Overall Risk of Vitamin D Deficiency Greatest This Time of Year

Published on March 12, 2026

Deficiency Signs and Symptoms, and Why Now Is the Time to Test

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By this time of year, many people have been struggling with the effects of winter, including months of limited sun exposure. What many don’t realize is that this is also the time of year when vitamin D levels are typically at their lowest.

Vitamin D is primarily produced in the skin through exposure to sunlight. During the winter months, shorter days, colder temperatures, indoor lifestyles, and reduced UVB intensity, especially at northern latitudes. These factors significantly limit the body’s ability to produce vitamin D. By the time late winter and early spring arrive, especially among those not supplementing properly, many people’s vitamin D levels have dropped drastically.

This is why data has consistently shown that population vitamin D levels reach their annual low point at the end of winter and early spring. In fact, an analysis of almost 4,000 GrassrootsHealth participants who tested their vitamin D levels and reported that they did not take vitamin D supplements showed that vitamin D levels peaked in the month of August and declined to their lowest levels during the winter months of January through March.

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For clinicians and individuals alike, this makes now one of the most important times of the year to test vitamin D levels.

Often Subtle and Easily Missed Signs of Deficiency

Vitamin D deficiency remains one of the most common nutritional deficiencies worldwide. Yet its symptoms are often subtle, nonspecific, and easily overlooked because they can be non-specific and easily attributed to other conditions such as aging, stress, depression, or chronic fatigue. In reality, vitamin D plays an important regulatory role in many systems of the body, including:

• Immune function
• Muscle metabolism
• Bone health
• Brain function and mood regulation
• Hormonal and metabolic processes

When vitamin D levels fall too low, these systems can begin to show signs of dysfunction. Research has identified a wide range of clinical manifestations associated with low vitamin D status.

Common Early Signs of Vitamin D Insufficiency

In the early stages of deficiency, symptoms may be subtle and easily overlooked. Some of the most commonly reported early signs include:

• Persistent fatigue or low energy
• Reduced exercise tolerance
• Poor sleep quality or insomnia
• Muscle aches or diffuse body pain
• Frequent infections
• Brain fog or difficulty concentrating
• Mood changes such as depression or anxiety

Fatigue and generalized muscle pain are among the most common presenting symptoms of vitamin D deficiency in clinical practice. Because these symptoms overlap with many other health issues, vitamin D deficiency is often not considered, and testing is overlooked.

Musculoskeletal Effects of Vitamin D Deficiency

Vitamin D plays a direct role in muscle strength and skeletal integrity through its activation of vitamin D receptors (VDR) in muscle and bone tissue. As deficiency progresses, symptoms may include:

• Proximal muscle weakness (difficulty climbing stairs or rising from a chair)
• Recurrent muscle strains or injuries
• Impaired balance
• Increased risk of falls
• Bone pain

Severe or prolonged deficiency can lead to osteomalacia in adults, a condition characterized by defective bone mineralization and increased fracture risk.

Effects on the Brain and Mood

Vitamin D receptors are widely distributed throughout the brain, including regions involved in memory and executive function. Vitamin D also acts as a neurosteroid hormone, influencing brain development, neurotransmitter regulation, and neuroprotection. Low vitamin D levels have been associated with:

• Depression
• Anxiety
• Reduced cognitive processing speed
• Difficulty with focus and concentration
• Memory complaints
• “Brain fog” or reduced mental clarity

Immune Function and Infection Risk

One of vitamin D’s most important roles is its regulation of the immune system.

Vitamin D stimulates immune cells to produce antimicrobial peptides, natural substances that help the body destroy bacteria and viruses. (Watch a recording of our recent Vitamin D Study Hour on this topic!) When vitamin D levels are low, this protective mechanism may be impaired. Individuals with vitamin D deficiency can experience:

• Frequent respiratory and other infections
• Slower recovery from illness
• Delayed wound healing

When Should Vitamin D Levels Be Tested?

Testing serum 25-hydroxyvitamin D [25(OH)D] is the most reliable way to assess vitamin D status. Clinicians should consider testing when patients present with symptoms such as:

• Persistent fatigue
• Diffuse musculoskeletal pain
• Muscle weakness
• Frequent infections
• Depression or cognitive complaints
• Sleep disturbances

These symptoms often appear long before obvious bone disease develops, which is one reason vitamin D deficiency can go undetected for years, especially when receiving care from under educated providers or when following generalized guidelines.

Why Now Is the Best Time to Check Your Level

Vitamin D levels fluctuate throughout the year. In most populations:

• Levels peak in late summer or early fall
• Levels are lowest in late winter or early spring

Testing during this seasonal low point provides valuable insight into whether your vitamin D intake or supplementation has been sufficient to maintain healthy levels year-round.

For individuals working to optimize their health, and for practitioners guiding patient care, this seasonal checkpoint can provide important information about vitamin D status and whether adjustments are needed throughout the seasons.

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Measure Your Vitamin D and Other Important Nutrients

If you haven’t had your vitamin D levels checked recently, now is the time! With so many Americans still falling short, awareness is the first step toward change.

Measuring and calculating a supplementation amount to help reach and maintain a target level, or taking loading doses to correct deficiency faster, could possibly make all the difference in overall health, wellbeing, and how a current disease situation progresses. Test your level now!

Create your custom home blood spot kit by adding any of the following measurements, along with your vitamin D:

• Omega-3 Index (with or without Ratios AA:EPA and Omega-6:Omega-3)
• Magnesium (with additional Elements copper, zinc, selenium, mercury, and cadmium)
• hsCRP as a marker of inflammation and HbA1c as a marker of blood sugar health

Having and maintaining healthy vitamin D levels and other nutrient levels can help improve your health, now and for the future. Enroll and test your levels today, learn what steps to take to improve your status of vitamin D (see below) and other nutrients and blood markers, and take action! By enrolling in the GrassrootsHealth projects, you are not only contributing valuable information to everyone, you are also gaining knowledge about how you could improve your own health through measuring and tracking your nutrient status, and educating yourself on how to improve it.