Carbon dioxide (CO2) levels this century will alter the protein, micronutrients, and vitamin content of rice grains with potential health consequences for the poorest rice-dependent countries

Abstract

Declines of protein and minerals essential for humans, including iron and zinc, have been reported for crops in response to rising atmospheric carbon dioxide concentration, [CO₂]. For the current century, estimates of the potential human health impact of these declines range from 138 million to 1.4 billion, depending on the nutrient. However, changes in plant-based vitamin content in response to [CO₂] have not been elucidated. Inclusion of vitamin information would substantially improve estimates of health risks. Among crop species, rice is the primary food source for more than 2 billion people. We used multiyear, multilocation in situ FACE (free-air CO₂ enrichment) experiments for 18 genetically diverse rice lines, including Japonica, Indica, and hybrids currently grown throughout Asia. We report for the first time the integrated nutritional impact of those changes (protein, micronutrients, and vitamins) for the 10 countries that consume the most rice as part of their daily caloric supply. Whereas our results confirm the declines in protein, iron, and zinc, we also find consistent declines in vitamins B1, B2, B5, and B9 and, conversely, an increase in vitamin E. A strong correlation between the impacts of elevated [CO₂] on vitamin content based on the molecular fraction of nitrogen within the vitamin was observed. Finally, potential health risks associated with anticipated CO₂-induced deficits of protein, minerals, and vitamins in rice were correlated to the lowest overall gross domestic product per capita for the highest rice-consuming countries, suggesting potential consequences for a global population of approximately 600 million.

Fig. 1. Average reduction in grain protein at elevated relative to ambient [CO₂] for 18 cultivated rice lines of contrasting genetic backgrounds grown in China and Japan using FACE technology.

A country by [CO₂] effect on protein reduction was not significant (P = 0.26). Bars are ±SE. *P < 0.05 and **P < 0.01 (see Methods for additional details).

Fig. 2. Average reduction in grain micronutrients, iron (Fe), and zinc (Zn) concentration at elevated relative to ambient [CO₂] for 18 cultivated rice lines of contrasting genetic backgrounds grown in China and Japan using FACE technology.

A country by [CO₂] effect was not significant for either micronutrient [P = 0.17 and 0.10 for iron (Fe) and zinc (Zn), respectively] so data from both locations are shown. Bars are ±SE. *P < 0.05 and **P < 0.01 for a given cultivar. CO₂; **P < 0.01 is based on all cultivars (see Methods for additional details).

Fig. 3. CO₂-induced reductions in vitamins B1 (thiamine), B2 (riboflavin), B5 (pantothenic acid), and B9 (folate) by cultivar.

No significant effect was observed for vitamin B6 (pyridoxine), and results are not shown. Analysis was conducted only for the China FACE location. Bars are ±SE. *P < 0.05 and **P < 0.01 for a given cultivar. CO₂; **P < 0.01 is based on all cultivars (see Methods for additional details).

Fig. 4. Projected [CO₂]-induced deficits in protein and minerals (Fe and Zn) and cumulative changes in vitamin B and cumulative changes in vitamin E derived from rice as a function of GDP per capita.

Data are based on 2011/2013 FAO food balance sheets for rice consumption and 2011/2013 World Bank estimates of GDP per capita per country.

Fig. 5. Average change in vitamin concentration (as percentage) in response to anticipated, relative to current, [CO₂] ±SE as a function of the ratio of the molecular weight of nitrogen (N) to the molecular weight of the vitamin.

There was a highly significant correlation between the amount of N present in the vitamin and the overall decrease or increase in response to higher [CO₂].