Vitamin D supplementation has become one of those topics where the advice you get depends entirely on who you ask. Official guidelines tend to be conservative. Some practitioners recommend doses ten times higher. And online wellness culture sometimes treats vitamin D as a cure-all that you simply cannot get enough of.

The truth, as usual, sits somewhere in the middle. Vitamin D deficiency is genuinely common and worth addressing. But more is not always better, and excessive intake can cause real harm. This article explains how to get your vitamin D levels right without overdoing it.

For a more comprehensive look at vitamin D’s health effects, see our main article on vitamin D benefits and precautions.

What is vitamin D and why does it matter?

Vitamin D is technically not a vitamin at all. It’s a hormone precursor that your body manufactures when sunlight hits your skin. The ultraviolet B (UVB) rays trigger a chemical reaction in your skin cells, converting a cholesterol derivative into vitamin D3.

Your liver then converts this to 25-hydroxyvitamin D (written as 25(OH)D), which is what blood tests measure. Your kidneys do the final conversion into calcitriol, the active hormonal form that actually does things in your body.

This active form influences over 200 genes. Its most established function is helping your gut absorb calcium, which is why vitamin D deficiency leads to bone problems. Without adequate vitamin D, you could eat all the calcium in the world and still not absorb enough for healthy bones.

But vitamin D receptors exist throughout the body, not just in the intestines. They’re found in immune cells, heart tissue, brain cells, and many other organs. This is why researchers have explored vitamin D’s role in everything from respiratory infections to depression to cancer. The evidence for these connections varies considerably.

How common is vitamin D deficiency?

More common than you might expect. The NHS estimates that about 1 in 5 adults in the UK has low vitamin D levels 1. In certain groups, the numbers are much higher.

People at increased risk include:

Those who spend most of their time indoors or cover their skin when outside
People living at northern latitudes (above 37°N, which includes most of the UK, northern Europe, Canada, and northern US states)
Individuals with darker skin, since melanin reduces vitamin D production
Older adults, whose skin produces vitamin D less efficiently
People who are overweight or obese, since vitamin D gets sequestered in fat tissue
Those with conditions affecting fat absorption (such as coeliac disease or Crohn’s disease)

Winter makes things worse. From October to March in the UK, the sun sits too low in the sky to provide useful UVB exposure. Even on sunny winter days, you’re not making vitamin D.

How much vitamin D do you actually need?

This is where things get contentious. Different organisations give different recommendations, and the gap between them is surprisingly wide.

Official recommendations

The UK’s NHS recommends 400 IU (10 micrograms) daily for most adults, with this dose increased during autumn and winter when sun exposure is limited. The US National Institutes of Health recommends 600-800 IU daily for adults 2.

These recommendations are designed to maintain bone health in the general population. They’re not personalised to your current blood levels or individual circumstances.

What many practitioners suggest

The Endocrine Society, which represents specialists who deal with hormone-related conditions, takes a different view. Their clinical guidelines suggest 1,500-2,000 IU daily may be needed to maintain blood levels consistently above 30 ng/mL (75 nmol/L) 3.

For people who are actually deficient, even higher doses may be needed temporarily to correct the problem. A common approach involves loading doses of 50,000 IU weekly for 6-8 weeks, then dropping to maintenance doses.

My honest assessment

The official recommendations err on the side of caution and are designed to prevent deficiency diseases (rickets, osteomalacia) in healthy populations. They’re not necessarily optimal for everyone.

If you’re genuinely deficient, 400 IU daily probably won’t fix the problem quickly. If you’re already at good levels, you don’t need mega-doses. The sensible approach is to check your blood levels and dose accordingly.

For most adults who suspect deficiency but haven’t been tested, something in the range of 1,000-2,000 IU daily is reasonable and unlikely to cause harm. But if you want to know what you actually need, get a blood test.

What blood levels should you aim for?

Blood tests measure 25-hydroxyvitamin D, usually reported in either ng/mL (nanograms per millilitre) or nmol/L (nanomoles per litre). To convert: multiply ng/mL by 2.5 to get nmol/L.

Here’s how different organisations classify vitamin D status:

Deficient: Below 20 ng/mL (50 nmol/L) according to most definitions, though the NHS uses a stricter threshold of below 10 ng/mL (25 nmol/L) for frank deficiency.

Insufficient: Between 20-30 ng/mL (50-75 nmol/L) according to many researchers, though this category is debated.

Sufficient: 30-50 ng/mL (75-125 nmol/L) is the range many practitioners target.

Potentially high: Above 50 ng/mL (125 nmol/L) is where some experts start expressing concern, though toxicity typically requires much higher levels.

If you’re taking vitamin D supplements, it’s worth getting tested after a few months to see where you’ve ended up. Blood levels reach steady state after about 2-3 months of consistent supplementation.

How to take vitamin D for better absorption

Vitamin D is fat-soluble, which has practical implications for how you take it.

Take it with food containing fat. A study found that taking vitamin D with a meal containing fat increased absorption by about 50% compared to taking it on an empty stomach 4. The type of fat (saturated or unsaturated) doesn’t seem to matter much.

D3 versus D2: Most supplements contain vitamin D3 (cholecalciferol), which is the form your skin makes naturally. Some plant-based supplements use D2 (ergocalciferol). Research suggests D3 raises blood levels more effectively 5, so D3 is generally preferred.

Daily versus weekly dosing: Both approaches work, but daily dosing produces more stable blood levels. Some evidence suggests that large, infrequent doses (like 50,000 IU monthly) may be less effective for certain health outcomes than equivalent daily doses 6.

Morning or evening: It doesn’t matter much. Take it when you’ll remember to take it consistently.

How much is too much? Understanding vitamin D toxicity

This is the crucial question, and the answer is more nuanced than you might expect.

The upper limits

The NHS and most health agencies set the tolerable upper intake level at 4,000 IU (100 micrograms) daily for adults. This doesn’t mean 4,001 IU will harm you. It means that at doses above this level, the risk of adverse effects starts to increase.

For children, the limits are lower:

Infants 0-6 months: 1,000 IU maximum
Infants 7-12 months: 1,500 IU maximum
Children 1-3 years: 2,500 IU maximum
Children 4-8 years: 3,000 IU maximum
Over 9 years: 4,000 IU maximum

What actually causes toxicity

Vitamin D toxicity (hypervitaminosis D) almost never happens from sun exposure or diet alone. Your body has feedback mechanisms that limit how much vitamin D your skin produces. And even fortified foods don’t contain enough to cause problems at normal consumption levels.

Toxicity comes from supplement mega-dosing. Case reports typically involve people taking 50,000-100,000 IU daily for months, or making errors with concentrated preparations.

The problem with excessive vitamin D isn’t the vitamin itself but what it does to calcium metabolism. High vitamin D levels increase calcium absorption from your gut. If your blood calcium rises too high (hypercalcaemia), you can experience:

Nausea, vomiting, and loss of appetite
Excessive thirst and frequent urination
Weakness and fatigue
Confusion and difficulty concentrating
Kidney problems, including stones
Heart rhythm abnormalities in severe cases

Blood levels above 150 ng/mL (375 nmol/L) are generally considered potentially toxic. Most cases of vitamin D toxicity involve levels well above this.

The grey zone

What about long-term use of doses between 4,000 and 10,000 IU daily? The evidence is mixed. Some studies show no apparent harm. Others raise concerns about increased fall risk or other adverse effects at high blood levels.

A randomised trial of monthly high-dose vitamin D (60,000 IU, equivalent to about 2,000 IU daily) actually found increased fall rates compared to lower doses 7. The mechanism isn’t entirely clear, but it suggests that more is not necessarily better.

My suggestion: if you want to take doses above 4,000 IU daily, do it under medical supervision with periodic blood monitoring. It’s not that such doses are definitely harmful, but the risk-benefit calculation becomes less favourable without a specific medical reason.

Signs that you might need vitamin D

Testing is the only way to know for certain, but some symptoms can suggest deficiency:

Bone pain or tenderness, particularly in the lower back, hips, or legs
Muscle weakness or cramps
Fatigue that doesn’t improve with rest
Frequent infections or slow wound healing
Low mood, particularly in winter (though this could have many causes)

These symptoms are non-specific and could indicate many things. But if you have several of them plus risk factors for deficiency, testing makes sense.

Who should be particularly careful?

Some conditions and medications interact with vitamin D supplementation:

Granulomatous diseases (sarcoidosis, tuberculosis, certain fungal infections): These conditions can cause your body to convert too much vitamin D to its active form, increasing the risk of hypercalcaemia even at normal supplement doses.

Kidney disease: The kidneys control vitamin D activation. People with kidney problems may need different forms of vitamin D (like calcitriol) rather than standard supplements.

People taking certain medications: Thiazide diuretics can increase calcium retention. Some seizure medications and corticosteroids can reduce vitamin D effectiveness. If you’re on multiple medications, discuss vitamin D supplementation with your doctor.

Those taking calcium supplements: Combining high-dose vitamin D with high-dose calcium may increase cardiovascular risk in some studies, though this remains controversial. The osteoporosis foods article discusses how these nutrients work together.

Putting it together: practical recommendations

For most healthy adults who want to supplement vitamin D:

If you haven’t been tested: 1,000-2,000 IU daily is reasonable during autumn and winter months, particularly if you have risk factors for deficiency.
If you’ve tested deficient: Your doctor may recommend higher loading doses (like 50,000 IU weekly for 6-8 weeks) followed by maintenance dosing. Follow their guidance and retest after a few months.
If your levels are adequate: You may not need supplements at all, or a lower maintenance dose of 600-1,000 IU may suffice.
If you’re taking high doses long-term: Get periodic blood tests to ensure you’re in the optimal range, not overshooting.

Take your vitamin D with a meal containing some fat. Choose D3 over D2 if you have the option. And don’t assume that if some is good, more must be better. That logic doesn’t apply here.

References

NHS. Vitamin D. https://www.nhs.uk/conditions/vitamins-and-minerals/vitamin-d/
National Institutes of Health Office of Dietary Supplements. Vitamin D Fact Sheet for Consumers. https://ods.od.nih.gov/factsheets/VitaminD-Consumer/
Holick MF, et al. Evaluation, Treatment, and Prevention of Vitamin D Deficiency: an Endocrine Society Clinical Practice Guideline. J Clin Endocrinol Metab. 2011;96(7):1911-1930. https://pubmed.ncbi.nlm.nih.gov/21646368/
Dawson-Hughes B, et al. Dietary fat increases vitamin D-3 absorption. J Acad Nutr Diet. 2015;115(2):225-230. https://pubmed.ncbi.nlm.nih.gov/25441954/
Tripkovic L, et al. Comparison of vitamin D2 and vitamin D3 supplementation in raising serum 25-hydroxyvitamin D status: a systematic review and meta-analysis. Am J Clin Nutr. 2012;95(6):1357-1364. https://pubmed.ncbi.nlm.nih.gov/22552031/
Waterhouse M, et al. Vitamin D and bone health in adults in Australia and New Zealand: a position statement. Med J Aust. 2019;211(1):29-34. https://pubmed.ncbi.nlm.nih.gov/31405774/
Bischoff-Ferrari HA, et al. Monthly High-Dose Vitamin D Treatment for the Prevention of Functional Decline: A Randomized Clinical Trial. JAMA Intern Med. 2016;176(2):175-183. https://pubmed.ncbi.nlm.nih.gov/26747333/