If you notice your mood dipping every winter — or you just feel flat, unmotivated, and not quite yourself — there is a good chance your vitamin D levels have something to do with it.
It is not just a feeling. There is real research behind the connection between vitamin D and mental health, and it is one of the most overlooked reasons people in Canada struggle with their mood, especially through the fall and winter months.
Why Vitamin D and Your Brain Are Connected
Most people think of vitamin D as a bone health nutrient. And while that is true, it does so much more than that — especially when it comes to your brain.
Vitamin D receptors are found throughout the brain and nervous system. That means your brain actively uses vitamin D to function. One of the most significant ways it does this is by supporting the production of serotonin — the neurotransmitter most closely linked to feelings of wellbeing, calm, and happiness.
When vitamin D levels drop, serotonin synthesis can slow down. And when serotonin is low, you might notice it in your mood, your motivation, your sleep, and even your ability to manage stress.
The Canadian Problem
Here is why this matters so much if you live in Canada.
Your body makes vitamin D when your skin is exposed to UVB rays from the sun. But from roughly October through April, the sun sits too low in the sky for most Canadians — including those of us in Windsor — to produce any meaningful amount of vitamin D from sunlight alone.
That is up to six months of the year where your body is not replenishing vitamin D through its most natural source. If you are also spending most of your time indoors, eating a diet low in vitamin D–rich foods, or have darker skin, which naturally filters more UV, your levels can drop significantly.
The result? A lot of Canadians are walking around with low vitamin D — and many of them attribute how they feel to stress, burnout, or just "winter."
What the Research Says About Vitamin D and Depression
The link between vitamin D deficiency and depression is one of the most studied areas in nutritional psychiatry right now.
Multiple studies have found that people with depression tend to have lower vitamin D levels than those without. A large-scale review published in the Journal of Affective Disorders found a significant association between low vitamin D and depressive symptoms across different age groups.
Researchers have also looked at Seasonal Affective Disorder (SAD) — a type of depression that follows a seasonal pattern, typically worsening in fall and winter and lifting in spring. While SAD has multiple contributing factors, low vitamin D is considered one of the key biological drivers.
It is worth noting: the research here shows an association, not a simple cause-and-effect. Low vitamin D does not guarantee you will feel depressed, and supplementing will not cure depression. But addressing a deficiency can be a meaningful piece of the puzzle, especially if your mood tends to track with the seasons.
Signs Your Mood May Be Related to Low Vitamin D
You might want to investigate your vitamin D levels if you notice:
- Persistent low mood, especially in fall and winter
- Feeling unmotivated or emotionally flat
- Fatigue that does not improve with rest
- Difficulty concentrating
- Increased anxiety or irritability
- Disrupted sleep
These symptoms overlap with a lot of things, which is why it is easy to dismiss them. But if they tend to show up or worsen between October and March, low vitamin D is absolutely worth looking into.
For a broader look at what low vitamin D can do to your body, our post on Symptoms of Vitamin D Deficiency: Signs, Causes & How to Fix It covers the full picture.
What You Can Do About It
Get Your Levels Tested
The most useful first step is knowing where you actually stand. Ask your doctor for a 25(OH)D blood test — this measures your circulating vitamin D. Most functional health practitioners recommend aiming for levels between 100–150 nmol/L for optimal function, not just the bare minimum.
Supplement Through the Fall and Winter
For most Canadians, supplementation from October through April is essentially non-negotiable if you want to maintain adequate levels. The exact amount varies depending on your baseline and health history, but a commonly suggested range for adults is 1,000–4,000 IU per day.
Always check with your healthcare provider before starting a higher-dose protocol, especially if you have any underlying health conditions.
Pair It With the Right Co-Factors
Vitamin D works best when it has a few key partners: magnesium, which helps activate vitamin D in the body, and vitamin K2, which works alongside D3 to direct calcium to the right places.
If you are supplementing with vitamin D but not noticing a difference, it is worth looking at whether you are getting enough magnesium in particular — it is one of the most common nutrient gaps and plays a direct role in how well your body uses vitamin D.
For a full breakdown of what to pair with your vitamin D, check out our post: Best Supplements to Take With Vitamin D (Magnesium, K2 & More).
Do Not Underestimate Sunlight in the Warmer Months
When spring and summer arrive, getting outside between 10am–2pm, even for 15–20 minutes a few times a week, can make a real difference in rebuilding your levels. You do not need to sunbathe — just get some skin exposure without sunscreen for a short window, then protect yourself.
Choosing a Vitamin D Supplement
Not all vitamin D supplements are created equal. You want to look for D3 (cholecalciferol) rather than D2 — D3 is the form your body naturally produces from sunlight and is more effective at raising and maintaining blood levels.
You also want a supplement from a brand that uses third-party testing and pharmaceutical-grade manufacturing. A few of our go-to options at Wellness Market include Thorne, NFH, AOR, and Natural Factors — brands we trust because they actually test what is on the label.
You can find our full Vitamin D collection here — and if you are not sure which format or dose is right for you, we are happy to help. Just reach out.
The Bottom Line
Your mood is not just about mindset or willpower. There are real physiological factors at play — and vitamin D is one of them. If you feel like yourself in the summer but quietly struggle through the darker months, it is worth asking whether your levels are where they should be. Shop vitamin D supplements
Getting a blood test is simple, supplementing is straightforward, and the difference it can make in how you feel is significant for a lot of people.
For a full overview of everything vitamin D does — from immunity to bone health to energy — our Vitamin D: The Complete Guide is a great place to start.
Wellness Market is a natural health supplement store in Windsor, Ontario. This blog is for educational purposes and does not constitute medical advice. Please consult your healthcare provider before starting any new supplement.
