Vitamin D

How much you need

The official Recommended Dietary Allowance for adults, per day:

People aged 18 fall in the official 14–18 group, which has the same values — 15 mcg (600 IU) a day, with an upper limit of 100 mcg (4,000 IU). Pregnancy and breastfeeding change what you need — confirm yours with your midwife, obstetrician or doctor.

Upper limit

The upper limit covers vitamin D from food, drink and supplements combined. It is the most that is unlikely to cause harm, not a target.

What it does

Vitamin D helps the gut absorb calcium and keeps blood calcium and phosphate at the levels bones need to mineralise; without enough, bones can become thin, brittle or misshapen ^[1]. In adults, enough vitamin D prevents osteomalacia, and together with calcium it helps protect older adults from osteoporosis ^[1]. The skin makes vitamin D3 when ultraviolet B light strikes it — though not through a window, because glass blocks those rays ^[1].

Forms

In food and supplements, vitamin D comes in two main forms: D2 (ergocalciferol) and D3 (cholecalciferol), which differ chemically only in one side chain ^[1]. The skin makes D3 ^[1]. Animal foods — including fish, egg yolks and cheese — provide mainly D3, along with a related compound, 25-hydroxyvitamin D3; mushrooms provide D2, and some are treated with UV light to raise it ^[1]. Supplements can contain either form ^[6]. Both raise the amount of vitamin D in the blood, but D3 may raise it higher and keep it there longer than D2 ^[6].

Where it comes from

1. Trout (rainbow), farmed, cookedD33 ounces16.2 mcg
2. Salmon (sockeye), cookedD33 ounces14.2 mcg
3. White mushrooms, exposed to UV light, rawD2½ cup, sliced9.2 mcg
4. Soy, almond or oat milkfortified1 cup2.5–3.6 mcg
5. Milk, 2% milkfatfortified1 cup2.9 mcg

How these five were chosen: The five foods with the most vitamin D per serving in the NIH fact sheet's food table. Fortified foods are marked. Fish liver oils, which are sold and used like supplements, are left out.

Getting the most from it

Because vitamin D dissolves in fat, eating some fat at the same time improves how much is absorbed, although some is absorbed even without it ^[1]. The NIH advises taking a supplement with a meal or snack that contains some fat ^[6]. Neither age nor obesity changes how well the gut absorbs vitamin D ^[1], but conditions that limit fat absorption — such as Crohn's disease, celiac disease and ulcerative colitis — reduce it ^[6].

What about vitamin K?

Vitamin D3 is often sold together with vitamin K2. The claim made for the pairing is not that K2 helps you absorb vitamin D, but that it helps direct calcium into bone. The evidence is limited: a review describes a meta-analysis of 16 trials in postmenopausal women in which K2 improved spine bone density in 10, but in subgroup analysis only K2 combined with other treatments had a significant effect — and many studies used high, pharmacological doses ^[7]. Another review called the evidence promising but still inconclusive ^[8]. One risk matters more than any benefit: vitamin K can interact seriously and dangerously with blood thinners such as warfarin (Coumadin), and people taking them need to keep their vitamin K intake consistent ^[9]. If you take a blood thinner, talk to your doctor before taking any supplement that contains vitamin K.

In the United States

Most people in the United States get less than the recommended amount from food: in national survey data from 2015–2016, average intake from food and drink was 5.1 mcg (204 IU) a day for men and 4.2 mcg (168 IU) for women ^[1]. Blood levels are better than intake suggests, largely because of sun exposure — in 2011–2014 data, 18% of people were at risk of inadequacy and 5% at risk of deficiency ^[1]. Fortified foods supply most of the vitamin D in American diets; almost all U.S. milk is voluntarily fortified with about 3 mcg (120 IU) per cup ^[1].

Who is most likely to run low

• Older adults — the skin makes less vitamin D with age, and many spend more time indoors ^[1]
• People with limited sun exposure, including the homebound and those whose clothing or work keeps skin covered ^[1]
• People with darker skin, which makes less vitamin D from sunlight; whether the lower blood levels harm health is not clear ^[1]
• People with conditions that limit fat absorption, such as Crohn's disease, celiac disease or cystic fibrosis ^[1]
• People with obesity or who have had gastric bypass surgery ^[1]

What deficiency looks like

In adults, long-term deficiency causes osteomalacia, in which bone is poorly mineralised and weakens, bringing bone pain and deformity ^[1]. Status is judged by a blood test for 25-hydroxyvitamin D. The National Academies' committee put the risk of deficiency below 30 nmol/L (12 ng/mL) and considered 50 nmol/L (20 ng/mL) or more sufficient for most people ^[1]. The U.S. Preventive Services Task Force found insufficient evidence to assess screening adults who have no symptoms ^[1].

What the evidence supports, and what it doesn't

Established. Bone health and normal calcium metabolism. The official values were set to maintain them ^[2], and the committee that set them found the evidence inadequate or too contradictory to conclude that vitamin D affects any other outcome ^[1]. A later government review of nearly 250 studies reached the same conclusion ^[1].

Not supported by trials. In VITAL, a U.S. trial in which 25,871 adults aged 50 and over took 50 mcg (2,000 IU) a day or a placebo for a median of 5.3 years, rates of breast, prostate and colorectal cancer did not differ, and heart attacks, strokes and cardiovascular deaths were not significantly reduced; fracture risk was not lowered and depression was not prevented ^[1]. The U.S. Preventive Services Task Force concluded with moderate certainty that supplements do not reduce falls in community-dwelling adults 65 and older ^[1]. In a trial of 2,423 adults with prediabetes given 100 mcg (4,000 IU) a day, supplements did not significantly prevent diabetes ^[1]. They do not promote weight loss ^[1].

Mixed. Meta-analyses of trials find that supplements do not change how many people develop cancer, but may reduce cancer deaths by 12% to 13% ^[1]. In older adults, vitamin D with calcium slightly increases bone density and reduces fractures in people living in care institutions, while results for people living independently are inconsistent ^[1].

Where official guidance disagrees

The United Kingdom recommends 10 mcg (400 IU) a day for everyone aged 4 and over, less than the U.S. figure ^[3]. The Endocrine Society recommends routine supplements for children and teenagers, pregnant women, adults with prediabetes and adults 75 and older — but not for healthy adults aged 19 to 74 — while advising that everyone meet the U.S. recommended amount ^[4]. A peer-reviewed analysis argues the U.S. recommended amount rests on a statistical error and should be reconsidered ^[5]. The NIH attributes such differences to an incomplete understanding of vitamin D's biology and to guidelines written for different purposes ^[1].

Too much

Too much vitamin D raises blood calcium, which can cause nausea, vomiting, muscle weakness, confusion, excessive thirst and kidney stones; in extreme cases it leads to kidney failure, calcium deposits in soft tissue, irregular heart rhythm and death ^[1]. It almost always comes from supplements — the sun cannot cause it, though frequent tanning-bed use can drive levels very high ^[1]. The National Academies' committee noted that symptoms are unlikely below 250 mcg (10,000 IU) a day, but that intakes below the upper limit may still have adverse effects over time ^[1].

Medicines that interact

The weight-loss drug orlistat reduces vitamin D absorption. Statins may reduce the body's own vitamin D production, and high intakes of vitamin D may make atorvastatin, lovastatin and simvastatin less effective. Corticosteroids such as prednisone impair vitamin D metabolism, and thiazide diuretics taken with vitamin D can raise blood calcium too high, especially in older adults ^[1]. If you take any of these regularly, talk to your doctor or pharmacist before taking a vitamin D supplement.