The heritability of the before-meal stomach content effects on food intake was investigated with 110 identical and 102 fraternal same-sex and 53 fraternal mixed-gender adult twin pairs who were paid to maintain 7-day food intake diaries. From the diary reports, the total and meal intakes of food energy and the amounts of the macronutrients ingested were estimated. A simple computer model of stomach emptying was used to estimate the contents of the stomach at the beginning and end of the meals. Linear structural modeling was applied to investigate the nature and degree of genetic and environmental influences and revealed significant genetic influences on the amount of food energy and macronutrients estimated to be present in the stomach at the beginning and end of meals. However, these influences were found to be secondary to genetic influences on overall intake and meal size such that when these were considered in the model the heritabilities for stomach contents vanished. The genes were also found to affect the magnitude of the negative correlations between the before meal stomach contents and the amounts of nutrients ingested, and the magnitude of the negative slope of the best-fitting regression line between before meal stomach contents and meal size. These results suggest that although the amount in the stomach found at meal time is only affected indirectly by the genes, the impact of the content of the stomach on the amount ingested is to some extent inherited. This suggests that responses to internal stimuli are influenced by the genes and become part of the total package of genetically determined physiological and psychological processes that regulate energy balance.