Most adults need 600 IU (15 mcg) of vitamin D per day, and that number applies whether you’re taking D2 or D3. If you’re over 70, the recommendation rises to 800 IU (20 mcg). These are the amounts set by the National Academies of Sciences, Engineering, and Medicine to maintain healthy bones and adequate blood levels in the general population. But because D2 is less potent than D3 at raising your blood levels, there are some practical details worth understanding before you settle on a dose.
Recommended Daily Amounts by Age
The official recommendations don’t distinguish between D2 and D3. They’re expressed as total vitamin D intake from food and supplements combined:
- Infants (0–12 months): 400 IU (10 mcg)
- Children and teens (1–18 years): 600 IU (15 mcg)
- Adults (19–70 years): 600 IU (15 mcg)
- Adults over 70: 800 IU (20 mcg)
- Pregnant or breastfeeding women: 600 IU (15 mcg)
For unit conversions on supplement labels, 1 IU of vitamin D equals 0.025 mcg. So a 1,000 IU supplement provides 25 mcg.
Why D2 and D3 Aren’t Equally Effective
Vitamin D2 (ergocalciferol) comes from plants and fungi. Vitamin D3 (cholecalciferol) comes from animal sources and is the form your skin makes from sunlight. Both forms are biologically active, but they don’t raise your blood levels by the same amount.
A systematic review and meta-analysis published in The American Journal of Clinical Nutrition found that D3 supplementation raises blood levels of 25-hydroxyvitamin D significantly more than D2 at the same dose. The liver enzyme that converts vitamin D into its usable form appears to prefer D3 as a starting material. D2 also gets broken down and deactivated faster in the body through an additional chemical step that D3 doesn’t undergo.
What this means in practice: if you’re relying exclusively on D2, you may need a somewhat higher dose to achieve the same blood levels as someone taking D3. Some practitioners suggest that daily D2 doses in the range of 1,000 to 2,000 IU are more realistic for maintaining adequate levels, particularly if you have limited sun exposure or are already running low. The only way to know for sure whether your current dose is working is a blood test measuring your 25-hydroxyvitamin D level.
Who Takes D2 and Why
D2 is the go-to form for people following a vegan or strict plant-based diet because it’s derived from fungi rather than animals. Most D3 supplements are made from lanolin (sheep’s wool oil) or fish oil, which rules them out for vegans. While vegan-certified D3 sourced from lichen does exist, D2 remains more widely available and is the form used in most plant-based fortified foods.
Mushrooms are one of the few whole foods that naturally contain D2. Varieties grown under ultraviolet light can provide roughly 450 IU per 100-gram serving, which is a meaningful contribution. Mushrooms grown in the dark, however, contain very little. If you eat UV-exposed mushrooms regularly, that intake counts toward your daily target.
D2 is also the form most commonly used in prescription-strength vitamin D. When doctors treat a confirmed deficiency, they often prescribe high-dose D2 capsules, typically 50,000 IU taken once a week for several weeks. This is a targeted, time-limited treatment protocol, not a daily maintenance dose. If you’ve been prescribed this, the dosing schedule your provider gave you supersedes general guidelines.
Upper Limits and Safety
The tolerable upper intake level for adults is 4,000 IU (100 mcg) per day from all sources. Going above this on a regular basis without medical supervision increases your risk of toxicity, which causes too much calcium to build up in your blood. Symptoms include nausea, excessive thirst, frequent urination, and in severe cases, kidney damage.
Toxicity from food alone is essentially impossible. It happens from supplements, specifically from taking very high doses (often well above 10,000 IU daily) for extended periods. Standard over-the-counter D2 supplements in the 400 to 2,000 IU range carry minimal risk for healthy adults.
Getting the Most From Your D2 Supplement
Vitamin D is fat-soluble, meaning your body absorbs it much better when you take it alongside a meal that contains some fat. This doesn’t need to be a heavy meal. A handful of nuts, avocado on toast, or a drizzle of olive oil on vegetables is enough to improve absorption noticeably. Taking your supplement on an empty stomach means a portion of it passes through without being absorbed.
Consistency matters more than timing. Whether you take it in the morning or evening doesn’t affect how well it works, but taking it daily (rather than sporadically remembering) is what keeps your blood levels stable. If you’re on D2 specifically because of dietary restrictions, pairing it with fortified foods like plant milks or cereals can help you reach your target without relying on a single high-dose pill.
If you suspect you’re deficient, or if you’ve been supplementing for months without knowing whether it’s working, a simple blood test can give you a clear answer. Levels below 20 ng/mL are generally considered deficient, 20 to 29 ng/mL is insufficient, and 30 ng/mL or above is where most people want to be. Your result will tell you whether your current D2 dose needs adjusting far more reliably than any general guideline can.