Fish Diet Makes Medieval Bones Appear Centuries Too Old
Researchers found freshwater fish consumption can skew radiocarbon dates of human skeletons from medieval Denmark.
Updated on Nov. 10, 2026 in Nutrition
A Danish-Dutch research team reexamined radiocarbon dates for 27 medieval skeletons and found that eating freshwater fish can make bones appear hundreds of years too old. Five of the 27 skeletons were incorrectly dated.
Why it matters
Radiocarbon dating is a core tool for building historical timelines, so misdated remains can distort conclusions about when monasteries and burial practices existed. The findings show dietary carbon must be accounted for before dates are trusted.
Radiocarbon dates for five of 27 reexamined skeletons were found to be wrong, with the freshwater reservoir effect ranging from zero to 1,000 years compared with roughly 400 years in the North Sea marine effect. The study does not yet establish how many other dated remains across the approximately 140 medieval Danish monasteries may be affected.
The players
npj Heritage Science
A peer-reviewed journal published by Nature Portfolio that covers the scientific study of cultural heritage and archaeological materials.
The details
Freshwater fish can contain less carbon-14 than land-based foods because old carbon released from lake and river sediments enters the food chain. When people eat these fish, that carbon becomes incorporated into their bones, making them test as older than they are. One Viborg skeleton's previous date range ended in A.D. 976, more than 200 years before the monastery was built.
Timeline
A.D. 976: Latest end of a previous date range for a Viborg skeleton.
A.D. 1100 to 1155: Previous radiocarbon date range for skeleton III9B from Svendborg.
1236: Svendborg Franciscan monastery was founded.
November 10, 2026: Research findings published in npj Heritage Science.
Big Picture
Archaeologists have long corrected radiocarbon dates for the roughly 400-year marine reservoir effect observed in the North Sea. This study shows freshwater diets can shift dates by anywhere from zero to 1,000 years, meaning those corrections cannot simply be borrowed from marine settings.
Readers interested in ancient diets or historical dating methods can see how eating habits can alter scientific measurements of the past. Anyone studying the past through bone or tooth samples should account for local diet before trusting radiocarbon results.
The takeaway
Scientific dates are only as reliable as the assumptions behind them, and diet can quietly shift a skeleton's apparent age by centuries. Checking local food sources before interpreting radiocarbon results helps keep historical timelines accurate.
Further reading
For more on how diet shapes what the body reveals about the past, explore our Nutrition coverage.
Source note: This article includes information reported by Phys.
