OBJECTIVE: To evaluate the potential of compound-specific isotope analysis of amino acids (CSIA-AA) for investigating infant feeding practices, we conducted a long-term study that compared infant and maternal amino acid (AA) nitrogen isotope ratios. MATERIALS AND METHODS: Fingernail samples were collected from a single mother-infant dyad over 19 months postpartum. Carbon and nitrogen stable isotope ratios were measured in the bulk keratin of the fingernail samples. Selected samples were then hydrolyzed and derivatized for compound-specific nitrogen isotope analysis of keratin AAs. RESULTS: As in previous studies, infant bulk keratin nitrogen isotope values increased during exclusive breastfeeding and fell with the introduction of complementary foods and eventual cessation of breastfeeding. Infant trophic AAs had elevated nitrogen isotope values relative to the mother, while the source AAs were similar between the mother and infant. Proline and threonine appeared to track the presence of human milk in the infant's diet as the isotopic composition of these AAs remained offset from maternal isotope values until the cessation of breastfeeding. DISCUSSION: Although CSIA-AA is costly and labor intensive, it appears to hold potential for estimating the duration of breastfeeding, even after the introduction of complementary foods. Through the analysis of a full suite of AAs, it may also yield insights into infant physiology and AA synthesis.

Arne Faculty of Arts Masaryk University Brno Czechia

Climate Change and History Research Initiative Princeton University Princeton New Jersey USA

Department of Archaeology Max Planck Institute of Geoanthropology Jena Germany

Department of Archaeology Memorial University of Newfoundland and Labrador St John's Canada

Department of Bioarchaeology Faculty of Archaeology University of Warsaw Warsaw Poland

Keck Carbon Cycle AMS Facility University of California Irvine Irvine California USA

Marine Microbiology and Biogeochemistry Department NIOZ Royal Netherlands Institute for Sea Research Den Burg The Netherlands

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Ambrose, S. H., & Norr, L. (1993). Experimental evidence for the relationship of the carbon isotope ratios of whole diet and dietary protein to those of bone collagen and carbonate. In J. Lambert & G. Grupe (Eds.), Prehistoric human bone (pp. 1–37). Springer.

American College of Obstetricians and Gynecologists. (2013). Definition of Term Pregnancy. Committee Opinion. https://www.acog.org/clinical/clinical-guidance/committee-opinion/articles/2013/11/definition-of-term-pregnancy

Ayliffe, L. K., Cerling, T. E., Robinson, T., West, A. G., Sponheimer, M., Passey, B. H., Hammer, J., Roeder, B., Dearing, M. D., & Ehleringer, J. R. (2004). Turnover of carbon isotopes in tail hair and breath CO2 of horses fed an isotopically varied diet. Oecologia, 139(1), 11–22.

Beaumont, J., Montgomery, J., Buckberry, J., & Jay, M. (2015). Infant mortality and isotopic complexity: New approaches to stress, maternal health, and weaning. American Journal of Biological Anthropology, 157, 441–457.

Borrell, A., Gómez‐Campos, E., & Aguilar, A. (2016). Influence of reproduction on stable‐isotope ratios: Nitrogen and carbon isotope discrimination between mothers, fetuses, and milk in the fin whale, a capital breeder. Physiological and Biochemical Zoology: PBZ, 89(1), 41–50.

Brignon, G., Chtourou, A., & Ribadeau‐Dumas, B. (1985). Preparation and amino acid sequence of human kappa‐casein. FEBS Letters, 188(1), 48–54.

Britton, K., McManus‐Fry, E., Cameron, A., Duffy, P., Masson‐MacLean, E., Czére, O., Smith, N., Stones, J., Winfield, A., & Muldner, G. (2018). Isotopes and new norms: Investigating the emergence of early modern U.K. breastfeeding practices at St. Nicholas Kirk, Aberdeen. International Journal of Osteoarchaeology, 28(5), 510–522.

Carstairs, S. A., Marais, D., Craig, L. C. A., & Kiezebrink, K. (2017). How important are the influencing factors to the decision on whether to provide seafood in infant and young child feeding? Appetite, 117, 224–233.

Chen, M. H. Y., Kendall, I. P., Evershed, R. P., Bogaard, A., & Styring, A. K. (2023). Reconstructing herbivore diets: A multivariate statistical approach to interpreting compound‐specific isotope values. Oecologia, 201(3), 599–609.

Cherel, Y., Hobson, K. A., & Guinet, C. (2015). Milk isotopic values demonstrate that nursing fur seal pups are a full trophic level higher than their mothers. Rapid Communications in Mass Spectrometry, 29(16), 1485–1490.

Chernikova, D. A., Madan, J. C., Housman, M. L., Zain‐ul‐abideen, M., Lundgren, S. N., Morrison, H. G., Sogin, M. L., Williams, S. M., Moore, J. H., Karagas, M. R., & Hoen, A. G. (2018). The premature infant gut microbiome during the first 6 weeks of life differs based on gestational maturity at birth. Pediatric Research, 84, 71–79. https://doi.org/10.1038/s41390-018-0022-z

Cheung, C., Herrscher, E., & Thomas, A. (2022). Compound specific isotope evidence points to use of freshwater resources as weaning food in Middle Neolithic Paris Basin. American Journal of Biological Anthropology, 179(1), 118–133.

Chikaraishi, Y., Kashiyama, Y., Ogawa, N. O., Kitazato, H., & Ohkouchi, N. (2007). Metabolic control of nitrogen isotope composition of amino acids in macroalgae and gastropods: Implications for aquatic food web studies. Marine Ecology Progress Series, 342, 85–90.

Chikaraishi, Y., Ogawa, N. O., Kashiyama, Y., Takano, Y., Suga, H., Tomitani, A., Miyashita, H., Kitazato, H., & Ohkouchi, N. (2009). Determination of aquatic food‐web structure based on compound‐specific nitrogen isotopic composition of amino acids. Limnology and Oceanography, Methods / ASLO, 7(11), 740–750.

Chikaraishi, Y., Ogawa, N. O., & Ohkouchi, N. (2010). Further evaluation of the trophic level estimation based on nitrogen isotopic composition of amino acids. Earth, Life, and Isotopes, 415, 37–51.

Cocozza, C., Fernandes, R., Ughi, A., Groß, M., & Alexander, M. M. (2021). Investigating infant feeding strategies at Roman Bainesse through Bayesian modelling of incremental dentine isotopic data. International Journal of Osteoarchaeology, 31(3), 429–439.

Darling, P. B., Dunn, M., Sarwar, G., Brookes, S., Ball, R. O., & Pencharz, P. B. (1999). Threonine kinetics in preterm infants fed their mothers' milk or formula with various ratios of whey to casein. The American Journal of Clinical Nutrition, 69(1), 105–114.

Darragh, A. J., & Moughan, P. J. (1998). The amino acid composition of human milk corrected for amino acid digestibility. British Journal of Nutrition, 80, 25–34.

De Luca, A., Bernardo, K., Frasquet‐Darrieux, M., Christin, P., Schiphorst, A.‐M., Grand, M., Ingrand, P., Robins, R. J., & Hankard, R. (2019). Maternal obesity does not influence human milk protein 15N natural isotope abundance. Isotopes in Environmental and Health Studies, 55(4), 385–393.

De Luca, A., Boisseau, N., Tea, I., Louvet, I., Robins, R. J., Forhan, A., Charles, M.‐A., & Hankard, R. (2012). δ15N and δ13C in hair from newborn infants and their mothers: A cohort study. Pediatric Research, 71(5), 598–604.

De Luca, A., Hankard, R., Alexandre‐Gouabau, M. C., Ferchaud‐Roucher, V., Darmaun, D., & Boquien, C. Y. (2016). Higher concentrations of branched‐chain amino acids in breast milk of obese mothers. Nutrition, 32(11–12), 1295–1298.

Donovan, S. M., & Lönnerdal, B. (1989). Non‐protein nitrogen and true protein in infant formulas. Acta Paediatrica Scandinavica, 78(4), 497–504.

D'Ortenzio, L., Brickley, M., Schwarcz, H., & Prowse, T. (2015). You are not what you eat during physiological stress: Isotopic evaluation of human hair. American Journal of Physical Anthropology, 157(3), 374–388.

Fernandes, R., Grootes, P., Nadeau, M.‐J., & Nehlich, O. (2015). Quantitative diet reconstruction of a Neolithic population using a Bayesian mixing model (FRUITS): The case study of Ostorf (Germany). American Journal of Physical Anthropology, 158, 325–340.

Fernandes, R., Nadeau, M.‐J., & Grootes, P. M. (2012). Macronutrient‐based model for dietary carbon routing in bone collagen and bioapatite. Archaeological and Anthropological Sciences, 4(4), 291–301.

Findlay, J. B., & Brew, K. (1972). The complete amino‐acid sequence of human ‐lactalbumin. European Journal of Biochemistry, 27(1), 65–86.

Fogel, M. L., Tuross, N., & Owsley, D. W. (1989). Nitrogen isotope tracers of human lactation in modern and archaeological populations. Carnegie Institution of Washington Yearbook, 88, 111–117.

Fomon, S. J., Bier, D. M., Matthews, D. E., Rogers, R. R., Edwards, B. B., Ziegler, E. E., & Nelson, S. E. (1988). Bioavailability of dietary urea nitrogen in the breast‐fed infant. The Journal of Pediatrics, 113(3), 515–517.

Fraser, I., Meier‐Augenstein, W., & Kalin, R. M. (2006). The role of stable isotopes in human identification: A longitudinal study into the variability of isotopic signals in human hair and nails. Rapid Communications in Mass Spectrometry, 20(7), 1109–1116.

Froehle, A. W., Kellner, C. M., & Schoeninger, M. J. (2010). FOCUS: Effect of diet and protein source on carbon stable isotope ratios in collagen: Follow up to Warinner and Tuross (2009). Journal of Archaeological Science, 37(10), 2662–2670.

Fuller, B. T., Fuller, J. L., Harris, D. A., & Hedges, R. E. M. (2006). Detection of breastfeeding and weaning in modern human infants with carbon and nitrogen stable isotope ratios. American Journal of Physical Anthropology, 129(2), 279–293.

Fuller, B. T., Fuller, J. L., Sage, N. E., Harris, D. A., O'Connell, T. C., & Hedges, R. E. M. (2005). Nitrogen balance and δ15N: Why you're not what you eat during nutritional stress. Rapid Communications in Mass Spectrometry, 19(18), 2497–2506.

Fuller, B. T., Molleson, T. I., Harris, D. A., Gilmour, L. T., & Hedges, R. E. (2006). Isotopic evidence for breastfeeding and possible adult dietary differences from Late/Sub‐Roman Britain. American Journal of Biological Anthropology, 129(1), 45–54.

Fuller, B. T., & Petzke, K. J. (2017). The dietary protein paradox and threonine 15N‐depletion: Pyridoxal‐5′‐phosphate enzyme activity as a mechanism for the δ15N trophic level effect. Rapid Communications in Mass Spectrometry, 31(8), 705–718.

Fuller, B. T., Richards, M. P., & Mays, S. A. (2003). Stable carbon and nitrogen isotope variations in tooth dentine serial sections from Wharram Percy. Journal of Archaeological Science, 30, 1673–1684.

Greenberg, R., Groves, M. L., & Dower, H. J. (1984). Human beta‐casein. Amino acid sequence and identification of phosphorylation sites. The Journal of Biological Chemistry, 259(8), 5132–5138.

Gupta, G. R., Dhruw, V. K., Athawal, B. K., Siddiqui, P., Agrawal, H. K., & Chandra, H. (2005). Human nail growth pattern and medicolegal aspect. Journal of Indian Academy of Forensic Medicine, 27(2), 87–91.

Habran, S., Debier, C., Crocker, D. E., Houser, D. S., Lepoint, G., Bouquegneau, J.‐M., & Das, K. (2010). Assessment of gestation, lactation and fasting on stable isotope ratios in northern elephant seals (Mirounga angustirostris). Marine Mammal Science, 26(4), 880–895.

Hare, P., Fogel, M. L., Stafford, T. W., Mitchell, A. D., & Hoering, T. C. (1991). The isotopic composition of carbon and nitrogen in individual amino acids isolated from modern and fossil proteins. Journal of Archaeological Science, 18(3), 277–292.

Hedges, R., Rush, E., & Aalbersberg, W. (2009). Correspondence between human diet, body composition and stable isotopic composition of hair and breath in Fijian villagers. Isotopes in Environmental and Health Studies, 45(1), 1–17.

Hedges, R. E., & Reynard, L. M. (2007). Nitrogen isotopes and the trophic level of humans in archaeology. Journal of Archaeological Science, 34(8), 1240–1251.

Herrscher, E., Goude, G., & Metz, L. (2017). Longitudinal study of stable isotope compositions of maternal milk and implications for the palaeo‐diet of infants. Bulletins et Mémoires de la Société d'Anthropologie de Paris, 29(3–4), 131–139.

Hobson, K. A., Alisauskas, R. T., & Clark, R. G. (1993). Stable‐nitrogen isotope enrichment in avian tissues due to fasting and nutritional stress: Implications for isotopic analyses of diet. Ornithological Applications, 95(2), 388–394.

Howcroft, R., Eriksson, G., & Lidén, K. (2012). Conformity in diversity? Isotopic investigations of infant feeding practices in two iron age populations from Southern Öland, Sweden. American Journal of Physical Anthropology, 149(2), 217–230.

Hülsemann, F., Flenker, U., Koehler, K., & Schänzer, W. (2009). Effect of a controlled dietary change on carbon and nitrogen stable isotope ratios of human hair. Rapid Communications in Mass Spectrometry, 23(16), 2448–2454.

Ishler, V. A., Rosemond, R., Beck, T., & Becker, C. (2023). Total mixed rations for dairy cows. Retrieved May 4, 2023, from https://extension.psu.edu/total-mixed-rations-for-dairy-cows

Itahashi, Y., Erdal, Y. S., Tekin, H., Omar, L., Miyake, Y., Chikaraishi, Y., Ohkouchi, N., & Yoneda, M. (2019). Amino acid 15N analysis reveals change in the importance of freshwater resources between the hunter‐gatherer and farmer in the Neolithic upper Tigris. American Journal of Physical Anthropology, 168(4), 676–686.

Jay, M. (2009). Breastfeeding and weaning behaviour in archaeological populations: Evidence from the isotopic analysis of skeletal materials. Childhood in the Past, 2(1), 163–178.

Jay, M., Fuller, B. T., Richards, M. P., Knüsel, C. J., & King, S. S. (2008). Iron age breastfeeding practices in Britain: Isotopic evidence from Wetwang Slack, East Yorkshire. American Journal of Biological Anthropology, 136, 327–337.

Jenkins, S. G., Partridge, S. T., Stephenson, T. R., Farley, S. D., & Robbins, C. T. (2001). Nitrogen and carbon isotope fractionation between mothers, neonates, and nursing offspring. Oecologia, 129(3), 336–341.

Jim, S., Ambrose, S. H., & Evershed, R. P. (2004). Stable carbon isotopic evidence for differences in the dietary origin of bone cholesterol, collagen and apatite: Implications for their use in palaeodietary reconstruction. Geochimica et Cosmochimica Acta, 68(1), 61–72.

Kunz, C., & Lönnerdal, B. (1992). Re‐evaluation of the whey protein/casein ratio of human milk. Acta Paediatrica, 81(2), 107–112.

Lawrence, R. A., & Lawrence, R. M. (2011). Biochemistry of human Milk. In R. A. Lawrence & R. M. Lawrence (Eds.), Breastfeeding (7th ed., pp. 98–152). W.B. Saunders.

Lehn, C., Mützel, E., & Rossmann, A. (2011). Multi‐element stable isotope analysis of H, C, N and S in hair and nails of contemporary human remains. International Journal of Legal Medicine, 125, 695–706.

Liao, Y., Weber, D., Xu, W., Durbin‐Johnson, B. P., Phinney, B. S., & Lönnerdal, B. (2017). Absolute quantification of human milk caseins and the whey/casein ratio during the first year of lactation. Journal of Proteome Research, 16(11), 4113–4121.

Lönnerdal, B., Erdmann, P., Thakkar, S. K., Sauser, J., & Destaillats, F. (2017). Longitudinal evolution of true protein, amino acids and bioactive proteins in breast milk: A developmental perspective. The Journal of Nutritional Biochemistry, 41, 1–11.

Lübcker, N., Whiteman, J. P., Millar, R. P., de Bruyn, P. J. N., & Newsome, S. D. (2020). Fasting affects amino acid nitrogen isotope values: A new tool for identifying nitrogen balance of free‐ranging mammals. Oecologia, 193, 53–65.

Macko, S. A., Fogel, M. L., Hare, P. E., & Hoering, T. C. (1987). Isotopic fractionation of nitrogen and carbon in the synthesis of amino acids by microorganisms. Chemical Geology: Isotope Geoscience Section, 65(1), 79–92.

McClelland, J. W., Holl, C. M., & Montoya, J. P. (2003). Relating low δ15N values of zooplankton to N2‐fixation in the tropical North Atlantic: Insights provided by stable isotope ratios of amino acids. Deep Sea Research Part I: Oceanographic Research Papers, 50(7), 849–861.

McClelland, J. W., & Montoya, J. P. (2002). Trophic relationships and the nitrogen isotopic composition of amino acids in plankton. Ecology, 83(8), 2173–2180.

McCutchan, J. H., Jr., Lewis, W. M., Jr., Kendall, C., & McGrath, C. C. (2003). Variation in trophic shift for stable isotope ratios of carbon, nitrogen, and sulfur. Oikos, 102, 378–390.

McMahon, K. W., & McCarthy, M. D. (2016). Embracing variability in amino acid δ15N fractionation: Mechanisms, implications, and applications for trophic ecology. Ecosphere, 7(12), 1–26. https://doi.org/10.1002/ecs2.1511

Mekota, A. M., Grupe, G., Ufer, S., & Cuntz, U. (2006). Serial analysis of stable nitrogen and carbon isotopes in hair: Monitoring starvation and recovery phases of patients suffering from anorexia nervosa. Rapid Communications in Mass Spectrometry, 20(10), 1604–1610.

Mekota, A. M., Grupe, G., Ufer, S., & Cuntz, U. (2009). Identifying starvation episodes using stable isotopes in hair. Rechtsmedizin, 19(6), 431–440.

Metges, C. (2000). Contribution of microbial amino acids to amino acid homeostasis of the host. The Journal of Nutrition, 130(7), 1857S–1864S.

Metges, C., Kempe, K., & Schmidt, H. L. (1990). Dependence of the carbon‐isotope contents of breath carbon dioxide, milk, serum and rumen fermentation products on the δ13C value of food in dairy cows. The British Journal of Nutrition, 63(2), 187–196.

Metz‐Boutigue, M. H., Jollès, J., Mazurier, J., Schoentgen, F., Legrand, D., Spik, G., Montreuil, J., & Jollès, P. (1984). Human lactotransferrin: Amino acid sequence and structural comparisons with other transferrins. European Journal of Biochemistry, 145(3), 659–676.

Minagawa, M., & Wada, E. (1984). Stepwise enrichment of 15N along food chains: Further evidence and the relation between δ15N and animal age. Geochimica et Cosmochimica Acta, 48(5), 1135–1140.

Miura, K., & Goto, A. S. (2012). Stable nitrogen isotopic fractionation associated with transamination of glutamic acid to aspartic acid: Implications for understanding 15N trophic enrichment in ecological food webs. Researches in Organic Geochemistry, 28, 13–17.

Mole, J. E., Bhown, A. S., & Bennett, J. C. (1977). Primary structure of human J chain: Alignment of peptides from chemical and enzymatic hydrolyses. Biochemistry, 16(16), 3507–3513.

Mora, A., Smith, C., Standen, V. G., & Arriaza, B. T. (2022). Bulk and amino acid isotope analyses of hair detail adult diets and infant feeding practices among pre‐ and post‐maize populations of the northern Chilean coast of the Atacama Desert. Journal of Anthropological Archaeology, 67, 101435.

Nardoto, G. B., Silva, S., Kendall, C., Ehleringer, J. R., Chesson, L. A., Ferraz, E. S. B., Moreira, M. Z., Ometto, J. P. H. B., & Martinelli, L. A. (2006). Geographical patterns of human diet derived from stable‐isotope analysis of fingernails. American Journal of Physical Anthropology, 131(1), 137–146.

Neuberger, F. M., Jopp, E., Graw, M., Püschel, K., & Grupe, G. (2013). Signs of malnutrition and starvation—Reconstruction of nutritional life histories by serial isotopic analyses of hair. Forensic Science International, 226(1–3), 22–32.

O'Connell, T. C. (2017). “Trophic” and “source” amino acids in trophic estimation: A likely metabolic explanation. Oecologia, 184(2), 317–326.

O'Connell, T. C., & Hedges, R. E. M. (1999). Investigations into the effect of diet on modern human hair isotopic values. American Journal of Biological Anthropology, 108, 409–425.

O'Connell, T. C., Kneale, C. J., Tasevska, N., & Kuhnle, G. G. C. (2012). The diet‐body offset in human nitrogen isotopic values: A controlled dietary study. American Journal of Physical Anthropology, 149(3), 426–434.

O'Leary, M. H. (1981). Carbon isotope fractionation in plants. Phytochemistry, 20(4), 553–567.

Palmer, G. (2009). The politics of breastfeeding: When breasts are bad for business. Pinter & Martin Publishers.

Petzke, K. J., Fuller, B. T., & Metges, C. C. (2010). Advances in natural stable isotope ratio analysis of human hair to determine nutritional and metabolic status. Current Opinion in Clinical Nutrition and Metabolic Care, 13(5), 532–540.

Polischuk, S. C., Hobson, K. A., & Ramsay, M. A. (2001). Use of stable‐carbon and ‐nitrogen isotopes to assess weaning and fasting in female polar bears and their cubs. Canadian Journal of Zoology, 79(3), 499–511.

Popp, B. N., Graham, B. S., Olson, R. J., Hannides, C. C. S., Lott, M. J., López‐Ibarra, G. A., Lott, M. J., & Fry, B. (2007). Insight into the trophic ecology of yellowfin tuna, Thunnus albacares, from compound‐specific nitrogen isotope analysis of proteinaceous amino acids. In Terrestrial ecology (Vol. 1, pp. 173–190). Elsevier.

Rasmussen, L. K., Due, H. A., & Petersen, T. E. (1995). Human alpha s1‐casein: Purification and characterization. Comparative Biochemistry and Physiology. Part B, Biochemistry & Molecular Biology, 111(1), 75–81.

Reynard, L., & Tuross, N. (2015). The known, the unknown and the unknowable: Weaning times from archaeological bones using nitrogen isotope ratios. Journal of Archaeological Science, 53, 618–625.

Richards, M. P., Mays, S., & Fuller, B. T. (2002). Stable carbon and nitrogen isotope values of bone and teeth reflect weaning age at the Medieval Wharram Percy site, Yorkshire, U.K. American Journal of Physical Anthropology, 119(3), 205–210.

Riekenberg, P. M., van der Meer, M., & Schouten, S. (2020). Practical considerations for improved reliability and precision during determination of δ15N values in amino acids using a single combined oxidation‐reduction reactor. Rapid Communications in Mass Spectrometry: RCM, 34(14), e8797.

Romek, K. M., Julien, M., Frasquet‐Darrieux, M., Tea, I., Antheaume, I., Hankard, R., & Robins, R. J. (2013). Human baby hair amino acid natural abundance 15N‐isotope values are not related to the 15N‐isotope values of amino acids in mother's breast milk protein. Amino Acids, 45(6), 1365–1372.

Runne, U., & Orfanos, C. E. (1981). The human nail: Structure, growth and pathological changes. Current Problems in Dermatology, 9, 102–149.

Santos, G. M., De La Torre, H. A. M., Boudin, M., Bonafini, M., & Saverwyns, S. (2015). Improved radiocarbon analyses of modern human hair to determine the year‐of‐death by cross‐flow nanofiltered amino acids: Common contaminants, implications for isotopic analysis, and recommendations. Rapid Communications in Mass Spectrometry, 29(19), 1765–1773.

Schoeninger, M. J., & DeNiro, M. J. (1984). Nitrogen and carbon isotopic composition of bone collagen from marine and terrestrial animals. Geochimica et Cosmochimica Acta, 48(4), 625–639.

Sharp, Z. D. (2007). Terminology, standards and mass spectrometry. In Z. D. Sharp (Ed.), Principles of stable isotope geochemistry. Pearson Education.

Soncin, S., Talbot, H. M., Fernandes, R., Harris, A., von Tersch, M., Robson, H. K., Bakker, J. K., Richter, K. K., Alexander, M., Ellis, S., Thompson, G., Amoretti, V., Osanna, M., Caso, M., Sirano, F., Fattore, L., Colonese, A. C., Garnsey, P., Bondioli, L., & Craig, O. E. (2021). High‐resolution dietary reconstruction of victims of the 79 C.E. Vesuvius eruption at Herculaneum by compound‐specific isotope analysis. Science Advances, 7(35), 1–9. https://doi.org/10.1126/sciadv.abg5791

Steffan, S. A., Chikaraishi, Y., Horton, D. R., Ohkouchi, N., Singleton, M. E., Miliczky, E., Hogg, D. B., & Jones, V. P. (2013). Trophic hierarchies illuminated via amino acid isotopic analysis. PLoS One, 8(9), e76152.

Stegall, V. K., Farley, S. D., Rea, L. D., Pitcher, K. W., Rye, R. O., Kester, C. L., Stricker, C. A., & Bern, C. R. (2008). Discrimination of carbon and nitrogen isotopes from milk to serum and vibrissae in Alaska Steller sea lions (Eumetopias jubatus). Canadian Journal of Zoology, 86(1), 17–23.

Stricker, C. A., Christ, A. M., Wunder, M. B., Doll, A. C., Farley, S. D., Rea, L. D., Rosen, D. A. S., Scherer, R. D., & Tollit, D. J. (2015). Stable carbon and nitrogen isotope trophic enrichment factors for Steller sea lion vibrissae relative to milk and fish/invertebrate diets. Marine Ecology Progress Series, 523, 255–266.

Svensson, E., Schouten, S., Hopmans, E. C., Middelburg, J. J., & Sinninghe Damsté, J. S. (2016). Factors controlling the stable nitrogen isotopic composition (δ15n) of lipids in marine animals. PLoS One, 11(1), e0146321.

Thakkar, S. K., Giuffrida, F., Bertschy, E., De Castro, A., Destaillats, F., & Lee, L. Y. (2016). Protein evolution of human milk. Nestle Nutrition Institute Workshop Series, 86, 77–85.

Tomlinson, C., Rafii, M., Sgro, M., Ball, R. O., & Pencharz, P. (2011). Arginine is synthesized from proline, not glutamate, in enterally fed human preterm neonates. Pediatric Research, 69(1), 46–50.

Toraño, A., & Putnam, F. W. (1978). Complete amino acid sequence of the alpha 2 heavy chain of a human IgA2 immunoglobulin of the A2m (2) allotype. Proceedings of the National Academy of Sciences of the United States of America, 75(2), 966–969.

Treckel, P. A. (1989). Breastfeeding and maternal sexuality in colonial America. Journal of Interdisciplinary History, 20(1), 25–51.

Wallace, C. J. A., & Hedges, R. E. M. (2016). Nitrogen isotopic discrimination in dietary amino acids: The threonine anomaly. Rapid Communications in Mass Spectrometry, 30(22), 2442–2446.

Walter, B. S., DeWitte, S. N., Dupras, T., & Beaumont, J. (2020). Assessment of nutritional stress in famine burials using stable isotope analysis. American Journal of Biological Anthropology, 172(2), 214–226.

Waters‐Rist, A. L., Bazaliiskii, V. I., Weber, A. W., & Katzenberg, M. A. (2011). Infant and child diet in Neolithic hunter‐fisher‐gatherers from cis‐baikal, Siberia: Intra‐long bone stable nitrogen and carbon isotope ratios. American Journal of Physical Anthropology, 146(2), 225–241.

Waterlow, J. C. (2006). Protein turnover. CABI. https://doi.org/10.1079/9780851996134.0000

Whiteman, J. P., Rodriguez Curras, M., Feeser, K. L., & Newsome, S. D. (2021). Dietary protein content and digestibility influences discrimination of amino acid nitrogen isotope values in a terrestrial omnivorous mammal. Rapid Communications in Mass Spectrometry, 31(11), e9073. https://doi.org/10.1002/rcm.9073

World Health Organization. (2008). Indicators for assessing infant and young child feeding practices, part I definitions: Conclusions of a consensus meeting held 6–8 November 2007 in Washington. DC.

Wright, L. E., & Schwarcz, H. P. (1999). Correspondence between stable carbon, oxygen and nitrogen isotopes in human tooth enamel and dentine: Infant diets at Kaminaljuyu. Journal of Archaeological Science, 26(9), 1159–1170.

Wu, G. (1997). Synthesis of citrulline and arginine from proline in enterocytes of postnatal pigs. The American Journal of Physiology, 272(6 Pt 1), G1382–G1390.

Wu, G., Bazer, F. W., Burghardt, R. C., Johnson, G. A., Kim, S. W., Knabe, D. A., Li, P., Li, X., McKnight, J. R., Satterfield, M. C., & Spencer, T. E. (2011). Proline and hydroxyproline metabolism: Implications for animal and human nutrition. Amino Acids, 40(4), 1053–1063.

Yaemsiri, S., Hou, N., Slining, M. M., & He, K. (2010). Growth rate of human fingernails and toenails in healthy American young adults. Journal of the European Academy of Dermatology and Venereology: JEADV, 24(4), 420–423.

Yarnes, C., & Herszage, J. (2017). The relative influence of derivatization and normalization procedures on the compound‐specific stable isotope analysis of nitrogen in amino acids. Rapid Communications in Mass Spectrometry, 31(8), 693–704. https://doi.org/10.1002/rcm.7832

You, X., Rani, A., Özcan, E., Lyu, Y., & Sela, D. A. (2023). Bifidobacterium longum subsp. infantis utilizes human milk urea to recycle nitrogen within the infant gut microbiome. Gut Microbes, 15(1), 2192546.