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Climate Change

Could increasing CO2 be decreasing human nutrition?

Article: Samuel S. Myers; Antonella Zanobetti; Itai Kloog; Peter Huybers; Andrew D. B. Leakey; Arnold J. Bloom; Eli Carlisle; Lee H. Dietterich; Glenn Fitzgerald; Toshihiro Hasegawa; N. Michele Holbrook; Randall L. Nelson; Michael J. Ottman; Victor Raboy; Hidemitsu Sakai; Karla A. Sartor; Joel Schwartz; Saman Seneweera; Michael Tausz & Yasuhiro Usui: Increasing CO2 threatens human nutrition. Nature, Vol. 510, pp. 139-143 doi:10.1038/nature13179, 2014

 

Background Information

Everyone knows to eat their vegetables to be healthy. Grains and legumes are just as important to eat in order to get minerals zinc and iron. Zinc is an important nutrient for cellular metabolism (which allows our bodies to make proteins, enzymes, etc) and immune function while iron is essential in carrying oxygen around our body. Likewise, grains (like rice) and legumes (like soybeans) are major protein sources.

 

Table 1: Countries whose populations receive at least 60% of dietary iron and/or zinc from C3 grains and legumes. Source: United Nations Food and Agriculture Organization 2010.

Table 1: Countries whose populations receive at least 60% of dietary iron and/or zinc from C3 grains and legumes. Source: United Nations Food and Agriculture Organization 2010.

It is estimated that 2 billion people world-wide suffer from zinc and/or iron deficiencies, which contribute up to 63 million deaths annually. Approximately 2.3 billion people get at least 60% of their zinc and/or iron from grain and legume crops (Table 1). Any reduction in zinc and iron concentrations in these food crops could have serious implications for human health.

Since the late 1990’s, laboratory studies have suggested that increasing CO2 concentrations decrease the amount of nutrients in plants. These controlled experiments have led scientists to hypothesize that as atmospheric CO2 levels increase, the nutrition of our major food crops will decrease. Previous studies, however, were often too data limited to produce any real conclusions. Myers et al. took on the task of assessing the nutrition of 6 different crops under free-air CO2 enrichment (FACE) conditions.

The Approach

Six of the world’s major crops were selected for the study and were grown for 1 to 6 growing seasons under FACE (elevated CO2) conditions. The first two plants were wheat and rice, both of which are C3 grasses. A C3 plant uses the Calvin-Benson cycle, which is the most common way for plants to photosynthesize. The next two crops tested were soybeans and field peas, both of which are C3 legumes. Finally, maize and sorghum are C4 grasses; C4 plants use the Hatch-Slack cycle to photosynthesize, which is better adapted for drought, high temperature, and/or low CO2 conditions.

An example of a wheat crop. Source: http://www.uky.edu

An example of a wheat crop. Source: http://www.uky.edu

The meta-analysis was conducted in seven study sites in Japan, Australia, and the United States. All crops were grown under ambient conditions (normal conditions) at an atmospheric CO2 concentration of 363-386 ppm and then compared to the same plants under elevated CO2 concentrations of 546-586 ppm. It is currently projected that the atmospheric CO2 concentration will rise to 550 ppm in the next 40-60 years.

The edible portions of these six crops were analyzed for various nutrients including zinc, iron, protein concentration, and phytate. Phytate is phosphate storage molecule in plants that can curb the adsorption of zinc in animals (such as humans).

 

The Findings

Figure 1: The percentage of change (%) of zinc (Zn), iron (Fe), protein, and phytate in the 6 crops tested. The number in parenthesis refers to the number of replicates tested. The error bars represent the 95% confidence interval.

Figure 1: The percentage of change (%) of zinc (Zn), iron (Fe), protein, and phytate in the 6 crops tested. The number in parenthesis refers to the number of replicates tested. The error bars represent the 95% confidence interval.

Myers et al., found a statistically significant decrease in zinc and iron in all of the C3 grasses and C3 legumes. The researchers propose that since the C4 plants (maize and sorghum) internally concentrate CO2, these plant species are not as susceptible to elevated CO2 as the C3 plants.

 

Under increased CO2, zinc decreased by 9.3% in wheat, 3.3% in rice, 6.8% in field peas, and 5.1% in soybeans. Similarly, iron decreased in wheat by 5.1%, rice by 5.2%, and by 4.1% in both field peas and soybeans. In the wheat, there was also a significant decrease in phytate, which may offset a portion of the zinc decrease. Additionally, the C3 grasses had a large decrease in protein: 6.3% in wheat and 7.8% in rice.

 

The mechanism that causes a nutrient decrease in some crops is not well known. Myers et al., suggest that dilution is the cause. Under elevated CO2, C3 plants produce more carbohydrates, which dilutes the amount of zinc, iron, and protein as a consequence. This effect is species dependent, meaning that the associated nutrient decrease will vary from plant to plant.

 

Significance

This study demonstrated that increasing CO2 concentrations could have negative effects not yet understood or predicted. In addition to the global threat of climate change and ocean acidification is a decrease in food nutrition. Many globally important crops showed a significant decrease in important nutrients such as iron and zinc. Communities and regions dependent on crops, such as wheat, may suffer negative health effects as CO2 increases in the atmosphere.

 

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