As winter approaches and daylight hours shorten, health experts are warning that maintaining adequate levels of vitamin D, often called the “sunshine vitamin,” can be challenging for many people. The fat-soluble nutrient is produced by the body after skin is exposed to sunlight and is also found in foods such as fatty fish, egg yolks, mushrooms, and fortified dairy products.
A 2023 study from Slovenia found that 63% of adults who were not taking supplements had insufficient vitamin D levels during the winter, compared to just 6% in summer. Kelsey Higgins, a nutrition support dietitian at Vanderbilt University Medical Center, said limited sun exposure in colder months reduces the body’s ability to absorb vitamin D. “When you are bundled up from head to toe and have limited sun exposure, your body is not absorbing as much vitamin D as it may in warmer conditions,” she said.
Adequate vitamin D is crucial for overall health. It aids calcium and phosphorus absorption, supporting bone density, and contributes to immune function, inflammation reduction, and muscle coordination, according to Carla Bouwmeester, a clinical professor at Northeastern University. In children, vitamin D deficiency can lead to rickets, a condition that softens bones, while in adults it may affect mood regulation.
The recommended daily intake of vitamin D varies by age: 600 I.U. for children and adults up to age 70, and 800 I.U. for adults over 70. Elizabeth Ko, an associate clinical professor at UCLA, said sun exposure alone may not be sufficient for everyone. Factors such as season, time of day, cloud cover, skin pigmentation, and sunscreen use all affect how much vitamin D the body can produce naturally.
Dietary sources of vitamin D are limited. Joan Salge Blake, a clinical professor at Boston University, said fatty fish such as salmon, sardines, and trout, as well as egg yolks, mushrooms, fortified milks, and cereals, are the primary sources, but few other foods contain significant amounts.
Certain groups face a higher risk of deficiency year-round, including older adults, people with digestive disorders like Crohn’s disease or celiac disease, individuals who have undergone gastric bypass surgery, and those with darker skin due to higher melanin levels. Some medications, including corticosteroids and certain weight-loss drugs, can also reduce absorption.
Experts advise considering supplementation, especially during winter months. Jacqueline Vernarelli, a public health nutritionist, said supplements should be taken with meals containing fat for optimal absorption. Vitamin D3, or cholecalciferol, is preferred over D2, or ergocalciferol, as it raises blood levels more effectively and maintains them longer.
While supplements can help prevent deficiency, Higgins emphasized caution. The tolerable upper limit for vitamin D intake is 4,000 I.U. per day. Excessive intake can lead to high calcium levels, kidney stones, or cardiac issues, making it important to aim for recommended levels rather than over-supplement.
Health professionals recommend checking vitamin D levels through a blood test and adjusting intake as needed to maintain optimal levels for bone, muscle, and immune health throughout the year.
