Using isothiocyanate excretion as a biological marker of Brassica vegetable consumption in epidemiological studies: evaluating the sources of variability

Intake of total cruciferous vegetable and its contents of glucosinolates and isothiocyanates, glutathione S-transferases polymorphisms and breast cancer risk: a case-control study in China

Cruciferous vegetables contain high levels of glucosinolates (GSL) and isothiocyanates (ITC). ITC are known to induce glutathione S-transferases (GST) and thus exert their anticarcinogenic effects. This study explored the combined effects of cruciferous vegetable, GSL and ITC intake and GST polymorphisms on breast cancer risk. A total of 737 breast cancer cases and 756 controls were recruited into this case-control study. OR and 95 % CI were assessed by multivariable logistic regression. Higher cruciferous vegetable, GSL and ITC intakes were inversely associated with breast cancer risk, with adjusted OR of 0·48 (95 % CI 0·35, 0·65), 0·54 (95 % CI 0·40, 0·74) and 0·62 (95 % CI 0·45, 0·84), respectively. Compared with women carrying the GSTP1 rs1695 wild AA genotype and high cruciferous vegetable, GSL or ITC intake, carriers of the AA genotype with low cruciferous vegetable, GSL and ITC intake had greater risk of breast cancer, with adjusted OR of 1·43 (95 % CI 1·01, 1·87), 1·34 (95 % CI 1·02, 1·75) and 1·37 (95 % CI 1·05, 1·80), respectively. Persons with the GSTM1-null genotype and lower intake of cruciferous vegetables, GSL and ITC had higher risk of breast cancer than those with the GSTM1-present genotype and higher intake, with OR of 1·42 (95 % CI 1·04, 1·95), 1·43 (95 % CI 1·05, 1·96) and 1·45 (95 % CI 1·06, 1·98), respectively. Among women possessing the GSTT1-present genotype, low intake of cruciferous vegetables, GSL or ITC was associated with higher risk of breast cancer. But these interactions were non-significant. This study indicated that there were no significant interactions between cruciferous vegetable, GSL or ITC intake and GST polymorphisms on breast cancer risk.

Urinary biomarkers of dietary intake: a review

Dietary intakes are commonly assessed by established methods including food frequency questionnaires, food records, or recalls. These self-report methods have limitations impacting validity and reliability. Dietary biomarkers provide objective verification of self-reported food intakes, and represent a rapidly evolving area. This review aims to summarize the urinary biomarkers of individual foods, food groups, dietary patterns, or nutritional supplements that have been evaluated to date. Six electronic databases were searched. Included studies involved healthy populations, were published from 2000, and compared measured dietary intake with urinary markers. The initial search identified 9985 studies; of these, 616 full texts were retrieved and 109 full texts were included. Of the included studies, 67 foods and food components were studied, and 347 unique urinary biomarkers were identified. The most reliable biomarkers identified were whole grains (alkylresorcinols), soy (isoflavones), and sugar (sucrose and fructose). While numerous novel urinary biomarkers have been identified, further validation studies are warranted to verify the accuracy of self-reported intakes and utility within practice.

Glucosinolates: Molecular structure, breakdown, genetic, bioavailability, properties and healthy and adverse effects

Glucosinolates are a large group of plant secondary metabolites with nutritional effects and biologically active compounds. Glucosinolates are mainly found in cruciferous plants such as Brassicaceae family, including common edible plants such as broccoli (Brassica oleracea var. italica), cabbage (B. oleracea var. capitata f. alba), cauliflower (B. oleracea var. botrytis), rapeseed (Brassica napus), mustard (Brassica nigra), and horseradish (Armoracia rusticana). If cruciferous plants are consumed without processing, myrosinase enzyme will hydrolyze the glucosinolates to various metabolites, such as isothiocyanates, nitriles, oxazolidine-2-thiones, and indole-3-carbinols. On the other hand, when cruciferous are cooked before consumption, myrosinase is inactivated and glucosinolates could be partially absorbed in their intact form through the gastrointestinal mucosa. This review paper summarizes the glucosinolate molecular breakdown, their genetic aspects from biosynthesis to precursors, their bioavailability (assimilation, absorption, and elimination of these molecules), their sensory properties, identified healthy and adverse effects, as well as the impact of processing on their bioavailability.

Glucosinolate and isothiocyanate intakes are inversely associated with breast cancer risk: a case–control study in China

Although previous studies have investigated the association of cruciferous vegetable consumption with breast cancer risk, few studies focused on the association between bioactive components in cruciferous vegetables, glucosinolates (GSL) and isothiocyanates (ITC), and breast cancer risk. This study aimed to examine the association between consumption of cruciferous vegetables and breast cancer risk according to GSL and ITC contents in a Chinese population. A total of 1485 cases and 1506 controls were recruited into this case–control study from June 2007 to March 2017. Consumption of cruciferous vegetables was assessed using a validated FFQ. Dietary GSL and ITC were computed by using two food composition databases linking GSL and ITC contents in cruciferous vegetables with responses to the FFQ. The OR and 95 % CI were assessed by unconditional logistic regression after adjusting for the potential confounders. Significant inverse associations were found between consumption of cruciferous vegetables, GSL and ITC and breast cancer risk. The adjusted OR comparing the highest with the lowest quartile were 0·51 (95 % CI 0·41, 0·63) for cruciferous vegetables, 0·54 (95 % CI 0·44, 0·67) for GSL and 0·62 (95 % CI 0·50, 0·76) for ITC, respectively. These inverse associations were also observed in both premenopausal and postmenopausal women. Subgroup analysis by hormone receptor status found inverse associations between cruciferous vegetables, GSL and ITC and both hormone-receptor-positive or hormone-receptor-negative breast cancer. This study indicated that consumption of cruciferous vegetables, GSL and ITC was inversely associated with breast cancer risk among Chinese women.

Dietary glucosinolates and risk of type 2 diabetes in 3 prospective cohort studies

Background

Glucosinolates are a group of phytochemicals that are abundant in cruciferous vegetables and precursors of the potentially chemopreventive isothiocyanates. Isothiocyanates may reduce oxidative stress and inflammation, but little is known regarding the association between glucosinolate intake and risk of type 2 diabetes (T2D).

Objective

To evaluate the association between the intake of glucosinolates and the incidence of T2D in US men and women.

Design

This prospective cohort study investigated 200,907 women and men [71,256 women from the Nurses' Health Study (NHS; 1984-2012), 88,293 women from the NHS II (1991-2013), and 41,358 men from the Health Professionals Follow-Up Study (1986-2012)] who were free of diabetes, cardiovascular disease, and cancer at baseline. Diet was assessed using validated semiquantitative food frequency questionnaires. Self-reported T2D incidence was confirmed by a supplementary questionnaire.

Results

During follow-up in the 3 cohorts, we accumulated 4,303,750 person-years and 16,567 incident cases of T2D. After adjustment for major lifestyle and dietary risk factors for T2D, participants in the highest quintile of total glucosinolate intake had a 19% higher risk (95% CI: 13%, 25%; Ptrend < 0.001) of T2D than did those in the lowest quintile. The intake of 3 major glucosinolate subtypes was consistently and significantly associated with T2D risk, with pooled HRs ranging from 1.13 to 1.18 (all Ptrend < 0.001). A significant association was also observed between total cruciferous vegetable consumption and T2D (HR: 1.16; 95% CI :1.07, 1.25; Ptrend < 0.001). These associations persisted in subgroups defined by demographic, lifestyle, and other dietary factors.

Conclusions

Dietary glucosinolate intake was associated with a moderately higher risk of T2D in US adults. These results need to be replicated in further investigations, including biomarker-based studies. Mechanistic research is also needed to understand the relation between exposures to glucosinolates, isothiocyanates, and other metabolites with T2D risk. This trial was registered at clinicaltrials.gov as NCT03366532.

A randomized pilot trial of dietary modification for the chemoprevention of noninvasive bladder cancer: the dietary intervention in bladder cancer study

Epidemiological data suggest robust associations of high vegetable intake with decreased risks of bladder cancer incidence and mortality, but translational prevention studies have yet to be conducted. We designed and tested a novel intervention to increase vegetable intake in patients with noninvasive bladder cancer. We randomized 48 patients aged 50 to 80 years with biopsy-proven noninvasive (Ta, T1, or carcinoma in situ) urothelial cell carcinoma to telephone- and Skype-based dietary counseling or a control condition that provided print materials only. The intervention behavioral goals promoted seven daily vegetable servings, with at least two of these as cruciferous vegetables. Outcome variables were self-reported diet and plasma carotenoid and 24-hour urinary isothiocyanate (ITC) concentrations. We used two-sample t tests to assess between-group differences at 6-month follow-up. After 6 months, intervention patients had higher daily intakes of vegetable juice (P = 0.02), total vegetables (P = 0.02), and cruciferous vegetables (P = 0.07); lower daily intakes of energy (P = 0.007), fat (P = 0.002) and energy from fat (P = 0.06); and higher plasma α-carotene concentrations (P = 0.03). Self-reported cruciferous vegetable intake correlated with urinary ITC concentrations at baseline (P < 0.001) and at 6 months (P = 0.03). Although urinary ITC concentrations increased in the intervention group and decreased in the control group, these changes did not attain between-group significance (P = 0.32). In patients with noninvasive bladder cancer, our novel intervention induced diet changes associated with protective effects against bladder cancer. These data show the feasibility of implementing therapeutic dietary modifications to prevent recurrent and progressive bladder cancer.

Isothiocyanate exposure, glutathione S-transferase polymorphisms, and colorectal cancer risk

BACKGROUND

Isothiocyanates, compounds found primarily in cruciferous vegetables, have been shown in laboratory studies to possess anticarcinogenic activity. Glutathione S-transferases (GSTs) are involved in the metabolism and elimination of isothiocyanates; thus, genetic variations in these enzymes may affect in vivo bioavailability and the activity of isothiocyanates.

OBJECTIVE

The objective was to prospectively evaluate the association between urinary isothiocyanate concentrations and colorectal cancer risk as well as the potential modifying effect of GST genotypes on the association.

DESIGN

A nested case-control study of 322 cases and 1251 controls identified from the Shanghai Women's Health Study was conducted.

RESULTS

Urinary isothiocyanate concentrations were inversely associated with colorectal cancer risk; the inverse association was statistically significant or nearly significant in the GSTM1-null (P for trend = 0.04) and the GSTT1-null (P for trend = 0.07) genotype groups. The strongest inverse association was found among individuals with both the GSTM1-null and the GSTT1-null genotypes, with an adjusted odds ratio of 0.51 (95% CI: 0.27, 0.95), in a comparison of the highest with the lowest tertile of urinary isothiocyanates. No apparent associations between isothiocyanate concentration and colorectal cancer risk were found among individuals who carried either the GSTM1 or GSTT1 gene (P for interaction < 0.05).

CONCLUSION

This study suggests that isothiocyanate exposure may reduce the risk of colorectal cancer, and this protective effect may be modified by the GSTM1 and GSTT1 genes.

Iodine nutritional status of children on the island of Tanna, Republic of Vanuatu

OBJECTIVE

To evaluate the iodine nutritional status of children living on the island of Tanna, Republic of Vanuatu.

DESIGN

Cross-sectional study. Urine and household salt samples were collected for iodine measurement. Thyroid volumes were measured by ultrasound. A food consumption frequency survey was carried out, particularly in relation to salt, iodine-rich foods and foods that containing thiocyanate, a potentially goitrogenic substance. Urinary thiocyanate levels were also measured.

SETTING

Island rural communities in Tanna, Vanuatu.

SUBJECTS

One hundred and fifty-three schoolchildren between 8 and 10 years of age from four locations on the island participated.

RESULTS

The median urinary iodine excretion (UIE) among the children was 49 mg/l, indicating moderate iodine deficiency. This was corroborated by 27% of boys and 33% of girls having thyroid glands greater than the international standard for their age, and 36% of boys and 45% of girls having thyroid glands greater than the international standard for their body surface area based on ultrasonography. There was a highly statistically significant inverse correlation between thyroid volume and UIE for boys and girls (r = -0.444, P = 0.001 and r = -0.319, P = 0.005, respectively). There was no correlation between thiocyanate and UIE or thyroid volume. Only 34% of children reported to consume fish (tinned or fresh) on a weekly basis.

CONCLUSIONS

Against the common perception, the study has demonstrated that the children on the island of Tanna were in a state of moderate iodine deficiency. More data need to be collected from other Pacific Island countries in order to provide evidence for formulating public policy in prevention and control of iodine deficiency disorders in these nations.

Evaluation of the safety and bioactivity of purified and semi-purified glucoraphanin

The anti-carcinogenic effects of broccoli have been attributed to sulforaphane, the hydrolysis product of glucoraphanin (GRP). Here we determined if purified GRP, in the absence of the plant-derived hydrolyzing enzyme myrosinase, could affect pulmonary and hepatic ethoxyresorufin O-deethylase (EROD) and/or NAD(P)H-quinone oxidoreductase 1 (NQO1) activity. Male F344 rats were administered semi-synthetic, semi-purified or purified GRP (240 mg/kg: 550 micromol/kg rat daily for 4 days) by gavage. Hepatic and pulmonary NQO1 activity increased ( approximately 20%), but not EROD. Varying doses of semi-purified GRP (30, 60, or 120 mg/kg rat daily for 4 days) again caused no change in EROD activity, although a dose-dependent increase in NQO1 was seen. Urinary excretion of mercapturic acids showed no difference between preparations, and recovery increased with decreasing dose. Histopathologic examination revealed no abnormal tissues other than cecum, where inflammation was dose dependent; mild at 120 mg/kg and severe at 240 mg/kg, a greatly supra-physiological dose. We conclude that GRP 30-60 mg/kg p.o. is safe and effectively enhances NQO1 in all tissues evaluated.

Cruciferous vegetable intake questionnaire improves cruciferous vegetable intake estimates

OBJECTIVE

To develop a validated, focused Cruciferous Vegetable Food Frequency Questionnaire (FFQ) as an assessment tool for specific quantification of dietary cruciferous vegetable exposure.

DESIGN/METHODS

Participants (n=107; 18 to 76 years old) completed a standard FFQ and the Cruciferous Vegetable FFQ twice over a 2-week period. Repeat dietary recalls were collected on 3 days over the same 2-week period. Urinary dithiocarbamate was determined as a biomarker of cruciferous vegetable intake.

STATISTICAL ANALYSES

Descriptive statistics of intake; paired t tests and sign tests for comparison of intake estimates between instruments; Spearman correlations to assess reliability and associations between diet instruments and urinary dithiocarbamate.

RESULTS

Cruciferous vegetable intake was significantly correlated between the two FFQs (r(s)=0.58), although the Cruciferous Vegetable FFQ estimated intake 35 g higher than the standard FFQ. The Cruciferous Vegetable FFQ was reliable, with a repeated measures correlation of 0.69 (P</=0.01). Urinary dithiocarbamate excretion correlated with cruciferous vegetable intake from the Cruciferous Vegetable FFQ (r(s)=0.26, P<0.01), and from the standard FFQ (r(s)=0.19, P=0.06).

CONCLUSIONS

The Cruciferous Vegetable FFQ provided a reproducible, valid estimate of cruciferous vegetable exposure and improved the relationship between crucifer consumption and urinary dithiocarbamate, a biomarker of cruciferous vegetable exposure. The nearly twofold difference in exposure estimates between the Cruciferous Vegetable FFQ and the standard FFQ could change the statistical significance of risk estimates in the context of epidemiological research. This questionnaire is an appropriate research tool to evaluate cruciferous vegetable intake more accurately than a standard FFQ, particularly in the context of dietary intervention studies that promote increased vegetable intake to reduce the risk for chronic disease.

GSTM1, GSTT1, GSTP1, and GSTA1 polymorphisms and urinary isothiocyanate metabolites following broccoli consumption in humans

Isothiocyanates (ITC) are potentially anticarcinogenic phytochemicals formed from the metabolism of glucosinolates and are found in cruciferous vegetables as well as a select number of other foods. ITC are both substrates for and inducers of glutathione S-transferase (GST) phase II metabolizing enzymes involved in carcinogen detoxification as well as effectors of phase I pathways. Previous studies report mixed results on the interaction between cruciferous vegetable intake, GST polymorphisms, and risk of cancer. We conducted a study of 114 healthy human subjects between 18 and 50 y of age to examine the biologic mechanism underlying the associations, specifically, to assess whether GST genotype is associated with urinary ITC metabolites following a known dose of broccoli. After 48 h of abstaining from all sources of glucosinolates, participants provided a blood sample, consumed 1 meal containing 2.5 g broccoli/kg body weight, and collected urine for 24 h. ITC metabolites were measured in the urine using a HPLC cyclocondensation assay. DNA was extracted from blood samples, and GSTM1 deletion, GSTT1 deletion, GSTP1 Ile105Val, and GSTA1*A/*B were genotyped by matrix-assisted laser desorption/ionization time-of-flight. A chi-square test was used to compare high and low ITC excretion levels across genotypes. ITC levels were regressed on genotype, adjusting for gender. There were no substantial differences in ITC levels among genotypes, either individually or in combination. Contrary to our hypothesis, a higher proportion of GSTM1 null individuals had high ITC excretion (62%) compared with the proportion of GSTM1 present with high ITC excretion (39%) (P = 0.03). These results are in agreement with another feeding study, and lend support to the idea of alternative routes of ITC metabolism.

Induction of quinone reductase by allylisothiocyanate (AITC) and the N-acetylcysteine conjugate of AITC in Hepa1c1c7 mouse hepatoma cells

Cruciferous vegetables contain a series of relatively unique secondary metabolites of amino acids, called glucosinolates, from which isothiocyanates (ITC) can be generated. While glucosinolates are not thought to be bioactive directly, ITC appear to have anticarcinogenic activity. Sinigrin, the predominant aliphatic glucosinolate in cruciferous vegetables, is hydrolyzed to yield allylisothiocyanate (AITC), which, following absorption and metabolism in humans, is excreted in the urine as an N-acetyl-cysteine (NAC) conjugate. AITC possesses numerous biochemical and physiological activities. This study examined the induction of quinine reductase (QR) by AITC and synthetic AITC-NAC in Hepa1c1c7 murine hepatoma cells. AITC and AITC-NAC inhibited cell growth in a dose-dependent manner. The induction of QR activity and QR mRNA expression was dose-responsive over a range of 0.1-2.5 microM. AITC caused 2.0- and 3.1-fold inductions of QR with 1- and 2-microM treatments, respectively. By comparison, 1 and 2 microM AITC-NAC caused 2.9- and 3.7-fold inductions of QR, respectively. Considering the potential of ITC to prevent cancer, these results provide a basis for the use of NAC-ITC conjugates as chemopreventive agents.

Isothiocyanates in cancer prevention

Isothiocyanates are sulfur-containing compounds which are largely responsible for the typical flavor of cruciferous vegetables. In animals and humans they are conjugated with glutathione; the first product of this reaction is a dithiocarbamate, which can be ultimately metabolized to the corresponding mercapturic acid, excreted in urine. Aliphatic and aromatic isothiocyanates, as well as dithiocarbamates, can react quantitatively with an excess of 1,2-benzenedithiol giving rise to a cyclic condensation product readily measured in biological fluids. The use of this assay as a biomarker of isothiocyanate or cruciferous vegetable intake can offer several advantages over food frequency questionnaires, mainly in the objectivity and accuracy of the measure and the lack of bias. Epidemiological studies on the relationship between cancer risk with isothiocyanate intake or excretion have shown inverse associations, mainly with lung cancer. The studies are also consistent in reporting a gene-environment interaction, with a stronger protective effect in persons null for the GSTM1 or GSTT1 genotype. However, further research is needed to better clarify the importance of these polymorphisms in modulating the effect of isothiocyanates in cancer development.

Evaluation of urinary N-acetyl cysteinyl allyl isothiocyanate as a biomarker for intake and bioactivity of Brussels sprouts

Isothiocyanates (ITC), glucosinolate hydrolysis products from Brussels sprouts (BS) and other cruciferous vegetables, are considered to protect the body from cancer by induction of detoxification enzymes such as quinone reductase (QR). Urinary N-acetyl-cysteine (NAC) conjugates of ITC have been proposed as biomarkers of crucifer intake. Here we asked if dietary intake and induction of detoxification enzymes are dose-related to urinary NAC conjugate appearance. Male F344 rats (4/group) received an AIN 76B-40 diet containing 0, 10 or 20% freeze-dried BS for 6 days. A human subject ingested 500 g BS. Urinary AITC-NAC was identified in human and rat urine. Ten and 20% BS diets caused a 1.4- and 2.3-fold induction of QR in the pancreas, a 1.5- and 2.5-fold induction in liver and a 3.1- and 3.6-fold induction in colonic epithelium, respectively. Liver and pancreatic QR induction was dose-related, whereas induction of QR in colon was less different between the two doses. Excretion of the conjugate was dose-related only on day 1, and unrelated to dose after day 2. These results suggest that urinary NAC-AITC is a qualitative biomarker for ingestion and bioactivity of BS, but that it may not be dose-related when rats are fed continuously for 2 or more days.

Food matrix effects on bioactivity of broccoli-derived sulforaphane in liver and colon of F344 rats

Sulforaphane (SF) is considered to be the major anticarcinogenic component in broccoli. The effects of feeding rats purified SF (5 mmol/kg of diet), broccoli containing SF formed in situ during laboratory hydrolysis (broccoli-HP; 20% freeze-dried broccoli diet, 0.16 mmol of SF/kg of diet), and broccoli containing intact glucosinolates (broccoli-GS; 20% freeze-dried broccoli diet, 2.2 mmol of glucoraphanin/kg of diet) were compared. Rats (male F344 rats, five per group) were fed control (modified AIN-76 B-40), SF, broccoli-HP, or broccoli-GS for 5 days. In rats fed broccoli-GS, quinone reductase activities (QR) in the colon and liver were greater (4.5- and 1.4-fold over control, respectively) than in rats fed broccoli-HP (3.2- and 1.1-fold over control, respectively). Broccoli-GS and SF diets increased QR to the same extent, even though the broccoli-GS diet contained far less SF (as the unhydrolyzed glucosinolate, glucoraphanin) than the purified SF diet. In a second experiment, rats were fed one of six diets for 5 days: (1) control; (2) 20% broccoli-GS; (3) diet 2 + low SF (0.16 mmol/kg of diet); (4) diet 2 + high SF (5 mmol/kg of diet); (5) low SF (0.32 mmol/kg of diet); or (6) high SF (5.16 mmol/kg of diet). In both liver and colon, QR was increased most by high SF plus broccoli-GS; individually, high SF and broccoli-GS had similar effects, and adding the low-dose SF to broccoli-GS had either no effect or a negative effect. In both experiments, urinary SF-mercapturic acid correlated with QR activity, not with dietary intake. It was concluded that all diets were substantially more effective in the colon than in the liver and that broccoli-GS was more potent than SF or broccoli-HP.

Urinary excretion of dithiocarbamates and self-reported Cruciferous vegetable intake: application of the 'method of triads' to a food-specific biomarker

OBJECTIVE

Greater intake of Cruciferous vegetables (e.g. broccoli) may prevent cancer at several sites. Urinary excretion of isothiocyanate conjugates (dithiocarbamates, DTC) provides a specific biomarker of Cruciferous vegetable consumption suitable for epidemiological investigations. However, no gold-standard referent is available for evaluating urinary DTC levels as an estimator of Cruciferous vegetable consumption. We compared urinary DTC levels to intake as measured by two self-reported dietary assessment techniques.

DESIGN

Cruciferous vegetable consumption was measured before and after a behavioural dietary intervention using multiple 24-hour recalls (24HR), a food-counting questionnaire (VFQ) and urinary DTC excretion levels. Analysis included a structural equation approach (Method of Triads) combining these three assessment techniques to estimate the relationship between DTC level and the study population's 'true' Cruciferous vegetable intake.

SETTING

The intervention curriculum assisted participants in consuming about 2 servings per day for a 6-week period. Participants attended four classes emphasising problem-solving skills, dietary counselling and vegetable preparation skills. There were no dietary restrictions.

SUBJECTS

Thirty-three healthy, free-living, post-menopausal women.

RESULTS

Although few participants reported Cruciferae consumption prior to the intervention, 30 participants reported Cruciferae consumption after the intervention (Post-intervention). Urinary DTC levels were correlated with estimated intake derived from either the 24HR (r = 0.57; 95% confidence interval (95% CI) 0.28, 0.76) or VFQ (r =0.49; 95% CI 0.17, 0.71). The validity coefficient (Method of Triads) between urinary DTC excretion and an index of true Cruciferous intake was stronger than the Pearson correlation (r(v) = 0.65; 95% CI 0.35, 0.90), and comparable to estimates derived from the 24HR (r(v) = 0.82; 95% CI 0.65, 1.00) or VFQ (r(v) = 0.76; 95% CI 0.47, 0.92) method. These associations were not affected by adjustment for body mass index, energy intake, or social approval or desirability response sets.

CONCLUSIONS

Food-frequency questionnaires (FFQ) suitable for large epidemiological studies may not be designed to measure all Cruciferae, and cannot capture exposure to phytochemicals derived from those vegetables. Urinary DTC measurement was significantly correlated with Cruciferae intake derived from two dietary assessment approaches, and urinary DTC levels could supplement traditional FFQ data by providing an index of recent Cruciferous vegetable intake not susceptible to reporting biases.

Influence of temperature and ontogeny on the levels of glucosinolates in broccoli (Brassica oleracea Var. italica) sprouts and their effect on the induction of mammalian phase 2 enzymes

Broccoli inflorescences have been recognized as components of healthy diets on the basis of their high content of fiber, vitamin C, carotenoids, and glucosinolates/isothiocyanates. Broccoli sprouts have been recently shown to have high levels of glucoraphanin (4-methylsulfinylbutyl glucosinolate), the precursor of the chemoprotective isothiocyanate, sulforaphane. This study evaluated the effects of temperature and developmental stage on the glucosinolate content of broccoli sprouts. Seedlings cultivated using a 30/15 degrees C (day/night) temperature regime had significantly higher glucosinolate levels (measured at six consecutive days postemergence) than did sprouts cultivated at lower temperatures (22/15 and 18/12 degrees C; p < 0.001). Both higher (33.1 degrees C) and lower (11.3 degrees C) constant temperatures induced higher glucosinolate levels in sprouts grown to a uniform size. Glucosinolate levels were highest in cotyledons and lowest in roots of sprouts dissected both early and late in the 11 day developmental span investigated. Nongerminated seeds have the highest glucosinolate levels and concordantly greater induction of mammalian phase 2 detoxication enzymes. Levels decline as sprouts germinate and develop, with consistently higher glucosinolate content in younger developmental stages, independent of the temperature regime. Temperature stress or its associated developmental anomalies induce higher glucosinolate levels, specific elevations in glucoraphanin content, and parallel induction of phase 2 chemoprotective enzymes.

Quantitative determination of dithiocarbamates in human plasma, serum, erythrocytes and urine: pharmacokinetics of broccoli sprout isothiocyanates in humans

BACKGROUND

Humans are exposed to substantial quantities of isothiocyanates and glucosinolates from vegetables. Since dietary isothiocyanates are widely regarded as potentially important chemoprotectors against cancer, reliable methods for measuring the plasma and tissue pharmacokinetics of isothiocyanates and their dithiocarbamate metabolites are essential for defining dosing regimens.

METHODS

Isothiocyanates (ITC) and dithiocarbamates (DTC) react quantitatively with 1,2-benzenedithiol to produce 1,3-benzodithiole-2-thione that can be quantified spectroscopically. Although this cyclocondensation reaction has been highly useful for analyzing plant material and urine samples, the determination of DTC/ITC (the total quantity of DTC and ITC components in a sample that react in the cyclocondensation reaction) in blood and tissues has been hampered by their low levels and the high concentrations of proteins that interfere with the cyclocondensation reaction. The protein content of blood and tissues was reduced by the precipitation with polyethylene glycol (PEG) or ultrafiltration, and the sensitivity of the method was increased substantially by the solid phase extraction of the cyclocondensation product.

RESULTS

Pharmacokinetic measurements were made in four human volunteers who received single doses of about 200 micromol of broccoli sprout isothiocyanates (largely sulforaphane, with lesser amounts of iberin and erucin). Isothiocyanates were absorbed rapidly, reached peak concentrations of 0.943-2.27 micromol/l in plasma, serum and erythrocytes at 1 h after feeding and declined with first-order kinetics (half-life of 1.77+/-0.13 h). The cumulative excretion at 8 h was 58.3+/-2.8% of the dose. Clearance was 369+/-53 ml/min, indicating active renal tubular secretion.

CONCLUSION

A sensitive and specific method for quantifying DTC levels in human plasma, serum, and erythrocytes has been devised. Determinations of ITC/DTC levels are important because: (i) dietary isothiocyanates are of potential value in reducing the risk of cancer, and (ii) humans are extensively exposed to DTC as fungicides, insecticides, pesticides and rubber vulcanization accelerators.

Phytochemicals from cruciferous plants protect against cancer by modulating carcinogen metabolism

Several epidemiologic studies suggest that consumption of cruciferous vegetables may be particularly effective (compared with total fruit and vegetable consumption) in reducing cancer risk at several organ sites. Crucifers that are widely consumed are especially rich in glucosinolates, which are converted by plant myrosinase and gastrointestinal microflora to isothiocyanates. A number of isothiocyanates and a limited number of glucosinolates that were examined effectively block chemical carcinogenesis in animal models. Many isothiocyanates are also potent inducers of phase 2 proteins. Substantial evidence supports the view that phase 2 enzyme induction is a highly effective strategy for reducing susceptibility to carcinogens. This conclusion has recently received strong molecular support from experiments on mice in which the specific transcription factor, nrf2, which is essential for induction of phase 2 proteins, was deleted. In these knock-out mice, the basal levels of phase 2 enzymes are very low and not inducible. Accordingly, these mice are much more susceptible than their wild-type counterparts to benzo[a]pyrene forestomach carcinogenesis and are not protected by phase 2 inducers. These experiments provide very strong evidence for a major role of phase 2 enzymes in controlling the risk of exposure to carcinogens. An increasing number of phase 2 proteins that exert a variety of protective mechanisms are being identified. Thus, in addition to detoxifying electrophiles, these proteins exercise versatile, long-lasting and catalytic antioxidant protection.

The chemical diversity and distribution of glucosinolates and isothiocyanates among plants

Glucosinolates (beta-thioglucoside-N-hydroxysulfates), the precursors of isothiocyanates, are present in sixteen families of dicotyledonous angiosperms including a large number of edible species. At least 120 different glucosinolates have been identified in these plants, although closely related taxonomic groups typically contain only a small number of such compounds. Glucosinolates and/or their breakdown products have long been known for their fungicidal, bacteriocidal, nematocidal and allelopathic properties and have recently attracted intense research interest because of their cancer chemoprotective attributes. Numerous reviews have addressed the occurrence of glucosinolates in vegetables, primarily the family Brassicaceae (syn. Cruciferae; including Brassica spp and Raphanus spp). The major focus of much previous research has been on the negative aspects of these compounds because of the prevalence of certain "antinutritional" or goitrogenic glucosinolates in the protein-rich defatted meal from widely grown oilseed crops and in some domesticated vegetable crops. There is, however, an opposite and positive side of this picture represented by the therapeutic and prophylactic properties of other "nutritional" or "functional" glucosinolates. This review addresses the complex array of these biologically active and chemically diverse compounds many of which have been identified during the past three decades in other families. In addition to the Brassica vegetables, these glucosinolates have been found in hundreds of species, many of which are edible or could provide substantial quantities of glucosinolates for isolation, for biological evaluation, and potential application as chemoprotective or other dietary or pharmacological agents.