Science - USA (2022-01-28)

Our results provide evidence that the gut
microbiome plays a functional role during the
hibernation season, building upon earlier re-
sults that suggest a possible role for the micro-

biome in pre-hibernation fattening ( 17 ). Our

demonstration of monogastric urea nitrogen

salvage as a mechanism facilitating protein

synthesis under nitrogen limitation has potential

implications beyond hibernation. For example,

muscle wasting affects hundreds of millions of

people globally as a result of nitrogen-limited

diets ( 18 ) and sarcopenia ( 19 ), and there is

evidence for humans possessing the neces-

sary machinery for urea nitrogen salvage ( 20 ).

Understanding the mechanisms by which

hibernators maintain protein balance and

mitigate muscle wasting under severe nitro-

gen limitation may inform strategies for muscle

preservation in humans.

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ACKNOWLEDGMENTS

We thank A. Steinberg, S. Cailey, W. Porter, S. Martin, and M. T. Grahn

for assistance, and the reviewers for insightful comments.

Funding:Work was supported by NSF Grant IOS-1558044 to H.V.C.,

F.M.A.-P., and G.S.; NIH Grants P41GM136463, P41GM103399

(NIGMS), and P41RR002301 to the National Magnetic Resonance

Facility at Madison; National Institute of General Medical Sciences of

the NIH traineeship T32GM008349 and NSF Graduate Research

Fellowship DGE-1747503 to E.C.; NSERC Canada Postdoctoral

Fellowship to M.D.R.Author contributions:H.V.C., F.M.A.-P., G.S.,

M.D.R., and M.T. designed the study; M.D.R., E.C., M.T., S.G., and K.V.

collected data; M.D.R., F.M.A.-P., G.S., E.C., and Y.L. conducted

analyses; M.D.R. wrote the manuscript with input from all authors.

Competing interests:F.M.A.-P. is the founder of MetResponse, LLC

and Isomark Health Inc. The other authors declare no competing

interests.Data and materials availability:Metagenomic data

are available at NCBI (PRJNA693524); other data are available as

supplementary materials.

SUPPLEMENTARY MATERIALS

science.org/doi/10.1126/science.abh2950

Materials and Methods

Figs. S1 to S8

Tables S1 to S3

References ( 21 – 49 )

MDAR Reproducibility Checklist

Data S1

1 March 2021; resubmitted 15 July 2021

Accepted 2 December 2021

10.1126/science.abh2950

SCIENCEscience.org 28 JANUARY 2022¥VOL 375 ISSUE 6579 463

A

B

C

Fig. 4.^15 N-urea nitrogen incorporation into metabolite and protein pools of gut microbiome and host
tissues.(A) Cecal content, (B) liver, and (C) muscle (quadriceps) of microbiome-intact (filled bars) and
microbiome-depleted (open bars) squirrels. For each panel,^15 N-Protein portrays^15 N-incorporation into
protein,^15 N-Metabolome portrays^15 N-incorporation into the total pool of metabolites (identifiable and
nonidentifiable), and the remaining plots portray^15 N-incorporation into the metabolite named above the plot.
Metabolomic data are relative abundances in arbitrary units. Asterisks indicate a significant difference
between microbiome groups within a season, and different lowercase letters indicate significant seasonal
difference among microbiome-intact groups. See tables S1 and S2 for statistical results.n= 5 for all data,
except for early winter microbiome depleted (n= 4) and cecum content^15 N-protein [n= 1 (pooled samples)].

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