Every winter, immune-support supplements begin flying off the shelves. At the first sign of a scratchy throat or an illness making its way through the family, people reach for vitamin C, zinc, and other nutrients that might help. While targeted support has its place, resilience during cold and flu season is built long before symptoms appear.
A prime example of this is Vitamin D. It contributes to immune regulation while also supporting bone strength, muscle function, and other systems that are central to healthy aging. Yet vitamin D levels often begin declining as the days shorten and we spend less time outdoors. But because vitamin D status reflects our sunlight exposure, food intake, supplementation, absorption, and other health factors over time, it’s unfortunately not something that can usually be corrected overnight.
Early fall provides a valuable window to find out where you stand, take advantage of the remaining sunlight, and make a thoughtful plan before winter begins!
Why Vitamin D Does More Than Support Bone Health
Most people associate vitamin D with strong bones (and for good reason). Vitamin D helps the intestines absorb calcium and phosphorus, which allows these minerals to be used for normal bone growth and remodeling. Even when calcium intake is sufficient, a severe vitamin D deficiency can prevent bones from mineralizing properly. This is why vitamin D deficiency is classically associated with rickets, a condition that causes soft, weak, and sometimes misshapen bones in children. In adults, the corresponding condition is known as osteomalacia, though vitamin D’s effect extends well beyond skeletal health.1
Although we call it a vitamin, the body actually converts vitamin D into an active hormone called calcitriol. This hormone binds to vitamin D receptors in tissues throughout the body, where it helps regulate gene activity and cellular function. Immune cells also contain vitamin D receptors, allowing vitamin D to influence both early immune defenses and the regulation of inflammation.
Vitamin D also contributes to muscle function, strength, and balance that strongly influence whether someone can remain active, mobile, and independent with age. Maintaining adequate vitamin D is but one, and yet an essential piece of protecting physical resilience that supports a longer, healthier life.
As Immune cells rely on vitamin D signaling to help recognize and respond to pathogens, produce antimicrobial compounds, and regulate inflammation, vitamin D status is part of the physiological foundation we bring into an infection.
Research conducted during the COVID-19 pandemic helped exhibit this relationship. A meta-analysis of 23 studies involving 2,692 patients found that people with vitamin D deficiency, defined as a 25-hydroxyvitamin D level below 20 ng/mL, were twice as likely to develop severe COVID-19 and 2.45 times as likely to die from the illness.2 While researchers cautioned that these findings do not prove that vitamin D deficiency caused the poorer outcomes (correlations are not causations) the findings reinforce a practical point: entering cold and flu season with a known nutrient deficiency may leave the body with fewer resources to draw upon when the immune system is challenged.
Adequate vitamin D does not guarantee that you will avoid illness, but identifying and correcting a low level ahead of time is a more thoughtful strategy than waiting until you are already sick - especially as vitamin D status takes time to build in the body.
Vitamins are broadly classified as either water-soluble or fat-soluble. Many water-soluble vitamins circulate for a relatively short time, with excess amounts excreted through the urine. As a result, we generally need a regular supply from food. Fat-soluble vitamins, including vitamins A, D, E, and K, are absorbed and then stored in the liver and body fat. This allows the body to build reserves, but it also means that vitamin D levels reflect what has been happening over time.
The form measured in the blood, 25-hydroxyvitamin D, has a circulating half-life of approximately 15 days. Your result is therefore influenced by your recent sunlight exposure, food intake, supplementation, absorption, and the amount of vitamin D already stored in the body.
In one randomized controlled trial, older adults taking 800 IU of vitamin D3 daily gradually increased their average 25-hydroxyvitamin D level toward 28 ng/mL over 16 weeks.3 The response differs considerably between individuals, but the study illustrates why rebuilding vitamin D status often requires consistent support over several weeks or months to intentionally increase levels. Starting in early fall leaves time to assess your current level, begin an appropriate supplement if needed, and monitor how your body responds.
The answer depends partly on what we mean by “optimal.” Many conventional reference ranges are designed to identify overt deficiency and prevent its most recognizable consequences, such as rickets and osteomalacia. A result can fall within the laboratory’s standard range without necessarily being the level a clinician would prefer for long-term health.
A 25-hydroxyvitamin D level below 20 ng/mL is widely considered deficient. The Endocrine Society’s 2011 clinical guideline classified levels between 21 and 29 ng/mL as insufficient and levels of 30 ng/mL or higher as sufficient. The guideline also noted that parathyroid hormone, which increases when the body is struggling to maintain calcium balance, often begins to stabilize when vitamin D reaches approximately 30 to 40 ng/mL.4
At the California Center for Functional Medicine, we generally consider a level below 30 ng/mL suboptimal and often prefer to see vitamin D closer to 50 to 60 ng/mL, depending on the individual. This provides some buffer against seasonal declines and falls within the range commonly used in functional medicine.5,6 Observational research also supports looking beyond the minimum cutoff. A meta-analysis of 32 studies found that levels at or below 30 ng/mL were associated with higher all-cause mortality, with risk continuing to decline as levels increased to approximately 36 ng/mL.7
It is important to note that because vitamin D is fat-soluble, higher is not always better. If you are supplementing with vitamin D, particularly at higher doses, monitoring your level can help ensure it does not become unnecessarily elevated.
Vitamin D levels vary considerably between individuals, even when their diets and supplement routines look similar. Sunlight exposure is a major part of the equation, but age, skin pigmentation, body composition, digestion, medications, and other health factors can all affect vitamin D status.
People are more likely to have suboptimal levels if they:8
Several of these factors can occur at the same time. Someone who works indoors, lives in a northern climate, and has a history of digestive problems may have a very different vitamin D requirement than someone who spends much of the year outside in a sunny climate.
Vitamin D levels tend to rise during the sunnier months and decline as we move through fall and winter. In one study examining seasonal variation, average vitamin D levels were approximately 6 ng/mL lower during the winter than during the summer.9 For someone beginning fall with a borderline or already suboptimal level, that seasonal decline may be enough to move them into deficiency.
The skin produces vitamin D when it is exposed to UVB rays, but the amount produced varies widely. Latitude, season, time of day, cloud cover, skin pigmentation, age, clothing, and the amount of skin exposed all influence vitamin D production. Additionally, sunlight through a window does not have the same effect because glass blocks most UVB radiation. Early fall is a good time to continue spending time outdoors while stronger sunlight is still available. A walk outside during the day, outdoor exercise, or simply moving part of your routine outdoors can contribute to vitamin D production while also supporting mood and circadian rhythms.
Vitamin D is unusual because relatively few foods contain substantial amounts naturally. Fatty fish are among the best sources. A 3-ounce serving of cooked trout provides approximately 645 IU, while the same amount of sockeye salmon provides about 570 IU. UV-exposed mushrooms can also contribute meaningful amounts, although their vitamin D content varies.
Most other foods provide much less. One egg contains approximately 44 IU, two sardines provide about 46 IU, and cheddar cheese contains only about 17 IU per serving. Fortified milk and plant-based milks typically provide around 100 to 144 IU per cup.10 T
Including vitamin D-rich foods is still worthwhile, especially because fatty fish also provide protein, omega-3 fats, and other nutrients that support long-term health. However, food is usually not enough to correct an existing deficiency or maintain an optimal level in someone with limited sunlight exposure, impaired absorption, or other risk factors. This is why supplementation is often necessary, but the appropriate amount depends on where your level is starting.
Once food and sunlight are no longer enough, the next question is usually how much vitamin D to take. There is no single dose that works equally well for everyone. The amount needed to correct a low level depends on where that level is starting, along with body size, absorption, genetics, sun exposure, medications, and all the other factors that have been mentioned thus far.
A randomized trial demonstrated this variability by giving adults either 1,000, 5,000, or 10,000 IU of vitamin D3 daily for 21 weeks. Vitamin D levels increased in a clear dose-dependent pattern, but the amount required to produce a specific increase was also closely related to body size.11 In practice, a dose that corrects one person’s deficiency may be inadequate for someone else or unnecessarily high for a person who already has sufficient levels.
Testing 25-hydroxyvitamin D provides a starting point for choosing a more appropriate dose. Then, follow-up testing can then show whether the plan is working and whether the dose should be maintained, increased, or reduced. This is especially important for anyone taking higher amounts, since vitamin D can accumulate over time.
Overall, a vitamin D result is most useful when it leads to a personalized plan, rather than choosing a supplement based on a general recommendation. Testing allows the dose to be matched to the individual and adjusted as their level, health, and seasonal sun exposure change.
Checking your vitamin D before winter is a practical place to start, but long-term health is rarely defined by one nutrient or one lab result. Immune resilience, energy, metabolic health, cardiovascular function, and healthy aging are influenced by patterns across multiple systems. Understanding those patterns makes it easier to identify what deserves attention now and what can be monitored over time.
This broader view is central to the VitOS Blueprint. The Blueprint combines advanced biomarker testing with a detailed health history and clinical intake, your VitOS Index score, and a personalized Roadmap developed by clinicians. Instead of receiving a list of disconnected lab results, you receive guidance on what those results mean and which steps are most relevant to your health.
Early fall is an ideal time to establish that baseline. It gives you an opportunity to identify nutritional gaps, address suboptimal markers, and build a stronger foundation before winter arrives. If you want to understand where you stand with vitamin D and the other markers that influence your long-term health, the VitOS Blueprint can help you turn that information into a clear plan.
References: