Ask A Doc: What is Vitamin D and why is it important?

by Dr. Rahul Gupta

First, the basics! Vitamin D is a hormone, meaning that it acts as a signal to stimulate different cells and tissues in the body into action. There are two forms of Vitamin D: D2 or ergocalciferol, made by plants, and D3 or cholecalciferol, made by mammals. The main source of vitamin D for humans is D3 made in the skin from a precursor substance via the energy of the sun’s ultraviolet light; it is then further modified in the liver and then the kidney to become its active form. Vitamin D can also be obtained from the diet – look for foods fortified with vitamin D such as milk, cereal, orange juice, yogurt, and margarine, as well as foods in the chart below.

Vitamin D’s major role is to facilitate absorption of calcium and phosphorus from food, both of which are essential for bone health. New evidence suggests a role for vitamin D in maintaining cognitive function and our immune system as well as preventing autoimmune diseases such as multiple sclerosis and rheumatoid arthritis, some forms of cancer, type 2 diabetes mellitus, and heart disease. Low vitamin D has some association with seasonal affective disorder (SAD) and possibly depression, but more research is needed to sort out the cause-effect relationship.

Because of lifestyle changes and sunscreen usage, the majority of the population is now at risk for low vitamin D, which comes in two forms: “insufficiency” (mild, common) and “deficiency” (more severe, less common). Here in upstate NY we are especially susceptible due to low sunlight exposure during our long winters! Those at greatest risk of low vitamin D are people with dark-pigmented skin, poor nutrition, certain chronic illnesses, people taking certain medications, and infants who are exclusively breastfed. Below we will provide information and guidelines about vitamin D as it relates to both child and adult health.


Recent studies show that most children are not getting enough of this essential vitamin. Vitamin D deficiency can lead to the bone-softening diseases of rickets (generally occurs in the first two years of life) and osteomalacia (occurs in older children and adolescents, different from osteoporosis) – these conditions cause bowed-legs, bone pain, muscle weakness, and increased risk of fractures.

According to the American Academy of Pediatrics (AAP), vitamin D requirements are 400 International Units (IU) per day for infants under 12 months old versus 600 IU per day beyond that age. Both breastfed and formula-fed babies (those that are taking less than 32 ounces of formula per day) should be on an oral vitamin D supplement of 400 IU daily. Supplemental vitamin D is recommended for children who may not be getting enough from sunlight or diet – 400 IU daily for young children and 600 IU daily for adolescents. Chewable vitamins are generally regarded as safe for children over the age of three who are able to chew hard foods and candy. Children with an increased risk of vitamin D deficiency due to a medical condition or specific medication may need higher doses of vitamin D under the guidance of a pediatrician. As with any medication or supplement, vitamin D should be kept out of reach of children.


Vitamin D insufficiency/deficiency without obvious symptoms, also known as “subclinical” deficiency, is quite common in adults, whereas severe deficiency is uncommon except in people with risk factors (see below). For low-risk people who may not be getting enough from sunlight or diet, experts suggest supplementation with 600-800 IU of D3 daily.

Should we routinely screen for vitamin D deficiency?

The short answer is, no. For adults who do not have symptoms or a medical condition for which vitamin D treatment is recommended, there is not enough scientific evidence to routinely screen for deficiency with blood testing. According to the US Preventive Services Task Force (USPSTF), this recommendation applies to non-pregnant adults, who live in community dwellings, seen in primary care settings.  

Who is at risk of getting vitamin  D deficiency in the adult


Those at risk include people who are: elderly and confined indoors, dark-skinned, obese, institutionalized, limited in sun exposure due to protective clothes during winter or excessive sunscreen use, hospitalized on a general medical service, taking medications that accelerate metabolism of vitamin D (such as phenytoin, an anti-seizure medication), and also people with certain diseases such as osteoporosis, inflammatory bowel disease (Crohn’s or ulcerative colitis), celiac disease, and others.

How is vitamin D deficiency defined in adults and what are

the effects?

The degree of low vitamin D is based on results of blood testing: less than 10 ng/ml is considered severe deficiency, whereas the cutoff for less severe deficiency is reported as less than 20-30 ng/ml according to most sources or less than 30-50 ng/ml according to other sources. Usually levels of 15-20 ng/ml do not cause symptoms. Prolonged severe vitamin D deficiency can result in osteomalacia, and in this case a panel of tests is usually ordered which includes testing for celiac disease and possibly X-rays if there is also bone pain.  

How is vitamin D supplemented?

For severe vitamin D deficiency (less than 10 ng/ml), patients are treated with 50,000 IU of D2 (only available by prescription) weekly for 6-8 weeks followed by 800 IU of D3 (over-the-counter) daily, thereafter.

For patients with levels between 10-20 ng/ml, we treat with 800-1000 IU of D3 daily and we re-check the level after 3 months.

For patients with levels between 20-30 ng/ml, we usually recommend 600-800 IU of D3 daily to maintain an adequate level.

Maintaining normal vitamin D levels in adults helps to prevent falls and fractures, as well as all the benefits outlined at the beginning of the article!