Plants that photosynthesize using the C3 pathway — which includes most trees, shrubs, and temperate grasses — incorporate carbon-13 at a lower ratio than plants using the C4 pathway, which includes maize (corn), sorghum, and tropical grasses.

In the Maya diet, maize was a critical staple crop, and the relative importance of maize in an individual’s diet is reflected in the carbon isotope ratios of their bones.

Higher carbon-13 values indicate greater maize consumption; lower values indicate more reliance on C3 foods.

Nitrogen stable isotopes, expressed as the ratio of nitrogen-15 to nitrogen-14, increase with trophic level — with each step up the food chain, the nitrogen-15 to nitrogen-14 ratio increases slightly.

Individuals who consume large amounts of animal protein show higher nitrogen-15 values than those who rely primarily on plant foods.