Article: Declining Fruit and Vegetable Nutrient Composition: What is the Evidence?
Author: Davis, D.R.
Publication: HortScience Vol. 44(1) February 2009
We often hear claims that minerals in both our food supply and our soil have been declining for the past several decades. Soils poor in minerals are a consequence of poor land stewardship, the argument goes, and as a result the quantities of vitamins and minerals in our food have also declined. What research evidence exists to support this argument?
The article cited above is one of the few peer-reviewed articles that directly address this topic. This article “summarizes three kinds of evidence pointing toward declines during the last 50 to 100 years in the concentration of some nutrients in vegetables and perhaps also fruits available in the United States and the United Kingdom.”
The first type of evidence presented deals with the “dilution effect” of synthetic fertilizers. Research trials have compared mineral content of fertilized plants and unfertilized plants, concluding: “…fertilized plants contained larger amounts of minerals than the unfertilized plants, but these amounts were sufficiently diluted by the increased dry matter that all mineral concentrations declined, except for P.”
Comparisons of historical measurements are another type of evidence presented. Statistical comparisons of older data (50 to 70 years) with current data reveal a similar trend of decline:
The strongest evidence for declines occurs for minerals in vegetables, especially calcium and copper (Cu), with median declines of ~17% and 80%…The one study that considered protein and vitamins found apparent median declines in 43 garden crops (nearly all vegetables) amounting to 6% for protein and 15% to 28% for three of the five vitamins studied.
Similar studies show statistically significant declines in potassium, manganese, zinc, copper, magnesium, calcium, phosphorous, and iron.
Data analysis reveals, however, that these trends are not necessarily well explained by soil mineral depletion. Natural variability between cultivars may be responsible for declines in nutrient densities. The author compares four side-by-side studies in order to test this hypothesis. He concludes that these studies:
show uniformly inverse associations between yield and nutrient concentrations for every nutrient studied so far (other than carbohydrate) – two minerals in broccoli; six minerals in wheat, plus protein, oil; and three amino acids in maize. These four studies suggest to me that genetic dilution effects may be common when selective breeding successfully increases crop yields.
The author explains the apparent reason for these consistent declines in mineral and nutrient concentrations:
In fruits, vegetables, and grains, usually 80% to 90% of the dry weight yield is carbohydrate. Thus, when breeders select for high yield, they are, in effect, selecting mostly for high carbohydrate with no assurance that dozens of other nutrients and thousands of phytochemicals will all increase in proportion to yield. Thus, genetic dilution effects seem unsurprising.
Davis’ explanation for declines in food nutrient densities is certainly plausible, and supported by the evidence. While current available data does little to support the hypothesis that declines in soil minerals are causing the decline in food quality, more research is required in this area to ascertain the true nature of the relationship between land management and nutrient densities in food. The research of Dr. Kristine Jones on the pasture cropping techniques of Colin Seis indicate massive changes in soil mineral content associated with a shift to pasture cropping; the effect that these changes may have on our food supply is still poorly understood.
What is clear, however, is that plant breeding programs have had the unintended consequence of reducing our food quality; this evidence highlights yet another reason why our agricultural biodiversity is such an important resource for the future of food production.