Physiological Role
The body obtains vitamin D in three ways: the skin produces it under the sun’s UVB rays, diet provides some and supplementation can add to these sources. The liver then turns it into vitamin D (25-OH), the form that circulates in the blood. The test combines the forms derived from vitamins D2 and D3. The kidneys finally produce the active form used by the body.
Vitamin D contributes to normal calcium and phosphorus absorption and use. It therefore supports the maintenance of normal bones and teeth, normal muscle function and normal blood calcium levels.
It also contributes to the normal function of the immune system and has a role in cell division. These functions explain why vitamin D status is followed beyond bone health alone.
Reference Ranges
Depending on the biomarker, Singular ranges are based on a synthesis of nutritional or clinical reference points and longevity research. They do not replace your laboratory's reference values or your healthcare professional's advice.
Biological Significance
A vitamin D (25-OH) test provides a picture of the body’s recent stores. It is the marker used to assess vitamin D status.
A result in the optimal range reflects a good balance between sun exposure, intake and the body’s needs. A low result may be linked to limited sun exposure, insufficient intake or greater needs. Conversely, a high result can reflect supplementation that has become too high.
Levels naturally vary with the seasons. Their evolution over time therefore often provides more information than a single measurement.
Influencing Factors
Sun exposure. Skin production of vitamin D3 depends on UVB intensity, which varies with latitude, season, time of day, clothing and the area of skin exposed.
Diet. Natural sources remain limited: fatty fish, egg yolk, cod liver and some UV-exposed mushrooms. Fortified foods can also contribute to intake.
Skin pigmentation. Melanin filters some UVB and slows vitamin D production under the same exposure.
Age. The skin’s ability to produce vitamin D gradually decreases with age.
Body composition. Higher body fat is often associated with a lower circulating level.
Supplementation. Vitamin D3 can raise and maintain blood levels. Dose, regularity and starting status influence how the level changes.
In the Singular Formula
Vitamin D is one of the markers most directly connected to the Singular formulation engine. Its result adjusts the amount of vitamin D3 included in the formula.
When the level is low, the formula increases vitamin D3 and provides guidance on sun exposure and diet. As the marker approaches the optimal range, the amount gradually decreases. In the upper part of that range and beyond, vitamin D3 is removed from the formula.
This calibration allows the formula to evolve with each blood test. The formula also combines vitamin K2-MK7, which contributes to the maintenance of normal bones, and magnesium, which supports normal muscle function and energy metabolism.
Linked Bioactives
Scientific Studies
| Authors | Year | Type | Journal | |
|---|---|---|---|---|
| NIH Office of Dietary Supplements | 2025 | Review | NIH Office of Dietary Supplements | View on PubMed |
Vitamin D — Fact Sheet for Health Professionals Institutional reference on vitamin D functions, sources, interpretation of blood status and supplementation precautions. | ||||
| Makris K et al. | 2021 | Review | Clinica Chimica Acta | View on PubMed |
Recommendations on the measurement and the clinical use of vitamin D metabolites and vitamin D binding protein — A position paper from the IFCC Committee on bone metabolism International position paper on vitamin D testing. It confirms vitamin D (25-OH) as the main overall status marker and explains why laboratory methods should be compared carefully. | ||||
| Gaksch M et al. | 2017 | Meta-analysis | PLOS One | View on PubMed |
Vitamin D and mortality: Individual participant data meta-analysis of standardized 25-hydroxyvitamin D in 26916 individuals from a European consortium Meta-analysis of eight European cohorts including 26,916 people. Low levels were associated with higher mortality, followed by a broad plateau. | ||||
| Autier P et al. | 2014 | Review | Lancet Diabetes and Endocrinology | View on PubMed |
Vitamin D status and ill health: a systematic review Systematic review putting population studies and clinical trials into perspective. It shows that a low level can also accompany an already impaired state of health. | ||||
| Manson JE et al. | 2019 | Randomised Controlled Trial | New England Journal of Medicine | View on PubMed |
Vitamin D Supplements and Prevention of Cancer and Cardiovascular Disease VITAL trial of 25,871 participants followed for more than five years. In this general population, vitamin D3 did not reduce the primary cancer or cardiovascular outcomes. | ||||
| Neale RE et al. | 2022 | Randomised Controlled Trial | Lancet Diabetes and Endocrinology | View on PubMed |
The D-Health Trial: a randomised controlled trial of the effect of vitamin D on mortality Five-year trial involving 21,315 adults. A high monthly dose of vitamin D3 did not reduce all-cause mortality in this general population. | ||||
| Demay MB et al. | 2024 | Review | Journal of Clinical Endocrinology and Metabolism | View on PubMed |
Vitamin D for the Prevention of Disease: An Endocrine Society Clinical Practice Guideline The 2024 prevention guideline for generally healthy people. It updates the situations in which supplementation or testing may be useful and replaces the 2011 guideline. | ||||