Seasonal Affective Disorder (SAD) is a variant of major depressive disorder that occurs during darker winter months when the days grow shorter. It’s fortunately far less common in sunny AZ. But it still does occur. This may be in part because, despite the sunshine, Vitamin D deficiency is still common in southern AZ. (I wrote here on why this is the case.)

SAD and Vitamin D deficiency are often discussed in the same breath, as if the latter is always THE cause of the former. This isn’t entirely the case, but they are very connected. Here’s what we know.

The Sun, Serotonin, and Vitamin D

We all feel better in blue skies, but what, specifically, is the sunlight doing for us?

Turns out, it’s more than just one thing. The brain responds to sunlight by producing serotonin, the feel-good neurotransmitter most associated with treatment of depression in the form of SSRIs. Studies show that the brain produces more of it on sunny days than on overcast days.

But what we all probably know best about Vitamin D is that it, too, is produced via sunlight exposure. In response to UVB rays, our skin creates 7-dehydrocholesterol. This enters the bloodstream and stops off of the liver, where it turns into 25-hydroxyvitamin D. This is the value tested on labs. Then this version travels to the kidneys and turns into the more metabolically active form called calcitriol, aka 1,25 dihydroxyvitamin D. Calcitriol’s main job is to increase calcium absorption from food (though high levels can indicate some more insidious processes in cases of chronic illness, as I wrote about here.)

Alternatively, you can eat 25-hydroxyvitamin D from fish, meat, eggs, and cheese (or get it from a supplement). 

So does sunlight make us feel better because of the serotonin, or the Vitamin D, or both?

Vitamin D Supplementation Can Lift Mood

Plenty of studies do show that low Vitamin D levels are connected with depression, and when Vitamin D is given orally to depressed, deficient individuals, depression lifts. So Vitamin D is doing something to directly improve mood, independent of the effects of sunlight.

This study makes that more explicit: although the cohort was small (15 participants), they were randomized into groups that either got a ridiculously high dose of Vitamin D one time (100,000 IU) or received a single dose of phototherapy. Those who received the Vitamin D experienced a much greater improvement than those who received the phototherapy.

So what’s going on here?

How Vitamin D Affects Neurotransmitters

In addition to its jobs regarding calcium absorption and immune function, Vitamin D also seems to both induce synthesis of serotonin, and block its breakdown, much like SSRIs do.

This may be part of the reason why Vitamin D receptors are present in areas of the brain linked to depression.

This is speculation on my part, but since chronic infections can suppress Vitamin D receptor function, I wonder if this is also the mechanism by which chronic infections can sometimes present as clinical depression.

Seasonal Affective Disorder Neurotransmitter Changes and Vitamin D

People with Seasonal Affective Disorder have been found to have a 5% increase in a serotonin transporter protein during darker months of the year, which means serotonin gets broken down more rapidly. Since serotonin production will also be lower with less sunlight, this compounds the problem.

Additionally, there’s a direct correlation between darkness and melatonin production—and as melatonin is the “sleep” hormone, this can mean hypersomnia, one of the hallmarks of depression.

And, of course, lower sun exposure means less Vitamin D production. Since Vitamin D indirectly raises serotonin levels as well, this too deepens the blues.

Traditional treatment for SAD is high intensity artificial light box treatment at 10,000 lux for half an hour daily. But studies suggest that those suffering from winter blues would do well to test for and treat low Vitamin D levels if present, as well. Optimal Vitamin D levels are between 50-80 ng/mL, though I always look for levels at least above 40. 

The Upshot

Even if you’re lucky enough to live in a sunnier climate year-round, if you struggle with low mood or depression, check your Vitamin D levels and treat, if appropriate.

Also, spend time in the sunshine for at least 15 minutes daily, if you have the opportunity to do so!