Vitamin B6 for Shrimp: The 69 mg/kg Sweet Spot in Soybean-Based Diets

If you are raising Pacific white shrimp, you have probably spent more time thinking about protein sources than about vitamins. That makes sense. Protein is the big line item. But when you swap out fish meal for soybean meal, the math on everything else changes too. Including vitamins you thought you had figured out.

A research team at Jeju National University recently took a hard look at one vitamin in particular: pyridoxine, better known as vitamin B6. They asked a question a lot of shrimp nutritionists have been wondering about. When you move to a soybean-based diet, does the optimal B6 level stay the same? The answer, it turns out, is no. And the difference might matter more than you think.

Why soybean meal changes the equation

The shrimp feed industry has been steadily moving away from fish meal for years. Prices keep climbing, supply is unreliable, and sustainability pressure is not going anywhere. Soybean meal has become the go-to replacement. It is digestible, widely available, and Pacific white shrimp handle it reasonably well.

But soybean meal is not just a protein swap. It changes the entire nutritional landscape of the diet. Amino acid profiles shift. Anti-nutritional factors come into play. And vitamins that were once supplied in sufficient amounts by fish meal may suddenly need to be topped up. Or, as this study shows, may not need as much as you thought.

Pyridoxine is one of those vitamins where the requirement depends heavily on what else is in the feed. Earlier studies using fish meal-based diets pegged the optimum somewhere between 110 and 152 mg per kilogram of feed. That is a wide range, but the takeaway was clear: shrimp need a decent amount of B6 to grow well. What nobody had tested properly was whether that number holds up when you replace most of the fish meal with soybean meal.

What pyridoxine actually does in shrimp

Before getting into the numbers, it is worth understanding why B6 matters in the first place. Pyridoxine gets converted in the body to pyridoxal 5′-phosphate, or PLP. That molecule is a coenzyme. It helps other enzymes do their jobs. And those jobs cover a lot of ground.

PLP is involved in amino acid metabolism. It helps shrimp break down and rebuild proteins, which is the whole point of feeding them. It plays a role in fatty acid metabolism. It regulates glycogen breakdown, which matters for energy management. And this is where it gets especially interesting for shrimp farmers: it sits at the center of antioxidant defense and immune function.

Specifically, PLP is required for two enzymes called cystathionine-β-synthase (CBS) and cystathionine-γ-lyase (CGL). These two run what is known as the transsulfuration pathway, which ultimately produces cysteine. Cysteine, in turn, is a building block for glutathione, one of the body’s most important antioxidants. No glutathione, and the shrimp’s ability to handle oxidative stress takes a serious hit.

The study also looked at antimicrobial peptides: penaeidine and crustin. These are part of the shrimp’s innate immune system. Shrimp do not have the kind of adaptive immunity that vertebrates rely on. Their defense is almost entirely innate. So anything that boosts those antimicrobial peptides is a big deal for disease resistance.

The experiment: what they actually tested

The researchers formulated a basal diet with 46% soybean meal and only 15% combined fish meal, sourced from sardine, tuna byproduct, and squid liver meal. Crude protein was set at around 33%, which is on the lower end. That reflects the kind of practical, cost-conscious formulation that commercial farms actually use.

They then supplemented that basal diet with seven graded levels of pyridoxine hydrochloride: 0, 25, 50, 75, 100, 125, and 150 mg per kilogram of feed. On top of that, they included a negative control. Adding 0.4% tetracycline hydrochloride to the basal diet was designed to suppress gut bacteria and figure out whether the shrimp’s microbiome was contributing any vitamin B6 of its own.

Six hundred and forty juvenile shrimp, each weighing about 0.38 grams at the start, were divided across 32 tanks and fed for 45 days. Six feedings a day. Every two weeks, the shrimp were weighed and the feeding rate was adjusted. This was not a quick-and-dirty trial. It was meticulous.

The sweet spot: 69 milligrams per kilogram

The clearest result came from growth performance. Weight gain, specific growth rate, feed conversion ratio, and protein efficiency all improved as pyridoxine levels increased. Up to a point. That point was 75 mg/kg, where the shrimp reached their highest final body weight and the best feed conversion ratio.

But here is the more precise number: broken-line regression analysis pegged the optimal dietary pyridoxine requirement at 69.0 mg/kg, based on weight gain. That is the number where the growth curve stops climbing and flattens out.

This is notably lower than the 110 to 152 mg/kg range reported in earlier studies that used fish meal-based diets. Why the difference? The most likely explanation is that soybean meal shifts how the shrimp processes amino acids. Since PLP is intimately involved in amino acid metabolism, the demand for B6 changes when the protein source changes.

There is another layer to this. In the tetracycline-treated group, the hepatopancreas pyridoxine concentration was significantly lower than in the untreated control. That tells you something important. The shrimp’s gut bacteria are synthesizing at least some vitamin B6 on their own. When you knock those bacteria out with an antibiotic, the shrimp’s tissue levels drop. This means the dietary requirement is not just about what is in the feed. It is also about what the microbiome is contributing behind the scenes.

More than just growth: immunity, antioxidants, and gut health

Growth gets the headline, but the study measured a lot more than weight. And those secondary findings paint a picture of what B6 is really doing inside the animal.

On the immune side, lysozyme activity peaked in the 75 mg/kg group. Lysozyme is an enzyme that breaks down bacterial cell walls. Anti-protease activity, which helps neutralize enzymes released by pathogens, was highest at the same level. The nitroblue tetrazolium assay, a measure of the respiratory burst that immune cells use to kill invaders, was also maximized around 50 to 75 mg/kg.

Antioxidant capacity followed the same pattern. Superoxide dismutase and glutathione peroxidase rose with pyridoxine supplementation and peaked at 75 mg/kg.

The digestive system got a boost too. Trypsin and lipase activities in the hepatopancreas were significantly higher in shrimp receiving adequate B6. The intestinal wall grew thicker, and the epithelial layer, where nutrient absorption happens, lengthened considerably. Under the microscope, the hepatopancreas of shrimp fed optimal pyridoxine showed more F-cells, which produce digestive enzymes, and B-cells, which handle absorption and secretion.

At the genetic level, the story held together. The genes for CBS, CGL, and pyridoxal kinase were all upregulated with B6 supplementation. So were the genes for penaeidine and crustin. This fits a coherent biological picture: more B6 means more PLP, which means more transsulfuration activity, which means more glutathione, which means better antioxidant defense and immune readiness.

Then there is the protein angle. Shrimp in the optimal B6 groups had higher whole-body protein levels and better protein efficiency ratios. PLP is a required cofactor for aminotransferases, the enzymes that shuffle amino groups around during protein synthesis and breakdown. B6 is not supporting growth in some vague nutritional sense. It is directly enabling the biochemical machinery that turns feed protein into shrimp protein.

What happens when you overshoot

One detail worth noting: shrimp fed the highest pyridoxine levels, at 150 mg/kg, actually performed worse than those at 75 mg/kg. Growth dropped. Feed conversion slipped. Enzyme activities and gene expression both trended downward.

This is consistent with something that has been observed in fish species. Too much B6 can paradoxically reduce B6 function. The hepatopancreas pyridoxine concentration plateaued after 75 mg/kg, suggesting the tissue was saturated. Excess B6 gets converted to pyridoxic acid and excreted. But at very high doses, the metabolic machinery may get overwhelmed in ways that are not fully understood yet.

The practical takeaway is simple. More is not better. There is a real optimum, and exceeding it wastes money and might even hurt performance.

What this means for feed formulation

If you are formulating shrimp feed with high levels of soybean meal, here is what matters.

First, the pyridoxine requirement for soybean-based diets, around 69 mg/kg, is lower than the requirement for traditional fish meal-based diets. That does not mean you can skimp on B6. It means the old numbers do not apply, and you should be targeting this new benchmark instead.

Second, the gut microbiome matters. The fact that tetracycline-treated shrimp showed lower tissue B6 levels confirms that gut bacteria contribute to the animal’s vitamin supply. This has practical implications. If you are using antibiotics, even at sub-therapeutic levels, you may be inadvertently reducing endogenous vitamin production and increasing dietary requirements. On the flip side, probiotic strategies that support B-vitamin-synthesizing bacteria could potentially reduce the need for supplementation.

Third, B6 is not a standalone concern. The gene expression data shows that pyridoxine status affects the transsulfuration pathway, which ties into sulfur amino acid metabolism. If your formulation is marginal on methionine or cysteine, suboptimal B6 will make that worse, and vice versa. These nutrient interactions are worth paying attention to.

Fourth, the benefits of getting B6 right go well beyond growth. Immune function, antioxidant capacity, gut morphology, digestive enzyme output: all of these improved at the optimal dose. In commercial production, where disease outbreaks and inconsistent growth are real economic threats, that kind of systemic improvement matters a lot.

The bigger picture

This work is part of a broader rethinking of shrimp nutrition. As the industry moves toward plant-based and alternative protein sources, every micronutrient requirement needs to be re-evaluated. The assumption that vitamin requirements stay constant regardless of diet formulation is convenient, but it is probably wrong.

The fact that pyridoxine requirements dropped by roughly 40% when moving from fish meal to soybean meal diets makes you stop and think. It suggests that soybean meal either supplies more bioavailable B6 precursors, or that the metabolic demand for PLP is lower when shrimp are processing plant proteins rather than animal proteins. Either way, the lesson is the same. Nutrient requirements are not fixed numbers. They depend on context.

There is also an ecological dimension here worth paying attention to. The evidence that gut bacteria synthesize B6 for their shrimp host opens up new possibilities for managing nutrition through the microbiome. We are used to thinking of feed formulation as a chemistry problem: mix this much protein, this much lipid, this much vitamin. But if the microbiome is an active participant in nutrition, then feed formulation might also be an ecology problem. What you feed does not just nourish the shrimp. It nourishes the bacteria that nourish the shrimp.

Where the research needs to go next

This was a single trial with a specific diet formulation, conducted under controlled laboratory conditions. Commercial ponds are messier. Variable temperatures, fluctuating oxygen, pathogen pressure, natural productivity. All of those factors could shift the optimal B6 level in ways this study cannot predict.

Commercial-scale validation is the obvious next step. Studies that test pyridoxine requirements across different soybean meal inclusion levels would help establish dose-response curves that feed mills can actually use. And research on microbiome manipulation, whether through probiotics, prebiotics, or dietary strategies that support B-vitamin-synthesizing bacteria, could open up entirely new approaches to vitamin nutrition in aquaculture.

But for now, the core finding stands. If you are feeding Pacific white shrimp a soybean meal-based diet, target around 69 mg of pyridoxine per kilogram of feed. That is the number that maximizes growth, supports immunity, boosts antioxidant defenses, and keeps the digestive system running smoothly.

Not bad for a vitamin most people never think about.