Detection of breastfeeding and weaning in modern human infants with carbon and nitrogen stable isotope ratios

Carbon (13C/12C) and nitrogen (15N/14N) stable isotope ratios were longitudinally measured in fingernail and hair samples from mother‐infant pairs where infants were exclusively breastfed (n = 5), breast‐ and formula‐fed (n = 2), or exclusively formula‐fed (n = 1) from birth. All exclusively breastfed infants had a dual enrichment in carbon (≈1‰) and nitrogen (≈2–3‰) when compared to maternal values. In contrast, breast‐ and formula‐fed subjects had reduced enrichments compared to exclusively breastfed subjects, and the exclusively formula‐fed infant showed no increase in δ13C or δ15N values. This finding of a carbon trophic level effect in breastfeeding infants suggests that 13C‐enrichments of approximately 1‰ in archaeological populations are not necessarily the result of the consumption of C4‐based weaning foods such as maize or millet. During the weaning process, the δ13C results for breastfed infants declined to maternal levels more rapidly than the δ15N results. This suggests that δ13C values have the potential to track the introduction of solid foods into the diet, whereas δ15N values monitor the length of time of breast milk consumption. These findings can be used to refine the isotopic analysis of breastfeeding and weaning patterns in past and modern populations. Am J Phys Anthropol, 2006. © 2005 Wiley‐Liss, Inc.