Vitamin D in Transition: The October Drop-Off You Don’t Want to Miss
Whether you’re an autumn lover, or mourning the end of summer, October is a marked time of transition. The air gets crisp, leaves start to fall, and our cozy sweaters finally come out of hiding. But there’s another shift happening right now that often slips under the radar: our vitamin D levels begin to take a sharp dive.
And because vitamin D plays such a wide-ranging role in health, this seasonal dip can leave you feeling more tired, more moody, and more vulnerable to illness right as cold and flu season ramps up.
Why Vitamin D is So Much More Than a ‘The Sunshine Vitamin’
Vitamin D is actually a prohormone, meaning it acts more like a hormone than a simple vitamin, this is why our body is able to produce it with sunlight.
Here’s what it influences:
Immune resilience: Low vitamin D has been linked with higher rates of respiratory infections, especially in the winter months. It helps your immune system respond appropriately, not too weakly and not too aggressively.
Mood and mental health: Get the winter blues? That’s because Vitamin D receptors are found in brain regions that regulate mood. Low levels are associated with seasonal affective disorder (SAD), brain fog, and even depression.
Energy and hormones: Adequate vitamin D supports thyroid function, fertility, and overall energy production.
Muscle and bone strength: Without vitamin D, your body can’t absorb and use calcium effectively, which puts your bones and muscles at risk of weakness over time.
The Sneaky Signs of Low Vitamin D
The tricky thing about vitamin D deficiency is that symptoms are vague. You might brush them off as ‘just fall tiredness’ or ‘winter blues,’ but nearly 32% of Canadians have insufficient vitamin D.
Some common clues include:
Constant fatigue, no matter how much you sleep
Low mood or irritability
Frequent colds
Slow recovery from infections
Muscle aches, joint pain, or bone tenderness
Hair loss
Foggy thinking or trouble concentrating
Loss of appetite
Pale skin
Because these symptoms overlap with so many other things, it’s easy to miss that low vitamin D is playing a role.
Why October Is the Drop-Off Point
If you live anywhere north of San Francisco, the sun’s angle in autumn and winter is too low for your skin to make enough vitamin D, no matter how much time you spend outside. That means October is the turning point where many people quietly move from “okay” into deficiency territory.
If you spent the summer soaking up the sun, you may have some vitamin D stored in your fat and liver, but by late fall, those reserves start running low. This timing is one reason why colds, flus, and low moods tend to spike around November and December.
Let’s Clear up Some Vitamin D Confusion: Myth vs. Fact
Myth #1: “If I go for a walk at noon everyday, I’ll get enough vitamin D.”
Fact: Once the sun sits lower in the sky (around October in Canada), UVB rays can’t penetrate the atmosphere strongly enough for your skin to synthesize vitamin D. You’ll still get the mood-boosting effects of daylight, but not the vitamin itself.
Myth #2: “Drinking milk gives me all the vitamin D I need.”
Fact: A cup of fortified cow’s milk has about 100 IU of vitamin D. Most adults in northern climates need 1,000–2,000 IU daily,sometimes more, so milk alone won’t cut it.
Myth #3: “I don’t need supplements because I was in the sun all summer.”
Fact: Your body does store vitamin D in fat and the liver, but those reserves only last a couple of months. By late fall, most people’s levels have dropped below optimal, especially if they didn’t spend hours outdoors every day without sunscreen (which you shouldn’t have!)
Myth #4: “If I eat lots of fish, I’m covered.”
Fact: Fatty fish like salmon and mackerel are fantastic sources of vitamin D, but you’d need to eat them several times per week to meet your needs. For most people, supplementation is still necessary through the darker months.
Your Fall Vitamin D Game Plan
1: Keep enjoying the sun for your circadian rhythm.
Even if you can’t make vitamin D, daylight exposure still matters for sleep, mood, and energy. Getting outside, even on cloudy days, helps keep your body clock on track.
2: Eat vitamin D–rich foods.
Excellent sources include wild salmon, sardines, mackerel, egg yolks, and fortified plant milks. These won’t replace what you’d make from the sun, but they help keep levels up.
3: Consider a supplement.
For most adults in northern climates, a vitamin D3 supplement is the easiest way to maintain healthy levels. Your dose depends on your individual needs and bloodwork, but many adults need at least 1,000–2,000 IU daily in fall and winter. Pairing vitamin D3 with a vitamin K2 supplement can be especially supportive for bone and cardiovascular health. As always, consult with your healthcare provider to find what's right for you!
4: Don’t forget vitamin D’s sidekicks.
Magnesium, zinc, and healthy fats all help your body absorb and use vitamin D effectively, so be sure to include foods rich in these in your diet, such as nuts, seeds, avocados, and leafy greens. Magnesium is also another common supplement that most people would benefit from. Look for a magnesium bisglycinate form to avoid any digestive upset caused by other forms of magnesium and aim for about 200- 400 mg per day.
While the sun may be retreating for the season, your energy, mood, and immune system don’t have to follow. Staying on top of your vitamin D now means you’ll have the resilience to enjoy everything fall and winter bring… All the cozy nights, crisp walks, and maybe even a snowball fight or two if we get lucky, without feeling like the season has drained you.