State of Science :: Nutritional Quality
Nutrient Decline Linked to the "Dilution" Effect
Using sophisticated statistical techniques and the U.S. Department of Agriculture's nutrient database, a team of scientists quantified the degree of nutrient decline in 43 garden crops from 1950 to 1999 (Donald Davis et al., 2005). Their findings are consistent with and reinforce the findings of an earlier study in Great Britain (Anne-Marie Meyer, 1997).
All comparisons of nutrient levels were made on a dry-weight adjusted basis, removing one source of "noise" in earlier studies. The ratio of nutrient levels in 1999 compared to 1950 was calculated for 13 nutrients and water for each food. The levels of six of the 13 nutrients show statistically significant declines: protein, calcium, phosphorous, iron, riboflavin, and ascorbic acid. The average levels of decline across the 43 foods fell within the range of a 6% decline in protein to a 38% drop in riboflavin.
The authors attribute the decline in nutrient density to a genetics-driven "dilution effect." Over many years plant breeders have used yield potential as a dominant selection criterion in developing improved varieties. While average yields have risen, plant root systems have apparently not been able to keep pace in drawing from the soil the micronutrients needed to synthesis certain proteins and vitamins.
Other possible explanations include gradual depletion of soil micronutrients and organic matter, changes in pest complexes and levels, and the impact of farming systems. Sorting out the interactions between these key variables - soil quality, pest pressure, farming systems, and plant genetics - is a top priority for the Organic Center and will make possible the design of organic farming systems that maximize nutrient density, and hence nutritional value per serving of food and calorie consumed.
Sources: "Changes in USDA Food Composition Data for 43 Garden Crops, 1950 to 1999."
Authors: Donald Davis, Melvin Epp, and Hugh Riordan.
Journal of the American College of Nutrition, Volume 23, Number 6, December 2005.
"Historical Changes in the Mineral Content of Fruits and Vegetables."
Author: Anne-Marie Mayer.
British Food Journal, Volume 99, Number 6, 1997.