The researchers set out to resolve a puzzle that has shadowed nutrition science for years. Protein is indispensable, fueling muscle growth, immune regulation, and bone development, yet large epidemiological studies have linked very high protein intake to elevated all-cause mortality and to age-related conditions such as chronic kidney disease and type 2 diabetes. Meanwhile, dozens of experiments in organisms from yeast to mice have shown that restricting protein extends life. What has been missing is a dynamic, mechanistic account of how excess protein actually promotes aging across the lifespan, rather than a snapshot taken at a single moment in time.

To fill that gap, the team turned to Drosophila melanogaster, the fruit fly, a classic aging model whose short life makes lifelong observation practical. They fed flies a series of six diets with identical total calories but protein energy supplies ranging from 5 to 30 percent, carefully adjusting glucose so that only the protein-to-carbohydrate ratio changed. The result was a striking inverted U-shaped curve. Flies on the 10 percent protein diet lived longest, with a median lifespan of 75 days. As protein climbed, lifespan fell in a dose-dependent manner, dropping to just 54 days at 30 percent protein. Even the extreme 5 percent diet shortened life, confirming that both too little and too much protein are harmful. Food intake measurements ruled out the possibility that flies on high-protein diets were simply eating more calories, and climbing tests showed the effect was not an artifact of general frailty at the start of the intervention.

With the lifespan effect established, the researchers deployed a formidable multi-omics arsenal. They sequenced the transcriptomes of flies at day 18, representing early life, and day 48, representing middle-to-late life, across all five protein levels, and complemented this with quantitative proteomics and amino acid metabolic profiling. By correlating the expression of every gene with lifespan grades across the dietary spectrum, they narrowed thousands of candidates down to a shortlist of genes that responded consistently to high protein at both life stages. Pathway analysis kept pointing to one metabolic hub: the glycine, serine, and threonine metabolism pathway, which was strongly and persistently activated by high-protein diets at both the RNA and protein levels, and whose activity scores correlated negatively with lifespan with Spearman coefficients as high as minus 0.97.

Within that pathway, a protein interaction network analysis flagged one gene as the top-ranked hub: CG6415, the fly homolog of the human AMT gene. Its expression rose steadily as dietary protein increased and lifespan fell, and it was also among a set of 150 genes whose expression changed in the same direction during natural aging, suggesting that the diet essentially hijacks a program the animal already runs as it grows old. To test causality, the team generated flies in which CG6415 was either deleted entirely or overexpressed throughout the body. The results were decisive. Knocking out CG6415 extended the median lifespan of flies on the 30 percent protein diet by 15.7 percent and restored the activity of antioxidant enzymes such as superoxide dismutase and catalase, which the high-protein diet had suppressed. Conversely, forcing CG6415 expression on a normal 10 percent protein diet shortened median lifespan by 22 percent, mimicking the effect of a high-protein diet without any change in nutrition. Combining overexpression with a high-protein diet produced the worst survival of all, a clear additive interaction confirmed statistically by two-way analysis of variance.

The mechanism, the omics data indicated, centered on oxidative phosphorylation, the mitochondrial process that converts nutrients into ATP. High-protein diets suppressed the oxidative phosphorylation pathway, activated senescence-related signaling through the p53 pathway, and dampened electron transport and ion transport, all changes that CG6415 knockout reversed. ATP levels told the same story: they fell significantly in high-protein-fed flies and were partially restored by deleting CG6415. Because oxidative phosphorylation is the core function of mitochondria, the team reasoned that CG6415 must be acting on mitochondrial homeostasis itself.

To test that idea in a mammalian context, the researchers moved to human embryonic kidney 293T cells, a validated model for mitochondrial and aging studies, and manipulated AMT, the human counterpart of CG6415. Overexpressing AMT reduced the activities of mitochondrial respiratory chain complexes I, II, III, and V, lowered cellular ATP, collapsed the mitochondrial membrane potential, and flooded the cells with reactive oxygen species, including mitochondria-specific superoxide detected with the MitoSOX probe. Senescence markers followed: senescence-associated beta-galactosidase staining increased, inflammatory genes such as TNF-alpha, IL-1 beta, and IL-6 were upregulated, and the canonical p53-p21 senescence pathway switched on at both the mRNA and protein levels. In the reverse experiment, silencing AMT with siRNA in cells stressed by hydrogen peroxide restored complex V activity, ATP production, and membrane potential, reduced ROS, and blunted the p53-p21 response, demonstrating that the AMT-ROS-p53-p21 axis is a genuine conduit from mitochondrial dysfunction to cellular senescence.

The final question was which component of dietary protein drives the effect. Proteins are built from twenty amino acids, so the team profiled the amino acid pools of high-protein-fed flies and then supplemented each amino acid individually onto the baseline 10 percent protein diet. Three of them, isoleucine, valine, and phenylalanine, shortened lifespan significantly, by roughly five to six days, while several others, including threonine, methionine, and glycine, actually extended it. Molecular docking showed that all three lifespan-shortening amino acids can bind the CG6415 protein, but only isoleucine reliably upregulated the gene in the fly thorax and abdomen and impaired both oxidative and heat stress resistance. Crucially, CG6415 knockout rescued the lifespan reduction caused by isoleucine supplementation, extending median lifespan by nine days, and lowering dietary isoleucine within a high-protein diet extended lifespan in control flies but not in CG6415-overexpressing ones. Isoleucine also reproduced the dose-response relationship of the full high-protein diet, with lifespan shortening progressively as supplementation increased. Together, these experiments position isoleucine as the key amino acid through which high-protein diets activate CG6415 and promote aging, a finding that resonates with earlier mouse studies showing that dietary isoleucine restriction extends healthspan and lifespan.

The authors are careful about the limits of their work. The study relied primarily on flies, with human cells used only for mechanistic verification, and organ-specific studies in mammals remain to be done. Individual amino acid supplementation only partially reproduced the high-protein effect, hinting that protein’s impact on aging emerges from the combined action of multiple amino acids rather than any single one. The researchers also focused on pathways uniformly regulated across two life stages, leaving stage-specific effects for future exploration. Still, the implications are hard to ignore. At a time when protein supplements are marketed aggressively to adolescents, fitness enthusiasts, and older adults alike, the study offers a cautionary counterweight: sustained excessive protein intake, particularly of isoleucine-rich foods, may quietly erode mitochondrial function and accelerate the very aging process that health-conscious consumers are trying to delay. The identification of CG6415/AMT as a druggable node in this pathway, and of isoleucine as its dietary trigger, opens a concrete path toward precision nutrition, in which protein quality and amino acid composition, not just quantity, become the targets of anti-aging dietary design.

Subject of Research: Mechanism by which high-protein diets accelerate aging through CG6415/AMT gene activation and mitochondrial dysfunction

Article Title: High-protein diet promotes aging by activating the CG6415 / AMT gene and disrupting mitochondrial homeostasis

Article References: Xu, X., Zheng, W., Liu, Q., Li, Y., Yang, S., Zhou, R., Wang, Y., Qi, X., Zhang, C., Zhang, F., Wu, Y., Li, Y., & Li, Y. (2026). High-protein diet promotes aging by activating the CG6415/AMT gene and disrupting mitochondrial homeostasis. Journal of Advanced Research, 88, 995-1012. https://doi.org/10.1016/j.jare.2026.01.063

Image Credits: AI Generated

DOI: 10.1016/j.jare.2026.01.063

Keywords: high-protein diet, aging, lifespan, Drosophila melanogaster, CG6415, AMT gene, mitochondria, oxidative phosphorylation, isoleucine, reactive oxygen species, p53-p21 pathway, nutrition

The study, led by researchers at Fujita Health University, set out to probe a long-standing puzzle in metabolic physiology. Insulin and glucagon, the two pancreatic hormones that jointly govern nutrient handling, respond very differently to protein than to carbohydrate. Glucose ingestion robustly stimulates insulin and suppresses glucagon in healthy individuals, but in type 2 diabetes the early insulin response is blunted and glucagon paradoxically rises. Protein, by contrast, stimulates both hormones in everyone, making protein a particularly potent glucagon secretagogue regardless of glycemic status. Because insulin drives amino acid uptake into peripheral tissues for protein synthesis while glucagon promotes hepatic gluconeogenesis, converting amino acids into glucose, the researchers hypothesized that the balance between these hormones, known as the insulin-to-glucagon ratio, would determine how the liver handles a sudden flood of dietary amino acids.

To test this, the team compared nondiabetic mice with mice made diabetic by injections of streptozotocin, a compound that destroys insulin-producing beta cells and produces a model of severe insulin deficiency. After an overnight fast, both groups received either a liquid normal control diet, containing roughly 67 percent carbohydrate and 15 percent protein, or a liquid high-protein diet with 58 percent protein and only 23 percent carbohydrate, matched for calories. Blood and liver samples were collected two hours after feeding, a window chosen to capture the immediate postprandial response rather than the slower transcriptional adaptation that emerges over days of high-protein feeding.

The hormonal results were revealing. In nondiabetic mice, the high-protein meal produced lower insulin and GIP secretion at 60 minutes than the control diet, an effect the authors attribute largely to the reduced carbohydrate content of the meal. Glucagon, however, rose significantly higher with the protein load. In diabetic mice, insulin secretion in response to the control diet was markedly attenuated, as expected, but the response to the high-protein diet was comparable to that of nondiabetic animals. Strikingly, despite lower insulin secretion, postprandial blood glucose was actually lower in diabetic mice given the high-protein meal than in those given the carbohydrate-rich control diet, suggesting that cutting carbohydrate from a meal can blunt glycemic excursions even when insulin secretion is impaired.

Plasma amino acid profiling then exposed a sharp divergence between the two metabolic states. The high-protein diet elevated nearly all essential amino acids, including the branched-chain amino acids leucine, isoleucine, and valine, in both diabetic and nondiabetic mice. But on the normal control diet, diabetic mice generally showed lower plasma amino acid levels than their nondiabetic counterparts, a pattern consistent with accelerated gluconeogenesis draining amino acids from the circulation. When the protein load arrived, several amino acids, including arginine, tyrosine, phenylalanine, tryptophan, and isoleucine, rose significantly higher in diabetic mice than in nondiabetic ones, hinting that the diabetic body handles a protein surge differently from the very first hours.

The liver told an even more dramatic story. In nondiabetic mice, two hours of high-protein feeding left hepatic amino acid content essentially unchanged, with glutamine the only significant increase. In diabetic mice, the same meal produced significant hepatic accumulation of alanine, aspartic acid, proline, threonine, and all three branched-chain amino acids, and the increase extended to nearly every essential amino acid except histidine. Glutamine was the lone exception, falling significantly below control levels. The combination of lower plasma levels for some amino acids and higher hepatic content in diabetic mice led the authors to conclude that amino acid uptake into the liver is accelerated in diabetic animals given a protein load, effectively overwhelming the organ’s metabolic capacity and leaving substrate pooled inside hepatic tissue.

Crucially, this accumulation occurred without any corresponding transcriptional program. Quantitative PCR of sixteen genes involved in glucose and amino acid metabolism, including gluconeogenic enzymes such as Pepck and G6pc, aminotransferases, urea cycle genes, and glutaminase, revealed almost no differences between diet groups within the two-hour window. The only significant diet-related change was Cps1 in diabetic mice. This stands in sharp contrast to the team’s earlier work showing that seven days of high-protein feeding induces robust glucagon-mediated upregulation of hepatic amino acid-catabolizing enzymes, which keeps plasma amino acids, apart from branched-chain amino acids, near normal. The new data indicate that the immediate postprandial amino acid excursion is governed by non-transcriptional mechanisms, such as altered tissue uptake and substrate diversion toward gluconeogenesis, rather than by rapid changes in enzyme gene expression.

Signaling analyses added further nuance. Western blotting of liver proteins showed that Akt phosphorylation, a readout of insulin signaling, was significantly reduced by the high-protein meal in diabetic mice, and reduced glycogen content followed the same pattern in both groups, likely reflecting the meal’s lower carbohydrate content. Yet phosphorylation of GSK3β, Akt’s downstream effector in glycogen synthesis, and of CREB, the glucagon-responsive transcription factor, were unchanged across all four groups, suggesting that the two-hour time point captures signaling dynamics whose temporal relationship to hormone levels is more complex than simple feed-forward regulation. The authors note that time-course studies will be needed to untangle these kinetics.

Metabolomics also uncovered a paradox in hepatic glycogen. Despite impaired insulin secretion and a glucagon-dominant hormonal profile, diabetic mice had higher liver glycogen than nondiabetic mice under both diets, echoing recent evidence that glucagon may contribute to postprandial glycogen repletion and that hyperglycemia itself, through sheer substrate availability, can drive glycogen accumulation even without insulin action. Meanwhile, diabetic livers showed elevated early glycolytic and gluconeogenic intermediates, including glucose 6-phosphate and fructose 6-phosphate, which the authors interpret not as inefficient metabolic flux, as reported in obese diabetic models, but as a distinct state in which overwhelming substrate availability from accumulated amino acids feeds into glucose production pathways.

The clinical implications are intriguing and carefully hedged. In type 1 diabetes, where insulin secretion is severely compromised, protein-rich meals can promote hyperglycemia by stimulating glucagon. But the present findings suggest that when residual insulin secretion is preserved, as in many cases of type 2 diabetes, high-protein, low-carbohydrate feeding enhances hepatic amino acid uptake and glucose production without raising postprandial blood glucose, because the limited carbohydrate content offsets the glucose-raising effect of glucagon-stimulated gluconeogenesis. The study also raises new questions about FGF21, a liver-derived hormone acutely lowered by the protein meal in nondiabetic mice, hinting that its secretion is rapidly tuned by dietary protein or insulin action. The authors acknowledge limitations, including the absence of fasting-baseline metabolomics, no direct measurement of glycogen turnover, and no assessment of intestinal nutrient absorption. Even so, the work delivers a memorable message: in diabetes, the liver’s first response to a protein feast is a silent traffic jam of amino acids, one that precedes and perhaps shapes the slower genetic adaptation that follows.

Subject of Research: Acute high-protein feeding and hepatic amino acid metabolism in diabetic versus nondiabetic mice

Article Title: Acute high‐protein feeding induces hepatic amino acid accumulation in diabetic male mice prior to transcriptional adaptation

Article References: Acute high‐protein feeding induces hepatic amino acid accumulation in diabetic male mice prior to transcriptional adaptation. (n.d.). https://doi.org/10.14814/phy2.71117

Image Credits: AI Generated

DOI: 10.14814/phy2.71117

Keywords: diabetes, glucagon, insulin, amino acids, liver metabolism, high-protein diet, branched-chain amino acids, gluconeogenesis, FGF21, metabolomics, streptozotocin, postprandial metabolism