How Do Fish Get Vitamin D? Uncovering Their Natural Sources

December 8, 2025 •

4 min read

Scientists have demonstrated that a fish's primary source of vitamin D comes from its diet, originating largely from plankton. This discovery challenges older assumptions and sheds light on how these aquatic vertebrates meet their nutritional needs, revealing the sophisticated pathways behind how fish get vitamin D.

Quick Summary

Fish obtain vitamin D mainly through their food chain, which begins with plankton that produces the nutrient from sunlight. Newer research also reveals some fish can synthesize it directly from certain light wavelengths.

Key Points

Food Chain Is Primary Source: The main way fish get vitamin D is by consuming plankton and other marine organisms that produce it from sunlight.
Plankton Produces Vitamin D: Microscopic phytoplankton and zooplankton absorb solar UV light near the water's surface, converting a precursor compound into vitamin D.
Endogenous Synthesis Exists: Some species, like rainbow trout and Atlantic salmon, can synthesize vitamin D directly in their skin when exposed to specific light wavelengths.
Blue Light Synthesis is Unique: Rainbow trout can use blue light, which penetrates deeper than UVB, for vitamin D production, a process possibly aided by their unique skin structure.
Farmed vs. Wild Differences: Farmed fish rely heavily on fortified feed for vitamin D, whereas wild fish acquire it from their natural diet and light exposure.
Deep Sea Fish Rely on Diet: Fish living in deep water, where sunlight cannot reach, obtain their vitamin D exclusively by eating organisms from higher up the food chain.
Stored in Fatty Tissues: Fish accumulate and store large quantities of vitamin D, particularly D3, in their liver and fatty tissues, making them a potent dietary source.

The question of how fish get vitamin D is more complex than previously thought, involving a combination of dietary intake and, for some species, direct synthesis. While humans rely on sunlight and diet, the aquatic environment presents unique challenges for vitamin D acquisition. For fish, the process primarily involves consuming organisms that have been exposed to sunlight, but recent studies have also uncovered the fascinating ability of certain species to create their own vitamin D using specific light wavelengths.

The Primary Dietary Source: The Aquatic Food Chain

For most fish, the dominant pathway for acquiring vitamin D is through their diet. The journey begins with microscopic organisms at the water's surface.

Plankton as the Origin

The aquatic food chain is the main source of vitamin D for fish. Plankton, which consists of microscopic phytoplankton (plant-like) and zooplankton (animal-like), floats near the water's surface where it is exposed to sunlight. Like terrestrial organisms, these planktonic species contain precursor compounds, such as 7-dehydrocholesterol, which are converted into vitamin D3 and D2 upon exposure to UV light. Studies have confirmed that both phytoplankton and zooplankton contain abundant amounts of vitamin D. The high concentrations of vitamin D found in certain fish are a direct consequence of consuming these nutrient-rich organisms.

Accumulation Up the Food Chain

As fish consume plankton and smaller fish, the vitamin D accumulates in their bodies, primarily in their liver and fatty tissues. This explains why deep-sea fish, which live far below where sunlight can penetrate, are still rich sources of vitamin D. They obtain the vitamin by preying on organisms from higher up in the water column, concentrating the nutrient in their own fat stores. This accumulation is a crucial aspect of why fatty fish are such a rich dietary source of vitamin D for humans.

Endogenous Synthesis via Light Exposure

While the food chain is the main source for most, it has been discovered that some fish can also produce vitamin D photochemically, similar to mammals. This process is particularly relevant for species living in shallower waters.

Synthesis from Blue Light

Groundbreaking research on rainbow trout revealed that they can produce vitamin D in their skin when exposed to visible blue light, in the 380–480 nm wavelength range. This is a key finding because blue light penetrates much deeper into the water than the UVB rays that humans use for synthesis. It is believed that guanine crystals in the fish's skin, which cause their silvery sheen, may play a role in this unique photochemical process.

Synthesis from UVB Light

For species like Atlantic salmon, research has confirmed that exposure to UV-B light stimulates the synthesis of vitamin D3 in their skin. This is especially significant for farmed salmon, where supplemental UV-B lighting can be used to increase vitamin D levels, bringing them closer to the levels found in wild fish. This endogenous synthesis provides a more reliable source of vitamin D than dietary intake alone.

Dietary Requirements in Aquaculture

For farmed fish, especially those raised indoors or in closed systems, the natural food chain and sunlight exposure are often limited or nonexistent. Therefore, their vitamin D levels are directly dependent on the feed provided by farmers. This practice ensures a consistent intake of the nutrient, but the vitamin D levels can vary significantly between wild and farmed fish.

The Evolutionary Context of Vitamin D in Fish

In contrast to the critical role vitamin D plays in calcium homeostasis for terrestrial vertebrates, its function in fish, which inhabit a calcium-rich environment, has been a subject of evolutionary study. While initially considered redundant, it's now understood that fish possess a functional vitamin D endocrine system that supports overall growth, bone development, immune response, and lipid metabolism.

Comparison: Dietary vs. Endogenous Sources


Feature	Dietary Intake (Food Chain)	Endogenous Synthesis (Light)
Mechanism	Consumption of plankton and smaller fish	Photochemical conversion in the skin
Energy Source	Vitamin D stored in food	Solar blue or UV-B light
Primary Species	Common for all fish, especially deep-sea species	Confirmed in certain species like rainbow trout and salmon
Location Dependence	Relies on food chain originating near surface	Depends on water depth and clarity, allowing light penetration
Consistency	Can be less reliable due to seasonal plankton cycles	Potentially more reliable for species in optimal light conditions
Role in Aquaculture	Essential for farmed fish via fortified feed	Can be enhanced using artificial lighting

Key Factors Influencing Vitamin D Levels

Diet: The type and abundance of plankton or other food sources containing vitamin D directly affects a fish's intake.
Light Exposure: Water depth, clarity, and season affect the penetration of sunlight, which impacts both plankton's vitamin D production and endogenous synthesis.
Species: Different fish species have varying capacities for endogenous synthesis and different feeding behaviors, influencing their vitamin D status.
Life Stage: Larval fish, with their high metabolic rates, may have different vitamin D requirements than juveniles or adults.
Aquaculture vs. Wild: The controlled diet of farmed fish means their vitamin D levels are manipulated, differing from the natural variations seen in wild populations.

Conclusion

Fish acquire their essential vitamin D through a dual system of dietary intake and, for some species, direct synthesis. The food chain, which begins with plankton capturing sunlight's energy, serves as the most widespread source, especially for fish in deep or murky waters. However, the discovery of endogenous synthesis, even with blue light, demonstrates a sophisticated adaptation in certain species. This comprehensive understanding of how fish get vitamin D not only illuminates marine biology but also holds implications for aquaculture practices and human nutrition, ensuring that we continue to benefit from these vitamin-rich resources. For a detailed review of this topic, refer to a comprehensive study on the subject ResearchGate Review on Vitamin D in Fish.

Frequently Asked Questions

Fish primarily get vitamin D by consuming smaller organisms like zooplankton and phytoplankton. These microscopic organisms synthesize vitamin D from sunlight and pass it up the food chain.

Yes, some species of fish can produce their own vitamin D in their skin through exposure to light. For example, rainbow trout can synthesize vitamin D from blue light, which penetrates deeper into the water than the UV-B light used by mammals.

Deep-sea fish obtain their vitamin D by eating smaller fish and plankton that live in sunlit surface waters. The vitamin D from these organisms accumulates as it moves up the aquatic food chain.

Yes, wild fish often have higher vitamin D levels due to their natural diet and sunlight exposure. The vitamin D content in farmed fish depends on the level of supplementation in their feed, especially if they are raised indoors with no natural light exposure.

While the function of vitamin D in fish, which live in a calcium-rich environment, was once considered less critical than in mammals, it is now known to be important for growth, bone development, and immune function. The vitamin D endocrine system in fish has similar functions to that of mammals.

Seasonal variations can affect vitamin D levels. In sunnier months, plankton produce more vitamin D, increasing the amount available in the food chain. For fish that synthesize their own vitamin D, stronger light intensity also increases production.

Fish like salmon are rich in vitamin D because they are oily fish and store the nutrient in their fatty tissues. Their diet, combined with any natural synthesis, leads to significant accumulation of vitamin D, especially in their liver.