Most adults need 600 IU (15 mcg) of vitamin D per day, and adults over 70 need 800 IU (20 mcg). These are the official Recommended Dietary Allowances set by the National Academies of Sciences, Engineering, and Medicine, and they assume you’re getting minimal sun exposure. But the right dose for you can vary significantly depending on your age, body weight, skin tone, and how much time you spend outdoors.
Recommended Daily Amounts by Age
The recommendations break down simply. Infants up to 12 months need 400 IU daily. Children and adults from age 1 through 70 need 600 IU. Adults over 70 need 800 IU, because aging skin produces less vitamin D from sunlight and the kidneys become less efficient at converting it to its active form.
These numbers represent the amount most people need from all sources combined: food, sunlight, and supplements. If you eat fatty fish a couple of times a week, drink fortified milk, and get regular sun exposure, you may already be close. If you rarely eat those foods or spend most of your day indoors, a supplement covering the full RDA makes sense.
When You Might Need More
The standard 600 to 800 IU recommendation works for the general population, but certain groups consistently end up with low blood levels even at those doses. People with higher body weight are one major example. Because vitamin D is fat-soluble, it gets stored in fat tissue instead of circulating in the blood. Research suggests that people with obesity may need two to three times the standard dose, with a minimum of about 2,000 IU per day, just to reach normal blood levels.
Other factors that increase your needs include darker skin (more melanin filters out the UV rays that trigger vitamin D production), living at higher latitudes where winter sunlight is too weak to produce vitamin D, and conditions that affect fat absorption like celiac disease or Crohn’s disease. Older adults, people who cover most of their skin, and those who work night shifts or stay indoors are also at higher risk for deficiency.
The Endocrine Society’s 2024 guidelines recommend against routine high-dose supplementation in healthy adults under 75 who aren’t deficient. Their position is straightforward: if you’re a generally healthy adult, stick with the standard RDA rather than taking large empiric doses. The higher doses should be reserved for people with a confirmed deficiency or specific risk factors.
How to Know If Your Levels Are Right
A simple blood test measures your level of 25-hydroxy vitamin D, the form your liver produces from whatever vitamin D enters your body. Most experts consider 20 to 40 ng/mL (50 to 100 nmol/L) a healthy range, though some recommend aiming for 30 to 50 ng/mL. Below 20 ng/mL is generally considered deficient, and below 12 ng/mL is severely deficient.
If you’re not sure whether you need a supplement or how much to take, a blood test gives you a concrete answer. It’s especially useful if you have risk factors for deficiency, because it lets you and your doctor choose a dose based on your actual levels rather than guessing.
D2 vs. D3: Which Form to Choose
Vitamin D supplements come in two forms. D3 (cholecalciferol) is the same form your skin makes from sunlight and is found in animal-based foods. D2 (ergocalciferol) comes from plants and fungi. Both raise blood levels of vitamin D, but D3 does it more effectively. D2 is cleared from the bloodstream faster because it binds less tightly to the protein that carries vitamin D through your system. When you’re choosing a supplement, D3 is the better option for most people. D2 supplements are available for those who prefer a plant-based source, but you may need a higher dose to achieve the same effect.
How to Take It for Better Absorption
Vitamin D is fat-soluble, which means your body absorbs it much better when you take it with a meal that contains some fat. This doesn’t require anything elaborate. A meal with olive oil, nuts, avocado, cheese, or even buttered toast provides enough fat to improve absorption. Taking your supplement on an empty stomach means a portion of it passes through without being absorbed, so pairing it with your largest meal of the day is a simple way to get more out of each dose.
Upper Limits and Toxicity Risk
Vitamin D toxicity is rare at normal supplement doses but very real at high ones. It typically occurs only at daily intakes above 10,000 IU taken over an extended period. The official tolerable upper intake level for adults is 4,000 IU per day, which represents the highest amount considered safe for long-term use without medical supervision.
Too much vitamin D causes dangerously high calcium levels in the blood. The symptoms build gradually and can include excessive thirst, frequent urination, nausea, vomiting, constipation, muscle weakness, fatigue, and confusion. In severe cases, it can damage the kidneys. Importantly, you cannot get vitamin D toxicity from sun exposure or food alone. It only happens from supplements.
If you’re taking more than 2,000 IU per day on your own, periodic blood testing is a reasonable precaution. Doses between 1,000 and 2,000 IU are widely used and well within the safety margin for most adults, but anything above 4,000 IU should be guided by a blood test confirming you actually need that much.
Practical Starting Points
For most healthy adults under 70 who don’t spend much time in the sun, a daily supplement of 600 to 1,000 IU of vitamin D3 taken with a meal is a reasonable starting point. Adults over 70 should aim for at least 800 IU. If you have obesity, very dark skin, or limited sun exposure, 1,000 to 2,000 IU is a more practical dose, though a blood test can help you fine-tune it.
For children, 600 IU covers ages 1 through 18. Breastfed infants should receive 400 IU daily as drops, since breast milk contains very little vitamin D. Formula-fed babies who drink at least 32 ounces of fortified formula per day are generally covered without additional supplementation.