The April issue of the
Tufts University Health and Nutrition Letter has an article on the benefits of
steaming vegetables. They cite research from the University of Illinois on
different ways of cooking broccoli.
Broccoli (like other cruciferous vegetables) contains a plant compound
called sulforaphane, which has shown ant-cancer action in laboratory tests. But
for sulforaphane to form, another broccoli compound must be present—an enzyme
called myrosinase.
The University of Illinois
scientists found that only when broccoli is steamed, for as long as five
minutes, did it retain the myrosinase necessary to form the cancer-fighting
sulforaphane. Boiling and microwaving broccoli, even for just one minute,
destroyed the enzyme.
Those findings follow a
2008 study published in the Journal of Agricultural and Food Chemistry, which
reported that steaming was the only cooking method to preserve glucosinolates—a
group of cancer-fighting compounds in broccoli and other cruciferous vegetables
In fact, steaming actually increased total glucosinolates by 30% compared to
raw broccoli. Boiling and frying both led to substantial degradation of
glucosinolates, with frying causing an 84% loss of these healthy compounds. To clarify – glucosinolate is a
precursor to sulforaphane – they are basically the same compound.
Sulforaphane is being
studied in the Brassica Chemoprotection Laboratory at Johns Hopkins. This unique
laboratory is exclusively dedicated to identifying edible plants that are
particularly rich in protective enzyme-inducer activity. This lab has an amazing story, to be
revealed in a future column.
Another study found that
steaming spinach and broccoli better preserved levels of folate, an important B
vitamin. Even after nearly five minutes of steaming spinach and 15 minutes
steaming broccoli, folate loss was negligible. Boiling the same vegetables,
however, lost more than half the folate in each.
A 2009 Chinese study found
that only steaming preserved the chlorophyll, vitamin C, soluble proteins and
soluble sugars in broccoli that were lost in microwaving, boiling, and
stir-frying.
Some nutrients, such as
water-soluble vitamin C and most B vitamins, are rapidly lost to boiling;
steaming retains more of these vitamins because the food comes into less
contact with the water
Heat destroys vitamin C by
speeding the rate at which the vitamin reacts with oxygen in the air, so raw
vegetables typically have more vitamin C than steamed ones. But since vitamin C
is so prevalent in fruits and vegetables, cooking is often worth the trade-off.
Many nutrients are
fat-soluble, including vitamin A, vitamin D, vitamin E, vitamin K and
carotenoids like beta-carotene and lycopene. So even if you steam your food to
minimize the loss of these nutrients, tossing steamed vegetables in a little
oil will help your body absorb these fat-soluble nutrients.
For easy and effective
steaming, use a steamer that keeps the food out of the water. You can add
herbs, spices or aromatic ingredients like garlic or onions to the steaming
water, or replace the water entirely with broth.
Testing
for Glyphosate (Roundup) in Food: The U.S. Food and
Drug Administration has confirmed that it would begin testing for residues of
the controversial herbicide glyphosate on foods sold in the U.S. for the first
time this year. Glyphosate, the active ingredient in Roundup, is widely sprayed
on American farms, and is the most-used agricultural chemical in the world. It
has been labeled “probably carcinogenic to humans” by the United Nations’ International
Agency for Research on Cancer, though a European food safety agency has disputed those claims.
The FDA says it
didn’t test food for glyphosate in the past because the “available methods”
would have been “very cost- and labor-intensive to implement.” The FDA has recently developed streamlined methods to test for
the chemical. “The agency is now preparing plans for Fiscal Year 2016 to
measure glyphosate in soybeans, corn, milk, and eggs, among other potential foods. Stay tuned for more on this
topic, including what is going on in the Senate - attempts to block the Vermont law to mandate GMO labeling
on all foods, due to go into effect July first. GMO foods are sprayed with glyphosate.
Sadja Greenwood, MD, MPH back columns on this blog
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