f all the vitamins required for human health, probably none have gotten more attention in recent years, or been subject to more misinformation and misunderstanding, than vitamin D.
To start, vitamin D (calciferol), is not really a vitamin at all. It’s actually a secosteroid – a steroid hormone. And unlike all other vitamins, which must be obtained from our diet, we can produce it ourselves when our skin is exposed to the sun. So why is it referred to as a vitamin? It all began with rickets.
Rickets, a disease of bone softening and deformity in children, is caused by extreme vitamin D deficiency. In adults the condition is referred to as osteomalacia. It was originally thought that because cod liver oil cured rickets, the vitamin A present in the oil was the reason.
But in 1922, biochemist Elmer McCollum found that heat-treated cod liver oil, a process which destroyed Vitamin A, still cured rickets. He assigned this newly discovered substance Vitamin D, the next letter of the alphabet after Vitamins A, B, and C. At around the same time it was discovered that sunlight exposure also cured rickets, which added to the mystery – how could both cod liver oil and sunlight cure rickets?
As more research was conducted over the years, the true nature of this remarkably important substance was determined, solving the mystery. But the “vitamin” label stuck, and is still used today.
Vitamin D is best known for its role in calcium and phosphorus balance. It helps the body absorb calcium and maintain blood levels that allow proper bone mineralization. In addition to the aforementioned rickets and osteomalacia, low circulating vitamin D levels can contribute to osteoporosis risk as we age.
Vitamin D receptors are found throughout the body, including on many immune cells, which is why vitamin D is also often discussed in the context of immunity. Observational studies frequently link low vitamin D levels with higher rates of certain infections, inflammatory conditions, and even cancer. Studies are ongoing, but for now the important takeaway regarding immune function is that correcting a true deficiency, and reaching optimal blood levels, supports normal immune function, but high-dose Vitamin D supplementation is not a guaranteed “immune booster”.
From a beauty perspective, vitamin D is closely tied to skin health and appearance. The skin not only helps produce vitamin D in the presence of sunlight, but derives direct benefit from it. Research shows that vitamin D has a number of positive effects on our skin. In addition to helping regulate inflammation and immune function, it also supports normal skin cell growth and differentiation, enhances collagen production, and promotes barrier integrity. Vitamin D may also help inhibit skin cancer formation, but it is important to note that increased sun exposure can have the opposite effect. Clinically, topical vitamin D medications are used to treat psoriasis and other skin conditions.
Vitamin D also supports neuromuscular function. In practical terms, maintaining adequate vitamin D can help preserve muscle strength and may lower fall risk in some older adults. This too is an important health and beauty benefit, since strength, posture, and mobility are foundational to looking and feeling vibrant.
Vitamin D is produced in the skin via ultraviolet-B (UVB) ray exposure from sunlight. While sun exposure is considered the natural source of vitamin D production, how much your body makes varies widely based on skin tone, age, time of day, season, latitude, clothing, and sunscreen use. Because ultraviolet radiation can accelerate skin aging, even at lower levels over time, and increases skin cancer risk, many do not view sun exposure as a viable option for their health and beauty strategy.
A better anti-aging approach to getting adequate vitamin D is a combination of sensible sun protection, food sources, and appropriately dosed supplements.
In foods and supplements, vitamin D is found in two forms - vitamin D3 (cholecalciferol) and vitamin D2 (ergocalciferol). D3, the form made by exposing skin to sunlight, is also found in some animal-derived foods, and in certain lichen species, the only known plant-based source. D2 is found in some plants and fungi. Both can raise blood vitamin D levels, but D3 is thought to be the more efficient form for raising physiological levels.
Dietary sources include fatty fish (such as salmon, sardines and mackerel), cod liver oil, egg yolks and liver. In many countries, vitamin D is also added to foods through fortification. These products can include milk and some plant milks, yogurt and breakfast cereals.
However it is obtained, vitamin D undergoes a two-step conversion process, first in the liver, and then in the kidneys. This pathway transforms vitamin D into its active form, calcitriol, which is responsible for its many important functions.
Vitamin D deficiency is common. The risk is higher in people with limited sun exposure, darker skin, older age, obesity, and conditions that impair fat absorption, as vitamin D is fat soluble.
The current RDA (Recommended Dietary Allowance) for most healthy adults is 600 IU per day, and 800 IU per day for adults over 70, assuming minimal sun exposure. While 600–800 IU is generally considered enough to prevent severe deficiency and maintain bone health, many experts argue that higher doses, in the range of 1,000–2,000 IU or higher, are needed for optimal, non-skeletal health, such as immune system function.
As with so many micronutrients, vitamin D follows a “too little vs. too much” pattern, referred to as a U-shaped curve in medicine. With a U-shaped curve, the ideal level is determined for maximum benefit, with too much or too little having deleterious effects. For vitamin D, it appears that the ideal blood level is around 40-60 ng/ml or 100-150 nmol/L (ng/ml are the units used to report results in the US, and nmol/L are the units used in other countries such as UK, Canada, and Australia).
Some experts are against routine vitamin D level testing for those supplementing with a moderate dose (1000-2000 IU), but this seems to mostly be driven by cost-containment strategies rather than true clinical data. The only way to be sure you are getting an adequate amount of vitamin D is to get tested. It is also important to point out that for many labs the lower normal range limit is 20 ng/ml (50 nmol/L). This is the minimum for bone health, and as previously stated, most likely too low for optimum overall health.
Vitamin D toxicity is rare from food, and with the sunlight-generated pathway there are mechanisms in place to prevent excess production. Toxicity can occur from excessive supplementation. The tolerable upper intake limit for adults is 4,000 IU per day; routinely exceeding this without medical supervision increases the risk of toxicity.
Too much vitamin D can dangerously raise blood calcium levels (hypercalcemia), which may lead to symptoms like nausea and weakness, and in more serious cases, kidney damage, cardiac arrhythmias and vascular calcification.
The best overall strategy then, to get to an optimum vitamin D level, is consistent, appropriate intake, based on accurate testing, and avoidance of supplement mega-doses.
As when starting any type of supplementation, it is important to talk to your doctor about testing and an individualized plan based on your specific needs and health status. This is especially critical for vitamin D, as people with certain health conditions such as sarcoidosis, kidney diseases, granulomatous disorders, or for those that have existing high blood calcium levels, should not supplement with vitamin D, unless specifically instructed by their physician to do so.
For most people, sensible vitamin D supplementation to reach an optimum blood level, is a good investment in health, and can help enhance both inner and outer beauty.