Adding whey protein to glucose lowered blood sugar spikes in healthy adults not by increasing glucose uptake, but by reducing early glucose absorption — despite a threefold increase in glucagon and enhanced GIP and insulin responses.
~3-fold glucagon increaseDespite this large glucagon stimulation from whey protein, blood sugar excursions were the lowest of all three conditions — the benefit came from reduced glucose absorption, not increased glucose disposal
What the researchers found
In 11 healthy adults, whey protein-glucose coingestion (25WG) produced the lowest glucose excursions compared to glucose alone (25G or 50G). Key peptide hormone responses: glucagon increased ~3-fold with 25WG (suppressed with glucose alone), GIP was significantly higher with 25WG versus both 25G and 50G, GLP-1 was similar across conditions, and insulin was higher for 25WG versus 25G.
Critically, despite greater insulin secretion, whole-body glucose disposal (Rd) was not enhanced. Endogenous glucose production was less suppressed with 25WG (~50%) versus 25G (~70%) or 50G (~80%). The net glycemic benefit stemmed from reduced early-phase (30-60 min) glucose absorption from the gut — a novel mechanistic finding.
Why it matters
This study resolves a long-standing paradox in nutrition science: why does protein improve blood sugar control despite stimulating glucagon? The answer — reduced glucose absorption rather than increased glucose uptake — has practical implications for dietary strategies in diabetes management and challenges assumptions about how protein and incretin peptides regulate postprandial glucose.
How the study worked
Eleven healthy adults completed three crossover trials ingesting: 25g glucose (25G), 50g glucose (50G), or 25g glucose plus 25g whey protein (25WG). The triple stable isotope glucose tracer technique was used to simultaneously measure glucose absorption from the gut, endogenous glucose production, and whole-body glucose disposal. Blood samples measured insulin, glucagon, GLP-1, and GIP responses.
What this study cannot tell us
Small sample size (n=11). Only healthy young adults were studied — results may differ in people with type 2 diabetes or impaired glucose tolerance. Only whey protein was tested; other protein sources may behave differently. The triple tracer technique measures net fluxes and may miss tissue-specific effects. A single protein dose (25g) was used, so dose-response relationships are unknown.
How to read the evidence
This is a well-designed crossover study in humans using the gold-standard triple stable isotope tracer technique. While the sample size is small (n=11), the within-subject crossover design and sophisticated measurement methods provide high-quality mechanistic data.
When this study was published
Published in 2025 in Diabetes, a top-tier journal, this is very recent research providing novel mechanistic insights into protein-glucose interactions.
The bigger picture
This study provides important mechanistic context for the incretin peptide field. The finding that GIP (not GLP-1) was the incretin response primarily enhanced by whey protein, and that the glucose benefit came from delayed absorption rather than increased disposal, challenges simplified models of how peptide hormones regulate blood sugar. This has implications for understanding GIP-based therapies like tirzepatide.
Questions still open
- Do people with type 2 diabetes show the same reduced glucose absorption benefit from protein-glucose coingestion?
- Does the GIP enhancement from whey protein explain part of the metabolic benefits attributed to high-protein diets?
- Would other protein sources (casein, plant proteins) produce similar effects on glucose absorption and incretin responses?
Common questions
Why does protein lower blood sugar if it also raises glucagon?
What is GIP and why was its response important in this study?
Read the original research
Mechanistic Insights Into Postprandial Insulin-Glucagon Interactions and Their Impact on Glucose Flux After Protein-Glucose Coingestion in Humans.
Diabetes, 74(11), 1946-1956
Citation
Dao, Giang M; Shaw, Chistopher S; Betik, Andrew C; Kuriel, Vicky; Bruce, Clinton R; Kowalski, Greg M. (2025). Mechanistic Insights Into Postprandial Insulin-Glucagon Interactions and Their Impact on Glucose Flux After Protein-Glucose Coingestion in Humans.. Diabetes, 74(11), 1946-1956. https://doi.org/10.2337/db25-0395