This is because a key phytochemical in these vegetables is poorly absorbed and of far less value if taken as a supplement.
The study, published in the Journal of Agricultural and Food Chemistry, was one of the first of its type to determine whether some of the healthy compounds found in cruciferous vegetables can be just as easily obtained through supplements and the answer is a firm no.
Emily Ho, principal investigator of the team from the Linus Pauling Institute at Oregon State University said “Some vitamins and nutrients, like the folic acid often recommended for pregnant women, are actually better-absorbed as a supplement than through food,”
“However, adequate levels of nutrients like vitamin D are often difficult to obtain in most diets. But the particular compounds that we believe give broccoli and related vegetables their health value need to come from the complete food.”
The reason, researchers concluded, is that a necessary enzyme called myrosinase is missing from most of the supplement forms of glucosinolates, a valuable phytochemical in cruciferous vegetables.
Without this enzyme found in the whole food, the study found that the body actually absorbs five times less of one important compound and eight times less of another.
Intensive cooking does pretty much the same thing, Ho said. If broccoli is cooked until it’s soft and mushy, its health value plummets.
However, it can be lightly cooked for two or three minutes, or steamed until it’s still a little crunchy and still retain adequate levels of the necessary enzyme.
Although broccoli has the highest levels of glucosinolates, they are also found in cauliflower, cabbage, kale and other cruciferous vegetables. The same cooking recommendations would apply to those foods to best retain their health benefits.
Broccoli has been of particular interest to scientists because it contains the highest levels of certain glucosinolates, a class of phytochemicals that many believe may reduce the risk of prostate, breast, lung and colorectal cancer.
When eaten as a raw or lightly cooked food, enzymes in the broccoli help to break down the glucosinolates into two valuable compounds of intensive research interest – sulforaphane and erucin.
Studies have indicated that sulforaphane, in particular, may help to detoxify carcinogens and also activate tumor suppressor genes so they can perform their proper function.