How much does a fish in heavy water weigh?
Answer: it depends what you feed it*. Replacement of finite marine ingredients in modern aquaculture diets relies on alternative protein sources, including plant-based ingredients. Although providing high quality protein, these also contain considerable amounts of carbohydrates such as starch. For carnivorous fish, such as barramundi or Asian seabass, we know from our past research that this can be detrimental to growth performance and feed efficiency, but we didn’t know how. This is where heavy water comes in.
In a series of recent publications from our group, in close collaboration with the research team of Dr Ivan Viegas at the University of Coimbra in Portugal, we were able to measure the metabolic impact of feeding starch to Australia’s iconic fish. This study fed a high protein or high starch diet to barramundi, then tracked the metabolic fate of dietary ingredients using deuterated (or heavy) water (2H2O or D2O).
The stable isotope of hydrogen (deuterium or 2H) provides a safe, non-radioactive and powerful marker to track the metabolic transformations that occur while fish eat normally over six days in heavy water. This provides significant advantages to other approaches that measure metabolic regulatory enzymes or gene expression at a single time point, but only if you have a finely tuned nuclear magnetic resonance (NMR) machine!
As it turns out, when compared to its closest cousin the European seabass, barramundi have a unique mechanism for dealing with dietary carbohydrates. These included increased rate of fatty acid synthesis in the liver and adipose tissue, while none occurs in the muscle, and increased conversion of dietary glucose directly to glycogen. This leads to the accumulation of fat in the liver and viscera instead of protein deposition in the muscle. This means that although growth is reduced the muscle remains lean and high quality.
These studies highlight the key roles of glycerol, glycogen and fatty acids in the normal metabolism of barramundi, and the metabolic plasticity that exists when adapting to plant-based ingredients. This knowledge will be applied to formulate advanced sustainable feeds that reduce reliance on our precious marine resources like fishmeal.
This work was recently published in the British Journal of Nutrition, Frontiers in Physiology and Comparative Biochemistry and Physiology.
*The actual answer: If the water content of a fish is ~70% of wet weight, after six days having equilibrated their body water with 3.5% deuterated water (D2O 1.1056g/mL vs H2O 0.9986g/mL) a 100g fish might be expected to weight 0.3765% (or 0.262g) more than a 100g fish that was in normal freshwater.
CSIRO Aquaculture contact
Dr Nick Wade – Senior Research Scientist