Every year around October, the same question starts appearing in my inbox from clients: "Now that the days are getting shorter, how do I make sure I'm getting enough vitamin D?"

It's a smart question. And if you're asking it, you're already ahead of most people who don't think about their vitamin D levels until they're dealing with the consequences—persistent fatigue, low mood, getting sick constantly, or that vague sense that something is just off.

Here's what's happening: as we transition from summer into fall and then winter, the angle of the sun changes. Even when it's sunny outside, the UVB rays that trigger vitamin D production in your skin become less intense. If you live above 37 degrees latitude (roughly the latitude of San Francisco or Athens), you might not be able to produce adequate vitamin D from sunlight at all between November and February, regardless of how much time you spend outside.

Add to this that most of us are spending more time indoors during colder months, covering more of our skin when we do go out, and the sun sets earlier, limiting the window for sun exposure anyway. The result is that vitamin D levels naturally drop during fall and winter for the majority of people in northern climates.

This isn't just about bone health, though that's important. Vitamin D affects your immune system (which is why colds and flu spike in winter), your mood (seasonal affective disorder is partially linked to low vitamin D), your muscle function, your inflammation levels, and even your sleep quality.

So yes, this matters. And yes, there are specific strategies that work when the sun isn't cooperating. Let me walk you through what you actually need to know.

Why Vitamin D Becomes a Problem in Fall and Winter

First, let's understand what's happening at a biological level so you understand why your summer strategies won't work year-round.

How vitamin D production actually works:

When UVB rays from the sun hit your skin, they interact with a cholesterol compound (7-dehydrocholesterol) in your skin cells. This triggers a chemical reaction that produces pre-vitamin D3, which then converts to vitamin D3 (cholecalciferol) through body heat.

This vitamin D3 travels to your liver, where it's converted to 25-hydroxyvitamin D—the form measured in blood tests. When your body needs active vitamin D, your kidneys convert it to the active form (1,25-dihydroxyvitamin D) that actually does the work in your cells.

What changes in fall and winter:

The sun's angle matters more than most people realize. During the summer months, the sun is high in the sky, and UVB rays can penetrate the atmosphere efficiently. During fall and winter, the sun is lower on the horizon, meaning UVB rays have to travel through more atmosphere to reach you. Much of the UVB radiation gets absorbed or scattered before it reaches your skin.

In practical terms: in Boston in December, you could stand outside in shorts at noon and still not produce meaningful amounts of vitamin D. The angle is wrong. The UVB simply isn't strong enough.

Your latitude determines how severe this problem is. If you live in Miami (26°N), you can still produce vitamin D year-round, though at reduced rates in winter. If you live in Seattle (47°N), London (51°N), or anywhere further north, your skin basically shuts down vitamin D production from November through February.

Why food alone usually isn't enough:

Very few foods naturally contain significant amounts of vitamin D. Fatty fish like salmon and mackerel are the best natural sources, but you'd need to eat them almost daily to meet your needs through food alone. Fortified foods help, but they typically contain modest amounts—a glass of fortified milk has about 100 IU of vitamin D, while your body might need 1,000-2,000 IU daily just to maintain adequate levels.

This is why vitamin D deficiency becomes so common in the fall and winter. Your primary source (sunlight) becomes unreliable or unavailable, and food sources alone can't fully compensate.

The Signs You're Not Getting Enough Vitamin D

Vitamin D deficiency is sneaky. The symptoms are nonspecific enough that you might attribute them to stress, busy schedules, or just "seasonal blues." But if you're experiencing several of these, especially during fall and winter, low vitamin D might be the culprit:

Persistent fatigue that doesn't improve with sleep. You're sleeping eight hours, but still feel exhausted. This is one of the most common signs of deficiency.

Getting sick more frequently. If you're catching every cold that goes around the office, low vitamin D might be compromising your immune function.

Bone pain or muscle aches. Unlike the sharp pain of injury, this feels more like a deep ache in your bones or generalized muscle discomfort.

Mood changes or depression. Seasonal affective disorder (SAD) has multiple causes, but low vitamin D is a significant contributor. If your mood noticeably dips as days get shorter, this is worth investigating.

Slow wound healing. If cuts and bruises take longer to heal than they used to, vitamin D deficiency can interfere with the healing process.

Hair loss. While many factors cause hair loss, severe vitamin D deficiency can contribute to hair thinning.

Muscle weakness. Difficulty with activities that used to be easy, or feeling like your muscles fatigue quickly.

Brain fog or difficulty concentrating. Vitamin D receptors are present in the brain, and deficiency can affect cognitive function.

Who's at higher risk:

Some groups are more vulnerable to vitamin D deficiency, especially during fall and winter:

• People with darker skin (melanin reduces vitamin D production)
• Older adults (skin becomes less efficient at producing vitamin D with age)
• People who are overweight or obese (vitamin D gets sequestered in fat tissue)
• People with limited sun exposure (office workers, homebound individuals)
• People with conditions affecting fat absorption (Crohn's disease, celiac disease)
• People living in northern latitudes
• People who consistently use sunscreen (blocks UVB rays)
• Strict vegans (most dietary sources are animal-based)

If you fall into any of these categories, you need to be especially proactive about vitamin D during the darker months.

Strategy 1: Maximize What Little Sun Exposure You Can Get

Even though sun exposure becomes less effective in fall and winter, you can optimize the limited opportunity you have.

Timing matters:

The strongest UVB rays occur during midday hours—roughly 10 AM to 2 PM. If you're going to get sun exposure, this is your window. A 15-minute walk at noon is more effective than an hour-long walk at 4 PM when the sun is lower.

Skin exposure matters:

You need direct sun exposure on your skin, not through windows (glass blocks UVB rays). Aim to expose your arms and face at minimum. More skin exposure means more vitamin D production, so if the weather permits, exposing your legs as well helps.

Duration guidelines:

During fall, aim for 10-15 minutes of midday sun exposure several times per week. In early fall, when the sun is still relatively strong, this can help maintain levels. By late fall and winter in northern climates, even this won't be sufficient, which is when you need to rely more heavily on food and supplements.

The sunscreen dilemma:

Yes, sunscreen blocks vitamin D production. But skin cancer risk is also real. The balanced approach: get your 10-15 minutes of sun exposure without sunscreen for vitamin D purposes, then apply sunscreen if you'll be out longer. You're not choosing between vitamin D and skin protection—you're doing both strategically.

Indoor light doesn't count:

Fluorescent lights and regular LED bulbs don't emit UVB radiation. Sitting by a sunny window feels nice but won't produce vitamin D. Some people use UVB lamps specifically designed for vitamin D production, but check with your doctor before using these, as they require careful usage to avoid skin damage.

Strategy 2: Eat Vitamin D-Rich Foods Strategically

Food alone usually can't meet all your vitamin D needs during fall and winter, but it can provide a foundation that reduces how much supplementation you need.

Fatty fish (the vitamin D powerhouses):

These are your best natural food sources:

• Salmon (wild-caught, 3.5 oz): 600-1,000 IU
• Salmon (farmed, 3.5 oz): 100-250 IU (farming practices reduce vitamin D content)
• Mackerel (3.5 oz): 360 IU
• Sardines (3.5 oz): 270 IU
• Tuna (canned, 3.5 oz): 230 IU
• Trout (3.5 oz): 645 IU

Practical approach: Aim for fatty fish 2-3 times per week during fall and winter. This alone can provide 1,200-3,000 IU weekly, which helps maintain baseline levels.

Egg yolks:

One large egg yolk: 40 IU

The vitamin D is in the yolk, not the white. While one egg doesn't provide much, if you regularly eat eggs for breakfast, it adds up. Three eggs give you about 120 IU.

Mushrooms:

Mushrooms are unique—they're one of the few plant sources of vitamin D, and they produce it when exposed to UV light, similar to human skin.

• UV-exposed mushrooms (3.5 oz): 400-1,000 IU
• Regular mushrooms (3.5 oz): 10-20 IU

Look for packages labeled "UVB-treated" or "high vitamin D." Some brands specifically expose mushrooms to UV light to increase their vitamin D content.

Fortified foods:

Many foods are fortified with vitamin D. While the amounts are modest, they add up when consumed regularly:

• Fortified milk (1 cup): 100 IU
• Fortified orange juice (1 cup): 100 IU
• Fortified cereal (1 serving): 40-100 IU (varies by brand)
• Fortified yogurt (6 oz): 80 IU
• Fortified plant milks (1 cup): 100-140 IU (varies by brand)

Cod liver oil:

One tablespoon: 1,360 IU

This is the most concentrated food source of vitamin D. One tablespoon daily can significantly boost your intake. The downside is that it also contains high amounts of vitamin A, so don't exceed the recommended dose without medical supervision.

Sample daily menu for maximizing dietary vitamin D:

• Breakfast: Scrambled eggs (3 eggs \= 120 IU) with fortified orange juice (100 IU)
• Lunch: Grilled salmon salad (600 IU)
• Snack: Fortified yogurt (80 IU)
• Dinner: Sautéed mushrooms (if UV-exposed, 400 IU)

Total from food: \~1,300 IU

This is close to the recommended daily intake, but most people can't or won't eat this way every single day, which is why supplements become important.

Strategy 3: Supplement Wisely (When Food and Sun Aren't Enough)

Here's what I tell my clients: during fall and winter in northern climates, most people need to supplement vitamin D. It's not a failure or a shortcut—it's recognizing that geography and modern lifestyles make it nearly impossible to maintain optimal levels through sun and food alone.

How much do you actually need?

The recommended dietary allowance (RDA) is 600 IU for adults up to age 70, and 800 IU for those over 70. However, many experts consider these amounts too low for maintaining optimal levels, especially during fall and winter.

Research suggests that maintaining blood levels in the optimal range (30-50 ng/mL) often requires 1,000-2,000 IU daily for most adults, and some people need even more depending on their baseline levels, body weight, and absorption capacity.

Get tested to know your baseline:

The best approach is to get your vitamin D levels tested (specifically, 25-hydroxyvitamin D) through your doctor. This gives you a baseline and helps determine your supplementation needs.

Optimal levels:

• Deficient: Less than 20 ng/mL
• Insufficient: 20-30 ng/mL
• Sufficient: 30-50 ng/mL
• Optimal: 40-60 ng/mL (many functional medicine practitioners aim for this range)
• Too high: Above 100 ng/mL (can cause toxicity)

If you're deficient, your doctor might prescribe a higher-dose supplement (50,000 IU weekly for 8 weeks) to bring levels up quickly, then transition to a maintenance dose.

Choosing the right supplement:

Vitamin D3 (cholecalciferol) vs. D2 (ergocalciferol): Always choose D3. It's more effective at raising blood levels and maintaining them. D2 is plant-derived and used in some prescription formulations, but D3 is superior.

Dosage: Most people do well with 1,000-2,000 IU daily during fall and winter. If you're deficient, you might need 3,000-5,000 IU daily, but check with your doctor for doses above 2,000 IU.

Form: Vitamin D is fat-soluble, so it's best absorbed when taken with a meal containing fat. Softgels with oil are slightly better absorbed than tablets, but the difference isn't huge.

Combination supplements: Some vitamin D supplements include vitamin K2, which works synergistically with vitamin D to direct calcium to bones rather than soft tissues. This combination can be beneficial, especially for bone health.

Quality matters: Look for third-party tested supplements (labels showing NSF, USP, or ConsumerLab certification). This ensures the supplement actually contains what the label claims.

Timing: Take vitamin D with your largest meal of the day, which typically contains the most fat for absorption. Morning or evening doesn't matter—consistency matters more.

When to supplement:

Start increasing your vitamin D intake in September or October, before levels start dropping significantly. Continue through March or April, depending on your latitude. Some people benefit from year-round supplementation, especially if they work indoors and get minimal sun exposure even in summer, but vitamin D can cause toxicity, so you need to consult with your doctor (see below).

Strategy 4: Adjust Your Expectations and Behaviors Seasonally

This is about working with your biology instead of against it. Our bodies evolved in environments with seasonal variation, and while we can't perfectly replicate ancestral sun exposure, we can be strategic.

Get outside during midday hours when possible:

Even on cloudy days, outdoor light exposure helps regulate circadian rhythms and mood. While it won't produce much vitamin D in winter, the light exposure itself benefits your mental health and helps with seasonal affective disorder.

Consider a fall/winter vitamin D testing schedule:

Get tested in early fall (September) to see where your levels are after summer. Get tested again in late winter (February/March) to see if your supplementation strategy is working. Adjust your dose based on results.

Be more aggressive if you're at higher risk:

If you have dark skin, are overweight, are over 60, or have conditions affecting absorption, you likely need higher doses than the average person. Work with your healthcare provider to determine your specific needs.

Pair vitamin D strategies with other winter wellness practices:

Vitamin D is one piece of winter health, but not the only piece. Also focus on:

• Getting adequate sleep (7-9 hours)
• Managing stress
• Eating nutrient-dense foods
• Staying physically active
• Getting light exposure for circadian rhythm support

Recognize that you might feel different in winter:

Even with optimal vitamin D levels, many people naturally have lower energy and prefer more rest during the winter months. This is normal. Vitamin D helps minimize the negative effects, but it doesn't make winter feel identical to summer.

What About Vitamin D Toxicity?

This is a common concern, but it's important to have perspective. Vitamin D toxicity is rare and typically only occurs with very high doses (over 10,000 IU daily) taken for extended periods.

You cannot get vitamin D toxicity from sun exposure—your body regulates production. You cannot get toxicity from food—even fatty fish consumed daily doesn't provide enough to cause problems.

Toxicity comes from excessive supplementation. Symptoms include nausea, vomiting, weakness, and frequent urination. Severe toxicity can cause kidney problems.

This is why testing is valuable. It prevents both deficiency and over-supplementation. If you're taking 2,000 IU or less daily, toxicity is extremely unlikely. If you're taking higher doses, periodic testing ensures you're in the safe range.

The Realistic Fall and Winter Vitamin D Plan

Here's what a practical approach looks like for most people living in northern climates:

September through October (early fall):

• Continue getting midday sun exposure when possible (10-15 minutes, arms and face exposed)
• Eat fatty fish 2 times per week
• Start daily vitamin D3 supplement: 1,000 IU if you had good sun exposure all summer; 2,000 IU if sun exposure was limited
• Consider getting baseline blood test to know your starting point

November through February (late fall and winter):

• Accept that sun exposure won't provide adequate vitamin D
• Continue or increase supplement to 2,000 IU daily (or higher if your doctor recommends based on testing)
• Eat fatty fish 2-3 times per week
• Include fortified foods regularly (fortified milk or plant milk, fortified orange juice)
• Get outside for light exposure and mood benefits, but don't count on it for vitamin D production
• Consider retesting in February to ensure your strategy is working

March through April (early spring):

• Gradually transition back to relying more on sun exposure as days get longer and sun angle improves
• Continue supplementation at a reduced dose (1,000 IU) until you're consistently getting sun exposure
• Retest if desired to see how levels rebounded with your winter strategy

May through August (summer):

• Get regular midday sun exposure (10-20 minutes several times per week)
• Continue eating vitamin D-rich foods
• Many people can reduce or eliminate supplementation during peak summer months if they get adequate sun
• Consider retesting in late summer to establish your new baseline going into fall

After years of working with clients on vitamin D optimization, here's what I've learned: most people underestimate how dramatic the seasonal drop in vitamin D levels can be, and most wait until they're already deficient to do anything about it.

The proactive approach—starting supplementation in early fall, maintaining it through winter, eating vitamin D-rich foods regularly, and getting tested to confirm your strategy works—prevents the energy crashes, frequent infections, mood dips, and muscle aches that so many people accept as "just winter."

You can't completely override the seasonal challenges of living far from the equator. Your body will still respond to shorter days and longer nights. But you can prevent vitamin D deficiency from making winter significantly harder than it needs to be.

Your immune system works better with adequate vitamin D. Your bones stay stronger. Your muscles function more efficiently. Your mood is more stable. You have more energy to do the things you want to do, even when it's cold and dark outside.

Start paying attention to your vitamin D levels now, before fall fully transitions into winter. Test if you can. Supplement strategically. Eat your fatty fish. Get outside during midday when possible.

Your winter self will thank you for the preparation.