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