According to the UN, wheat, rice, and maize—a mere three out of the Earth's 50,000+ edible plants—provide 60 percent of the world’s plant-derived food energy. In the developing world, they provide up to 70-80 percent of the energy in a person’s daily diet. These crops are key providers of micronutrients like zinc and iron, which they draw up from the soil.

As we continue to spew carbon dioxide into the atmosphere, however, we may be less able to rely on these crops. A meta-analysis indicates that grains and legumes grown at elevated CO 2 levels have lower concentrations of zinc and iron, and some crops also have reduced protein levels.

Data was collated from 143 comparisons of crops grown at current CO 2 concentrations—about 400 ppm—to those grown under free-air CO 2 enrichment conditions. These typically raised the concentrations to around 550 ppm, approximating the levels we are targeted to hit in the next 40-60 years. Crops included wheat, rice, maize, soybeans, field peas, and sorghum, and they were grown in Australia, Japan, and the US. Nutrient levels were measured over one to six growing seasons.

The increased CO 2 concentration was associated with significant decreases in zinc and iron in all C 3 grasses and legumes. However, it had only a modest effect on field peas and no significant effect on soybeans or C 4 crops like maize and sorghum. These more recently evolved plants are better able to thrive in hotter, drier conditions because they fixate CO 2 more efficiently than their C 3 cousins. Wheat and rice also had lower protein contents when grown in higher CO 2 concentrations.

The mechanisms underlying these effects remain unknown. It does not seem to be due to an increased production of carbohydrates at the expense of these other components when they are flooded with CO 2 . The mechanisms behind these nutritionally relevant changes may also be different in the different species.

Paleo dieters eschew all grains and legumes, as well as nuts and seeds, partially because they contain phytates. These molecules are used by plants as phosphate storage, but they also inhibit the absorption of dietary zinc, magnesium, calcium, and iron. If phytate levels went down in crops grown at elevated CO 2 concentrations, the amount of their bioavailable zinc might remain steady even though their total zinc went down. But this turned out to only be the case in wheat—in all other crops. phytate levels stayed the same, exacerbating the potential for zinc deficiency.

Continuously rising carbon dioxide levels in the atmosphere are going to have many effects. Rendering our food less nutritious might not be one that we saw coming, but now that we know, we might be able to prepare. Perhaps we need to start engineering or breeding crops with micronutrients that are less vulnerable to the increasing CO 2 concentrations.

Nature, 2014. DOI: 10.1038/nature13179 (About DOIs).