Most people get enough vitamin D through sunlight, food, and basic daily habits, but several groups have a genuinely harder time maintaining healthy levels. Breastfed infants, older adults, people with darker skin, those with digestive conditions, people living at northern latitudes, and individuals with obesity all face specific biological or environmental barriers that make supplementation worth considering.
Breastfed Infants
Breast milk is nutritionally excellent but consistently low in vitamin D. The CDC and American Academy of Pediatrics recommend that all breastfed and partially breastfed infants receive 400 IU of vitamin D daily, starting in the first few days of life. Infants who drink at least 32 ounces of formula per day typically get enough from the formula itself, since it’s fortified. But babies who split between breast milk and smaller amounts of formula often fall short.
Adults Over 50
Your skin manufactures vitamin D when UVB rays hit a precursor compound in the outer layers. As you age, the concentration of that precursor drops significantly. Between age 20 and 80, levels fall by roughly 50%, cutting the skin’s production capacity in half regardless of how much sun exposure you get. At the same time, the body breaks down the active form of vitamin D faster with age. This double hit means older adults are far more likely to become deficient even with the same sun habits they had when younger.
The current recommended dietary allowance for adults of all ages is 600 IU per day, though many clinicians suggest higher amounts for people over 65. Getting that amount from food alone is difficult since few foods are naturally rich in vitamin D (fatty fish and egg yolks are the main sources, along with fortified milk and cereal).
People With Darker Skin
Melanin, the pigment responsible for darker skin tones, absorbs the same UV radiation the body uses to produce vitamin D. The more melanin your skin contains, the less efficiently it converts sunlight into vitamin D. This doesn’t mean darker-skinned people can’t make vitamin D, but it does mean they need substantially more sun exposure to produce the same amount as someone with lighter skin. For people who live in less sunny climates or spend most of the day indoors, supplementation can bridge the gap.
People With Digestive Conditions
Vitamin D is fat-soluble, meaning your gut needs to absorb it alongside dietary fat. Several gastrointestinal conditions severely impair this process. Celiac disease damages the intestinal lining, reducing absorption of both vitamin D and calcium. Long-standing celiac disease commonly leads to low bone density because the body pulls calcium from bones when it can’t absorb enough from food. Treatment includes a gluten-free diet plus vitamin D and calcium supplementation.
Inflammatory bowel diseases like Crohn’s and ulcerative colitis create similar problems through a combination of intestinal damage, dietary restrictions, reduced sun exposure (people with flares often stay home more), and the use of corticosteroid medications that further interfere with vitamin D metabolism. Bariatric surgery, particularly gastric bypass procedures, reroutes the digestive tract in ways that reduce contact with the bile salts needed to absorb fat-soluble vitamins. Routine vitamin D monitoring and supplementation are standard care for anyone who has had weight-loss surgery.
People With Obesity
Body fat acts as a reservoir for vitamin D, pulling it out of circulation and locking it away. Research comparing vitamin D storage in obese and normal-weight women found that obese subjects had significantly greater total vitamin D stored in fat tissue (2.3 mg versus 0.4 mg) but lower levels circulating in the blood. The larger the fat mass, the more vitamin D it takes to “fill the tank” before levels in the bloodstream rise to a healthy range. For a given intake, someone with obesity will typically end up with lower blood levels than someone at a normal weight. This means people carrying significant extra weight often need higher doses to reach the same target.
People With Limited Sun Exposure
If you work indoors, live in a northern climate, wear clothing that covers most of your skin, or consistently use sunscreen, you produce less vitamin D through your skin. During winter months at higher latitudes, UVB rays are too weak to trigger meaningful vitamin D production even on clear days. Anyone who rarely gets unprotected midday sun exposure for at least 10 to 15 minutes several times a week is a reasonable candidate for supplementation, especially during fall and winter.
How to Know If You’re Deficient
A simple blood test measuring 25-hydroxyvitamin D tells you where you stand. The widely used thresholds are:
- Below 12 ng/mL: Deficiency, associated with rickets in children and soft, weakened bones in adults
- 12 to 20 ng/mL: Generally considered inadequate for bone and overall health
- 20 ng/mL or above: Sufficient for most people
- Above 50 ng/mL: Potentially harmful, especially above 60 ng/mL
Some experts advocate for a target above 30 ng/mL to capture the full range of vitamin D’s benefits, though the National Academies considers 20 ng/mL adequate for the general population. The US Preventive Services Task Force currently does not recommend routine screening for people without symptoms or known risk factors, so testing is most useful when you fall into one of the higher-risk groups described above or have symptoms like unexplained fatigue, bone pain, or frequent fractures.
How Much Is Too Much
Vitamin D toxicity is rare but real, and it only happens through supplements, never from sun exposure or food. When blood levels climb above 150 ng/mL, excess calcium can build up in the blood, causing nausea, kidney problems, and in severe cases, heart rhythm issues. The recommended daily amount of 600 IU for most children and adults is well within safe limits. If your doctor prescribes a higher dose based on bloodwork, periodic retesting ensures levels stay in a healthy range.