what is vitamin D3?

vitamin D is a fat-soluble vitamin that your body needs for several important functions. vitamin D3 (cholecalciferol) is the form your skin makes when exposed to sunlight, and it's also the form found in supplements.

unlike most vitamins that you mainly get from food, your body can produce vitamin D when your skin is exposed to sunlight. that's why it's sometimes called the "sunshine vitamin."

what does vitamin D do?

vitamin D plays several roles in the body:

• helps absorb calcium and phosphorus for bone health
• supports muscle function
• plays a role in immune system function
• may affect mood and energy levels
• involved in cell growth and repair

why is deficiency so common?

several factors contribute to widespread vitamin D deficiency:

• limited sun exposure (indoor lifestyles, office work)
• use of sunscreen (which blocks vitamin D production)
• living in areas with less sunlight
• darker skin (produces less vitamin D from sunlight)
• age (older adults produce less)
• limited dietary sources
• certain health conditions affecting absorption

general reference ranges

vitamin D is measured in ng/mL (nanograms per milliliter) or nmol/L. the test is called 25-hydroxyvitamin D or 25(OH)D.

• deficient: less than 20 ng/mL (50 nmol/L)
• insufficient: 20–29 ng/mL (50–72 nmol/L)
• sufficient: 30–100 ng/mL (75–250 nmol/L)
• potentially harmful: greater than 100 ng/mL (250 nmol/L)

note: optimal levels are debated among experts. your doctor will recommend what's appropriate for you.

symptoms that may be associated with deficiency

vitamin D deficiency often has no obvious symptoms, but may be associated with:

• fatigue and tiredness
• bone pain or achiness
• muscle weakness or cramps
• mood changes
• frequent infections
• slow wound healing

these symptoms are vague and can have many causes. testing is the only way to know your vitamin D level.

why do doctors check vitamin D?

• investigating fatigue or unexplained symptoms
• bone health assessment
• monitoring certain health conditions
• before starting supplements (to establish baseline)
• in people with risk factors for deficiency
• as part of a comprehensive health evaluation

questions doctors often ask

• how much time do you spend outdoors?
• do you use sunscreen regularly?
• what is your typical diet like?
• are you taking any vitamin D supplements?
• have you had bone fractures or bone pain?
• do you have any digestive issues?
• what is your energy level like?

what information to prepare

• your current supplement intake (including dosages)
• typical sun exposure and outdoor activities
• any symptoms you've been experiencing
• dietary habits (fish, fortified foods, etc.)
• previous vitamin D test results
• any digestive conditions that might affect absorption

frequently asked questions