Glucosinolates from Brassica vegetables:risks and benefits

Gary WilliamsonProcter Department of Food Science

University of LeedsUK

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Glucosinolates

Glucosinolates occur in Cruciferous plants.

On damage to the plant, including food processing,chewing, cooking, etc., the glucosinolates breakdowninto a variety of highly bioactive products.

The breakdown products are isothiocyanates, nitriles,epithionitriles and thiocyanates.

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Glucosinolate breakdown

O

O H

O H

O H

C H 2 O H

S

N

O S O 3-

S

O

sugarphytochemicalglucosinolates e.g.glucoraphanin

SN

O

C S

sulforaphane

myrosinaseMyrosinase in the plant or colon bacteria

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Compound Source Flavour/taste, quality

2-Propenyl isothiocyanate Horseradish,mustard

Pungency, lachrymatory, bitter

2-Propenyl thiocyanate Horseradish Horseradish, garlic-like, lacking inpungencyandlachrymatoryquality

3-Butenyl isothiocyanate Brownmustard Pungency, aroma4-Pentenyl isothiocyanate Horseradish Acrid, fragrant leaf3-Methylpropyl isothiocyanate Horseradish Acrid, leaf greenaroma2-Phenylethyl isothiocyanate Horseradish,

watercress, turnipStrongaromaof watercress, tinglingsensation

2-Phenylethylnitrile Watercress Veryweakwatercress, almost odourlessPungency, noaroma

p-Hydroxybenzyl isothiocyanate Radish Pungency, sulfuryaroma4-Methylthiobutyl isothiocyanate Rutabaga Radish-likeBenzes thiocyanate Cress Garlic-like5-vinyloxazolidine-2-thione Cabbage,

BrusselssproutsBitter

Methyl isothiocyanate Horseradish Powerful lachrymator, horseradish-likeEthyl isothiocyanate Extremelypungent, garlic-likeiso-Propyl isothiocyanate Pungent

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Several decades ago:

Glucosinolate breakdown products were considered onlyas natural toxicants.

Oil seed rape contained a high level of progoitrin, aglucosinolate which exhibited some toxic propertieswhen given in high amounts to farm animals.

This led to the development of varieties of oil seed rapelow in glucosinolates (and also erucic acid).

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Isothiocyanate-protein interactions

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50mg benzyl isothiocyanate/g protein reducedprotein intake and weight gain in growing rats

Protein intake and growth response of rats fed for 9 days withegg white proteins and its BITC derivatives (50mg/g protein)

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Toxicities of various dietary compounds

LD50 NOAEL

(mg/kg rat) (mg/kg/day)

allylisothiocyanate 112

allyl nitrile 115

phenethylisothiocyanate 700

quercetin 159

vitamin D 42

vitamin C 11900

vitamin K 500

proanthocyanidins >5000 1400

green tea flavanols 1000

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Toxicity at high doses of pure compounds

Thyroid gland, liver, kidney, and pancreas are the main targets.

In rats, toxicity at daily doses > 10 - 50 mg/kg body weight. At suchhigh concentrations, certain isothiocyanates and nitriles mayinitiate mutagenic, cytotoxic, and carcinogenic processes.

Promoter post-initiation effects of phenethyl isothiocyanate andbutyl isothiocyanate on liver and bladder carcinogenesis in ratspre-treated with nitrosamines.

Thyroid-toxicity in animal experiments: certain isothiocyanatesinterfere with the synthesis of thyroid hormones, whereasthiocyanates compete with iodine and inhibit iodine uptake by thethyroid gland.

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However…

The situation changed in 1992 after the identification andpurification of sulforaphane (from glucoraphanin), whichshowed anticarcinogenic properties.

Substantial research effort has now confirmed this, atleast in vitro and in animal models.

This highlights the dual nature of many naturallyoccurring compounds in plants, but the concept is notnew to nutrition.

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Beneficial effect of Cruciferous vegetables:

Isothiocyanates are potent inducers of Phase IIdetoxification enzymes, and increase the metabolismand detoxification of chemical carcinogens in vitro and inanimal models.

Some inhibit mitosis and stimulate apoptosis in tumourcells by blocking DNA damage, thus inhibiting thegrowth of tumour cells after initiation by chemicalcarcinogens.

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PhIP N-hydroxyPhIP

PhIP-DNA adducts

CYP1A2

Phase I activation

Some flavonoids are potentinhibitors e.g galangininhibits P4501A2 withKi = 8 nM; (Zhai et al 1998)

anticarcinogenesisby phytochemicals

antimitoticeffect

induction ofapoptosis

induction ofapoptosis

suppressionof angiogenesis

induction of detoxifyingand suppression ofactivating enzymes

DNA Hypermethylation

apc K-ras p53

N-hydroxy-PhIP-GSHGST a

Phase II detoxification

Detoxification rateincreased by inductionof phase II enzymes

normalcell

faster cellturnover

smalladenoma

largeadenoma

carcinomain situ

cancer metastasis

Synergistic effects may occur with other compounds ofthe diet at all stages of carcinogenesis

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14C-PhIP DNA adducts formed in human cultures HepG2cells after 24h treatment with PhIP– modification of doseresponse by sulforaphane or quercetin

*not significantly different to background

10 μM SFN

20 μM Quercetin

Bacon et al 2003 Carcinogenesis 24, 1903-1911

0.00

0.05

0.10

0.15

0.20

0.25

C SFN Q

100 pM PhIP

ad

du

cts

/10

9b

ase

s

**

0.00

0.05

0.10

0.15

0.20

0.25

C SFN Q

100 pM PhIP

ad

du

cts

/10

9b

ase

s

**

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Epidemiology: most studies support a protectiveeffects of Brassicas

6 cohort studies: inverse associations between brassicaconsumption and risk of lung cancer, stomach cancer, allcancers taken together.

64% of 74 case-control studies: inverse associationbetween consumption of one or more brassica vegetablesand risk of cancer at various sites.

Comparison of the diets of 2,832 postmenopausal womendiagnosed with invasive breast cancer, to the diet of 2,650healthy women of the same age. Result: no correlationbetween total fruit and vegetable consumption and breastcancer risk but women consuming 1 to 2 servings ofBrassica vegetables daily had a 20 to 40 % decreased riskof breast cancer

‘Health Professionals’ study followed over 47,000 men for6.3 years and compiled food intake data for 8 years.Brassica vegetable consumption was related to a 51 %reduction in the risk of bladder cancer

No effect ofbrassicaconsumption

e.g.

'pooled analysis ofcohort studies'conducted bySmith-Warner etal.

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U shaped curve

The U shaped dose-response curve is now a commonlyaccepted concept in nutrition and toxicity

Paracelsus said, more than 400 years ago, that the dosemakes the poison.

This implies that all substances which are biologicallyactive – whether from food, drugs or other chemicals –can have possible toxicity, but this depends on the dose.

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U shaped curve

This applies equally to vitamins, minerals, and naturallyoccurring plant products.

The high amounts are generally not achievable throughnormal dietary means such as food, but can only beobtained from heavily fortified foods or “mega-dose”supplements.

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Dose-response – the key!

dose

broadoptimum

Adverseresponse

Best effect on health at optimum levels

May 08 Brussels

Dose-response – the key!

dose

broadoptimum

Adverseresponse

Best effect on health at optimum levels

Higher risk ofchronic disease at“deficiency level”

Higher risk of chronic disease at“excess intake” – in most cases not

possible from food intake alone

May 08 Brussels

Dose-response – the key!

dose

broadoptimum

Adverseresponse

Best effect on health at optimum levels

Normal dietarylevels for most

of thepopulation

Higher risk ofchronic disease at“deficiency level”

Higher risk of chronic disease at“excess intake” – in most cases not

possible from food intake alone

May 08 Brussels

Dose-response – the key!

dose

broadoptimum

Adverseresponse

Best effect on health at optimum levels

Higher risk ofchronic disease at“deficiency level”

Higher risk of chronic disease at“excess intake” – in most cases not

possible from food intake alone

Desired dietarylevels for besthealth benefit

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0

10

20

30

40

50

60

70

80

90

choline folate niacin vitamin A B6 C D E

rati

ob

etw

een

UL

an

dR

DA

brooptbroopt

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Intake of glucosinolates

Daily intake of two glucosinolates, glucobrassicin andneoglucobrassicin, was 5.0 and 0.5 mg/capita/day in the Danishand 2.5 and 0.3 mg/day in the Finnish population respectively

These average values might be very different in individuals, wholike or dislike Brassicas.

In Potsdam, Germany, the average daily cabbage consumptionwas 54 g/capita/day, mainly white cabbage, cauliflower and redcabbage; consumption increased with age.

The intake was slightly higher in the winter compared to thesummer.

The uptake of progoitrin was relatively low with only 3 and 2mg/capita/day in winter and summer, respectively.

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Conclusions

Brassica vegetables in the diet are not toxic or dangerous in anyway: in fact, increased consumption is desirable

Supplements containing purified compounds, derived fromBrassicas, should be viewed with caution

Content-enhanced functional foods may be a way of increasingprotective effects from Brassica vegetables, but should be testedfor safety

Better ways of processing could be beneficial to optimise theoriginal and natural content of processed foods