Leguminous Fish Poison: Soy Saponins Injure Intestinal Structure of Breeding Fish.

Image 1: Soybean saponins turn out
to be an effective fish poison.
Bottom line: No Soy for Your Koi!

As a "student" of the SuppVersity, your source for nutrition and exercise science on the Internet, you already know about the "Soy Ploy". Probably, you are also aware that despite deliberately avoiding the consumption of soy products, many of you are exposed to meat and fish which has been raised on a soy-based chow. As questionable as the practice of feeding animals plants they naturally would not consume may be, the provision of soy-based diets to all sorts of livestock is still considered a highly economical way of fattening up fatstock. A recent study (Chen. 2011) from the Ocean University of China could however change this perspective and persuade fish farmers to use a more natural foodstuff for their breed.

Chen et al. found that a diet containing more than 3.2g/kg saponins damaged the gut lining of the fish and lead to a significant decrease in weight gain and thus economic damage, which (unfortunately) appears to be the only reason for the food industry to reconsider feeding practices:

High dietary saponins level (6.4 g saponins kg^-1 diet) significantly depressed the weight gain and FER of flounder at both day 28 and 56. The weight gain of fish fed the diet with 3.2 g sapoins kg^-1 diet (Diet 3), however, was significantly depressed at day 28, while no significant difference was observed at day 56 compared with the control diet. The integrity of histological structure in the distal intestine was gradually impaired as soybean saponins levels increased. 

The inverse relationship between saponin content and the impairment of the histological structure in the distal intestine should make you (re-)think your voluntary or involuntary soy consumption - especially in view of the fact that different brands of soy protein contain 3-25g/kg and whole soy beans poison you with a whopping 56g/kg saponins (cf. Fenwick. 1980).