Search

Chemopreventive effects of broccoli, a highly valued vegetable, have been known for a long time. Several studies have demonstrated that broccoli might be beneficial by reducing the risk for the development of certain forms of cancer. These effects are generally attributed to glucosinolate-derived degradation products like isothiocyanates and indoles which are formed by the hydrolytic action of plant myrosinase and/or glucosidases deriving from the human microbial flora. However, recent in vitro and experimental animal studies indicate that broccoli, its extracts and the glucosinolate-derived degradation products might also have undesirable effects, especially genotoxic activities. However, the relevance of the genotoxic activities to human health is not known yet. This paper gives an overview on genotoxic, anti-genotoxic/chemopreventive, nutritive and antinutritive properties of broccoli, its ingredients and their degradation products. A qualitative comparison of the benefit and risk of broccoli consumption benefit-risk assessment shows that the benefit from intake in modest quantities and in processed form outweighs potential risks. For other preparations (fortified broccoli-based dietary supplements, diets with extraordinary high daily intake, consumption as a raw vegetable) further studies both for potential risks and beneficial effects are needed in order to assess the benefit and risk in the future., (Copyright © 2011 Elsevier Ltd. All rights reserved.)

Sulfur-containing phytochemicals of two different kinds are present in all Brassica oleracea (Cruciferae) vegetables (cabbage, broccoli, etc.). They are glucosinolates (previously called thioglucosides) and S-methyl cysteine sulfoxide. These compounds, which are derived in plant tissue by amino acid biosynthesis, show quite different toxicological effects and appear to possess anticarcinogenic properties. Glucosinolates have been extensively studied since the mid-nineteenth century. They are present in plant foods besides Brassica vegetables with especially high levels in a number of seed meals fed to livestock. About 100 different kinds of glucosinolates are known to exist in the plant kingdom, but only about 10 are present in Brassica. The first toxic effects of isothiocyanates and other hydrolytic products from glucosinolates that were identified were goitre and a general inhibition of iodine uptake by the thyroid. Numerous studies have indicated that the hydrolytic products of at least three glucosinolates, 4-methyl-sulfinylbutyl (glucoraphanin), 2-phenylethyl (gluconasturtiin) and 3-indolylmethyl (glucobrassicin), have anticarcinogenic activity. Indole-3-carbinol, a metabolite of glucobrassicin, has shown inhibitory effects in studies of human breast and ovarian cancers. Kale poisoning, or a severe haemolytic anaemia, was discovered in cattle in Europe in the 1930s, but its link with the hydrolytic product of S-methyl cysteine sulfoxide was only shown about 35 years later. S-methyl cysteine sulfoxide and its metabolite methyl methane thiosulfinate were shown to inhibit chemically-induced genotoxicity in mice. Thus, the cancer chemopreventive effects of Brassica vegetables that have been shown in human and animal studies may be due to the presence of both types of sulfur-containing phytochemicals (i.e. certain glucosinolates and S-methyl cysteine sulfoxide).

Many plant secondary metabolites exhibit a wide range of dose-related physiological properties when included in the diet of animals and man. Within the brassica family, the glucosinolates, a group of sulphur-containing glucosides have been the subject of a vast amount of study and the effects of dietary inclusion of such compounds or their metabolites on the health of animals and man are briefly reviewed. Although excessive amounts of glucosinolates in animal feed formulations may reduce growth and performance as well as affecting thyroid, liver, and kidney function, there is little evidence of such effects in man. On the contrary, compounds derived from glucosinolates as a consequence of enzymic breakdown or metabolism have been shown to activate mechanisms which moderate or reverse carcinogenic processes. Such effects may contribute to the net anti-carcinogenic benefit attributed to these vegetables in numerous epidemiological studies.

This paper surveys the literature concerning biological properties of rapeseed glucosinolates, chiefly the goitrogenic activity of these compounds and their influence on the morphological and histological abnormalities of internal organs in animals. An attempt has been made to establish threshold glucosinolate levels in diet which trigger the onset or increase of internal organs impairment in animals depending on their species and breeding.

Glucosinolates are present in kale but also in rapeseed meal. Rapeseed meal diets with 0.2 to 8 g (0.5-19 mmol) glucosinolates/kg were tested with a total of 197 growing pigs. The iodine supply was different. In case of lacking iodine pigs consumed less of feed with rapeseed meal. The growth was strongly lowered. Clinical hypothyroidism had occurred. In the serum T4 was not detectable. The strongly enlarged thyroid contained only little iodine. The Zn status was impaired. The serum vitamin A concentration was increased and Cu has been accumulated in the liver. Giving iodine the glucosinolates were without any effect or the effects were weakened. In case of < 2 g glucosinolates/kg diet the feed intake was not impaired. 1 g glucosinolates/kg diet had not affected thyroid weight (reference range: 80-120 mg/kg body weight). However iodine dosage was above requirement. In people an estimated daily intake of 0.1 g glucosinolates may be a health risk only under iodine deficiency conditions.

To examine the possible harmful effects of feeding Brussels sprouts to rats, groups of 10 male, weanling rats received the non-dehydrated vegetable in moist diets at levels providing 2.5-30% of the dry matter for 4 wk. A first study comprised test diets with 15 and 30% of the dry matter as uncooked or cooked Brussels sprouts and control diets without and with 0.2% potassium thiocyanate (KSCN) for comparison. The second study comprised diets with 0, 2.5, 5, 10 and 20% of the cooked vegetable and diets with 0 and 20% of the cooked vegetable with extra iodine. Diets with the uncooked vegetable contained considerably less intact glucosinolates than did diets with the cooked product, probably as a result of more extensive enzymatic degradation in the uncooked product. Growth depression and decreased food intake, not accompanied by decreased food efficiency, occurred in rats fed 10% or more dry matter as Brussels sprouts. These findings were less marked with the cooked than with the uncooked vegetable, probably because of unpalatability. Decreased levels of blood haemoglobin and plasma thyroxin were found with 15% or more Brussels sprouts. Prothrombin times were increased if 2.5% or more was fed. Thyroid stimulating hormone was increased by feeding potassium thiocyanate, but not by feeding the vegetable. Increased kidney weights and impaired kidney function not accompanied by microscopic renal changes were observed in rats fed 10% or more Brussels sprouts. Increased liver weights, which occurred from the 5% level, were accompanied by microscopic hepatic changes only at feeding levels from 10% of the cooked vegetable. 'Morphological activation' of the thyroid was increased with 10% or more of the cooked vegetable and with 0.2% KSCN. Iodine supplementation of the diets did not influence the results obtained with the vegetable. These studies indicated that 2.5% Brussels sprouts dry matter in the diet was not without effect, and that the thyroid characteristics were less sensitive to Brussels sprouts than were other criteria examined.

Glucosinolates were removed from whole rapeseed by a hot-water extraction procedure or depleted by heat treatment. When laying hens were maintained for three months on diets containing about 300 g kg-1 of these rapeseed cakes, the incidence of liver haemorrhages detected at post mortem examination was similar to that in birds maintained on 300 g kg-1 commercial rapeseed meal and significantly greater than in control birds fed soya-based diets. The effectiveness of glucosinolate extraction or depletion was determined by chemical analysis and by histological examination of the thyroid glands. Histologically the haemorrhages were similar after feeding extracted and commercial rapeseed meals. Diets containing mixtures of nitriles and glucosinolates severely depressed food intake and egg production but did not cause a greater incidence of haemorrhages than the other rapeseed products tested. Mortality from causes other than liver haemorrhage was higher with the diets containing rapeseed and this suggests that rapeseed has a more generalised effect on the body's defence mechanisms. These observations suggest that other factors in rapeseed meal, alone or acting with glucosinolates, may be responsible for inducing liver haemorrhages in laying hens.

The composition of rapeseed meal and hulls is reviewed with emphasis on components of nutritional interest. Carbohydrates in rapeseed meal are mainly pectins, pentosans and cellulose and comprise nearly one-half the gross energy. The digestibility of energy is lower for rapeseed than for soybean meal. Protein is well digested, but the protein digestion coefficient and the availability of amino acids are lower than for soybean meal. Data on digestible and metabolizable energy are summarized. The kinds of glucosinolates present in high (HG) and low (LG) glucosinolate rapeseed meal and the nature of their hydrolytic products are discussed. The toxic effects of oxazolidinethione , isothiocyanates, thiocyanates and nitriles, and their relationship to their parent glucosinolates , to myrosinase action and to conditions during rapeseed processing are reviewed briefly. The effects of glucosinolates in animal tissues and products and the effects of converting to LG rapeseed meal are presented. Finally, a brief review is given of the responses of swine and cattle to the feeding of HG and LG rapeseed meal. The nutritional advantages of LG rapeseed meal, over the older type HG meal, are clearly evident from the feeding trials.