What protein carries vitamin D?

October 26, 2025 •

3 min read

Approximately 85-90% of vitamin D in the bloodstream is bound to a specific carrier, addressing the question of what protein carries vitamin D. This essential protein, known as vitamin D-binding protein (DBP), is critical for transporting vitamin D metabolites throughout the body to where they are needed.

Quick Summary

Vitamin D-binding protein (DBP), also known as Gc-globulin, is the main carrier protein for vitamin D and its metabolites in the blood. It transports vitamin D to the liver and kidneys for activation and regulates its availability to tissues.

Key Points

Primary Carrier: The main protein that carries vitamin D in the bloodstream is Vitamin D-binding protein (DBP), also called Gc-globulin.
Multifunctional Role: DBP not only transports vitamin D but also regulates its bioavailability, stabilizes its concentration, scavenges actin, and modulates immune responses.
Produced in the Liver: The synthesis of DBP primarily occurs in the liver, and its levels can be affected by certain disease states, like liver failure.
Bioavailability Regulation: DBP controls the amount of 'free' or unbound vitamin D that is available for direct use by cells, influencing its biological activity.
Genetic Variations: Genetic polymorphisms in the DBP gene can alter the protein's concentration and its affinity for vitamin D, leading to differences in vitamin D status among individuals.
Role in Metabolism: By transporting vitamin D to the liver and kidneys, DBP is essential for the metabolic activation of vitamin D into its active hormonal form.

Introduction to Vitamin D's Transport System

To understand the function of vitamin D, it is essential to first grasp how it travels throughout the body. As a fat-soluble vitamin, it cannot simply dissolve in the watery environment of the bloodstream. Instead, it relies on specific carrier molecules. While a small amount can be transported via lipoproteins and albumin, one protein is overwhelmingly responsible for carrying the bulk of vitamin D and its metabolites: vitamin D-binding protein, or DBP. Synthesized primarily by the liver, DBP acts as a critical intermediary in vitamin D metabolism, controlling its availability and delivery to various organs.

The Discovery and Identification of DBP

DBP was initially identified as a polymorphic serum protein in the 1960s, later confirmed to be the primary carrier for vitamin D. It is part of the albumin superfamily of proteins.

Functions of Vitamin D-Binding Protein (DBP)

DBP is a multifunctional protein with several critical roles in the body. For more detailed information on its functions, including transport, reservoir, stabilization, regulation of bioavailability, immune modulation, and actin scavenging, please refer to {Link: Frontiers in Endocrinology https://www.frontiersin.org/journals/endocrinology/articles/10.3389/fendo.2019.00718/full} and {Link: NCBI NIH PMC https://pmc.ncbi.nlm.nih.gov/articles/PMC6821678/}.

The Importance of DBP and the Free Hormone Hypothesis

The "free hormone hypothesis" for vitamin D suggests that only the free, unbound vitamin D is biologically active and can enter cells. Some tissues also have specialized receptors to internalize DBP-vitamin D complexes. For a comprehensive understanding of DBP's importance and the free hormone hypothesis, consult {Link: Frontiers in Endocrinology https://www.frontiersin.org/journals/endocrinology/articles/10.3389/fendo.2019.00718/full}.

DBP and Albumin: A Comparative Overview

A comparative overview of Vitamin D-Binding Protein (DBP) and Albumin regarding features like primary function, binding affinity, circulating concentration, contribution to transport, and role in bioavailability can be found at {Link: Frontiers in Endocrinology https://www.frontiersin.org/journals/endocrinology/articles/10.3389/fendo.2019.00718/full}.

Genetic Polymorphisms and Variations in DBP

Genetic variations (polymorphisms) in the DBP gene can influence DBP concentration and binding affinity, affecting circulating 25-hydroxyvitamin D levels. These genetic differences may impact bone density and immune responses.

Other Transport Pathways and DBP's Importance

While DBP is the primary carrier, other mechanisms are involved in vitamin D transport, including chylomicrons for dietary vitamin D absorption and lipoproteins. DBP is crucial for maintaining vitamin D homeostasis by transporting both newly synthesized and dietary vitamin D, as well as its metabolites through the activation process.

Conclusion

The vitamin D-binding protein (DBP), also known as Gc-globulin, is the main protein responsible for carrying vitamin D in the body. Produced primarily in the liver, DBP plays essential roles in transporting vitamin D metabolites, creating a circulating reservoir, stabilizing levels, and regulating bioavailability. Genetic variations in DBP can influence its levels and binding affinity, contributing to differences in vitamin D status among individuals. Understanding DBP is vital for comprehending the complex system that manages vitamin D's distribution and its effects on health.

Visit the NIH for more on DBP and its functions

Frequently Asked Questions

DBP stands for Vitamin D-binding protein, a carrier protein that transports vitamin D and its metabolites through the bloodstream to various organs and tissues.

DBP serves several purposes for vitamin D, including transporting it from its sources to the liver and kidneys for activation, stabilizing its levels in the blood, and controlling the amount of bioavailable vitamin D.

Yes, albumin also carries vitamin D, but it does so with a much lower binding affinity and transports a smaller portion compared to DBP, which is the primary carrier.

DBP regulates the concentration and bioavailability of vitamin D by creating a large circulating reservoir. The level of DBP in the blood determines how much vitamin D is bound versus freely available to cells.

DBP is primarily synthesized and secreted into the bloodstream by the liver.

Genetic variations in DBP can alter the protein's concentration and binding affinity, potentially affecting an individual's overall vitamin D status, bone mineral density, and immune responses.

Yes, according to the 'free hormone hypothesis,' the unbound, or free, portion of vitamin D is considered the biologically active form that can readily enter cells and initiate a response.