Dietary ME taken in as diet which is not dissipated as heat is retained in the body as new tissue constituents. The difference between the enthalpy of combustion (i.e., GE) of the body at the beginning and that at the end of a period of time is referred to as “recovered energy” (RE) according to the NRC (1981) nomenclature.
The most direct way of estimating RE is to determine the GE of a number of individuals (representative samples) from a group of experimental ani- mals at the beginning and at the end of a study. This method of determining RE is termed comparative carcass analysis or slaughter technique and was discussed earlier in the chapter. Alternatively, RE can be estimated by the difference between IE and FE, UE+ZE, and HE (Blaxter, 1989), which is also known as the energy balance technique.
RE can be either positive or negative and represents the enthalpy of com- bustion of organic compounds stored or lost by the animal. Protein and lipids are the main energy-yielding components of the body, whereas glyco- gen generally represents only a small proportion of the body of the animal (<1%) and consequently of RE. RE is not always a quantitative measure of weight gain because deposition of lipid reduces the water content of the body, thus changing the energy value per unit weight of the living animal.
The great difference in the energy value of lipid and protein also exagger- ates the difference in the energy content of body weight gain. However, RE remains a useful and simple measure of growth and nutrient deposition provided that one recognizes its limitations. RE is only an estimate of the deposition of energy-yielding nutrients (mostly protein and lipids) achieved through different systems and regulated by different factors. Consequently, RE measurements should always be accompanied by measurements of ni- trogen gain (RN) and interpreted with care.
There are 3–6 grams of water associated with each gram of protein tissue deposited (Cho and Kaushik, 1990). On a wet weight basis protein tissue is only about 25% dry matter and the dry matter has an energy value of 23.6 kJ/
g, giving a value for protein of 6 kJ/g wet tissue. Lipid is usually deposited in adipose tissue in association with relatively little water. Fat tissue is about
0 100 200 300 400 500
0 200 400 600 800
Live weight (g/fish)
Component (g/fish)
0 2000 4000 6000 8000
Gross energy (kJ/fish)
H2O CP Lipid GE
FIG. 1.3
Chemical composition (absolute; g/fish or kJ/fish) of rainbow trout of various sizes fed practical diets with 20–22 g DP/MJ DE. Data from Bureauet al.(unpublished).
Regressions: H2O (g/fish)=0.670 BW (g/fish) – 3.13 (R2=1.00); Crude protein (CP) (g/fish)=0.169 BW (g/fish) – 0.07 (R2=1.00); Lipid (g/fish)=0.125 BW (g/fish) – 2.52 (R2=0.96); Gross energy (GE) (kJ/fish)=8.6 BW (g/fish)−40.1 (R2=0.98).
85% dry matter, with an energy value of approximately 39 kJ/g. Fat tissue therefore contains about 33 kJ/g of tissue deposited (Smith, 1989). In ad- dition, lipid is stored in tissues generally substituting for water. RE varies greatly with the type of tissue being produced.
Figures 1.3 and 1.4 show the absolute (gram or kilojoule per fish) and relative (percentage or kilojoule per gram) compositions of rainbow trout of various sizes fed a practical diet with 20–22 g digestible protein (DP)/MJ digestible energy (DE). Figure 1.3 shows that the absolute contents of water, protein, lipid, and gross energy of fish increase linearly with the weight of the animal. Figure 1.4 shows that, expressed as the relative composition (e.g., percentage of body weight), the protein content of the fish increases slightly, then remains approximately constant. Moisture tends to decrease rapidly with fish size up to about 100 g live weight, then decreases more slowly.
Lipid and GE increase rapidly with size also up to 100 g live weight, then increase linearly very slowly. Similar observations and figures are reported by Shearer (1994) and Lupatschet al.(1998).
Shearer (1994) concluded that the protein content of growing salmonids is determined solely by fish size, that the lipid level is affected by both en- dogenous (fish size, growth rate) and exogenous (dietary, environmental) factors, and that the ash content is homeostatically controlled. The protein
0 20 40 60 80 100
0 200 400 600 800
Live weight (g/fish)
Composition (%)
0 5 10 15 20
Gross energy (kJ/g)
H2O CP Lipid GE
FIG. 1.4
Chemical composition (relative; % or kJ/g) of rainbow trout of various sizes fed practical diets with 20–22 g DP/MJ DE. d−1, day−1. Data from Bureauet al.
(unpublished, 2000).
or ash contents of the whole body appear to vary little with the growth of a given species of fish, whereas the whole-body energy content varies consider- ably over time. Whole-body GE content increase is due mainly to increasing lipid content with increasing body weight.
The results from a number of studies on protein and lipid deposition clearly show that this increase in lipid content of fish of increasing body weight, fed to near-satiety, is generally not due to enhanced deposition of lipid compared to that of protein as the animal increases in size. Azevedo et al.(1999) observed that rainbow trout fed the same diet at different ration levels (from 70 to 100% of near-satiety) deposited protein and lipid accord- ing to the same ratio regardless of fish size and water temperature. Their data showed a very good proportionality between protein and fat gain; for each unit of protein energy gain, the fat energy gain was 1.4 times higher.
A similar proportionality of protein and lipid deposition with increasing feed intake has been reported in domestic animals (Boekholtet al.,1994, 1997).
Severe feed restriction results in a significant alteration of the protein-to- lipid deposition ratio in fish. Protein deposition has, in general, priority over
REl = 0.46x - 6.70
REp = 0.18x + 1.68 R2 = 0.946 R2 = 0.967
-10 0 10 20 30 40
0 20 40 60 80 100
ME intake (kJ (kg0.824)-1d-1) RE (kJ (kg0.824 )-1 d-1 )
FIG. 1.5
Recovered energy (RE) as protein (REp; squares) and lipid (REl; diamonds) as a function of the metabolizable energy (ME) intake of rainbow trout reared at 8.5◦C.
d−1, day−1. Data from Bureauet al.(unpublished observations).
lipid deposition. As mentioned earlier, fish fed a ration allowing a RE=0 can still deposit body protein (positive protein energy gain) while mobilizing body lipids. Clearly, live weight gain is driven by protein deposition. Studies on the effect of feeding levels on fish have shown that protein and lipid depositions increase linearly with feed allocation but that protein and lipid energy depositions have different slopes and intercepts (Fig. 1.5). Under severe feed restriction, protein deposition greatly exceeds lipid deposition.
Energy deposition as lipids often exceeds that as protein at moderate to high feeding levels. A number of studies have also shown that protein deposition tends to plateau at high feeding levels, whereas lipid deposition does not appear to level off (increases linearly). A decreasing protein-to-lipid deposi- tion ratio can be observed at feeding levels approaching maximum protein deposition.
The relative importance of protein and lipid deposition depends on a great number of nutritional factors. The balance of available amino acids, particularly essential amino acids, in the dietary protein and the digestible protein-to-digestible energy ratio in the diet are the major factors. Proteins of high biological value may promote greater protein deposition than those of lower value. Large excesses of energy intake and improper balance of protein to energy result in the deposition of a larger proportion of RE as lipid. Seasonal changes in body composition, in relation to specific physio- logical stages or endocrine status, are also known to occur. That there are considerable interspecific differences in lipid deposition and tissue distribu- tion should also be recognized. Nutrient deposition and temporal changes
in body composition of fish, and effects of all the factors mentioned above, should be examined more closely.