Any home gardener will tell you supermarket produce is no match for freshly picked fruits and vegetables. But that’s more than lip service or homegrown pride. Actually, there’s plenty of scientific evidence to suggest that backyard produce doesn’t just taste better — it’s more nutritious, too.
In the United States, we consume an overabundance of calories, yet around a third of us don’t get the recommended levels of magnesium, vitamin A and vitamin C — all nutrients we get from plants, which we have more year-round access to than ever before. So what gives?
Part of the answer may lie in the plants themselves and how we process them. Instead of simply eating more servings of fruits and vegetables, we might want to focus on ensuring those servings are rich in the nutrients that do the hard work of keeping us healthy in the first place.
Large-scale agricultural producers utilize everything from fertilizers and pesticides to plowing and selective breeding to increase the uniformity and yield of crops. But in doing so, they may be inadvertently decreasing those crops’ ability to nourish us. Researchers have found a marked decline in the nutritional value of fresh food over the last 100 years. Today’s produce pales in nutritional value with fruits and vegetables grown before those agricultural practices became common, and many studies back that up.
A Brief History of Nutrition Research
The first federal funding for human nutrition research in the United States was earmarked in 1893, in large part because of the work of an American chemist named Wilbur Olin Atwater.
Atwater was one of few scientists at the time devoted to the studies of human nutrition and metabolism. His work in the late 19th and early 20th centuries informed much of what we now understand about food and nutrition.
Research by Robert Alexander McCance and Elsie Widdowson at King’s College University of London further advanced our understanding of food’s chemical compositions. Their findings were published in 1940 and provided more comprehensive data on the organic and mineral makeup of foods. This exhaustive body of work has been updated and republished every few years to reflect changes in nutrient values and diets.
In 2003, researcher and purveyor of mineral supplements David Thomas compared the various sets of nutrition data from this research and found that, between 1940 and 1991, many vegetables had undergone a significant drop in nutritional value. Potatoes had lost almost half of their copper and iron, and 35 percent of their calcium. Meanwhile, broccoli saw an 80-percent drop in copper, an essential nutrient required for human growth and development. Thomas also discovered a 75-percent reduction in broccoli’s calcium content over the years.
“You would need to have eaten 10 tomatoes in 1991 to have obtained the same copper intake as one tomato would have given you in 1940,” Thomas wrote in his report. Significantly, many of these nutrient drops were in foods considered good sources for those same vitamins and minerals.
Thomas theorized that changes in agricultural practices could be damaging the soil and contributing to the overall nutritional depletion of food. Not everyone agreed — although many similar conclusions were popping up from other research.
Abundance and Deficiency
Separate research in the late ’90s by nutrition researcher Anne-Marie Mayer and health writer Alex Jack similarly found that the nutritional value of fresh foods has dropped dramatically in the last half-century, likely due to the very methods the agriculture industry has worked to develop to increase efficiency and production.
Mayer analyzed government reports on vitamin and mineral levels in fresh produce from 1930 to 1980 in the United Kingdom. Her findings, published in a 1997 edition of the
British Food Journal, included a 19-percent drop in calcium content in 20 vegetables, a 22-percent drop in iron, and a 14-percent slump in potassium. She suggested this nutrient decline could be related to the agriculture industry’s over-reliance on fertilizers, pesticides, soil compaction and plowing. These factors, Mayer said, represent environmental and genetic effects that could have altered fresh produce significantly over the course of half a century.
Jack investigated USDA food composition tables from 1975 and 1997 and found that calcium levels in 12 fresh vegetables had dropped by 27 percent; iron by 37 percent; vitamin A by 21 percent; and vitamin C by 30 percent. Jack concluded that the nutritional loss was the probable result of declining quality of soil, air and water, in addition to lowered seed quality.
The “Dilution Effect”
Donald R. Davis, a research scientist from the Clayton Foundation Biochemical Institute of the University of Texas at Austin, saw all these studies and decided to follow up in the early 21st century with his own.
Davis and his team studied USDA nutritional data from 1950 and 1999 for 43 vegetables and fruits. Declines in protein, calcium, phosphorous, iron, riboflavin (vitamin B2) and ascorbic acid (vitamin C) were consistent. These findings were published in the Journal of the American College of Nutrition in 2004.
“Most people, including the authors of the first paper, tend to assume that these declines are caused by soil deficiencies and minerals,” Davis says. “But beginning with our paper, that didn’t really fit with the data. And although everybody believes soil decline affects the nutritional value of food, we didn’t find it.”
Primary causes of the nutrition deficit, Davis says, are actually twofold.
“One, we have more intensive agricultural growing methods that are designed to increase the yield, such as fertilizer and irrigation. The second factor is what plant breeders have been working hard to do, which is selective breeding and hybridization to also increase the yield by any means. Both of these are what I call the dilution effect.
You have the genetic dilution effect from selective breeding, and the environmental dilution effect from things like fertilizer. These are the major causes.”
Bigger Yields, Fewer Nutrients
Plants bred for size, such as commercially produced fruits and vegetables, expend their energy making big fruits, not on the absorption of micronutrients. Bigger fruits tend to mean fewer flavors, more water and fewer nutrients.
“One illustration of this is broccoli, which is one of the studies that has carefully documented the genetic dilution effect,” Davis says. “In a side-by-side study where they planted old and new varieties of broccoli — same field, same environment but different varieties — what they found, the larger the broccoli head, the lower the mineral concentrations in the head.”
Davis’ research unearthed the possibility that plants selected for mass production because of their quick seed-to-fruit ratio and yield may also possess reduced abilities to absorb minerals and process vitamins and nutrients. Heirloom (open-pollinated) seeds, on the other hand, are from older plant stocks that likely haven’t suffered nutritional declines.
It’s not just in the seeds themselves, either: It’s also how we cut, package and ship commercial produce that’s destroying their vitamin and mineral makeup. Catherine Barry-Ryan of the Dublin Institute of Technology and David O’Beirne from the University of Limerick performed a study to determine how various methods of processing affect the ascorbic acid (vitamin C) level in shredded iceberg lettuce. What they discovered is shocking: That machine-sliced bag of iceberg lettuce sitting on a supermarket shelf has up to 63 percent less vitamin C than a head of iceberg lettuce you take home and tear yourself.
Furthermore, bruised produce has an increased rate of nutrition loss and can affect a fruit’s ability to fully ripen. Damaged or injured tomato tissue, for example, has 15 percent less vitamin C than unbruised fruits. Mechanical harvesting has an inherently higher rate of bruising and damaging produce than handpicking.
Early Picking Cuts Down on Nutrition
A fruit only reaches its full nutritional value when it is fully ripe. That includes nutrients such as lycopene and beta-carotene, as well as carotenoids and vitamins A and C. The average commercially produced fruit or vegetable travels long distances to reach your table: One study found that many vegetables travel around 2,000 miles from vine to supermarket. Because of that time, and so the food will not rot on the journey, it must be picked well ahead of its ripeness. That means a tomato sold commercially is hardy, but not necessarily nutritious. It’s also going to be picked and shipped green, far before it’s reached its nutritional potential.