Protein and mineral concentrations in plant tissues are often affected by the atmospheric CO₂ concentrations at which plants grow. This suggests that rising atmospheric CO₂ over the 21^st century might affect the nutritional value of plant foods. We used meta-analysis techniques to synthesize data from the research literature on the effect of elevated atmospheric [CO₂] on the protein and mineral (Fe, Ca, Mg, Mn, K, Zn) concentrations of common food crops. Most crops had lower protein concentrations when grown at elevated (540- 958 ppm) compared to ambient (315-415 ppm) CO₂. For the C₃ cereal crops wheat, barley and rice, the reduction in grain protein concentration was 10-15% of the value at ambient CO₂. For potato the reduction in tuber protein concentration was 14%. For soy beans there was a much smaller, although still statistically significant reduction of protein content of 1.4%. Other legumes also showed only a small decrease in protein at elevated CO₂. Reductions were seen in mineral concentrations for most species. The magnitude of the CO₂ effect varied with experimental methodology. For both wheat and soy, studies performed in open-top chambers had a larger CO₂ effect than most other types of studies. There was also indication of a possible pot artifact. For both wheat and soy, studies performed in open-top chambers showed a significantly greater CO₂ effect when plants were rooted in pots rather than in the ground. While the magnitude of the reductions in protein and minerals varied with experimental technique, definite reductions were seen for most species, regardless of the experimental methodology. These findings suggest that increasing atmospheric CO₂ concentrations are likely to decrease the protein and mineral contents and hence the nutritional values of many important plant foods in the 21^st century.