammation in the digestive tract. Vitamin D may help the immune system to reduce levels of inflammatory proteins that get overproduced, according to the Vitamin D Council.

Low serum ferritin, which stores iron, and vitamin D2 are associated with hair loss in women, according to research. Screening and supplementing may be beneficial in treating the problem. Alopecia areata, an autoimmune disease that causes sudden and severe hear loss, has also been linked to low Vitamin D levels.

Wound infections and wound healing may benefit from adequate levels of Vitamin D, according to research. Other studies have shown that the nutrient helps with the production of compounds that are vital for forming new skin, which is what wound-healing is all about.

Researchers have found a strong correlation between depression and a lack of Vitamin D. The precise connection between Vitamin D and the brain is not fully understood. What is known is that the nutrient plays a role in the production of serotonin, also known as the “happy” hormone. It affects people’s feeling of happiness.

Vitamin D helps in the regulation of insulin, Dr. Franceschelli-Hosterman says, and this way it helps prevent diabetes as it also affects glucose metabolism. Studies have shown significant improvements in Fasting Plasma Glucose and insulin after treatment with Vitamin D, suggesting that Vitamin D supplementation could reduce insulin resistance.

Vitamin D receptors are widespread in brain tissue, and Vitamin D’s biologically active form has shown neuroprotective effects including the clearance of amyloid plaques, a trademark of Alzheimer’s disease. Associations have been noted between low levels of Vitamin D and Alzheimer’s as well as dementia. The risk of cognitive impairment was up to four times greater in severely deficient adults. Other large studies have indicated that low Vitamin D concentrations may increase the risk of cognitive decline.

Mood changes, similar to SAD or “winter blues,” can be a symptom of Vitamin D deficiency, Dr. Franceschelli-Hosterman says. “There are several different pathways through which Vitamin D improves mood and reduces chronic pain.” The vitamin acts as a hormone that helps release endorphins, including serotonin, in the brain, she adds.

Muscle weakness can be a very subtle symptom which you may confuse with simply feeling tired. Vitamin D receptors are located all over the body, including the muscles. Deficiency decreases calcium absorption and often leads to musculoskeletal pain. Lack of Vitamin D can also cause muscle twitching.

Research has shown that people with higher vitamin D levels tend to have lower blood pressure and are less likely to develop hypertension. Studies have also indicated, even though they are not conclusive, that taking a vitamin D supplement may help reduce blood pressure.

Studies have indicated that vitamin D has a protective effect against respiratory tract infections such as like colds, bronchitis and pneumonia.

This is perhaps the one symptom most people overlook because they contribute it to pretty much everything else. Vitamin D supplementation has been seen to help patients who stopped feeling tired all the time during the day. A separate study found that almost 90 percent of nurses who reported chronic fatigue had low levels of Vitamin D.

Studies have suggested a role of low vitamin D levels for heightened central sensitivity, particularly augmented pain processing upon mechanical stimulation in chronic pain patients. This is because a vitamin D receptor is present in nerve cells that sense pain. Over 70 percent of people with chronic pain were found to be deficient.

The lower the vitamin D levels, the higher the anti-thyroid antibodies, according to the National Academy of Hypothyroidism. They are found in patients with autoimmune thyroid disorders such as Graves’ disease and Hashimoto’s thyroiditis, according to research. In an unrelated study, researchers found a significantly higher rate of vitamin D deficiency in children with the condition –73.1 percent, compared to healthy children –17.6 percent.

Vitamin D receptors are found in human skeletal muscle tissue, including the heart, which means that vitamin D has a direct effect on muscle activity and can influence maximal oxygen uptake. Some studies have shown that athletes with higher levels had a significantly higher VO2max than those with low levels.

Patients with chronic kidney disease have an exceptionally high rate of severe vitamin D deficiency that is further exacerbated by the reduced ability of the kidneys to convert the nutrient into its active form, according to research. (That ability is also diminished as people age, even if they don’t have kidney problems.)

Vitamin D can't be metabolized without sufficient magnesium levels, meaning Vitamin D remains stored and inactive, a recent study shows.  Adequate magnesium supplementation should be considered as an important aspect of vitamin D therapy, according to the National Institutes of Health.

Sunlight is the best known source of Vitamin D. You only need about 15 minutes a day. Other sources of Vitamin D include supplements and, surprisingly, a lot of foods. It naturally occurs in a few, but it is added to many processed foods such as milk and fortified cereal.