Vitamin D Deficiency
Vitamin D is a hormone that is mainly produced in the skin as a response to sunlight.12 The dominant form of vitamin D that circulates throughout the body is 25- hydroxyvitamin D (25[OH]D).13 According to the Institute of Medicine (IOM), vitamin D deficiency is defined as a serum 25(OH)D level <12 ng/mL.14 The Endocrine Society defines vitamin D deficiency as 25(OH)D levels ranging from 21 to 29 ng/mL.15 The recommended measurement of vitamin D is the 25(OH)D level using a reliable assay.15 Vitamin D deficiency in the general population has numerous causes, including lack of sunlight, poor dietary choices, and comorbidities such as diabetes, inflammatory bowel disease, or gastric bypass.
In the United States, milk, orange juice, yogurts, cheese, breads, and cereals are fortified with vitamin D.15 Foods that naturally contain vitamin D include fatty fish, eggs, mushrooms, and fish oil.14 Individuals with IBS often admit to food avoidance due to unpleasant IBS symptoms, resulting in a general nutritional deficit and a diet that may exclude food sources high in calcium and vitamin D.16 It is also known that individuals with IBS commonly have lactose intolerance and are more likely to avoid all dairy products. Although diet plays a small role in maintaining normal vitamin D levels, it continues to be an important aspect in maintaining daily recommended levels.
Ultraviolet B (UVB) is a medium wavelength radiation that is emitted by the sun.17 Most vitamin D is produced from UVB that is absorbed through the epidermal layer of the skin where 7-dehydrocholesterol is present.4 According to the IOM, a few minutes of exposure to sunlight each day is enough to obtain sufficient vitamin D levels.14 This guidance can be conflicting for individuals, especially when they are advised to avoid the sun or to wear sunscreen due to the damaging effects of the sun that can lead to skin cancer.
Other barriers to obtaining enough sunlight to maintain sufficient vitamin D levels exist, such as geography, time of day, cloud coverage, smog, season, pollution, glass filters, and an individual’s mobility to access the outdoors.14 In 2009, the American Academy of Dermatology made a strong statement that advised protection from the sun at all times by using sunscreen and wearing protective clothing, and in 2014, a statement from Brett Colidron, MD, president of the American Academy of Dermatology, reinforced the recommendation of photoprotection.18 Due to the potential benefits of vitamin D, weighing the risk vs the benefits of sun exposure is a challenge for healthcare providers.
Symptoms of deficient vitamin D levels include fatigue, muscle weakness, chronic musculoskeletal pain, leg heaviness, and joint pain, all of which are common complaints in a primary care setting.19 Vitamin D deficiency has been linked to an increased risk of colon cancer, ovarian cancer, and prostate cancer.4
According to the US Preventive Services Task Force, there is currently insufficient evidence to assess the risks and benefits of screening for vitamin D deficiency in asymptomatic adults.20 As a result of this, healthcare providers often order vitamin D levels only when a patient has complaints related to vitamin D deficiency. There are no set recommendations on daily dosing of vitamin D supplementation; however, the IOM reports the upper limit of safe supplementation to be 4000 IU/d and that too much vitamin D can place an individual at risk for cancer, cardiovascular disease, falls, fractures, and mortality.14
Monitoring blood levels in patients taking a vitamin D supplement is essential. Vitamin D supplements can be purchased as over-the-counter agents, but prescription doses can also be prescribed by healthcare providers. The National Kidney Foundation provides the following guidance: severe vitamin D insufficiency at <5 ng/mL requires supplementation with vitamin D (ergocalciferol) 50,000 IU weekly for 12 weeks followed by monthly dosing thereafter; moderate vitamin D insufficiency at 5 ng/mL to 15 ng/mL requires vitamin D (ergocalciferol) 50,000 IU weekly for 4 weeks followed by monthly dosing; and mild vitamin D insufficiency at 16 ng/mL to 30 ng/mL requires monthly supplementation with vitamin D (ergocalciferol) 50,000 IU.21 Vitamin D (ergocalciferol) at a dose of 50,000 IU is available via prescription only and cannot be purchased over the counter.