The point of diminishing returns
One of the largest scientific studies to draw attention to this issue was published in the U.S., via the December 2004 issue of the Journal of the American College of Nutrition. Using USDA nutrient data published in 1950 and 1999, researchers at the University of Texas at Austin noted changes in 13 nutrients in 43 different garden crops—from asparagus and snap beans to strawberries and watermelon.
These raw fruits and veggies showed declines in protein, calcium, and phosphorus, which are essential for building and maintaining strong bones and teeth and for proper nerve function. There were also dips in iron, vital for carrying oxygen throughout the body, and in riboflavin, which is crucial for metabolism of fats and drugs. Levels of vitamin C—important for the growth and repair of various tissues in the body and for immune function—also fell.
The level of decline varied depending on the specific nutrients and the type of fruit or vegetable, but it generally ranged from 6 percent for protein to 38 percent for riboflavin. In particular, calcium dropped most dramatically in broccoli, kale, and mustard greens, while the iron content took a substantial hit in chard, cucumbers, and turnip greens. Asparagus, collard greens, mustard greens, and turnip greens lost considerable amounts of vitamin C.
Further studies since then have backed up the case that nutrient levels are dissipating. Research in the January 2022 issue of the journal Foods found that while most vegetables grown in Australia had relatively similar iron content between 1980 and 2010, there were noteworthy drops in certain veggies. Declines in iron content, ranging from 30 to 50 percent, occurred for sweet corn, red-skinned potatoes, cauliflower, green beans, green peas, and chickpeas. By contrast, Hass avocados, mushrooms, and silverbeet (another name for chard) actually gained in iron.
Grains have also experienced declines, experts say. A study in a 2020 issue of Scientific Reports found that protein content in wheat decreased by 23 percent from 1955 to 2016, and there were notable reductions in manganese, iron, zinc, and magnesium, as well.
The alarming declines have ripple effects for meat-eaters too. Cows, pigs, goats, and lambs are now feasting on less nutritious grasses and grains, Montgomery says, which in turn makes meat and other animal-derived products less nutritious than they used to be.