The Carnivore Paradox: Why this popular fish digests sugars so poorly
The rainbow trout (Oncorhynchus mykiss), an emblematic carnivore of rivers and aquaculture farms, presents a puzzling particularity: its organism utilizes dietary carbohydrates very poorly.
In a context where aquaculture seeks to replace fishmeal with plant-based ingredients rich in starch, this relative intolerance becomes a major scientific, economic, and environmental challenge. This article explores the complex mechanisms governing glucose metabolism in trout and recent discoveries that could revolutionize its feeding.
In mammals, a carbohydrate-rich meal leads to a transient increase in blood sugar, quickly regulated by insulin. In juvenile trout, the same meal causes prolonged hyperglycemia, which can last several hours, similar to a pre-diabetic state 2 6 .
If juveniles are poor utilizers of carbohydrates, a recent discovery has overturned this conception: breeders (reproductive adults) are much more efficient.
A study fed male and female trout during a complete reproductive cycle with either a carbohydrate-free diet (NC) or a high-carbohydrate diet (HC, 35%) and low in protein 7 .
Breeders fed the HC diet showed no persistent hyperglycemia, no growth retardation, unlike juveniles 7 . Their hepatic metabolism adapted by increasing energy storage in the form of glycogen (glycogenesis) and potentiating the pentose phosphate pathway.
| Parameter | Juvenile Trout | Breeder Trout |
|---|---|---|
| Postprandial Glycemia | Prolonged hyperglycemia 6 | Normal regulation 7 |
| Growth | Reduction beyond 20% carbohydrates 6 | Normal even at 35% carbohydrates 7 |
| Hepatic Metabolism | Uninhibited gluconeogenesis 1 | Increased glycogenesis 7 |
| Reproductive Performance | Not applicable | Slightly affected but viable 7 |
The story doesn't end there. Feeding breeders with an HC diet has consequences on their offspring, a phenomenon called nutritional programming.
Research shows that when both parents are fed an HC diet, the fry show modifications in their energy metabolism and in the methylation of their DNA (an epigenetic mechanism) 6 .
Fortunately, these effects seem to diminish in the long term, and after six months, the growth of juveniles is no longer affected 6 .
DNA methylation patterns in offspring are altered when parents consume high-carbohydrate diets, affecting metabolic gene expression.
| Group | Observed Effects |
|---|---|
| Adult Breeders |
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| Offspring (Fry) |
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The study of this complex metabolism requires advanced molecular and physiological tools.
Obtain sequences of genes of interest to study their expression.
Example: Partial cloning of genes for Glut2, FBPase, PK to study their nutritional regulation 1 .
Analyze all messenger RNAs (transcripts) to see which genes are activated/inhibited.
Example: Identify carbohydrate metabolism genes affected by hormonal treatment or diet 3 .
Study cellular mechanisms in a simplified and controlled system.
Example: Use hepatocytes (liver cells) to test the effect of insulin, amino acids, and glucose 2 .
Specifically block a receptor to understand its role.
Example: Use eplerenone (anti-MR) and mifepristone (anti-GR) to study the role of DOC in metabolism 3 .
Evaluate the impact of diets on the intestinal barrier without animal experimentation.
Example: Test the effect of soybean meal or feather meal-based diets on trout intestinal cells 5 .
Comprehensive analysis of small molecule metabolites to understand metabolic pathways.
Example: Profile metabolic changes in response to different dietary regimes.
The case of rainbow trout teaches us that biology is never simple. The regulation of its carbohydrate metabolism is a finely orchestrated process, varying according to age, developmental stage, and animal physiology.
The discovery of the adaptive capacities of breeders opens exciting perspectives for more sustainable aquaculture. Using cheaper and more ecological energy sources like carbohydrates for breeder fish would reduce dependence on fishmeal, decrease nitrogen discharges, and thus limit the environmental impact of farming 6 7 .
Research continues to unravel the last mysteries of this glucose intolerance and better exploit the metabolic flexibility of this emblematic fish.