An estimated 40 percent of American adults have clinically insufficient vitamin D levels. The figure climbs higher among older adults, people with darker skin, people who spend most of their time indoors, and people who live in northern latitudes, which is to say, a very large fraction of the population.

And yet the official Recommended Dietary Allowance for vitamin D in adults over 50 is just 600 to 800 IU per day.

Many researchers who study vitamin D metabolism specifically have, for years, argued that these recommendations are seriously outdated, based on the minimum dose needed to prevent rickets rather than the optimal dose for broader health. For adults over 50, a growing body of evidence suggests the actual therapeutic range may be several times the current RDA.

Why vitamin D matters more after 50

Vitamin D is poorly named. It is technically a hormone the body synthesizes from cholesterol when skin is exposed to UVB radiation. Its role extends far beyond bone health. Current research implicates it in immune function, muscle strength, mood regulation, and the regulation of more than a thousand different genes.

Three things change with age that make vitamin D sufficiency harder.

Skin synthesis declines. A 70-year-old produces roughly 25 percent of the vitamin D from sun exposure that a 20-year-old does, given the same amount of sun.

Time outdoors decreases. Most older adults spend more time indoors than they did in their younger years.

Kidney conversion slows. The kidneys convert inactive vitamin D into its active form, and this conversion becomes less efficient with age.

The cumulative effect: older adults often have quite low blood levels of 25-hydroxyvitamin D (the standard measured form), even when they are technically consuming enough according to the RDA.

What the blood test actually tells you

For anyone who has not had their vitamin D level checked, this is the single most useful action item in this article. A standard 25-hydroxyvitamin D blood test is inexpensive, usually covered by insurance, and tells you your actual status rather than letting you guess from intake.

Interpretation varies by researcher, but a common framework:

Below 20 ng/mL: clinically deficient.
20 to 30 ng/mL: insufficient.
30 to 50 ng/mL: sufficient for most purposes.
50 to 80 ng/mL: the range many vitamin D researchers consider optimal.
Above 100 ng/mL: approaching toxicity territory, worth backing off.

A reading below 30 puts a person in company with roughly 40 percent of American adults, but it is also a number worth addressing.

Getting levels up

The most reliable approach is a combination of modest sun exposure (when available and safe for the individual's skin type), dietary sources, and supplementation. Food sources are limited (fatty fish like salmon, egg yolks, and fortified dairy), so for most older adults, supplementation carries the bulk of the load.

Dosing is where things get interesting. For someone starting from a deficient baseline, many integrative physicians will prescribe a higher short-term dose (often 5,000 IU per day) for a few months, followed by a lower maintenance dose once levels are in range.

Two practical notes on supplementation:

Form matters. Vitamin D3 (cholecalciferol) is better absorbed and longer-lasting than D2 (ergocalciferol). Look for D3 specifically.

Pair it with K2. Vitamin D drives calcium absorption, but without sufficient vitamin K2, that calcium can end up in the wrong places (arteries, kidneys) rather than bones. Many vitamin D supplements are now formulated with K2 as MK-7 for this reason.

A word of caution

Vitamin D is fat-soluble, which means it is possible to over-supplement. Unlike water-soluble vitamins, the body cannot flush out excess quickly. Taking 10,000 IU daily for a year without testing levels is not a sensible approach for most people.

The reasonable approach is simple: get tested, talk to a doctor, pick a dose that brings levels into the sufficient-to-optimal range, and retest once or twice a year. For people on blood thinners, with kidney disease, or diagnosed with sarcoidosis or certain other conditions, the calculus changes. Self-dosing is not appropriate in those situations without medical guidance.

For most adults, the takeaway is straightforward. The current RDA was not written for optimal health in older adults. It was written for a narrowly defined baseline. For anyone who has been assuming the label on a multivitamin is enough, this is worth a conversation with a doctor and a simple blood test.

Disclaimer: This article is editorial coverage of public research. It is not medical advice. Consult a qualified healthcare provider before significantly changing any vitamin or supplement regimen, especially if you take medications or have chronic health conditions.