Why Vitamin D Deficiency Peaks in Autumn and What to Do About it

As the days get shorter, the sun shines less and the temperatures drop, there is another seasonal change that many people may not think about: low vitamin D levels. 

Most of us know that we can make vitamin D from the sun, but it’s so easy to forget that depending on where you live, many people don’t get enough sun to make adequate vitamin D during certain parts of the year, typically starting in fall. 

Technically, vitamin D deficiency does not “peak” in early autumn. Research shows that vitamin D levels often are at their lowest point in winter or early spring, but fall is the critical season to be thinking about vitamin D and taking proactive steps to prevent deficiency later on.¹

Why does this matter? Because vitamin D is critical for bone health, the immune system, inflammation, and so much more. Read on to get the tools you need to be prepared to prevent vitamin D deficiency this autumn and beyond. 

Why Autumn and Winter Dramatically Reduce Vitamin D Production

The amount of vitamin D your skin can produce depends on a few factors.

First there’s the sun. During fall and winter, when the sun sits lower in the sky, the UV rays are not as strong as they are in summer. Additionally, the days are shorter, offering less time during the day to get exposed to the sun’s light.

Second, we are typically more covered up in the colder months, so there’s less surface area of the skin getting direct sunlight. And third, we are indoors more. All of these factors mean less vitamin D production. 

There can be other factors as well, such as age, genetics, medical conditions and more that can influence vitamin D levels. People who are the most susceptible to low vitamin D may fit into one or more of these categories:²

• Liver at northern latitudes
• Spend little time outdoors
• Regularly cover most of their skin
• Have darker skin pigmentation (darker skin is better at blocking UV light)
• Are older adults
• Have conditions or medications affecting fat absorption (fat helps vitamin D absorb better)

And because relatively few foods naturally contain substantial amounts of vitamin D, it can be difficult to make up the difference from diet alone. 

Common Signs That Your Vitamin D Levels May Be Low

One of the challenges with vitamin D is that it does not always produce obvious symptoms. Many live with mild deficiency that may cause no noticeable symptoms at all.³

But if the deficiency gets worse, there are some common signs you might see:^3,4

• Muscle weakness
• Muscle aches
• Bone and back pain
• Fatigue
• Reduce physical performance
• Mood changes (sadness, depression, irritability)
• Frequent illness

Vitamin D is particularly important for bone health because it helps the intestines absorb calcium and support normal bone mineralization. This is especially critical during the menopause years when our bone density can take a hit. Research also associates adequate vitamin D levels with maintaining muscle function.⁴

Research also suggests vitamin D has a role to play in a properly functioning immune system. This is why you commonly see vitamin D included in vitamins for the immune system and other immune support supplements.⁵  

If you are one of those people that frequently get sick or experience any of these symptoms listed, considering getting your vitamin D levels checked. A high-quality vitamin D supplement can fill the gap and get you on the path to restoring those levels for optimal health. 

Why Vitamin D and K2 Work Better Together Than Alone

When it comes to bone health supplements, vitamin D often gets most of the attention, But vitamin K is also up there when it comes to important nutrients for bone health. They have separate yet complementary roles.

Think of their partnership this way:^4,6

• Vitamin D helps you absorb calcium: It increases the body’s ability to absorb calcium from the digestive tract so that calcium can get where it needs to go – namely the bones.
• Vitamin K helps activate proteins involved in binding calcium to the bone matrix. One of these is osteocalcin, a vitamin K-dependent protein needed for bone mineralization. 

This unique relationship is why the best bone health supplements have both.

As you might expect, Body Kitchen understands these roles and offers both nutrients combined conveniently in one capsule, helping you prevent deficiency and achieve optimal bone health and overall body support. 

But please note: anyone taking warfarin or other medications that can be affected by vitamin K need to talk with their doctor first before adding vitamin K2 or any other supplements. 

How to Choose the Right Vitamin D Supplement Heading into Winter

If a vitamin D supplement sounds like something you want to consider, here are a few things to look for so you make the right choice:

1. The type of vitamin D
   Typically we mention vitamin D3, but did you know there is also vitamin D2 in some supplements? Most people should choose D3 as it’s more potent and arguably better absorbed. A recent study found that D3 was more effective than D2 at raising total vitamin D blood levels.⁷ 
2. Check the dose
   The recommended intake for vitamin D is 600-800IU per day for adults, but some studies some studies suggest higher doses may be better.⁸  Taking moderate doses up to 5,000IU/ per day is safe, but always check with your doctor first to make sure it does not interfere with any medications or other treatment goals.
3. Consider having your vitamin D levels checked
   Whether you are deficient or not (and many people are) can help you determine the best product and dose. 

Final Thoughts

Autumn is a smart time to pay attention to vitamin D since levels start declining as soon as the sun begins to dip. It’s really important not to neglect this key nutrient as it’s involved in bone health, muscle function, the immune system and overall health. 

Be sure to check out Body Kitchens Vitamin D + K2 for the best combo for bone health and beyond. Proper vitamin D levels will make sure you go into autumn and winter fully armed and feeling your best. And if in doubt – get your vitamin D levels checked! 

Frequently Asked Questions 

Why does vitamin D deficiency become more common in autumn and winter?

Vitamin D is primarily synthesized in the skin through direct exposure to UVB sunlight. In autumn and winter, the sun sits lower in the sky, UVB rays travel through more of the atmosphere, and people spend significantly less time outdoors. In many northern latitudes, the angle of the sun from October through March means that UVB exposure is insufficient for meaningful vitamin D production even on clear days. Combined with shorter days and more time spent indoors, this seasonal shift causes vitamin D levels to drop for a large portion of the population.

What are the most common signs that vitamin D levels may be low?

Vitamin D deficiency is often subtle and easy to attribute to other causes. Common signs include persistent fatigue, low mood or seasonal blues, muscle weakness, frequent illness, and bone or joint discomfort. Because vitamin D plays a role in immune function, mood regulation, muscle health, and calcium metabolism, deficiency can affect multiple systems at once. A blood test measuring serum 25-hydroxyvitamin D is the most reliable way to confirm a deficiency rather than relying on symptoms alone.

What does vitamin K2 have to do with vitamin D supplementation?

Vitamin D increases the absorption of calcium from the gut, but calcium needs to be directed to the right places in the body. Vitamin K2 activates proteins that guide calcium into bones and teeth rather than allowing it to deposit in soft tissues like arteries and blood vessels. When vitamin D supplementation is taken without adequate K2, calcium absorption increases but without the directional guidance that K2 provides. Combining D3 with K2 supports both effective calcium utilization and cardiovascular safety.

How much vitamin D do most adults need during autumn and winter months?

The amount of vitamin D needed varies by individual based on baseline blood levels, body weight, age, and how much sun exposure a person gets. General guidance suggests that many adults benefit from 1,000 to 2,000 IU of vitamin D3 daily during low-sun months, with some individuals requiring higher amounts under the guidance of a healthcare provider. Because vitamin D is fat-soluble and accumulates in the body, it is worth checking blood levels before significantly increasing intake, and taking it with a meal that contains dietary fat improves absorption.

Who is most at risk for vitamin D deficiency heading into fall?

People at greatest risk of autumn vitamin D deficiency include those who live at higher latitudes, spend most of their time indoors, have darker skin tones that require more sun exposure for adequate synthesis, are over 65 (as skin becomes less efficient at producing vitamin D with age), and those who are overweight, since vitamin D is stored in fat tissue and may be less bioavailable. People who wore sunscreen consistently through summer may also have lower stored levels than expected, making a fall supplement routine particularly important for this group.