Nutrient density and trace elements
What a food really provides in vitamins, minerals and trace elements per mouthful, and why the way it is grown matters.
Explanation
Nutrient density measures how rich a food is in nutrients (vitamins, minerals, trace elements, fibre) relative to its weight or its calories. The subject often comes up since a study by the biochemist Donald Davis (University of Texas), which compared the American composition tables for vegetables and fruits from 1950 and 1999 (Davis, Epp & Riordan, Journal of the American College of Nutrition, 2004).
Refined definition
Trace elements are minerals the body needs only in very small amounts (a few milligrams or micrograms a day) but which are essential to it: iron, zinc, copper, manganese, iodine, selenium, molybdenum, fluoride. Health authorities set reference intakes for them (ANSES for France, EFSA for the European Union). On the plant side the vocabulary is close: boron, chlorine, copper, iron, manganese, molybdenum, nickel and zinc are the “micronutrients” essential to its growth (Marschner, Mineral Nutrition of Higher Plants). What a plant contains therefore depends on what its soil offers it, on its variety and on the moment of harvest.
Key points
What the Davis study (2004) says, and what it does not say
It compares composition tables (USDA 1950 and 1999) for 43 garden crops. Six nutrients show a reliable decline (protein, calcium, phosphorus, iron, vitamin B2, vitamin C); the other seven do not change significantly. The authors themselves recall that analytical methods and varieties changed between the two dates: part of the gap may come from the measurement (Marles, Journal of Food Composition and Analysis, 2017). Vegetables and fruits remain, by far, a major source of these nutrients: the conclusion is not “they are worthless now”, but “how they are produced matters”.
The dilution effect
Long known to agronomists (Jarrell & Beverly, Advances in Agronomy, 1981): when a plant produces more matter (bigger fruit, higher yield) without taking up more minerals, their concentration falls. Varietal selection geared to yield, size and transport resistance, irrigation and intensive nitrogen fertilisation all push that way. Davis (2009) judges this “genetic and environmental dilution” better supported than the hypothesis of a soil depleted in minerals.
Ripeness and freshness
Vitamin C, the most fragile, depends strongly on the harvest stage, the light received by the fruit and the time elapsed before eating (Lee & Kader, 2000). A fruit picked ripe and eaten in the following days keeps what a fruit picked green to travel has lost. It is one of the concrete advantages of the vegetable garden.
Trace elements and living soil
Mycorrhizal fungi widen the volume of soil explored by the roots and improve the uptake of phosphorus, zinc and copper (Marschner & Dell, Plant and Soil, 1994). A covered soil, rich in organic matter and without deep tillage, maintains these networks (see Living soil and Mycorrhiza). pH plays a part too: in very calcareous soil, iron and manganese become less available to the plant.
What the gardener can do
Diversify varieties, especially heritage or farm-saved ones (see Farmers' seeds); feed the soil rather than the plant (compost, mulch, green manures); avoid excess nitrogen, which swells the plant with water; harvest ripe and eat soon; vary the species on the plate, since no plant provides everything. The Ciqual composition tables (ANSES) make it possible to compare foods with one another.
Watch out for
- • This concept does not replace medical advice or a blood test: trace-element needs and deficiencies are diagnosed with a health professional.
- • The figures circulating online (“50 % fewer nutrients”, “it would take ten oranges to equal one from 1950”) are not those of the study: the measured declines range from 6 % to 38 % depending on the nutrient, and several nutrients do not decline.
- • More minerals in the soil does not mean more on the plate: beyond the plant's needs, the excess is of no use, and some trace elements (copper, zinc, boron) become toxic to it at high doses.
Examples
- • A garden tomato picked red, at ripeness, rather than a tomato harvested green to travel: same variety, more vitamin C at the moment of eating.
- • A vegetable garden that alternates heritage varieties of carrot, beetroot and cabbage: different compositions, so a more complete plate than with a single high-yield variety.
- • A mulched, undug soil where the mycorrhizae are active: better uptake of zinc and copper by the roots.
Sources
- Davis, D.R., Epp, M.D. & Riordan, H.D. (2004). Changes in USDA food composition data for 43 garden crops, 1950 to 1999. Journal of the American College of Nutrition, 23(6), 669-682.
- Davis, D.R. (2009). Declining fruit and vegetable nutrient composition: what is the evidence? HortScience, 44(1), 15-19.
- Lee, S.K. & Kader, A.A. (2000). Preharvest and postharvest factors influencing vitamin C content of horticultural crops. Postharvest Biology and Technology, 20(3), 207-220.
- Marles, R.J. (2017). Mineral nutrient composition of vegetables, fruits and grains: the context of reports of apparent historical declines. Journal of Food Composition and Analysis, 56, 93-103.
- Jarrell, W.M. & Beverly, R.B. (1981). The dilution effect in plant nutrition studies. Advances in Agronomy, 34, 197-224.
- Marschner, H. & Dell, B. (1994). Nutrient uptake in mycorrhizal symbiosis. Plant and Soil, 159(1), 89-102.
- Anses : table de composition nutritionnelle des aliments Ciqual (ciqual.anses.fr)