Vitamin D and Its Role in Depression

Seasonal Affective Disorder, often shortened to SAD, is a form of depression tied to a chemical imbalance in the brain that seems to follow the calendar. When daylight shrinks and the sun stays low through winter, some people slide into it. The signs vary from person to person, but they tend to cluster: constant tiredness, pulling away from friends, losing the pull toward things that normally feel good, a sudden appetite for bread, pasta, and other carbohydrates, and weight that creeps up. Getting more sunlight tends to ease the symptoms, and when the condition runs deeper, phototherapy (treatment with artificial light) has worked for a lot of patients.

A set of findings published in the November 2014 issue of Medical Hypotheses pointed to vitamin D as part of the picture. The work came out of a collaboration spanning three institutions: the University of Georgia College of Education, the University of Pittsburgh, and the Queensland University of Technology in Australia. Alan Stewart and Michael Kimlin, the latter from QUT's School of Public Health and Social Work, sifted through more than 100 of the better-regarded papers on the subject and kept finding a thread connecting vitamin D to seasonal depression.

"Vitamin D could have a regulative role in the development of SAD," Stewart said. The idea has some anatomy behind it. Vitamin D and the elements in the body that respond to it show up in high concentration in the hypothalamus, which houses the circadian timing systems and a good deal of the wiring that runs them. So the brain's clock and the vitamin are keeping close company.

Stewart laid out why the team suspected vitamin D was feeding into seasonal depression rather than just riding alongside it. "We believe there are several reasons for this, including that vitamin D levels fluctuate in the body seasonally in direct relation to seasonally available sunlight," he said. The timing is one of the more telling clues. Studies put roughly an eight-week gap between the point when ultraviolet radiation peaks and the point when SAD typically sets in. That delay lines up with how long the body needs to turn UV exposure into usable vitamin D.

There is also a chemical angle. Levels of vitamin D in the central nervous system influence how much serotonin and dopamine the body makes, and both of those are already known to track with depression. Kimlin, who holds the Cancer Council Queensland professorship in Cancer Prevention Research, put the logic plainly: "Evidence exists that low levels of dopamine and serotonin are linked to depression, therefore it is logical that there may be a relationship between low levels of vitamin D and depressive symptoms." He noted as well that patients who are depressed have often turned up with lower vitamin D in their systems.

One thing sets vitamin D apart from the other vitamins we need. The body makes its own supply when skin meets sunlight. That production is not even across everyone, though. Researchers found that people with darker skin tend to register lower vitamin D, since more pigment absorbs less of what sun exposure would otherwise generate compared with someone who has fairer skin. The same shortfall shows up in adults who routinely cover up or reach for sun protection outdoors. Kimlin drew the uncomfortable conclusion: people with more skin pigmentation may face not only a higher chance of deficiency but, as a result, a higher chance of psychological and psychiatric problems too.

Vitamin D earns its keep in plenty of other ways. Bones depend on it to form, grow, and repair themselves, and the body cannot absorb calcium properly or keep its immune defenses in order without it. The research connects it to a lower risk of Multiple Sclerosis, to holding onto cognitive sharpness, and to keeping body weight in a healthy range. It has been shown to blunt the frequency and severity of asthma attacks and to lower the odds that women develop rheumatoid arthritis. Reduced risk of cancer and of heart attack has turned up in the literature as well.

"What we know now is that there are strong indications that maintaining adequate levels of vitamin D are also important for good mental health," Kimlin said. "A few minutes of sunlight exposure each day should be enough for most people to maintain an adequate vitamin D status." For a nutrient the body can brew on its own with a little time outside, the reach of vitamin D extends well past the skeleton and into the brain.