After practicing in the gray skies of the Pacific Northwest and enduring the long, dark Alaskan winters, Grimes Family Medicine physician Dr. Brad Willenbring has seen firsthand how a lack of sunshine can affect overall health.
“Questions about vitamin D were a common topic among my patients in those regions,” says Dr. Willenbring. “Even here in central Iowa, with its cold but sunlit winters, it’s important to understand the vital role vitamin D plays in our well-being.”
What is vitamin D and why is it important?
Vitamin D is best known for its role in helping the body absorb calcium, supporting bone health, and preventing health conditions in adults and children. However, its benefits extend far beyond that.
“Nearly every day, we discover a new pathway or link between vitamin D and other critical body functions,” says Dr. Willenbring. “Recent research has shown connections between vitamin D and immune system function, brain health, and even cancer prevention.”
Vitamin D deficiency has been associated with a variety of health concerns. In older adults, deficiency has been associated with cognitive problems and conditions like Alzheimer’s. Research also indicates that adequate vitamin D is essential for brain development in babies and may reduce the risk of autoimmune disorders, such as rheumatoid arthritis.
What are the best sources of vitamin D?
Vitamin D can be sourced in several ways, including through diet, supplements, or sun exposure. Your body naturally produces vitamin D when your skin is exposed to UVB light, but there are also natural and fortified foods that can help boost your levels. The best natural sources of vitamin D include:
- Fish, such as trout or salmon
- Beef liver
- Egg yolks
- Cheese
- Mushrooms
In addition, cow’s milk in the U.S. is typically fortified with vitamin D, providing about 120 IU per cup. Vitamin D supplements, including D2 and D3, are also widely available on store shelves. D3 has been found to replace vitamin D more efficiently than D2, making it the preferred choice for most people.
Many people wonder if tanning beds are a good way to get vitamin D. While the UVB light from tanning beds can help increase vitamin D levels, it’s not a safe or recommended option because there is no clear limit for how much UVB exposure is safe.
“A better and safer way to boost vitamin D is through sun exposure or supplements,” says Dr. Willenbring.
Spending about 20 minutes in the sun, twice a week, with your arms, hands, and legs covered is usually enough. However, there’s a catch: in places like Iowa, where sunlight is limited during fall and winter, your skin can’t make much vitamin D. This is why levels peak in September and dip to their lowest in March.
For most people, incorporating vitamin D-rich foods and supplements into their routine is a reliable way to maintain healthy levels year-round.
How much vitamin D do I need?
The recommended daily intake of vitamin D varies by age and individual health needs:
| Age Group | Recommended Level of Vitamin D (IU) |
| Healthy adults | At least 600 IU per day |
| Adults over age 70 | At least 800 IU per day |
| Infants and Children | At least 400 IU per day, starting shortly after birth |
“For formula-fed babies, all formulas sold in the U.S. contain the recommended amounts of vitamin D, so supplementation isn’t needed,” Dr. Willenbring explains. “Breastfed babies, however, do require supplementation because breast milk alone doesn’t provide enough vitamin D, even if the mother is taking supplements.”
Children one year and older should receive at least 600 IU of vitamin D daily, and new or expecting mothers should ensure they are meeting their own vitamin D needs to support their baby’s health.
Certain groups may require higher doses of vitamin D to avoid deficiency. People with a BMI over 30, for example, may need additional supplementation due to how vitamin D is stored in body fat.
How do I know if I am vitamin D deficient?
Vitamin D’s most well-known role within the body is facilitating calcium absorption from the gut and regulating bone health. The most common symptoms of a vitamin D deficiency include:
- Bone pain
- Muscle weakness
- Bone fractures
- Difficulty walking
If left untreated, long-term deficiency can lead to serious health issues, including osteoporosis in adults and rickets in children.
“Fortunately, a simple blood test can check your vitamin D levels,” says Dr. Willenbring. “If your levels are low, your doctor can recommend the right dose of supplementation to bring them back to a healthy range.”
Although vitamin D toxicity is rare, it can occur when high doses of supplements are taken without medical supervision. Thankfully, the body naturally regulates how much vitamin D it produces from sunlight, so there’s no risk of getting too much from sun exposure.
Whether you’re soaking up the sun during the brighter months or relying on supplements during the winter, maintaining adequate levels of vitamin D is essential for overall health. If you have questions about your vitamin D intake or suspect a deficiency, schedule an appointment with your primary care provider to discuss your needs and guide you on the best way to maintain healthy levels.
BradWillenbring , DO
Dr. Brad Willenbring is a board-certified physician that joined The Iowa Clinic Family Medicine department in 2024. With more than a decade of experience, Dr. Willenbring enjoys caring for a diverse panel of patients, from newborn to geriatric. The ability to provide care for entire families and develop meaningful relationships is something Dr. Willenbring finds most rewarding about his practice.
After completing his undergraduate degree at the University of Iowa, Dr. Willenbring was sought out and selected for the US Army Health Professions Scholarship Program. After beginning medical school at Des Moines University, he was commissioned as a uniformed officer. He then made his way to Washington state, where he completed a rotating internship and earned an international diploma in mountain medicine. Dr. Willenbring then spent four years on active duty with the Army in Alaska, two years embedded with the infantry, and two years in aviation medicine. Afterwards, Dr. Willenbring finished residency through a Michigan State University affiliate outside of Detroit before returning to Iowa.
An Iowa native, Dr. Willenbring looks forward to providing exceptional care to the people of central Iowa. Outside of work, he enjoys spending time with his wife and children, paddleboarding, being outdoors, and reminiscing with 90s music.