Vitamin D and Seasonal Depression: What the Evidence Says

Key Takeaways

• Seasonal affective disorder is a type of major depressive disorder that typically appears around autumn and winter with symptoms like changes in appetite, weight gain, difficulty concentrating, withdrawal, and persistently feeling hopeless and sad.

• Although low vitamin D levels are correlated with depression in some research, vitamin D supplementation hasn’t been shown to reliably decrease depression symptoms, and more research is needed.

• Limited to no UVB exposure in winter makes it difficult to maintain vitamin D levels year-round. Eating foods that contain vitamin D and, when appropriate, taking vitamin D supplements can help maintain adequate vitamin D levels. 

I live in Chicago where the sun starts setting around 4 p.m. in the early winter. Despite the cold, my dog and I continue to go on regular walks, but I’m bundled up with only a sliver of eye showing. These months are really tough. It’s hard to keep your mood up when everything around you is dreary. 

And so, I, like many winter warriors, am concerned about my vitamin D levels for almost half the year. After all, there’s a link between vitamin D and seasonal depression. But the fix is rarely as simple as popping a supplement — the body’s physiology is just too complex for that. This article will take an honest look at the question of does vitamin D help with seasonal depression, plus three ways to raise your levels of vitamin D  to support your health and mood no matter the season or where you live.

What Is Seasonal Affective Disorder (SAD)?

Seasonal affective disorder, aptly named SAD, is a type of depression that’s related to lack of sunlight or a change in season, according to the Anxiety & Depression Association of America (ADAA). SAD is more than the winter blues because you’re more than just a little down or generally “blah,” you have symptoms of major depressive disorder, says the ADAA.

Interestingly, summer depression is also a mood condition, but it’s far less common. The type of depression that hits in the colder, darker months is also called winter depression, winter-onset SAD, or winter-pattern SAD. These all refer to the same set of symptoms.

Symptoms of winter-pattern SAD

If you live in a northern climate, the days will begin to turn darker and colder, and you may find that you don’t go outside as much. Or, if you do, you’re bundled up. And with that, mood symptoms start appearing with less daylight and sunlight. According to the ADAA, the following are symptoms of SAD:

• Changes in appetite, cravings, or weight gain

• Difficulty concentrating

• Feeling hopeless, worthless, or guilty

• Feeling sad or depressed most of the time

• Having low energy or feeling sluggish

• Losing interest in activities you once enjoyed

• Sleeping too much

• Thoughts of suicide or self-harm 

SAD is not defined as a standalone disorder in the “Diagnostic and Statistical Manual of Mental Disorders” (DSM-5). It is considered a major depressive disorder (MDD) with a seasonal pattern, according to the American Psychiatric Association. Typically, symptoms start to improve in spring as daylight gets longer.

Does Vitamin D Affect Serotonin?

Serotonin is a neurotransmitter. Your body uses the amino acid tryptophan to create serotonin. And although we think of serotonin as a mood-regulating hormone, that’s not the only thing it does. Serotonin is also involved in immunity, helping to stop bleeding, and promoting brain functions like memory, outlines a review in Health Science Reports.

Vitamin D and serotonin may be linked, but right now, their relationship isn’t completely clear. In the Health Science Reports research, the authors set out to look at how vitamin D supplementation affects serotonin levels compared to placebo. They could not come to a conclusion when analyzing six randomized clinical trials, though animal research suggests that there may be a plausible mechanism for vitamin D supplementation to improve serotonin communication between cells.

Instead, the connection may lie with UV light. When UVB light (a type of UV radiation) is used in a “rational and proper way,” there are many beneficial effects that go beyond vitamin D production, says Andrez Slominski, MD, PhD, a physician-scientist who specializes in dermapathology. As Slominski explains, vitamin D may be a marker of UVB exposure. But UVB may stimulate other mechanisms in skin important for health. Those include the production of hormones, neurotransmitters (including serotonin), and immune signals activated by UV exposure, as outlined by an article that Slomininski authored in PNAS. 

“There are multiple, multiple possibilities by which UVB can affect the body,” says Slominski. “This is why the use of oral vitamin D has its limitations. It’s only hitting on one pathway.” Expanding the scope beyond vitamin D is essential. In terms of treatment modalities, the rational use of measured UVB exposure that’s titrated to skin type, may make sense for activating the pathways mentioned above.

The Link Between Vitamin D Deficiency and Seasonal Depression

It’s easy to say that vitamin D deficiency is responsible for seasonal depression, but that connection is not so straightforward. Here’s what we know about vitamin D deficiency and depression.

• Higher vitamin D correlates with lower depression rates in observational studies. One study using National Health and Nutrition Examination Survey (NHANES) data found that elevated levels of vitamin D3 were linked to reduced odds of depression, per Frontiers in Nutrition. ( Other studies have questioned whether low vitamin D is a cause — or a consequence — of depression, according to a cross-sectional analysis in the Journal of Affective Disorders.

• However, vitamin D supplementation doesn’t reliably treat SAD. Research is conflicting and inconsistent, according to a review in the journal Nutrients, with some studies showing no difference in depressive symptoms between vitamin D supplementation and placebo groups, and others showing a benefit to well-being after supplementation. One problem may be that we don’t have reliable protocols across studies to show what people actually need in terms of vitamin D supplementation. 

• UVB exposure enables the skin to synthesize vitamin D. But intentional UV exposure is not recommended solely to increase vitamin D, because UV radiation increases skin cancer risk. UVB exposure is how your skin makes vitamin D from sunlight. Your skin also self-regulates vitamin D production so that you don’t get a toxic amount, notes an article in the American Journal of Lifestyle Medicine. 

• Sunlight may affect mood through pathways other than vitamin D alone. Sunlight is protective against physical and psychological conditions, and research shows that there’s clearly more than vitamin D at play. For example, sunlight regulates the body’s daily cortisol circadian rhythm, and cortisol levels are linked to mental health, points out research in Risk Management and Healthcare Policy, which looked at the impact of sunlight deficiency on operating room nurses. There is also the effect less sun has on our ability to be physically active, and exercise is protective against inflammation, oxidative stress, and brain and hormone changes that impact mood.

• Bright light therapy using visible light is an established treatment for winter-pattern SAD. UVB exposure can increase vitamin D production but has not been established as a treatment for SAD.

Association is not causation

One thing is for certain: We need more data on the link between seasonal depression, vitamin D, sunlight, and other potential factors in the development of winter-pattern depression. 

It’s possible that reverse causality is at play. There is an association between unhealthy lifestyle habits and depression that goes both ways: An unhealthy lifestyle increases depression risk and  depression usually worsens lifestyle habits, notes research in the Spanish Journal of Psychiatry and Mental Health. Being less active outdoors, having a weight in the obese range, or eating an unhealthy diet can all affect your vitamin D levels. 

Do Vitamin D Supplements Help Seasonal Depression?

Vitamin D supplements may not reliably treat SAD. As mentioned earlier, research is mixed. Trials have a major limitation because they mostly tested supplementation in people who weren’t deficient. 

One trial in JAMA looked at adults aged 50 and older without depressive symptoms and found that vitamin D supplementation compared to placebo did not help prevent depression over the median five-year follow-up. However, the mean starting vitamin D level of participants  was 31.1 ng/mL, in the sufficient range, and only 11.6% of participants had levels lower than 20 mg/mL.  As the authors point  out, their findings contrasted with observational studies that found that incremental increases in vitamin D levels was associated with a lower risk of depression. The fact that trials tend to show no benefit but observational studies show the opposite is likely due to other confounding variables. 

Earlier research that looked specifically at vitamin D supplementation and SAD found disappointing results, as the supplement didn’t affect symptoms for the 12-week study duration, per BMC Research Notes. One short-term trial (8 weeks) on 56 people with mild to moderate depression did find that taking a high dose of vitamin D every two weeks improved circulating levels of vitamin D as well as a noted decrease in depression severity scores, per BMC Psychiatry.

What is clear is that vitamin D supplements are not enough on their own. As the National Center for Complementary and Integrative Health (NCCIH) states: “At present, vitamin D supplementation by itself is not considered an effective SAD treatment.”

Vitamin D vs. Light Therapy for SAD — What’s the Difference?

Traditional light boxes designed to treat SAD, which have a 10,000 lux intensity in the visible light spectrum, filter out UV light, according to Mayo Clinic.  Bright light therapy works through the eyes and influences circadian timing. 

UVB light, however, works through the skin, activating a variety of neurotransmitters and hormones, including vitamin D. (The difference between UVA and UVB is important, as UVB stimulates vitamin D production.) Keep in mind that bright light therapy is a first-line SAD treatment, per The Einstein Journal of Biology and Medicine. Vitamin D is not a substitute for this therapy. Here’s a closer look at light therapy vs vitamin D:

How To Get and Maintain Healthy Vitamin D in Winter

If you are in a Northern latitude where winter hits hard with low levels of sunlight and scant time outdoors, you may need to work hard to help maintain healthy vitamin D levels in winter. There are three ways you can get vitamin D, including how to get vitamin D when sunlight isn’t an option:

Sunlight and UVB light therapy

Our bodies are smarter than sometimes we give them credit for. Not only is skin production the most efficient way for your body to make vitamin D, it’s also the most optimal. Your skin will shut production off, preventing toxic vitamin D levels. If you take too high a dose via oral supplement, you can increase your levels too much. 

Whether you can get enough vitamin D from the sun is a question for a good portion of the year in many climates. Due to the  angle of the sun, environmental factors, and winter behaviors, the skin often does not get enough UVB exposure to produce vitamin D in the winter months at northern latitudes. UVB devices can increase cutaneous vitamin D production. In one randomized controlled trial in Anticancer Research, the SOLIUS UVB system increased 25(OH)D concentrations in people with vitamin D deficiency or insufficiency. However, UVB exposure carries potential skin risks, and these devices are not established treatments for SAD.

Diet

It’s a good idea to make sure you’re getting foods that contain vitamin D. According to the National Institutes of Health, these include:

• Cod liver oil

• Egg yolk

• Fatty fish, such as trout, salmon, sardines, canned tuna

• Fortified ready-to-eat cereal

• Beef liver 

• UV-exposed mushrooms

• Vitamin D-fortified dairy and non-dairy substitutes

Unless you focus on fitting these foods into your diet often, it’s difficult to get all the vitamin D you need through diet alone, which is why diet is just one part of a vitamin D plan.

Supplements

Vitamin D supplements are sold as vitamin D2 andr vitamin D3. These can be helpful in correcting a diagnosed deficiency. Vitamin D is a fat-soluble vitamin, so make sure you take it with food that contains a healthy fat source. Watch for upper limits to avoid toxicity — talk to your doctor about a dose they recommend for you. 

Should You Test Your Vitamin D Levels?

Routine vitamin D testing is not recommended for most healthy adults. Testing may be appropriate when some clinical indications or conditions increase the risk of vitamin D deficiency. The 25-hydroxy vitamin D test is the most accurate testing for vitamin D, per the National Library of Medicine. This is a quick blood test. There are typically no signs of vitamin D deficiency in winter (or any time of year). When prolonged and severe, vitamin D deficiency can leads to bone pain, joint and muscle aches, fatigue, twitching, and weakness.

According to the National Institutes of Health (NIH), most people should have a level of 50 nmol/L (20 ng/mL) or more for sufficiency. That said, the Endocrine Society, which formerly endorsed a higher sufficient level (30 ng/mL or 75 nmol/L) no longer defines vitamin D sufficiency, insufficiency, and deficiency. For his part, Slominski says that above 30 ng/mL remains a good target. 

If you are being treated for documented vitamin D deficiency, your clinician can determine whether and when repeat testing is appropriate.

People most at risk for insufficiency or deficiency in vitamin D include older adults, those with limited sun exposure, those with darker skin tones, medical conditions that limit fat or nutrient absorption, and those who have obesity or have had gastric bypass surgery for weight loss, says the NIH.

How To Treat Seasonal Depression

Seasonal depression is treated with bright light therapy, cognitive behavioral therapy for SAD (CBT-SAD), antidepressant medications like SSRIs, as well as strategies like getting outdoor activity and maximizing sleep hygiene. It’s always a good idea to see a doctor for any type of depression, including one that arrives seasonally, especially if prescription medication may be needed. If symptoms persist, worsen, or involve thoughts of self-harm, see a medical provider right away.

The Bottom Line

Seasonal depression, or SAD, is a type of major depressive disorder that tends to arrive in fall and winter as daylight is shorter and we spend more time indoors, especially in northern winter latitudes. Although vitamin D has biological effects in the brain and low vitamin D levels have been associated with depression, there’s evidence that sunlight affects the body’s physiology and your psychiatry in ways that go beyond vitamin D. 

To maintain adequate vitamin D intake during winter, include vitamin D-containing or fortified foods and discuss supplementation with your healthcare professional when appropriate. Winter depression is a form of major depressive disorder, so talk to your doctor about treatment rather than trying to treat yourself. Therapies might include bright light therapy, cognitive behavioral therapy for SAD, medication, and mood-supporting lifestyle behaviors. 

Frequently Asked Questions (FAQs)

Does vitamin D3 help with anxiety? 

For people who have low vitamin D levels, there’s some evidence that vitamin D supplementation may improve anxiety symptoms, according to research in Brain and Behavior. However, vitamin D is not a verified treatment for anxiety. Generalized anxiety disorder is treated with psychotherapy and/or medication, per the National Institute of Mental Health.

How much vitamin D should I take in winter?

Ask your healthcare provider for a blood test to check your vitamin D levels. If you are insufficient or deficient, your clinician may recommend taking a vitamin D supplement and will let you know the dose that’s recommended for your situation. The Tolerable Upper Intake for vitamin D supplementation (the safe amount that won’t cause toxicity) is 4,000 IU per day for adults. To understand your correct vitamin D dosage for seasonal affective disorder, talk to your clinician. Vitamin D levels can generally be supported through dietary sources and, when appropriate, oral supplementation. UVB exposure can increase vitamin D production but carries skin-related risks and is not routinely recommended solely for this purpose.

Can you get enough vitamin D from sunlight in winter?

It is not likely you’ll get enough vitamin D from sunlight in winter if you live in a northern latitude. In these cases, you’ll want to support vitamin D levels with vitamin D-rich or fortified foods and, when appropriate, oral vitamin D supplementation can help maintain adequate intake.