Vitamin D and Immunity: Why Deficiency Spikes in Winter (and What You Can Do About It)
As the days get shorter and the weather cools down, many people start to feel run-down, fatigued, or catch more colds and viruses than usual. What’s to blame? While there isn’t one single cause, one major culprit is often low levels of vitamin D , the “sunshine vitamin.”
Vitamin D deficiencies tend to become more frequent during the winter months, and for good reason. Between spending more time indoors, shorter daylight hours, and weaker sun exposure, our bodies simply don’t produce enough. But here’s the good news: understanding how vitamin D impacts your immune system, and how to restore healthy levels, can make a big difference in how you feel all season long.
Why Vitamin D Matters for Immunity
Vitamin D is a powerhouse nutrient that plays a major role in immune defense. Essentially, Vitamin D helps regulate T-cell function and supports balanced immune responses, which play a role in defending against infections.
Vitamin D is unique because your body can produce it naturally, primarily when your skin is exposed to sunlight. When your vitamin D levels drop, your immune system can’t respond as efficiently, leaving you more vulnerable to colds, flu, and other illnesses.
Why Deficiency Spikes in Winter
Even in warm and sunny Fort Worth, vitamin D deficiency can become more common in winter. Here’s why:
Less Sunlight Exposure
During the colder months, shorter days and cooler weather mean we spend less time outside. Vitamin D is synthesized in your skin through sunlight, so less exposure = lower production.
Weaker UVB Rays
Even if you are outside, the angle of the winter sun reduces the strength of UVB rays that trigger vitamin D synthesis, especially early in the morning or late afternoon.
More Time Indoors
Between holiday gatherings, less than desirable weather, and indoor activities, many of us spend most of the day inside. Windows block UVB rays, so even sitting by a sunny window won’t boost vitamin D levels to the level needed.
Lifestyle & Nutrition
Low dietary intake can compound the problem. Few foods naturally contain vitamin D, so without supplementation or fortified foods, levels can quickly drop. It’s also no coincidence that the biggest holiday months - Halloween, Thanksgiving, Hanukkah, Christmas, and New Year’s Eve - all fall during this same time frame. Sugar and processed foods tend to be everywhere, while nutrient-dense meals and healthy eating habits often take a back seat.
These lifestyle shifts can leave your immune system more vulnerable right when your vitamin D levels are already dropping. Less sunlight, more sugar, and fewer whole foods create the perfect storm for sluggish energy, low mood, and increased susceptibility to illness.
Signs You Might Be Low in Vitamin D
Did you know Vitamin D deficiency is a common global issue. About 1 billion people worldwide have vitamin D deficiency, and approximately 35% of adults in the United States are deficient, according to the Cleveland Clinic.
Symptoms of low vitamin D can be subtle and often overlap with other conditions. Common signs include:
Frequent colds or infections
Fatigue or low energy
Muscle weakness or bone pain
Mood changes or “winter blues”
Slow wound healing
If you’re unsure, the best way to confirm deficiency is through a simple blood test.
How to Support Healthy Vitamin D Levels
At Restore + Revive, we take an individualized approach to restoring optimal Vitamin D and supporting immune health. Here’s what we recommend:
1. Spend Time Outside
Even on colder days, try to get at least 10–30 minutes of sun exposure daily, ideally around midday when UVB rays are strongest. A short walk, stretching, or simply stepping outside for fresh air can help your body produce vitamin D naturally.
2. Eat Vitamin D–Rich Foods
Vitamin D is fat-soluble, so your body can store it in fat and liver tissue. The “sunshine production” pathway creates vitamin D3 (cholecalciferol) in your skin when UVB rays hit. Once produced, it’s metabolized in the liver and kidneys into its active form that the body can use.
Some foods contain vitamin D naturally or are fortified with it. Eating these foods provides additional vitamin D that your body can absorb directly, supplementing what you might make from sunlight. While sunlight is the main source, certain foods can boost your intake:
Fatty fish like salmon, mackerel, sardines, and tuna
Egg yolks
Mushrooms- shiitake, portobello, cremini
Cows milk and fortified alternatives (soy, almond and oat milk)
Cod liver oil
Essentially, foods act as an external source of vitamin D to help maintain adequate levels when your body isn’t making enough from sunlight, which is especially important in winter months or for people who spend most of their time indoors.
3. Pair With Healthy Fats
Vitamin D is fat-soluble, meaning your body absorbs it better when eaten with healthy fats like avocado, olive oil, nuts, and seeds.
4. Get Tested
Test your vitamin D levels via a simple blood test, this provides a baseline for your provider to create a personalized supplementation plan. Everyone metabolizes vitamin D differently, so what works for one person may not be enough for another.
5. Supplement Smartly
Vitamin D3 is the preferred form for most people because it’s the same type your body naturally produces. The right dose depends on your levels, absorption, and overall health, which is why our team of functional medicine providers tailor supplementation plans to each individual.
6. Pair with Vitamin K2
Vitamin K2 helps direct calcium into your bones and out of your arteries, enhancing vitamin D’s benefits and supporting cardiovascular health.
7. Prioritize Whole-Body Wellness
Optimal immunity isn’t just about one nutrient. Adequate sleep, hydration, nutrient-dense foods, and balanced stress all strengthen your immune defense.
8. Consider Vitamin Shot Therapy
For those who are significantly deficient or need faster replenishment, Vitamin D shots can deliver vitamin D and other immune-boosting nutrients directly into your system for enhanced absorption.
Support Your Immune Health This Winter in Fort Worth
If you’re feeling run-down or want to strengthen your immune system naturally, our team can help. At Restore + Revive, we combine functional medicine with individualized testing to identify nutrient deficiencies and design a plan that supports your whole-body wellness.
Call (817) 720-6220 to book an appointment or learn more about how we can help you.