Determination of the effects of pulse starches (pea, lentil and faba bean) versus cornstarch on growth performance and glucose tolerance in rainbow trout (Oncorhynchus mykiss)
Carbohydrates are the major energy source for humans and terrestrial animals. However, in fish, protein and lipid are more efficiently used for energy than carbohydrates. Despite this, stud-ies have shown that diets with an appropriate amount of carbohydrate improve growth in fish. Therefore, as a first step in examining what is already known about carbohydrate use in fish, a meta-analysis was performed. Three parallel meta-analyses were conducted to determine the ef-fect dietary carbohydrate inclusion level on final weight (FW) of carnivorous finfish. The first meta-analysis examined the effect of starch as the primary component of carbohydrate from ge-latinized and precooked cornstarch, gelatinized tapioca starch and gelatinized potato starch on fish growth. The overall effect size was 0.42 and P value was 0.65. The second meta-analysis examined the effect of glucose, maltose and dextrin on fish growth. The overall effect size was 0.39 and P value was 0.69. The third meta-analysis examined the effects of native starches (corn, tapioca and potato starches) on fish growth. The overall effect size was 0.79 and P value was 0.43. In conclusion, all three studies showed no significant difference in final weight between control and diets with carbohydrate inclusion up to 280 g/kg in most carnivorous fish species. Based on these meta-analyses, work in this thesis then used fish growth trials using the common commercial species, rainbow trout (Oncorhynchus mykiss). The overall hypothesis of this thesis was inclusion of a better starch source like pulses would improve fish growth by improving post-prandial glycemic control. In this project, three experi-ments (digestibility, growth performance and glycemic index testing) were carried out to investi-gate utilization of pulse starches (pea, lentil and faba bean) versus modified cornstarch in rainbow trout diets. In the first experiment, there were significant differences in apparent digestibility coefficients (ADCs) of macronutrients among starch ingredients (P < 0.05). Protein digestibility was much higher than starch digestibility for pea, lentil and faba bean starches, while the ADC of crude protein in modified cornstarch was not measurable. Digestibility of most starches was low in rainbow trout (6% for pea starch, not detectable for faba bean, 36% for modified cornstarch), while lentil starch digestibility was inexplicably higher (55%). Similar trends were evident for protein, fat and energy digestibility in trout, with negligible digestibility for faba bean starch, low digestibility for modified cornstarch or pea starch (<30%), but high digestibility for lentil starch. Since digestibility of faba bean starch was not detectable in trout, faba bean starch was not examined in the subsequent growth trial. The effects of modified cornstarch, pea starch and lentil starch on growth performance were investigated at control, 100 and 200 g/kg starch inclusion. After 8 weeks, fish had doubled their weight and regression model analysis showed that starch inclusion up to 200 g/kg did not significantly affect the average daily gain (ADG), specific growth rate (SGR), average daily feed intake (ADFI) or feed conversion rate (FCR). The one ex-ception was the diet with lentil starch inclusion, which showed significant quadratic relationship (P = 0.03), with 100 g/kg inclusion having the best FCR at 0.90. The third experiment was aimed at establishing glycemic index (GI) values for pulse versus cornstarch in fish maintained on normal commercial diets, but fasted for 48 hr. Baseline plasma glucose (pre-feeding) ranged from 4.5 ± 0.29 mmol/l and increased to a peak of 12.6 ± 1.41 or 9.8 ± 0.86 mmol/L, respectively at 24 hours after force-feeding glucose (a reference) or unmodi-fied cornstarch. In contrast, plasma glucose levels continuously increased over 96 hours after feeding pulse starches or modified cornstarch, ranging from 20.3 mmol/l to 25.8 mmol/l at 96 h. There were significant differences among starches in peak, time to peak, and area under the curve or glycemic index among treatments. The glycemic index of unmodified cornstarch in rainbow trout was below 100 whereas GI values of pulse starches and modified cornstarch were all higher than 100, reflecting the severe and prolonged hyperglycemia that was observed. Taken together, digestibility of raw pulse starches and modified cornstarch was poor in rain-bow trout, but yet produced paradoxically high and prolonged hyperglycemia. The GI model from human and other related mammals does not fit rainbow trout. While the work in this thesis cannot explain benefits of using pulse starches in trout diets based on postprandial hyperglycemic or glycemic index, this thesis clearly showed that pea or lentil starches can be included in rainbow trout diets up to 200 g/kg. More studies are needed to find the upper tolerable limit for pulse starch inclusion in rainbow trout diets and to better characterize how trout utilize these starches for growth.
rainbow trout, pulse starches, growth, glycemic response
Master of Science (M.Sc.)
Animal and Poultry Science