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Hyperphagia is not the primary cause of fatness in broilers selected for high plasma lipoprotein concentration
Chapter 10
Hyperphagia is not the primary cause of fatness in broilers selected for high plasma lipoprotein concentration C. C. Whitehead
Introduction Selection on the basis of high or low plasma very low density lipoprotein (VLDL) has resulted in two lines of broilers differing considerably in body fat content (see Chapter 3). The body weights of these lines are very similar but the food intake of the fat line is about 15% higher than that of the lean line up to 7 weeks of age after seven generations of selection. In order to understand the metabolic basis for the body compositional differences between the lines, it is important to investigate the role of hyperphagia in the greater fatness of the high-VLDL line. There are two possible hypotheses concerning the hyperphagia: (1) the fat line has a naturally greater appetite than the lean line but only the same potential for lean tissue growth; hence the extra food (energy) consumed is deposited as fat, and (2) there is a fundamental difference between the lines in the utilization of nutrients for different components of growth and the fat line must consume more food (protein) to achieve the same lean tissue growth. To test which of these hypotheses was more likely, the following pair-feeding experiment was undertaken.
Materials and methods Thirty females of each line from the fifth generation were reared up to 12 days of age in a wire-floored brooder and fed ad libitum on the standard diet on which the lines had been selected (Whitehead and Griffin, 1984). The birds were then assigned to ten pairs on the basis of nearly equal body weights of the fat and lean pair mates. The birds were transferred to single-bird wire-floored cages, with pair mates being housed in adjacent cages. Feed was provided thereafter at regular intervals throughout each day in equal amounts to both members of a pair, at a rate equivalent to the intake of the lean pair mate. The experiment continued until 42 days of age when the birds were killed and the abdominal fat pads were excised and weighed. The carcasses (including fat pads) were frozen and minced to a homogeneous paste which was freeze-dried and extracted with petroleum spirit to measure its lipid content. 125
126
Hyperphagia is not the primary cause of fatness in broilers
Results and discussion The fat-line birds always ate their feed more quickly and hence were without food for varying periods until their lean-line pair mates had consumed their own allocations. Lean-line birds were also sometimes without feed if they had consumed theirs before feed allocation times. Two of the original ten pairs had to be abandoned because of ill health of an individual. The results from the others are summarized in Table 10.1. Overall mean feed intake was very similar in both lines and intakes within each pair were closely matched. Mean body weight of lean-line birds at 42 days was significantly (P<0.05) higher whereas the proportions of abdominal fat and total body lipid were significantly (P<0.001) lower in this line. These line differences were reflected consistently in the individual pairings: within every pair the lean-line bird was always heavier and contained a lower proportion of abdominal fat and total body lipid. Table 10.1 Body weight, feed consumption and body fat content in broilers from lean and fat lines pair-fed from 12 to 42 days of age
Body weight (g) at 12 days at 42 days Feed intake (g) 12-42 days Abdominal fat (g/kg) Body lipid (g/kg) a
Fat line
Lean line
Significance of difference
250 1474 3081 28.2 174
250 1622 3100 14.2 125
NSa P<0.05 NS P<0.001 P<0.001
NS, not significant
These observations indicate that hyperphagia is not the primary cause of fatness in broilers selected for high plasma VLDL levels and that hypothesis (2) is the more likely. There must be a fundamental metabolic difference between the lines in the partitioning of nutrients that results in the fat birds becoming fat, even at the expense of overall body growth, when feed is limited. Of course, hyperphagia in fat line birds fed ad libitum may well contribute to further compositional and metabolic differences between the lines.
Conclusion Hyperphagia is not the primary cause of fatness in birds selected for high plasma VLDL levels. References WHITEHEAD, c. c. and GRIFFIN, H. D. (1984) Development of divergent lines of lean and fat broilers using plasma very low density lipoprotein concentration as selection criterion: the first three generations. British Poultry Science, 25, 573-582
Hyperphagia is not the primary cause of fatness in broilers selected for high plasma lipoprotein concentration C. C. Whitehead
Introduction Selection on the basis of high or low plasma very low density lipoprotein (VLDL) has resulted in two lines of broilers differing considerably in body fat content (see Chapter 3). The body weights of these lines are very similar but the food intake of the fat line is about 15% higher than that of the lean line up to 7 weeks of age after seven generations of selection. In order to understand the metabolic basis for the body compositional differences between the lines, it is important to investigate the role of hyperphagia in the greater fatness of the high-VLDL line. There are two possible hypotheses concerning the hyperphagia: (1) the fat line has a naturally greater appetite than the lean line but only the same potential for lean tissue growth; hence the extra food (energy) consumed is deposited as fat, and (2) there is a fundamental difference between the lines in the utilization of nutrients for different components of growth and the fat line must consume more food (protein) to achieve the same lean tissue growth. To test which of these hypotheses was more likely, the following pair-feeding experiment was undertaken.
Materials and methods Thirty females of each line from the fifth generation were reared up to 12 days of age in a wire-floored brooder and fed ad libitum on the standard diet on which the lines had been selected (Whitehead and Griffin, 1984). The birds were then assigned to ten pairs on the basis of nearly equal body weights of the fat and lean pair mates. The birds were transferred to single-bird wire-floored cages, with pair mates being housed in adjacent cages. Feed was provided thereafter at regular intervals throughout each day in equal amounts to both members of a pair, at a rate equivalent to the intake of the lean pair mate. The experiment continued until 42 days of age when the birds were killed and the abdominal fat pads were excised and weighed. The carcasses (including fat pads) were frozen and minced to a homogeneous paste which was freeze-dried and extracted with petroleum spirit to measure its lipid content. 125
126
Hyperphagia is not the primary cause of fatness in broilers
Results and discussion The fat-line birds always ate their feed more quickly and hence were without food for varying periods until their lean-line pair mates had consumed their own allocations. Lean-line birds were also sometimes without feed if they had consumed theirs before feed allocation times. Two of the original ten pairs had to be abandoned because of ill health of an individual. The results from the others are summarized in Table 10.1. Overall mean feed intake was very similar in both lines and intakes within each pair were closely matched. Mean body weight of lean-line birds at 42 days was significantly (P<0.05) higher whereas the proportions of abdominal fat and total body lipid were significantly (P<0.001) lower in this line. These line differences were reflected consistently in the individual pairings: within every pair the lean-line bird was always heavier and contained a lower proportion of abdominal fat and total body lipid. Table 10.1 Body weight, feed consumption and body fat content in broilers from lean and fat lines pair-fed from 12 to 42 days of age
Body weight (g) at 12 days at 42 days Feed intake (g) 12-42 days Abdominal fat (g/kg) Body lipid (g/kg) a
Fat line
Lean line
Significance of difference
250 1474 3081 28.2 174
250 1622 3100 14.2 125
NSa P<0.05 NS P<0.001 P<0.001
NS, not significant
These observations indicate that hyperphagia is not the primary cause of fatness in broilers selected for high plasma VLDL levels and that hypothesis (2) is the more likely. There must be a fundamental metabolic difference between the lines in the partitioning of nutrients that results in the fat birds becoming fat, even at the expense of overall body growth, when feed is limited. Of course, hyperphagia in fat line birds fed ad libitum may well contribute to further compositional and metabolic differences between the lines.
Conclusion Hyperphagia is not the primary cause of fatness in birds selected for high plasma VLDL levels. References WHITEHEAD, c. c. and GRIFFIN, H. D. (1984) Development of divergent lines of lean and fat broilers using plasma very low density lipoprotein concentration as selection criterion: the first three generations. British Poultry Science, 25, 573-582