Vitamin C is a water-soluble micronutrient necessary for the proper functioning of many biological systems. This nutrient, also known as L-ascorbic acid, is a powerful antioxidant. Most fish species cannot produce the nutrient internally and must therefore acquire it through their diet. This inability classifies Vitamin C as an essential nutrient for fish.
The Biological Need for Dietary Vitamin C
The requirement for dietary Vitamin C stems from a genetic limitation shared by most teleost fish, humans, and guinea pigs. Many mammals and birds possess an active enzyme pathway that allows them to synthesize L-ascorbic acid from glucose. The final step in this internal production process is catalyzed by the enzyme L-gulonolactone oxidase (L-GLO).
The gene responsible for producing the L-GLO enzyme has become non-functional in the evolutionary history of most bony fish, resulting in a pseudogene. Because this final step of the synthesis pathway is blocked, these fish are unable to produce their own supply of Vitamin C. Consequently, any fish lacking a functional L-GLO gene must rely entirely on external sources for this compound.
Essential Functions in Fish Physiology
Once acquired, L-ascorbic acid serves as a cofactor and an antioxidant within the fish body. A key function is its role in the biosynthesis of collagen, a structural protein necessary for connective tissues, skin, bone, and cartilage. This action is important for maintaining the structural integrity of the fish’s skeleton and fins.
Vitamin C is an antioxidant that protects cells from damage caused by reactive oxygen species, especially in high-stress environments like aquaculture. Adequate Vitamin C supports the immune system, enhancing the fish’s ability to resist disease and cope with stressors like handling or temperature changes. It is also required for wound healing and supports the absorption of iron.
A prolonged deficiency in this nutrient leads to a condition often described as “fish scurvy.” Symptoms include structural deformities like scoliosis (curved spine) and lordosis (inward curvature), hemorrhages, and impaired growth. A lack of Vitamin C compromises skeletal development and overall health due to its involvement in bone and connective tissue formation.
Supply and Stability in Farmed Fish Diets
For fish raised in aquaculture, the reliance on external Vitamin C presents a significant challenge for feed manufacturers. Pure L-ascorbic acid is highly unstable; it is water-soluble and easily degraded by exposure to air, heat, and light. This instability means that much of the nutrient can be lost during the high-heat processes of feed manufacturing, such as steam-pelleting or extrusion.
To overcome this issue and ensure the fish receive the necessary dietary levels, commercial feeds utilize chemically or physically stabilized forms of Vitamin C. These stable forms, such as L-ascorbyl-2-polyphosphate or microencapsulated ascorbic acid, are designed to protect the compound from degradation during processing and storage. The coating or chemical modification ensures that the nutrient remains intact and biologically available until the fish consumes the feed pellet.
This technological necessity is paramount because the nutrient can also leach quickly into the water once the pellet is submerged, further reducing the amount available to the fish. The use of these stabilized formulations is essential for delivering a consistent, bioavailable dose to support the health and growth of farmed fish populations.
Vitamin C Levels in Fish Consumed by Humans
Although fish require Vitamin C in their diet, most edible fish are not considered a primary source of the nutrient for human consumption. Muscle tissue, the part typically eaten, naturally contains lower concentrations of ascorbic acid compared to plant-based sources. The vitamin is often concentrated in organs such as the liver or kidneys, which are generally not consumed.
The inherent instability of L-ascorbic acid means that a portion of the content present in raw fish is lost during cooking and processing. Studies comparing the nutritional content of fish flesh with common fruits and vegetables show that fish provide negligible or trace amounts of Vitamin C.
For example, a serving of fish like cod or salmon will contain significantly less Vitamin C than an orange, a bell pepper, or a small handful of strawberries. The nutritional value of fish for humans lies predominantly in other compounds, particularly high-quality protein, omega-3 fatty acids, and fat-soluble vitamins like Vitamin D. Therefore, while the fish needs a dietary supply of Vitamin C to thrive, humans should not rely on consuming fish as a significant source of their daily intake.