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A rose is a rose is a rose, as Gertrude Stein liked to put it. But two tomatoes or stalks of broccoli could be surprisingly different from each other. They may look the same but contain wildly divergent amounts of important vitamins and minerals. And there is some evidence that what's in the supermarket today does not measure up to what grandma used to grow in her garden. NPR's Dan Charles investigates.

DAN CHARLES: Let's say you want to know how much bone-strengthening calcium you'll get from a stalk of broccoli. You can go online to the U.S. Department of Agriculture's nutrient database and it will tell you an average-sized broccoli stalk delivers 55 milligrams of calcium. That's about five percent of a middle-aged person's recommended daily dose, which is a little disappointing, because 50 years ago those same USDA figures reported broccoli contained twice that much calcium. Donald Davis from the University of Texas says this has been the general trend. He compared USDA figures in 1950 and 1999 for a basket of 43 common fruits and vegetables.

Mr. DONALD DAVIS (University of Texas): Out of the 13 nutrients that we were able to study, we found apparently statistically reliable declines in six of the 13 nutrients.

CHARLES: The rest stayed the same. None went up. Davis's results read like a bad day on Wall Street, a Black Monday for nutrition. Riboflavin down 38 percent, calcium off 16 percent, phosphorous down nine percent, iron down 15 percent, vitamin C down 20 percent. So if you can trust the statistics, America's produce is less nutritious than it was 50 years ago, except maybe you can't trust those old numbers.

Ms. JOANNE HOLDEN (U.S. Department of Agriculture): The esteemed 1950 handbook is thrown on my shelf.

CHARLES: Joanne Holden is the USDA official who's in charge of the agency's nutrient database.

Ms. HOLDEN: For the record, here it's a blue paper volume which contains all the data in 147 pages.

CHARLES: Holden says scientists used to use somewhat different methods and that may have led to different results. Also, nobody knows whether the vegetables that they analyzed back then were an accurate sample of what Americans actually were eating. It took until 1997 for the USDA to apply what you might call modern polling techniques to the analysis of food. At that point, the agency began gathering random samples of produce from supermarkets across the nation. David Haytowitz, who helps run this program, says his veggie buyers have to follow strict rules to make sure they're picking fruit at random.

Mr. DAVID HAYTOWITZ (U.S. Department of Agriculture): Because we don't somebody just picking the one on top because oftentimes in the stores they're going to rearrange their bins and put, you know, the best looking ones on top. We don't want that best looking one on top. We want an average one, a representative one.

CHARLES: Because which tomato or cantaloupe you pick makes a big difference. For instance, the USDA found that some cantaloupes had four times as much Vitamin A as others. Or take broccoli. It's a good source of nutrients called glucosinilates, which can help prevent cancer. John Juvik is a plant geneticist at the University of Illinois. He discovered that some kinds of broccoli have 40 or 50 times more glucosinilates than others.

Professor JOHN JUVIK (University of Illinois): So you could go into the store one week and buy a head of broccoli, and it would provide a dose of glucosinilates that would more or less protect you from certain forms of cancer, for a period of time, maybe a week or so. And then you could go back a week later and pick up another variety of broccoli. You wouldn't see the difference and it would basically provide you with no health benefits, or very limited health benefits.

CHARLES: It turns out that vegetables are just like people; they can vary widely and the reason is a combination of their genes and their environment. First the genes. That's what John Juvik looked at in his experiments with broccoli. Plant breeders have created countless varieties, or genetic types, of vegetables. But until recently, he says they didn't really pay much attention to nutritional quality, and they ended up with nutrient levels that are all over the map.

Prof. JUVIK: They were selecting for yield, marketable yield, and they were selecting for appearance.

CHARLES: As for the influence of the environment - soil, farming practices, weather, the length of time a vegetable sits around before you eat it - that's more difficult to study. One organization that's trying is the Rodale Institute, located among rolling hills west of Kutztown, Pennsylvania. Paul Hepperly is the institute's research manager.

Dr. PAUL HEPPERLY (Rodale Institute): This is the entrance to the farming systems trial, which was started in 1981.

CHARLES: In front of us are 30 acres of corn and soybeans ready for harvest. Half of this land has been farmed using organic methods, such as compost for fertilizer. The rest is farmed conventionally, with synthetic fertilizer and pesticides. Hepperly admits the organic cornfields have more weeds.

Dr. HEPPERLY: But the other thing that you see is actually the corn is higher and more robust in the organic than in the conventional. You see the difference in the size of the plants?

CHARLES: Hepperly says that's because organic farming creates healthier soil. He also says there's evidence that this soil produces more nutritious harvests. In 2003, the institute planted oats across this whole area and farmed it all the same way.

Dr. HEPPERLY: And we could see that where we had a legacy of organic soil improvement, actually the oat tissues were higher in minerals.

CHARLES: Now, the Rodale Institute was set up to promote organic farming, so for skeptics this result won't be convincing or surprising. What is surprising, though, is how few scientists anywhere are conducting similar experiments. So we can't be certain that vegetables 50 years ago were more nutritious, and for the most part farmers and food companies don't know how best to grow nutritious crops. In the grocery store, tomatoes are just tomatoes, and every stalk of broccoli costs the same.

There are a few things, fortunately, that we do know. Donald Davis, the researcher who discovered the apparent decline in vegetable nutrients, often reminds people that depletion of the American diet is nothing compared to what people do to themselves.

Dr. DAVIS: If you're really concerned about loss of nutrients in your diet, you probably ought to be looking first at how much of your calories are coming from added fats, added sugars and white flour and white rice.

CHARLES: So eat lots of vegetables and whole grains, he says, even if you don't know which stalks of broccoli have all the glucosinilates. If you eat a lot of them, you're sure to find a good one.

For NPR News, I'm Dan Charles.

SIMON: Now, you may not be able to tell which broccoli stalk is healthiest, but you could still choose wisely when deciding on vegetables to eat. Our Web site offers tips. Come to npr.org. And eat your vegetables.

You're listening to WEEKEND EDITION from NPR News.

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