Getting Sick or Infected Often
One of vitamin D’s most important roles is keeping your immune system strong so you’re able to fight off the viruses and bacteria that cause illness.
It directly interacts with the cells that are responsible for fighting infection (4).
If you become sick often, especially with colds or the flu, low vitamin D levels may be a contributing factor.
Several large observational studies have shown a link between a deficiency and respiratory tract infections like colds, bronchitis and pneumonia (5, 6).
A number of studies have found that taking vitamin D supplements at dosages of up to 4,000 IU daily may reduce the risk of respiratory tract infections (7, 8, 9).
In one study of people with the chronic lung disorder COPD, only those who were severely deficient in vitamin D experienced a significant benefit after taking a high-dose supplement for one year (10).
Feeling tired can have many causes and vitamin D deficiency may be one of them.
Unfortunately, it’s often overlooked as a potential cause.
Case studies have shown that very low blood levels can cause fatigue that has a severe negative effect on quality of life (11, 12).
In one case, a woman who complained of chronic daytime fatigue and headaches was found to have a blood level of only 5.9 ng/ml. This is extremely low, as anything under 20 ng/ml is considered to be deficient.
When the woman took a vitamin D supplement, her level increased to 39 ng/ml and her symptoms resolved (12).
However, even blood levels that aren’t extremely low may have a negative impact on energy levels.
A large observational study looked at the relationship between vitamin D and fatigue in young women.
Bone and Back Pain
Bottle of Vitamin D Capsules
Vitamin D is involved in maintaining bone health through a number of mechanisms.
For one, it improves your body’s absorption of calcium.
Bone pain and lower back pain may be signs of inadequate vitamin D levels in the blood.
Large observational studies have found a relationship between a deficiency and chronic lower back pain (15, 16, 17).
One study examined the association between vitamin D levels and back pain in more than 9,000 older women.
The researchers found that those with a deficiency were more likely to have back pain, including severe back pain that limited their daily activities (17).
In one controlled study, people with vitamin D deficiency were nearly twice as likely to experience bone pain in their legs, ribs or joints compared to those with blood levels in the normal range (18).
Depression
A depressed mood may also be a sign of deficiency.
In review studies, researchers have linked vitamin D deficiency to depression, particularly in older adults (19, 20).
In one analysis, 65% of the observational studies found a relationship between low blood levels and depression.
On the other hand, most of the controlled trials, which carry more scientific weight than observational studies, didn’t show a link between the two (19).
However, the researchers who analyzed the studies noted that the dosages of vitamin D in controlled studies were often very low.
In addition, they noted that some of the studies may not have lasted long enough to see the effect of taking supplements on mood.
Some controlled studies have shown that giving vitamin D to people who are deficient helps improve depression, including seasonal depression that occurs during the colder months