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Vitamin D deficiency is one of the most common nutritional deficiencies worldwide, yet it does not affect everyone equally. While seasonal changes, especially the winter months, play a major role in lowering vitamin D levels, there are several well-established factors that can significantly increase an individual’s risk of deficiency year-round.

Last week, we focused on the influence of the seasons on vitamin D levels, and how this time of year is when a majority of the population is at their highest risk of deficiency. Understanding the additional following risk factors for vitamin D deficiency is essential for both individuals and practitioners, as it helps identify who may need higher intake, closer monitoring, or more frequent testing year-round.

Vitamin D levels are influenced by a combination of:

• Sun exposure
• Skin pigmentation
• Body composition
• Age
• Diet and supplementation
• Genetics
• Overall health status
• And more

Because of this, two people taking the same amount of vitamin D—or getting the same amount of sun—can have very different blood levels.

1. Limited Sun Exposure, Sun Avoidance, and Sunscreen Use

Vitamin D is produced in the skin when exposed to UVB radiation from sunlight, and for those who do not supplement, sunlight becomes a key determinant of their vitamin D status. Individuals at higher risk include:

• Those who spend most of their time indoors
• People who live in northern latitudes
• Individuals experiencing winter months with minimal UVB availability
• Those who consistently avoid sun exposure or cover most of their skin
• Individuals who regularly use sunscreen during all sun exposure

Even in sunny regions, modern indoor lifestyles, sun avoidance, and protective behaviors can significantly limit vitamin D production.

2. Darker Skin Pigmentation

Melanin reduces the skin’s ability to produce vitamin D from sunlight. As a result:

• Individuals with darker skin require more sun exposure to produce the same amount of vitamin D
• Risk of deficiency is significantly higher, particularly in regions with limited sunlight

3. Aging

As we age, the skin becomes less efficient at producing vitamin D. Older adults may also:

• Spend less time outdoors
• Have reduced dietary intake
• Have an increased incidence of chronic disease
• Be on medications affecting vitamin D (see below)
• Experience decreased liver or kidney function, which affects vitamin D activation

4. Overweight and Obesity

Body fat can sequester vitamin D, reducing its availability in circulation. Research has shown that:

• Individuals with higher body weight often require 2–3 times more vitamin D to achieve the same blood levels
• Obesity is consistently associated with lower 25(OH)D levels

5. Inadequate Supplementation or Intake

Many individuals rely on standard recommendations that may be too low to achieve optimal levels. Most also fail to retest vitamin D levels after adjusting intake to ensure that intake is the correct dose for them. Without sufficient intake and monitoring of serum levels:

• Vitamin D stores decline, especially during winter
• Blood levels may fall into deficient or insufficient ranges

6. Genetics and Individual Variability

Because vitamin D status varies so widely between individuals, testing is the only way to know your level. Testing serum 25-hydroxyvitamin D [25(OH)D] allows you to:

• Identify deficiency or insufficiency
• Understand your personal response to supplementation
• Adjust intake based on your individual needs
• Monitor changes over time

For individuals with one or more risk factors, testing becomes even more important. Measure your risk of vitamin D deficiency with our quiz here.

Vitamin D deficiency is not just a seasonal issue—it is influenced by a wide range of personal and environmental factors.

Understanding your individual risk can help guide:

• When to test
• How much vitamin D you may need
• How to maintain healthy levels throughout the year

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.

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