Introduction
The sustainability of the aquaculture industry depends on how efficient we are in using available resources to supply an increasing population with affordable and nutritious protein. Put simply, we need to produce more, and better, with less. There is, therefore, an important opportunity for industry and academia to work together on the specifications of fish feed formulas that cost-efficiently optimise the nutrition and health of fish throughout their entire production cycle, for example, using methionine.Changing diets
Modern aquaculture diets are produced with low or no fish meal and higher amounts of plant protein sources. This change in diet formulation results in methionine being typically the first limiting amino acid - one that is in insufficient amounts in a food - in the fish and shrimp diets.
Methionine has a central role as a building block in protein synthesis, along with several other functions. Recent research shows it to be a key regulator of antioxidant defence, immune response, overall fish health status and carbohydrate and lipid metabolism. Additionally, as a precursor of S-adenosylmethionine (SAM) and S-adenosylhomocysteine (SAH), methionine can modulate the expression of genes related to the growth and health of animals by DNA methylation reactions.
Protein turnover
Inside the body of fish and shrimp, protein mass gain is the net gain of body protein deposition over protein loss, under the biological process called ‘protein turnover’. This is a continuous process in all tissues, involving both protein synthesis and protein breakdown (proteolysis).
Protein turnover is affected by the nutritional composition of the diet and the nutritional status of animal. For a growing animal, it is important to keep the diet balanced for amino acids while providing enough energy from non-protein energy sources.
Commercial feed producers have found that supplementing methionine levels in aqua feed results in better animal production. Recent studies in fish have demonstrated that methionine deficiency modulates a key intracellular signalling pathway, resulting in increased expression of several genes involved in protein degradation and inhibition of protein synthesis. For example, a methionine deficient diet (formulated to supply 32 percent less methionine than the normal diet) in rainbow trout (40g, initial body weight) was demonstrated to affect the phosphorylation of translation initiation factor (eif2α), and induce the expression of several factors involved in the two major muscle proteolytic pathways (proteasome and autophagy).Overall, recent studies have improved our understanding of the role of methionine in fish growth at cellular level. This has provided important biomarkers to assess fish performance in methionine deficient diet.
Oxidative stress
Intensive farming is becoming more common to meet the growing demand for global fish consumption. Under this method, animals are often exposed to various biotic (eg high stocking density, pathogen exposure) and abiotic (eg poor water quality, transportation, imbalanced diet) stressors. These lead to constant production of reactive oxygen species (ROS). ROS are basically chemical substances containing oxygen (eg superoxide, hydrogen peroxide, hydroxy radicals) and give rise to reactive free radicals (molecules with unpaired electrons). These free radicals participate in oxidative reactions that damage biological molecules such as lipids, proteins and the DNA of animals. Animals undergo oxidative stress when they don’t have the capacity to detoxify the free radicals or to repair the resulting damage.
Methionine is an essential compound for host defence against oxidative stress. It assists in the formation of glutathione (GSH) and taurine; essential compounds for host defence against oxidative stress. Methionine is also easily oxidised by ROS-forming methionine sulphoxide, which can readily be repaired by methionine sulphoxide reductase. It therefore constitutes an important antioxidant defence mechanism that may play a role in redox signalling; a form of cellular communication.
Several studies have shown that dietary methionine deficiency may reduce reservoirs of GSH, the most important intracellular antioxidant, and thus increase oxidative damages in several tissues. Yellow catfish fed a methionine deficient diet (formulated to supply 37 per cent less methionine than the normal diet) displayed peroxidative damage by ROS accompanied by an increase in antioxidant enzyme activities (SOD and GPX), indicating that a methionine-deficient diet causes oxidative stress in the yellow catfish. In contrast, a previous study showed that dietary methionine restriction (formulated to supply 45 percent less methionine than the control) does not affect the amount of total glutathione in the liver tissue of juvenile salmon under laboratory conditions.
Research has also demonstrated that dietary methionine deficiency (formulated to supply 55 percent less methionine than the control), decreased the oxidative status of the liver of rainbow trout. This indicates that the negative effects of methionine deficiency on the oxidative status of fish depend on the factors including the level of methionine deficiency in relation to the need of the animal and the level of oxidative stress.Factors including the life stage of fish, production intensity and growing conditions can all influence oxidative stress. Under intensive farming conditions, where animals are prone to oxidative stress, the methionine requirement of fish and shrimp can be higher than the values determined under favourable laboratory conditions.
Immune response
Deficiency of certain amino acids has been known to impair immune function and increase the susceptibility of animals to infectious disease. Amino acids affect the immune response of an animal, either directly or indirectly, through their metabolites.
Dietary methionine deficiency has been shown to decrease innate immune response and thus the protection against the infection of bacteria (A. hydrophila) in juvenile yellow catfish.
In addition, during infection or inflammation, fish fed the methionine supplemented diet are better protected against micro-organisms (such as bacteria) and bactericidal activities. This response might be due to the role of methionine on cell proliferation.
Intermediary metabolism
The liver is the main site for intermediary metabolism (including, lipids and carbohydrate metabolism). Hepatic expression of genes involved in lipogenesis and gluconeogenesis has been shown to respond to dietary methionine imbalance in fish. These disturbances are also reflected in fish at the phenotype level. When salmonids were fed low methionine diets, hepatic triglyceride (TAG) accumulation was recorded. It is believed that liver TAG accumulation following low methionine diets is due to the reduced availability of phosphatidylcholine involved in transport of lipids from liver to peripheral organs. Phosphatidylcholine is synthesised from phosphatidylethanolamine, where SAM produced from methionine acts as an important methyl donor. Therefore, methionine deficiency can affect the endogenous synthesis of phosphatidylcholine.
Another hypothesis is that methionine deficiency can decrease the availability of taurine, which is an important precursor in the synthesis of bile salt (taurocholic acid). Bile salt is highly important in the digestion and absorption of fat and fat-soluble vitamins. In addition, bile acid synthesis is the major route of cholesterol metabolism, as about half of the cholesterol produced in the body is used for bile acid synthesis.
Overall, methionine has important roles in the proper intermediary metabolism of nutrients, and long-term deficiency of methionine may be detrimental to fish growth and health.
Nutritional programming
Early nutritional events exerted during critical developmental windows may result in permanent changes in later life of animals, namely affecting their growth potential, health and metabolic status. Those events are the result of the so-called nutritional programming, and open a new field of research and opportunities towards the optimisation of nutrition and health of farmed animals, with obvious economic implications.
Nutritional programming through epigenetic mechanisms of methylation reactions, such as the DNA and histone methylation, is directly dependent on SAM, and thus of its major precursor: methionine. Dietary methionine levels and their adequacy in meeting animals’ requirements at early stages may therefore constitute a critical factor in modulating animals’ phenotype throughout their production cycle. Recent studies with rainbow trout show that feeding broodstock with diets limiting methionine at 50 percent affects various traits in the offspring, some of which persisted during the first weeks of exogenous feeding. Whether epigenetic mechanisms are behind those effects requires further investigation.
Overall health
Methionine deficiency can have different impacts on overall health. For example, it can lead to the development of cataracts in several fish species, such as rainbow trout, hybrid striped bass and Arctic charr. Although the mechanism behind this is not well understood, it is thought that glutathione, synthesized endogenously from methionine or cysteine, is likely to play a role in preventing the formation of disulphide bonds, which lead to the insolubility of lens protein and the development of ocular opacity.
Moreover, methionine has been shown to play a role in gut health in Jian carp. Microbial populations in the digestive tracts of fish (such as Firmicutes) have been shown to be closely correlated to fish health and nutrition. Bacteria of the fish digestive tract could secrete digestive enzymes which promote digestion of nutrient substances and synthesise nutrient substance that the fish need. It has also been demonstrated that methionine could influence the balance of intestinal microflora by promoting the growth of beneficial bacterium and depressing the growth of harmful bacterium.
Changing the way we look at methionine nutrition and fish health.
Considering the number of metabolic pathways requiring methionine and/or its derivatives, it is not surprising that dietary methionine not only affects growth but also metabolism and overall health status of fish.
Evidence of the functional role played by methionine in fish is growing and suggests we need to change the way we look at methionine nutrition and the health of fish. It has been proved that the first-feeding stage in fish is a critical window for nutritional programming and that there is a positive impact of the early-feeding of a plant-based diet on its future acceptance and utilisation.
Working together, industry and academia need to establish whether it is possible to programme the future growth and health of farmed fish through optimised nutrition. And, if so, the response criteria that should be (re)considered to define methionine (nutrient) recommendations that best optimise nutrition and health of fish.
