Glucosinolates as undesirable substances in animal feed(1) Scientific Panel on Contaminants in the Food Chain

Glucosinolates (alkyl aldoxime-O-sulphate esters with a beta-D-thioglucopyranoside group) occur in important oil-and protein-rich agricultural crops, including among others Brassica napus (rapeseed of Canola), B. campestris (turnip rape) and Sinapis alba (white mustard), all belonging to the plant family of Brassicaceae. They are present in all parts of these plants, with the highest concentrations often found in seeds. Several of these Brassica species are important feed ingredients and some species are also commonly used in human nutrition such as cauliflower, cabbages, broccoli and Brussels sprouts. Glucosinolates and their breakdown products determine the typical flavour and (bitter) taste of these vegetables. The individual glucosinolates vary in structure and the configuration of their side chain. They are hydrophilic and rather stable and remain in the press cake of oilseeds when these are processed and de-oiled. However, glucosinolate producing plants as well as some microorganisms contain specific beta-thioglucosidases (denoted as myrosinases). In the intact plant these enzymes are separated from the glucosinolates and sequestered in aqueous vacuoles. Upon plant damage (including chewing during ingestion) the myrosinases are released, and initiate in the presence of water the conversion of glucosinolates into diverse breakdown products, including isothiocyanates, oxazolidinethiones (5-vinyl-2-oxazolidinethione and 5-vinyl-1,3 oxazolidine-2thione), thiocyanates, nitriles, epithionitriles and other indol-3-ylmethyl derivatives. The biological effects of plant glucosinolates in mammalian species are predominantly related to these glucosinolate-derived compounds. They interfere with iodine uptake (thiocyanate ion) and the synthesis of thyroid hormones triiodothyronine (T3) and plasma thyroxine (T4) (5vinyloxazolidine- 2 thione), leading eventually to hypothyroidism and enlargement of the thyroid gland (goitre). As a consequence of these changes in thyroid function, clinical signs of toxicity described in farm animals include growth retardation, reduction in performance (milk and egg production), impaired reproductive activity, and impairment of liver and kidney functions, the latter are likely being associated with the formation of nitriles. Data on the toxicity of individual glucosinolates for food-producing animal species are very limited, and in most cases only the total glucosinolate content in a given feed material, measured indirectly though the quantification of hydrolysable glucose, is available. Only for rapeseed meal or press cakes comprehensive feeding trials in farm animals have been conducted, resulting in the recommendation to restrict the total glucosinolate content to 1 - 1.5 mmol per kg feed for monogastric animals, and to even lower concentrations in feeds for young animals. In recognition of potential adverse effects exerted at high concentrations of glucosinolates, selection of plant varieties with low glucosinolate content (in addition to a low content of erucic acid in the oil) commenced more than three decades ago, resulting in the use of varieties, particularly rapeseeds, with a low glucosinolate content(2). The common practices of selecting lowglucosinolate plant varieties as forage plants and processing crops with a potential high glucosinolate concentration prior to use, together with experience- based recommendations for maximal inclusion rates into animal diets given in textbooks, have proven to be effective measures to avoid intoxications and production losses in farm animals, and the undesirable fishy taint in animal-derived products. However, it is recommended that the available advanced analytical techniques should be applied to quantify the major glucosinolates in these forage plants with the aim to more accurately define animal exposure. This applies particularly to new or reemerging oilseed crops, such as Camelina sativa, that may contain long-side chain glucosinolates, which are not detected in other common Brassica species. Following exposure of farm animals to forages and concentrates containing glucosinolates, a carry over of glucosinolates and their associated breakdown products into edible tissues, milk and eggs has been described, but the rate of carry-over is very low. The measurable residues in dairy milk corresponding to approximately 0.1 % of the given glucosinolate dose, the residues in muscle tissues and organs were even lower. In certain breeds of laying hens, excretion of glucosinolate-derived compounds may convey an undesirable fishy taint to the eggs. However, all measured concentrations in animal-derived products are much lower than those found in vegetables for human consumption, and are unlikely to induce adverse health effects in consumers.