Vitamin DVitamin

• Observational studies versus randomized trialsPossibly helps

  This is the through-line of the whole vitamin D literature, and it is unresolved.
  
  Higher blood vitamin D tracks with lower risk of heart disease, cancer, depression, multiple sclerosis and type 2 diabetes across observational cohorts [1]. Randomized trials of supplementation have largely not reproduced those findings [1].
  
  NASEM's Food and Nutrition Board judged the evidence inadequate or too contradictory for any outcome except bone measures [1]. A 2014 AHRQ review of nearly 250 studies landed in the same place [1]. ^[1]

• Correcting a diagnosed deficiencyLikely helps

  Rickets in children and osteomalacia in adults are vitamin D deficiency diseases, and vitamin D resolves them [1].
  
  Per NIH ODS, D2 and D3 both do this, and appear equally able to [1].
  
  Keep this separate from the question the trials below asked. Whether supplements help people whose levels are already adequate is a different experiment, and mostly a negative one [1]. ^[1]

• Bone density and fracture riskPossibly helps

  Bone is the one area NASEM accepted evidence for, and even there the trials pull in different directions [1].
  
  In institutionalized older adults, vitamin D plus calcium produced small increases in bone mineral density and lower fracture rates [1]. In community-dwelling adults the results are inconsistent [1].
  
  A meta-analysis of 6 trials (49,282 people, about 5.9 years of daily D plus calcium) found 6% fewer fractures of any kind and 16% fewer hip fractures [1].
  
  The USPSTF found the evidence insufficient to weigh benefits against harms, and recommended against low-dose supplementation of 400 IU or less with 1,000 mg calcium or less for this purpose [1].
  
  The 2022 VITAL ancillary fracture study — 25,871 participants, 2,000 IU/day D3, most already vitamin-D-sufficient — found no reduction in total, hip or nonvertebral fractures [1]. ^[1]

• Cancer — incidence versus mortalityPossibly helps

  The split matters here. Trials generally do not show vitamin D lowering cancer incidence, but meta-analyses of those same trials do find lower cancer mortality [1].
  
  Pooling 10 RCTs, including VITAL, daily vitamin D3 went with a 13% lower cancer mortality rate [1].
  
  Observational work links higher serum vitamin D to lower incidence too. The trials support only the mortality half of that [1]. ^[1]

• Cardiovascular diseaseNot shown to help

  Two large randomized trials looked and found nothing. A New Zealand trial (5,110 people) and VITAL (25,871 people) reported no reduction in heart attacks, strokes or cardiovascular death [1].
  
  That held even among participants whose baseline vitamin D was low [1].
  
  The observational picture is the opposite. Pooling 34 studies (180,667 people), each 25 nmol/L (10 ng/mL) higher serum 25(OH)D went with 10% lower CVD event risk [1]. ^[1]

• Depressive symptomsNot shown to help

  Low vitamin D is observationally associated with depression, but supplementation has not moved the outcome in trials [1].
  
  A meta-analysis of 9 trials (4,923 people) found no significant effect on depressive symptoms, and VITAL agreed [1]. ^[1]

• Multiple sclerosisPossibly helps

  Observational and genetic studies tie low vitamin D to higher MS risk [1].
  
  A 2018 Cochrane review of 12 trials (933 participants) found no effect of supplementation on relapse or disability [1].
  
  Cochrane rated all 12 trials low quality. That makes this an absence of good evidence rather than a settled negative [1]. ^[1]

• Type 2 diabetesPossibly helps

  The largest trial gave 4,000 IU/day to 2,423 adults with prediabetes and found no significant overall reduction in progression to diabetes [1].
  
  A post hoc subgroup — participants whose baseline 25(OH)D was under 30 nmol/L — showed a 62% lower incidence than placebo [1].
  
  Post hoc subgroups generate hypotheses, not conclusions. It is still the clearest hint in this literature that correcting a deficiency and dosing a replete person are different experiments [1]. ^[1]

• Weight and fat lossNot shown to help

  Obesity is observationally linked to lower vitamin D status, which is where the idea came from [1].
  
  Clinical trials do not support vitamin D on its own, without calorie restriction, as a way to reduce body weight or fat mass [1]. ^[1]

• Rickets in children — bone tissue that fails to mineralize, giving soft, bowed or bent bones, joint deformities, muscle weakness and bone pain [1].
• Very severe childhood cases add failure to thrive, developmental delay, hypocalcemic seizures, tetanic spasms, cardiomyopathy and dental abnormalities [1].
• Osteomalacia in adults and adolescents — bone that mineralizes incompletely during remodelling, leaving it weak and painful [1].
• Milder shortfalls are vague: fatigue, bone pain, muscle weakness, aches or cramps, and mood changes including depression [1].
• Those milder symptoms trace back to the low blood calcium and secondary hyperparathyroidism that low vitamin D can trigger [1].
• Many people with low vitamin D have no symptoms at all [1].
• Status is read from serum 25-hydroxyvitamin D. NASEM puts the deficiency-risk threshold at 30 nmol/L (12 ng/mL) or below [1].
• Between 30 and 50 nmol/L (12–20 ng/mL) counts as potentially inadequate; 50 nmol/L (20 ng/mL) or above is adequate for most people [1].
• Highest-risk groups include breastfed infants given no supplement, older adults, people with little sun exposure, and people with darker skin [1].
• Fat-malabsorption conditions, obesity at a BMI of 30 or above, gastric bypass surgery, lactose intolerance, milk allergy and vegan diets also raise risk [1].
• NHANES 2011–2014 found 5% of Americans age 1 and up at risk of deficiency, and 18% at risk of inadequacy [1].
• That risk was not evenly spread: 17.5% among non-Hispanic Black Americans versus 2.1% among non-Hispanic White Americans [1].
• Cleveland Clinic puts the global figure near 1 billion people, and about 35% of US adults, by one clinical estimate [3].^[1]

• Vitamin D3 (cholecalciferol)The default over-the-counter form .

  Elemental content

  Made by UV-irradiating 7-dehydrocholesterol from sheep's-wool lanolin. A lichen-derived, animal-free D3 is also sold [1].

  Absorption

  Most evidence has D3 raising serum 25(OH)D higher than D2, and holding it there longer [1].
  
  Both forms resolve rickets, and most metabolic steps are identical [1].

  Best for

  The default over-the-counter form [1]. It is what most of the large supplementation trials used.

• Vitamin D2 (ergocalciferol)Vegan-suitable without needing a lichen source.

  Elemental content

  Made by UV-irradiating ergosterol in yeast, so it is inherently plant-sourced [1].

  Absorption

  Raises serum 25(OH)D, but by less and for a shorter stretch than D3 in most evidence [1].

  Best for

  Vegan-suitable without needing a lichen source. Expect a smaller and shorter-lived rise than the same microgram dose of D3 [1].

• Calcifediol, 25(OH)D3Worth recognising on a label. NIH ODS notes it is not currently sold as an over-the-counter US supplement .

  Elemental content

  Pre-hydroxylated, so it skips the liver's conversion step [1].

  Absorption

  Roughly 3 to 5 times as potent per microgram as standard D3 at raising serum levels [1].

  Best for

  Worth recognising on a label. NIH ODS notes it is not currently sold as an over-the-counter US supplement [1].

• Softgels versus liquid dropsWhichever one you will actually take with food.

  Elemental content

  Same nutrient either way. Vitamin D is fat-soluble, so both are carried in oil [1].

  Absorption

  With normal gut function and a fat-containing meal, the two are generally read as equivalent [1].
  
  Formulation studies disagree on micellar and nanoemulsion products: some reach higher peak blood concentrations than softgels or tablets, others find capsules comparable or slightly better [1].

  Best for

  Whichever one you will actually take with food. The liquid advantage looks clearest in fat-malabsorption conditions, not the general population [1].

• Too much becomes hypercalcemia

  Vitamin D toxicity is real, and it comes almost entirely from supplements — not from food, and not from sun exposure [1].
  
  The mechanism is hypercalcemia, too much calcium in the blood. It shows up as nausea, vomiting, poor appetite, weakness, confusion and kidney stones [1].
  
  In extreme cases it extends to kidney failure, soft-tissue calcification and cardiac arrhythmias [1]. ^[1]

• Fat malabsorption and gastric bypass

  Celiac disease, Crohn's disease, ulcerative colitis, cystic fibrosis and some liver disease all impair fat absorption, and vitamin D needs dietary fat to be absorbed [1].
  
  Gastric bypass surgery and a BMI of 30 or above also raise the risk of inadequacy [1]. ^[1]

• Thiazide diuretics

  Thiazides cut urinary calcium excretion, while vitamin D increases how much calcium the gut absorbs [1].
  
  Combining them raises hypercalcemia risk, particularly in older adults and in people with impaired kidney function or hyperparathyroidism [1]. ^[1]

• Statins, in both directions

  Statins lower cholesterol synthesis, and since vitamin D derives from cholesterol, they may reduce how much your body makes for itself [1].
  
  Running the other way, high vitamin D intake — especially from supplements — may reduce the potency of atorvastatin, lovastatin and simvastatin [1].
  
  The two appear to compete for the same metabolizing enzyme [1]. ^[1]

• Orlistat

  The weight-loss drug orlistat reduces absorption of vitamin D, and of the other fat-soluble vitamins, from both food and supplements [1].
  
  The effect appears alongside the reduced-fat diet orlistat is taken with, and lowers serum 25(OH)D [1]. ^[1]

• Oral corticosteroids

  Prednisone and similar steroids reduce calcium absorption and impair vitamin D metabolism [1].
  
  In NHANES 2001–2006, deficiency below 25 nmol/L (10 ng/mL) affected 11% of oral steroid users against 5% of non-users [1]. ^[1]

• Absorption blockers and enzyme inducers

  Bile-acid sequestrants — cholestyramine, colesevelam, colestipol — and mineral oil both reduce how much vitamin D you absorb [1].
  
  The anti-seizure drugs phenytoin, fosphenytoin and phenobarbital, plus the antibiotic rifampin, alter its metabolism [1].
  
  Antacids, magnesium supplements and phosphorus supplements are listed as potential interactions too [1]. ^[1]

Barely any food carries much vitamin D naturally, which is why fortification does the heavy lifting in American diets [1].

Cod liver oil is the outlier at 34.0 mcg (1,360 IU) per tablespoon [1]. Cooked farmed rainbow trout gives 16.2 mcg (645 IU) per 3 oz, and cooked sockeye salmon 14.2 mcg (570 IU) [1].

UV-exposed white mushrooms supply 9.2 mcg (366 IU) per half cup — that UV step is deliberate, and ordinary mushrooms carry variable, much smaller amounts of D2 [1].

Fortified 2% milk runs 2.9 mcg (120 IU) per cup, fortified soy, almond or oat milk 2.5–3.6 mcg (100–144 IU), and a cereal serving fortified to 10% DV 2.0 mcg (80 IU) [1].

Smaller contributors: two canned sardines at 1.2 mcg (46 IU), one large scrambled egg at 1.1 mcg (44 IU), and braised beef liver at 1.0 mcg (42 IU) per 3 oz [1].

Broccoli, carrots, almonds, apples, bananas, rice, whole wheat bread, lentils, sunflower seeds and edamame contain essentially none [1].

Sunlight is the other route. UVB converts a skin compound into previtamin D3, but it needs direct uncovered skin, since glass blocks the relevant wavelengths [1].

Season, time of day, latitude, cloud cover, skin melanin and sunscreen all change how much you make [1].

Take it with a meal or snack containing fat. That is the one timing point with measurable evidence behind it [8].

A small study of 17 people found pairing the dose with the day's largest meal raised blood levels about 50% over 2–3 months [8]. In 50 older adults, a high-fat meal produced roughly 32% higher 12-hour levels than a fat-free one [8].

Beyond food pairing, no strong evidence names a best hour. Morning or early afternoon gets recommended mostly for convenience, and because sun-driven synthesis peaks during the day [7].

Evening dosing is genuinely unsettled. One line of research suggests vitamin D near bedtime could increase serotonin-to-melatonin conversion and disturb sleep; other reviews instead associate supplementation with better sleep quality [8].

Reviewers agree consistency beats clock time. Pick the mealtime you will keep, and move off nighttime dosing only if you notice your own sleep suffering [7].