Toxicity of Canola-Derived Glucosinolate Degradation Products in Pigs—A Review
Department of Animal Science, South Dakota State University, Brookings, SD 57007, USA
Department of Animal Resource and Science, Dankook University, Cheonan-si, Chungnam 31116, Korea
Department of Animal Science, Aarhus University, Blichers Allé 20, DK-8830 Tjele, Denmark
Author to whom correspondence should be addressed.
Animals 2020, 10(12), 2337; https://doi.org/10.3390/ani10122337
Received: 20 November 2020 / Revised: 4 December 2020 / Accepted: 4 December 2020 / Published: 9 December 2020
(This article belongs to the Special Issue Feeds and Feeding Programs in Functional Meat Production)
Canola co-products, which are included in swine diets as a source of amino acids, contain glucosinolates that limit the inclusion of these co-products in swine diets. Aliphatic and aromatic glucosinolates are two major canola co-product-derived glucosinolates. Aliphatic glucosinolates include progoitrin and gluconapin, whereas aromatic glucosinolates include 4-hydroxyglucobrassicin. Glucosinolates are non-toxic, but they are degraded into isothiocyanates, thiocyanate, and nitriles. Isothiocyanates produce goitrin, leading to reduced serum tetraiodothyronine concentration; thiocyanates lead to increased hypothyroidism; nitriles result in hepatic hypertrophy and hyperplasia. Canola-derived glucosinolates are degraded by heat during feed processing, in stomach acid (in the presence of iron), and by myrosinase in various sections of the gastrointestinal tract. Myrosinase is heat-labile and hence most of the myrosinase in canola co-products is inactivated during oil extraction. Notably, microorganisms are highly concentrated in the hindgut of pigs. Thus, the stomach and hindgut are the major sites of glucosinolate degradation in pigs. Most of the glucosinolates that escape degradation by acid in the stomach are degraded in the lower parts of the gastrointestinal tract. Practical swine diets contain iron; hence, degradation of glucosinolates in the stomach may not be limited by iron and may not be easily modified through changes in diet composition. Since the hindgut pH can be modified by diets fed to pigs, the composition of glucosinolate degradation products in the hindgut can be modified through diet modification. A reduction in hindgut pH of pigs due to dietary inclusion of highly fermentable dietary fiber can potentially favor the production of less toxic glucosinolate degradation products derived from canola co-products.
Canola co-products are widely included in swine diets as sources of proteins. However, inclusion of canola co-products in diets for pigs is limited by toxicity of glucosinolate degradation products. Aliphatic and aromatic glucosinolates are two major classes of glucosinolates. Glucosinolate degradation products derived from aliphatic glucosinolates (progoitrin) include crambene, epithionitriles, and goitrin, whereas indole-3-acetonitrile, thiocyanate, and indole-3-carbinol are the major aromatic glucosinolates (glucobrassicin)-derived degradation products. At acidic pH (<5.7), progoitrin is degraded by myrosinases to crambene and epithionitriles in the presence of iron, regardless of the presence of epithiospecifier protein (ESP), whereas progoitrin is degraded by myrosinases to goitrin in the absence of ESP, regardless of the presence of iron at neutral pH (6.5). Indole-3-acetonitrile is the major degradation product derived from glucobrassicin in the absence of ESP, regardless of the presence of iron at acidic pH (<4.0), whereas thiocyanate and indole-3-carbinol are the major glucobrassicin-derived degradation products in the absence of ESP, regardless of the presence of iron at neutral pH (7.0). In conclusion, the composition of glucosinolate degradation products is affected by parent glucosinolate composition and hindgut pH. Thus, toxicity of canola co-product-derived glucosinolates can be potentially alleviated by modifying the hindgut pH of pigs.