Dr. Snead’s Healthcare Secrets Newsletter

Vitamin D, Sunlight, and Lung Health

Dr. Snead’s Healthcare Secrets Newsletter is designed to make complex medical information easier to understand and easier to discuss with a healthcare professional.

Main lab test: 25-hydroxyvitamin D (25[OH]D)
Useful cutoff: 20 ng/mL is commonly used as adequate for most
Seasonality: Levels often run lower in winter
Higher risk: Low sun exposure, darker skin, indoor lifestyle

Why checking vitamin D matters

Vitamin D is important for calcium absorption, bone strength, muscle function, and overall health. In cooler climates with limited sunlight, vitamin D levels often drop because the skin cannot make enough vitamin D from UVB exposure alone.

Testing vitamin D can be especially important in people with limited sun exposure, darker skin, older age, obesity, chronic illness, malabsorption, osteoporosis, bone pain, muscle weakness, or a history of fractures and falls.

Why levels may be low

  • Living in northern climates with long winters
  • Indoor work and limited midday sunlight
  • Darker skin, which reduces UVB-driven vitamin D production
  • Low intake of vitamin D-rich or fortified foods
  • Certain medical conditions or medications

Why levels are worth knowing

  • Helps evaluate bone and muscle health risk
  • Can guide safe supplementation decisions
  • Provides context in patients with frequent illness or fatigue
  • Can support broader prevention and wellness discussions
  • May be particularly relevant in higher-risk groups
Practical point: In northern states such as Illinois, vitamin D levels are often lower in winter, but many patients may have low levels all year—especially patients of color who are not taking supplements.

Vitamin D and lung health

Vitamin D has been studied for its relationship to lung disease, asthma, and respiratory infections because it plays a role in immune function and inflammatory balance.

Asthma

  • Low vitamin D levels are frequently found in people with asthma.
  • Some studies suggest low levels may be associated with more asthma symptoms or complications.
  • Vitamin D may be useful as part of a broader plan, but it should not replace standard asthma treatment.

Lung infections

  • Low vitamin D has been associated in some studies with a higher risk of respiratory infections.
  • Correcting true deficiency is reasonable, especially in high-risk individuals.
  • Vitamin D is supportive, not a stand-alone substitute for vaccines, prescribed treatment, or other prevention steps.

COVID-era perspective

During the COVID outbreak, vitamin D received major attention because people with low levels appeared in some reports to have worse outcomes. Geographic area, time of year, sun exposure, and skin tone may all influence vitamin D levels. While vitamin D is not the only factor involved in health outcomes, maintaining adequate levels remains an important part of good overall health.

Food sources of vitamin D and vitamin K

Vitamin D foods

  • Fatty fish such as salmon, sardines, trout, mackerel, and tuna
  • Fortified milk and many fortified plant milks
  • Fortified cereals and some yogurts
  • Egg yolks
  • UV-exposed mushrooms

Vitamin K foods

  • Leafy greens such as spinach, kale, and collards
  • Broccoli and some vegetable oils
  • Natto and some fermented foods
  • Certain cheeses, eggs, and dairy products
Bottom line: Diet alone may not be enough for many people—especially in winter or in those with persistently low vitamin D—so supplementation may be appropriate when guided by testing and clinical judgment.

Dr. Snead’s Healthcare Secrets Newsletter is designed to make complex medical information easier to understand and easier to discuss with a healthcare professional.

This newsletter is not designed to treat, diagnose or cure any condition, It’s purpose is informational only!