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In 1950, your parents, grandparents, or a perhaps a younger version of you could eat a handful of string beans — about three-and-a-half ounces — and get about 9 percent of the calcium you needed for the day. Almost 50 years later, in 1999, the amount of calcium in string beans dropped by 43 percent, leaving you with only 5 percent of your daily calcium. You could eat more string beans — except you might not want to, because they wouldn’t be as flavorful as in the past. So you could eat more of other vegetables, but it’s likely other vegetables wouldon’t have as much calcium or flavor as they used to, either. And it’s not just calcium: Preliminary research shows that many vegetables have lost significant amounts of nutritional value.

Donald Davis, a scientist retired from the University of Texas at Austin, and his colleagues published a study in 2004 comparing U.S. Department of Agriculture data on vegetable nutrients from 1950 to data from 1999, and found notable decreases, particularly for key nutrients like calcium, iron, phosphorus, riboflavin, and ascorbic acid.

Davis believes that the primary reason for the decrease is selective breeding: As growers and researchers have spent the last 50 years trying to produce varieties of crops that yield more fruit, they’ve been ignoring the effects on nutrient content. Davis cites a few studies that compared high-yield varieties to non-high-yield varieties in the same soil and growing conditions, and found decreased nutrient content in the former.

“It’s early evidence, but that’s very powerful evidence because the soil is the same; the only difference is the genetics of the plant,” Davis says.

The studies show that as fruits and vegetables get bigger and more plentiful, nutrients get diluted. Some high-yield varieties are “dwarf” plants, meaning the plants themselves are smaller. But plants draw in and store minerals in their stalks, and when it comes time to create a fruit or vegetable, the plants extract the minerals and transform them into beneficial nutrients in the fruit. A lack of storage space for nutrients in the stalk translates to a loss of nutrients in the fruits. (Because taste is so subjective, not much research has been done flavor loss in high-yield varieties. But some people anecdotally claim that flavors in high-yield varieties also seem diluted compared to heirlooms — varieties that have been passed down, unaltered, through generations.)