The wheat that we eat today has changed a great deal over the past century. The “Green Revolution“, the breeding of semi-dwarf, higher-yielding cultivars of wheat, and other cereal crops, has led to greatly increased grain production and crop yields since the mid-1960s. This undoubtedly contributed to the alleviation of global food shortages, however, modern plant breeding over the past sixty years has aimed for higher crop yields with little attention paid to the nutritional quality of the resulting cereal grains (Morris and Sands 2006). Increased yields of grain may have caused a reduction in the amounts of minerals in the grain, although this is difficult to test as the mineral contents can vary due to a wide range of factors including the plant variety, soil conditions, climate, and fertilizer treatment.

The Broadbalk Wheat Experiment

A study in 2008 aimed to limit these variable factors with the use of historic stored wheat samples from the Broadbalk Wheat Experiment. Started in 1843 and continuing to the present day this fascinating experiment has occupied 5 hectares of rural Hertfordshire, divided in the parallel plots, and tested different fertilizers and manure treatments on the most popularly grown wheat varieties of the day. All the while recording rainfall, temperatures, and storing away wheat and soil samples each year. In some of these plots, acting as controls, have varied little in their fertilizer treatments since the beginnings of the experiment.

Making use of these stored samples of grain, in their study in the Journal of Trace Elements in Medicine and Biology, Ming-Sheng Fan and her colleagues analysed the amounts of zinc, copper, iron, magnesium and phytate over the many years of the experiment.

Declining mineral content

Figure 1. Trends in wheat grain yield (a), harvest index (b), Zn (zinc) (c), Fe (iron) (d), Cu (copper) (e), and Mg (magnesium) (f) concentrations in wheat grain from three plots of the Broadbalk Experiment since 1845.

Beginning in the 1960s there is a clear trend for reductions in zinc, iron, copper, and magnesium. Between 1840 and 1960 the levels of these minerals had remained constant. This coincided with the increased yields due to the switch to new semi-dwarf wheat varieties. Between 1968–2005 in semi-dwarf cultivars, grain zinc, copper and magnesium concentrations decreased significantly in all plots. To quote the authors of the study,

“Mean concentrations in 2000–2005 were lower than the means of the long-straw cultivars by 33–49% for Zn, 25–39% for Cu and 20–27% for Mg. Grain Fe concentration did not show a significant decreasing trend during 1968–2005, but the mean concentrations were 23–27% lower than those of the 1845–1967 period. The trends of grain P, Mn, S and Ca (data not shown) were broadly similar to those of Zn, Cu and Mg…”

This could be due to the dilution effect, meaning that the grain is growing larger with the same mineral content, however the yields in Figure 1 suggest this is not the case. This shows that even in unfertilised wheat, where wheat yield did not increase, there was still the same drop in mineral content since the 1960s.

Direct direct comparison between the old and new cultivars

Between 1988 and 1990 in the  Broadbalk Experiment, an old tall variety of wheat called Squarehead’s Master was grown side-by-side with a modern semi-dwarf variety of wheat called Brimstone. This confirmed the previous trends as the Brimstone wheat had 18–29% lower concentrations of zinc, copper, iron and magnesium than Squarehead’s Master wheat.

Soil mineral concentrations

A common notion when discussing the nutrient content of modern food is that conventional farming causes a depletion of mineral nutrients in the soil, resulting in lower mineral concentrations in grain. To test for this the study authors measured stored soil samples from the past 160 years. Perhaps surprisingly they found that the concentrations of the minerals studied either remained the same or actually increased over the last 160 years. For sake of completeness they also measured the bioavailability of the minerals to the plants and found that the concentrations of bioavailable zinc, copper, and magnesium had all increased substantially over the last 160 years.

Phytate content of the wheat

The trends in the amount of phytate in the wheat was also measured as this is one of the best known modulators of mineral bioavailability in cereal grains. The ratio of phytate to minerals in the wheat declined over time.  This means there was more phytate relative to the amount of minerals and suggests that those minerals may be less well absorbed.

Conclusions

The declining mineral content of wheat reported in this study suggest that the Green Revolution of the 1960s has had an unintentional side effect decreasing mineral density in the wheat grain. This was due to the changes in the varieties of wheat grown, rather than farming practices, or use of extra fertiliser. The authors speculate one potential reason for this,

“Dwarfing of wheat cultivars is achieved by the introduction of the gibberellin-insensitive Rht genes [24]; as a result, proportionally more photosynthates are distributed to the grain. It is unlikely that the dwarfing genes would have a pleiotropic effect on the uptake of several mineral nutrients from the soil. A more plausible explanation is that the re-distribution of minerals from the vegetative tissues to grain does not catch up with the much enhanced re-distribution of photosynthates in the short-straw cultivars.”

Translated into English:

The reduced height (Rht) genes cause the wheat grow to a shorter height by making the wheat unresponsive to gibberellin, the equivalent of a plant growth hormone. Photosynthates are the sugars produced in the plant through photosynthesis, using energy from the sun. Growing shorter means less energy is needed for growth and more of those sugars are transported into the grain where they are formed into starch.

The authors speculate that the faster rate of sugars being transported in the grain and the resulting faster starch accumulating in the grain is not matched by an increase in minerals transported through the plant. So the overall density of minerals is reduced.

A limitation to this study was that only whole grain samples were analysed, as many people consume only refined white flour with much of the bran removed, where much the minerals are found. It would be interesting to know how much these decreases in mineral content has affected the minerals found in the starchy endosperm of the wheat, the part used to make white flour.

Still, even if you eat whole grain wheat products, due to the use of modern high-yielding wheat cultivars, you are unlikely to be consuming the same amounts of minerals as you would have done eating wheat 100 years ago.

References:

Fan MS et al (2008) Evidence of decreasing mineral density in wheat grain over the last 160 years. Journal of Trace Elements in Medicine and Biology. 2008;22(4):315-24

“Wheat is an important source of minerals such as iron, zinc, copper and magnesium in the UK diet. The dietary intake of these nutrients has fallen in recent years because of a combination of reduced energy requirements associated with sedentary lifestyles and changes in dietary patterns associated with lower micronutrient density in the diet. Recent publications using data from food composition tables indicate a downward trend in the mineral content of foods and it has been suggested that intensive farming practices may result in soil depletion of minerals. The aim of our study was to evaluate changes in the mineral concentration of wheat using a robust approach to establish whether trends are due to plant factors (e.g. cultivar, yield) or changes in soil nutrient concentration. The mineral concentration of archived wheat grain and soil samples from the Broadbalk Wheat Experiment (established in 1843 at Rothamsted, UK) was determined and trends over time examined in relation to cultivar, yield, and harvest index. The concentrations of zinc, iron, copper and magnesium remained stable between 1845 and the mid 1960s, but since then have decreased significantly, which coincided with the introduction of semi-dwarf, high-yielding cultivars. In comparison, the concentrations in soil have either increased or remained stable. Similarly decreasing trends were observed in different treatments receiving no fertilizers, inorganic fertilizers or organic manure. Multiple regression analysis showed that both increasing yield and harvest index were highly significant factors that explained the downward trend in grain mineral concentration.”

Morris CE. and Sands DC. (2006) The breeder’s dilemma – yield or nutrition? Nature Biotechnology. 24(9):1078–1080

“Plant breeders, challenged to create more nutritious crops, face seemingly radical choices that constitute a ‘breeder’s dilemma’. In the search for higher yields and lower farming costs, have breeders inadvertently selected for crops with reduced nutritional quality? To create foods that keep pace with our growing understanding of what constitutes healthy diets, plant breeders may need to make a significant shift away from traditional selection criteria. Subsidizing crop nutritional value rather than yield could be an important and economical driver for this shift in perspective.”