快猫短视频

IF turkeys do not relish becoming part of the festive feast, neither do
Brussels sprouts. You know that slightly bitter, sulphurous taste sprouts have,
which some people so dislike? Well, it is meant to discourage would-be diners.
This is vegetable chemical warfare. Yet, mystery of mysteries, the bitter stuff
is not toxic to humans. In fact, it turns out to be just the thing for us.

It鈥檚 no secret, of course, that sprouts and their close relatives are among
the most nutritious of leafy vegetables, rich in minerals, fibre, protein,
carotene and vitamin C. What鈥檚 new is the discovery that these plants, and lots
of other fruits and vegetables besides, contain a range of 鈥渘on-nutrient鈥�
compounds that seem to protect against a wide range of cancers, including
breast, lung and colon cancer.

A few Christmases from now, you may find varieties of Brussels sprouts and
broccoli on supermarket shelves being promoted for guaranteed high levels of
these beneficial compounds. Genetic engineers may even be able to introduce the
genes for these anticancer compounds into other plants. Sprouts could be swept
along with the trend that is giving the label 鈥渉ealth food鈥� a whole new meaning
(see 鈥淎 plateful of medicine鈥�, 快猫短视频, 2 November, p 12).

The health-promoting qualities seem to reside in hundreds of 鈥渟econdary鈥�
compounds, which have no vital role in the essential, or primary metabolism that
keeps the plant alive. Many secondary compounds are toxic, and form part of a
plant鈥檚 defences against voracious herbivores. Sprouts and broccoli are rich in
one family of secondary compounds, called glucosinolates. At the Institute of
Food Research in Norwich, nutritional physiologist Ian Johnson is focusing on a
glucosinolate called sinigrin, which is found at high levels in Brussels
sprouts.

The good news, according to Johnson and his colleagues, is that sinigrin can
suppress the development of precancerous cells. These are cells that have
somehow become damaged, and so may eventually develop into full-blown tumours.
Johnson鈥檚 idea was to see if a dose of sinigrin could protect laboratory rats
from cancers of the colon. Sure enough, six weeks after giving just one dose of
sinigrin they found that many precancerous cells had been destroyed. 鈥淚t is a
new finding that anything in the diet can do this in the gut,鈥� says Johnson.

The protective chemical seems to be a breakdown product of sinigrin, called
allyl isothiocyanate. It is a volatile molecule, and is largely responsible for
the smell and taste of sprouts. Johnson and his colleagues have found that allyl
isothiocyanate can trigger damaged, precancerous gut cells to 鈥渃ommit
suicide鈥濃�攁 natural process called apoptosis (see 鈥淟ife and death in the
condemned cell鈥�, 快猫短视频, 25 January 1992, p 34). The compound鈥檚
effect is so powerful that Johnson suspects even an occasional dose of sprouts
might suffice to knock out potentially cancerous cells in the colon.

But there鈥檚 a twist to this tale. Some varieties of sprouts contain much more
sinigrin than others, and the more they contain the more bitter they taste. Yet
supermarkets are offering us sprouts with milder and milder flavours. A December
press release from Marks & Spencer, for example, announces that the sprouts
in its stores 鈥渉ave undergone a revolution in eating quality and flavour in
recent years鈥�. Gone is the bitter taste, claims Andrew Sharp, M&S鈥檚
vegetable technologist. New varieties have 鈥渁 sweet nutty flavour鈥�.

There is a dilemma here. Breed low levels of sinigrin into the plants to
produce the popular, milder-tasting sprouts and you risk reducing the health
benefits and perhaps make the plants more vulnerable to certain pests as well.
There is an upper limit too: if sinigrin concentrations go above 200 milligrams
per 100 grams of fresh vegetable, the sprouts become so bitter that no one will
eat them. 鈥淲e aim to work with plant breeders to optimise all three
factors鈥攈ealth, flavour and resistance to pests,鈥� says Johnson, who hopes
this new, integrated thinking will head off the supermarkets鈥� search for ever
sweeter sprouts.

Why bother to eat sprouts at all? Why not just take tablets of sinigrin? It鈥檚
a possibility, admits Johnson, though he suspects it will not do as much good as
the real thing. 鈥淚t鈥檚 the cocktail of compounds in a wide variety of real
vegetables that is probably so beneficial,鈥� he says. Eat a diet rich in a
diverse range of vegetables, and you gain a goodly measure of protection against
many different types of cancer. 鈥淭here are hundreds of compounds, working
through a whole range of mechanisms that we still hardly understand,鈥� he
stresses. 鈥淲hat all this research is telling us is that vegetables are essential
to health.鈥� (See 鈥淓at up those greens, they鈥檙e good for you鈥�.)

Good for your liver

Gary Williamson, a colleague of Johnson鈥檚 at the Institute of Food Research,
is hot on the heels of another glucosinolate. This one, called glucoraphanin, is
present at high levels in broccoli. It breaks down into an isothiocyanate called
sulphoraphane that also has powerful anticancer effects. But unlike sinigrin, it
works by blocking cancer rather than suppressing it: that is, by neutralising
cancer-causing substances or stopping them forming, so that no damage is
done.

Research on sulphoraphane continues, but the results to date are striking.
Add the compound to human cells in tissue culture and you stimulate production
of liver enzymes that destroy cancer-causing chemicals. Give it to laboratory
animals, and you can discourage the formation of liver and colon cancer.
Sulphoraphane seems to fight cancer by stimulating the part of the body鈥檚 own
detoxifying system known as phase II enzymes. A diet rich in broccoli (and other
brassicas) actually seems to help to keep our chemical detoxifying systems in
good working order. Williamson suspects that such a diet may even boost enzyme
activity in people with low levels of these protective enzymes.

Already, there are signs that a vegetable-rich diet can improve our chemical
defence systems remarkably quickly. In a recent study, researchers at the Dutch
Nutrition and Food Research Institute in Zeist recruited volunteers to eat 300
grams of cooked Brussels sprouts a day for a week, and then eat a 鈥渘ormal鈥�,
sprout-free diet. While feasting on the sprouts, which also contain
sulphoraphane, levels of phase II enzymes in the stomach, liver, intestine and
kidney increased by 50 per cent. 鈥淭he enhancement of such detoxifying enzyme
systems could potentially increase the capacity to withstand the burden of
toxicants and (pre)carcinogens we are exposed to daily,鈥� the researchers
conclude.

So is it broccoli and sprouts with everything? Like Johnson, Williamson
stresses that variety is the thing. 鈥淚 don鈥檛 believe there is one magic
compound. Just increasing glucosinolates in your diet wouldn鈥檛 do you that much
good,鈥� he says. 鈥淏ut I now eat a lot more fruit and veg than I used to, and try
to eat a portion of brassica vegetable each day.鈥�

Take one wild cabbage . . .

Williamson鈥檚 colleagues, geneticists Richard Mithen and Kathy Faulkner at the
John Innes Centre in Norwich, have gone one better: they grow their own
broccoli, in a quest for improved commercial strains. 鈥淧eople tend to like
broccoli, and it has a very mild taste. So we are trying to breed plants with
increased levels of the beneficial glucosinolate.鈥� Luckily, sulphoraphane does
not have the strong, unpleasant taste of its counterpart in sprouts, so there is
no need to worry about generating a variety that is healthy but unpalatable.

Broccoli evolved from wild cabbages, which Mithen hopes to recruit into his
breeding programme. Some have extremely high levels of glucoraphanin鈥攗p to
ten times as much as the typical levels in cultivated broccoli. 鈥淭he aim is to
end up with enhanced levels in the broccoli, so it should do us more good if we
eat it,鈥� he says.

In four to five years, he predicts, they will have a new commercial variety
of broccoli with extra-healthy qualities. 鈥淎bout 95 per cent of the broccoli
grown commercially in Britain is Marathon variety, which is in the middle range
of anticancer levels.鈥� This is similar to levels in two other cultivars Mercedes
and Emperor, grown by amateur gardeners, which have about 60 to 70 micromoles of
glucoraphanin per gram of plant. By contrast, another cultivar, Green Comet, has
less than 10 micromoles per gram, while Trixie has the highest levels Mithen and
his colleagues have yet measured in any commercially available cultivar鈥攁n
impressive 150 micromoles per gram. If supermarkets so desired, they could
request a variety at the high end of the range and so oblige growers to switch
to more health-giving cultivars.

Mithen is also cloning the genes that produce the desired glucosinolate,
using thale cress, Arabidopsis thaliana. This small, easy-to-breed
species has a tiny genome, and for geneticists it has become the plant
equivalent of the fruit fly Drosophila. He and his team started out
ahead of the game: they were already working on the genetics of these compounds
in 1991, several years before there was any hint of their anticancer role. 鈥淲e
were studying these compounds as part of our basic research into the chemical
interactions between plants and their pests,鈥� says Mithen. 鈥淭here鈥檚 a very
useful moral here, about the importance of fundamental research.鈥�

The aim now is to introduce the genes that broccoli uses to make its
glucosinolate into other vegetables, such as Chinese cabbage, salad rocket and
turnips. 鈥淭hey should improve the flavour and help to give these crops
anticancer activity too,鈥� he says.

There鈥檚 a deep irony in all this: these plant compounds have until recently
been regarded with suspicion by some of the very researchers who now celebrate
them. 鈥淟ots of them are very toxic,鈥� says Johnson. After all, they are natural
pesticides, meant to discourage predators. So how is it that we cannot only eat
them with impunity, but actually benefit from the experience?

The answer must lie in our evolutionary past, researchers suspect. Thanks to
our long history as omnivores, consuming a panoply of fruits and vegetables, we
find ourselves today biologically adapted to an intake of plant toxins鈥攕o
well adapted that, paradoxically, our health suffers if we are deprived of a
steady supply of the plants鈥� chemical weapons. So eat up your gently steamed
sprouts, the more the better. And give broccoli a try, too: look for named
varieties and new recipes. Be bold with brassicas.

* * *

One-upvegetable-ship

CABBAGES and their pests are locked in an endless evolutionary battle, in
which each tries to outmanoeuvre the other鈥檚 defences. Field research along the
coast of southern England, near Swanage in Dorset, shows just what this battle
has done to wild brassicas.

Richard Mithen from the John Innes Centre in Norwich and Alan Raybould of the
Institute of Terrestrial Ecology鈥檚 Furzebrook Research Station in Dorset looked
at wild cabbages growing along sea cliffs. One population, at Kimmeridge, grows
on the cliff face and cannot be reached by generalist predators such as slugs,
snails and rabbits. This colony has quite low levels of one chemical weapon, the
glucosinolates. But a couple of kilometres along the coast, at Winspit, the wild
cabbages grow in grassland on top of the cliff. Here, the plants make huge
amounts of butenyl glucosinolate鈥攁nd they taste terrible as a result.

Mithen reckons that the wild cabbages at Winspit have stepped up
glucosinolate production to try to deter the slugs, snails and rabbits that
thrive in the grassland. But the plants 鈥済et hammered by insects鈥�, especially
specialist cabbage pests such as flea beetles and caterpillars of cabbage white
butterflies. These have evolved detoxifying systems that enable them to overcome
the brassicas鈥� chemical defences. They even find the taste and smell of
glucosinolates attractive.

Yet it鈥檚 not all bad news for the plant. The glucosinolates beloved of the
specialist insects also attract parasitic wasps that prey on the pests. 鈥淵ou
begin to see how complicated the whole system is,鈥� Mithen says, 鈥渨ith plants
producing such different solutions, just two kilometres apart. Both are
extremely happy in their habitats.鈥�

* * *

Eat up those greens, they鈥檙e good for you

NUTRITIONISTS now advise us to eat at least five servings of fruit and
vegetables every day. Whatever else you do that is unhealthy, do this and you
may substantially reduce your risk of dying prematurely from cancer or heart
disease (see 鈥淔ive apples a day鈥︹��, 快猫短视频, 2 November 1996, p
50).

This idea springs from a revolution in nutritional thinking. For decades,
scientists have struggled to understand the ways foods can harm us, pinpointing
saturated fats from meat and dairy products, sugar and salt as particularly
dangerous in excess. Now nutritionists are increasingly 鈥渢hinking positive鈥�, as
research at the cutting edge focuses on how food can improve our health.

The turning point came four years ago, when epidemiologist Gladys Block of
the University of California at Berkeley published a major review of more than
200 nutritional studies in the journal Nutrition and Cancer (vol 18, p
1). For an area normally dogged by complex and contradictory findings, the
evidence was unusually clear: people who regularly ate substantially more fruit
and vegetables than average were up to four times less likely to succumb to a
huge range of cancers, including breast, lung and colon cancer. A diet rich in
fruit and vegetables also seemed to protect against heart disease.

Nutritionist Philip James, director of the Rowett Research Institute in
Aberdeen, points out just how impressive the evidence is. He says, this 鈥渁mazing
consistency of evidence鈥� over so many studies makes this 鈥渢he single most
convincing dietary relationship鈥� known to science.