1. Isotopic ratios of nitrogen are often used in food-web studies to determine trophic position (including food chain length) and food sources, with greater ratios of 15N/14N (d15N) usually considered indicative of higher trophic position. However, fasting and starving animals may also show a progressive increase in d15N over time as they catabolise their own tissues.

2. To determine the importance of starvation, we conducted a 4-month laboratory experiment testing effects of starvation on body condition and isotope ratios in the muscle tissue of freshwater guppies (Poecilia reticulata). We also compared laboratory results and conclusions with analyses of body condition and isotope ratios in various small species of fish collected in four seasons from the Kansas River in north-eastern Kansas, U.S.A.

3. Fish starved in our laboratory experiment had significantly higher 15N values and poorer body condition than those fed more regularly. The diverse group of fish species collected in summer (July) from the Kansas River had higher weight-to-length ratios and lower 15N values than those retrieved in other seasons. Overall body condition resulting from reduced food consumption explained 44 and 53% of the variability in 15N for field and laboratory fish, respectively.

4. These results are applicable to a wide variety of food-web research but are especially pertinent to studies of organisms that undergo large changes in life history, dormancy, extended fasts or periods of significant nutritional allocation to young.