Quick question: Are vegetables less nutritious than they used to be?
You're free to argue about this, because scientists haven't managed to come up with a clear answer.
There's some new data out this week in the journal Crop Science, and at least for broccoli, the answer seems to be no. But keep reading, because the story gets a little more complicated.
Scientists at the U.S. Department of Agriculture took 13 different genetic lines of broccoli that were released by seed companies over the past 46 years, grew them side-by-side, and measured levels of many micronutrients, including calcium, magnesium, iron, and zinc. They also grew an older variety of the vegetable, called Waltham 29, that was created by plant breeders in Massachusetts in 1950.
They found no clear trend of rising or falling nutrients among the commercial broccoli lines. Older hybrids did not, on average, have higher or lower concentrations than more recent ones. But the researchers did find bigger is worse; the genetic lines that produced larger heads of broccoli also had lower concentrations of nutrients — that is, there was less nutrition per ounce of vegetable.
And then there's the geezer broccoli: Waltham 29. It does have significantly higher levels of many nutrients. But there's a catch. Waltham 29 is a very different version of broccoli. The flower is looser, more bush-like, with bigger flower buds. It's less predictable; many plants don't produce any flowers at all. It doesn't keep as long, and it's harder to ship.
As a result, broccoli like Waltham 29 never was very widely consumed. Mark Farnham, a co-author of the new study, says that back in the day when plant breeders created Waltham 29, Americans ate less than one pound per person per year, on average. In the 1960s and 1970s, breeders came up with hybrid lines that could be more easily stored and shipped. "It's really after these dramatic changes that people started eating it," Farnham says. Today, the average American eats seven or eight pounds per year.
Could plant breeders have kept Waltham 29's high nutrition levels, while still making broccoli popular? No one knows, at least not yet.
This study certainly won't end the debate over nutrients in modern vegetables. It started in earnest in 2004, when researcher Donald Davis at the University of Texas took a look at data published by the USDA, over many years, on the nutritional content of 43 common vegetables. Davis discovered that in 1950, the USDA reported higher levels of most nutrients than in 1999.
There are many possible reasons for this apparent decline. The USDA may simply have changed its survey methods. But Davis said at the time that he suspected it resulted from the introduction of new varieties.
Wheat and rice breeders have created varieties that yield more grain per acre. That's more calories and protein, but a few studies, especially of wheat, do suggest that there's a tradeoff between quantity and quality: The more grain, the less the concentration of important stuff like iron.
But genetics — what plant breeders control — probably isn't the major factor determining nutrient levels. Those levels can vary widely depending on what the soil a plant grew in, what time a vegetable was harvested, and how long it sat in storage. And how you cook broccoli can also have some effect on how much good they do you.
The bottom line? No matter what, all of those vegetables are good for you. So just listen to that echo of your mother's voice in your head, and eat them.