Combating Stress: The Use of Isoflavones as Nutraceuticals to Improve Immunity and Growth in Nile Tilapia (Oreochromis niloticus)

Stressors in the aquaculture environment can lead to negative impacts on growth and immune health, resulting in susceptibility to infectious diseases. These stressors are expected to increase as the growth of aquaculture continues to rise to meet demands for quality fish protein. Isoflavones, as a crude extract or as a pure isolate, may be effective in modulating the stress response, promoting growth and immunity. The objective of these studies was to examine the effect of various pure isoflavone isolates and crude isoflavone extracts on stress, growth, and immunity. Nile tilapia (Oreochromis niloticus) were stressed by adding hydrocortisone to the feed. In a 7-week study, pure isoflavone isolates of genistein and puerarin were evaluated to determine their respective effects on stress, growth, and immunity. A separate 10-day physiological and 6-week growth study focused on crude isoflavone extracts from kudzu (Pueraria lobata), red clover (Trifolium pratense), and soybean (Glycine max) was performed to determine their respective effects on stress, growth, and immunity. Numerous physiological parameters of the fish were measured (serum cortisol concentration, blood glucose concentration, hematocrit, hepatosomatic index, plasma protein concentration, lysozyme activity, and spleensomatic
index) to determine the effects of these pure isoflavone and crude isoflavone extracts on the modulation of stress and immunity. Many growth parameters were examined (length, weight, condition factor, weight gain, specific growth rate, feed intake, feed conversion ratio, and protein efficiency ratio) as well to determine the effects of these pure isoflavones and isoflavone extracts on growth. The addition of isoflavone and crude isoflavone extracts to the diet of Nile tilapia ameliorated some of the negative consequences of stress. Compared to stressed fish fed commercial feed, genistein and puerarin added to the diet appeared to improve serum cortisol concentrations, which resulted in increased plasma protein, albeit at different durations of stress. Puerarin, as well as all three crude isoflavone extracts, significantly increased spleen-somatic index compared to non-supplemented stressed fish, although the crude isoflavone extracts did not appear to improve serum cortisol concentrations. Crude isoflavone extracts also showed overall increases in lysozyme activity compared to non-supplemented stressed fish, although this was not significant. Genistein, puerarin, and red clover showed increased growth rates, feed conversion ratio, and protein efficiency. Overall, pure isolates of isoflavone appear to be more effective in modulating stress, immunity, and growth than the crude isoflavone extracts, although red clover extract showed promises in the ability to modulate the stress response and improve growth and immunity. There are likely substantial interactions between the isoflavones in the crude extracts that cannot be fully understood by measuring the effects of single isoflavones. Regardless, isoflavone supplementation (pure or crude) appeared to generally have an overall positive impact on stressed Nile tilapia, requiring more research to better understand the effects and mechanisms behind these isoflavones.