The archaeology of ancient diet has been transformed in recent decades by the application of isotope analysis to human and animal remains, enabling reconstruction of what ancient people actually ate rather than what elite sources claim they ate.

Isotope analysis works because different foods contain different ratios of stable isotopes that are incorporated into bone and dental enamel over a person's lifetime. Carbon isotopes distinguish between C3 plants (most temperate crops: wheat, barley, legumes) and C4 plants (maize, millet, sorghum). Nitrogen isotopes indicate how much animal protein (fish, meat, dairy) a person consumed; higher nitrogen values indicate more animal protein. Strontium and oxygen isotopes can indicate geographic origins.

The results have repeatedly surprised archaeologists. Roman soldiers in Britain, contrary to the literary image of legionaries sustained by grain rations, consumed substantial amounts of fish and animal protein. Medieval peasants ate more varied diets than the grain-and-turnip stereotype suggests. Elite individuals in many ancient societies consumed less meat and more grain than their status would predict — possibly reflecting the reality that stored grain was the primary form of wealth display, while fresh meat was available mainly at festivals.

The combination of isotope analysis with zooarchaeology (the study of animal remains from excavations) and archaeobotany (plant remains) is producing a detailed picture of ancient food systems that supplements and sometimes contradicts the literary and documentary evidence. The systematic study of dietary change over time within single sites reveals the impact of political change, trade intensification, and climate variation on what ordinary people ate.