Effects of Dietary Supplementation with Glycyrrhiza Polysaccharides on Growth Performance, Histomorphology, Immunity, and Oxidative Stress in Largemouth Bass (Micropterus salmoides)

In the world of aquaculture, raising fish is a delicate balancing act between providing enough food for rapid growth and protecting the animals from the harsh realities of their environment. Farmed fish often face challenges that wild populations do not, such as crowded living conditions and water contaminated by industrial pollutants. One of the most common and dangerous pollutants is cadmium, a heavy metal that seeps into waterways and enters the bodies of fish, causing invisible damage at the cellular level. When a fish is exposed to cadmium, its internal systems produce an excess of harmful molecules that attack healthy cells, a process known as oxidative stress. To fight this, fish rely on a natural defense system of enzymes and proteins that neutralize these threats. However, when the pollution is too heavy, this defense can collapse, leading to poor growth, damaged organs, and disease. Scientists have long looked to nature for solutions, exploring whether certain plant extracts could act as a shield, boosting the fish's own defenses without the side effects of chemical drugs.

A team of researchers from Henan University of Science and Technology decided to test this idea using the largemouth bass, a popular and valuable freshwater fish in China. They focused on a specific substance derived from licorice root, known as Glycyrrhiza polysaccharide. This natural compound is made of long chains of sugar molecules and is known in traditional medicine for its ability to reduce inflammation and fight oxidative damage. The researchers wanted to see if adding this licorice extract to the fish's food would help them grow better, stay healthier, and withstand the stress of heavy metal pollution. They set up a controlled experiment with 450 young bass, dividing them into five groups. One group ate a standard diet, while the other four groups received the same diet but with different amounts of the licorice extract added: 500, 1000, 1500, or 2000 milligrams per kilogram of feed. The fish were raised for eight weeks, a period long enough to see how their bodies changed in response to the new ingredients.

The results of the eight-week feeding trial were clear and pointed toward a specific sweet spot for the supplement. The fish that received the licorice extract generally grew faster and heavier than those on the standard diet. The most significant improvements were seen in the groups fed 1000 and 1500 milligrams of the extract per kilogram of food. These fish gained the most weight and converted their food into body mass most efficiently. Their muscles also changed; the fish in the 1000 milligram group had higher protein content, while those in the 1500 milligram group had lower fat content, suggesting the supplement helped the fish build lean muscle rather than storing excess energy as fat. However, the benefits did not continue to increase with higher doses. The group fed the highest amount, 2000 milligrams, actually performed worse than the middle groups. Their growth slowed, and their bodies showed signs of strain, indicating that too much of a good thing can become harmful.

To understand why this happened, the scientists looked inside the fish at the cellular level. They examined the liver and intestines, two organs critical for digestion and detoxification. In the fish that received the optimal amount of the extract, the tissues looked healthy and well-organized. The liver cells were neatly arranged, and the tiny finger-like projections in the intestines, which absorb nutrients, were long and intact. In contrast, the fish fed the highest dose of 2000 milligrams showed damaged liver cells with empty spaces inside them, a sign of cellular stress and dysfunction. Their intestinal walls also showed signs of wear, with cells peeling away. This physical damage explained why the high-dose group did not grow as well; their bodies were struggling to process the food and manage the metabolic load.

The researchers also measured the chemical activity within the fish to see how the supplement affected their internal defense systems. They found that the fish fed the moderate doses of licorice extract had much higher levels of antioxidant enzymes. These are the proteins that act as the body's cleanup crew, neutralizing the harmful molecules that cause damage. The fish also had lower levels of a substance called malondialdehyde, which is a marker for damage caused by oxidation. Essentially, the supplement turned up the volume on the fish's natural immune and repair systems. The study traced this effect to a specific genetic pathway in the liver, known as the Nrf2-Keap1 pathway. In simple terms, the supplement helped switch on the genes that produce the antioxidant enzymes while switching off the genes that would normally suppress them. This molecular switch worked best at the 1000 to 1500 milligram dose, providing a clear biological reason for the improved health and growth observed in those groups.

To test the resilience of these fish, the researchers subjected them to a final challenge: exposure to water containing cadmium. This simulated a real-world scenario where fish might swim in polluted water. The fish that had been fed the optimal amount of licorice extract handled this stress much better than the others. When exposed to the heavy metal, their antioxidant levels remained high, and their bodies produced fewer signs of inflammation. The genes responsible for fighting infection and reducing swelling were kept in a balanced state, whereas the fish on the high-dose diet showed signs of an overactive immune response, which can be just as damaging as the pollution itself. The fish on the standard diet, without the supplement, suffered the most, with their antioxidant defenses overwhelmed by the toxic exposure.

The study concludes that adding licorice extract to the diet of largemouth bass is a viable strategy for improving their health and growth, but only if the amount is carefully controlled. The optimal range is between 1000 and 1500 milligrams per kilogram of feed. Within this range, the supplement acts as a powerful booster for the fish's natural defenses, helping them grow faster, build better muscle, and resist the damaging effects of heavy metal pollution. It strengthens their internal organs and keeps their immune systems balanced. However, exceeding this limit causes the benefits to fade and can even lead to organ damage. This research provides a practical guide for farmers, showing that nature-derived additives can be effective tools for sustainable aquaculture, provided they are used with precision. The findings suggest that by understanding the specific needs of the fish, we can use simple plant compounds to create a safer and more productive environment for the animals we rely on for food.