Calorie Restriction Rewires Muscle Proteins in Sex-Specific Ways to Improve Blood Sugar Control

New research from teams at the University of Michigan and the University of Sydney has uncovered a surprisingly large sex-based difference in how aging muscles respond to a reduced-calorie diet. While both male and female rats experienced better insulin sensitivity and improved glucose uptake after several weeks of calorie restriction, the underlying molecular changes were dramatically different between the sexes. This discovery adds an important layer to how scientists understand diet, metabolism, and aging — and it highlights why studying both sexes is essential when developing strategies to prevent or treat type 2 diabetes.

Researchers worked with 24-month-old rats, an age roughly equivalent to elderly humans, and fed them 35% fewer calories than usual for eight weeks. The main goal was to figure out exactly which protein-level changes inside muscle cells explain why calorie restriction so consistently improves insulin-stimulated glucose uptake — a key part of healthy blood sugar control that often declines with age.

While the outcome was similar for both sexes — increased glucose uptake in skeletal muscle — the internal “routes” their muscles used were very different. The study mapped changes in protein phosphorylation, a chemical process that works like a switch to turn protein functions on, off, or somewhere in between. These phosphorylation shifts reflect how muscle cells rewire themselves in response to new conditions such as diet changes or insulin exposure.

Major Findings: Same Benefits, Different Biological Pathways

The researchers found that nearly 70% of the calorie restriction–induced molecular changes were sex-dependent. That means male and female muscles relied on different sets of proteins and pathways to produce the same final benefit.

More specifically:

Calorie restriction boosted insulin-stimulated glucose uptake in both sexes.
Females had higher glucose uptake overall, regardless of whether their diet was restricted or not.
Insulin altered phosphorylation at more than twice as many protein sites in females as in males, with females showing around 193 affected sites compared to 107 in males.
Only about 60 phosphorylation sites overlapped between males and females in response to insulin.
When it came to calorie restriction alone, males actually showed about 30% more diet-responsive phosphorylation changes than females.

In simpler terms, both sexes improved their muscle sugar handling — but they did so through different molecular strategies. The researchers compared this to using Google Maps: the destination is the same, but the routes differ.

Two Key Proteins Stand Out

Among hundreds of phosphorylation changes, two proteins emerged as particularly important:

Lmod1 (Leiomodin-1)
Ehbp1l1 (EH domain-binding protein 1-like 1)

These two proteins contained insulin-responsive phosphorylation sites that strongly correlated with insulin-stimulated glucose uptake across individual animals. This makes them especially interesting because both proteins already have known genetic links to blood sugar regulation in humans.

That means the molecular behavior observed in aged rats might also matter in human metabolism — potentially opening new therapeutic directions.

Metabolite Analysis Shows Even More Sex Differences

The research team also conducted a separate experiment using the same animals, analyzing about 1,000 metabolites — chemicals produced by diet or metabolism. Their findings showed:

Around 40% of these metabolites were altered by calorie restriction within each sex.
Many were changed in both sexes, but a large number were altered only in one sex, reinforcing the idea that males and females adapt differently on a biochemical level.

These additional results strengthen the case that biological sex needs to be considered not just in physiology, but in metabolomics, proteomics, and therapeutic design.

Why This Study Matters for Diabetes and Aging Research

Muscle tissue plays a major role in regulating blood sugar because it’s the main site where glucose is absorbed during insulin signaling. As people age, muscle insulin sensitivity often declines, increasing the risk of type 2 diabetes.

Calorie restriction has long been known to improve insulin sensitivity in animals, and even in humans under controlled conditions. But scientists have not fully understood the molecular mechanisms behind this improvement, especially in older individuals.

This study helps fill that gap by showing:

Aging muscles undergo extensive protein-level rewiring in response to calorie restriction.
These changes differ substantially between males and females.
The improvements in glucose uptake rely on sex-specific phosphorylation patterns, rather than identical biochemical pathways.

These findings suggest that future treatments for age-related metabolic decline may need to be sex-tailored. Therapies that work well for one sex may not work the same way for the other — or may need different dosages or molecular targets.

What to Know About Phosphorylation and Muscle Metabolism

Protein phosphorylation is a central way that cells regulate thousands of biological processes. In skeletal muscle, phosphorylation controls:

Glucose transport
Insulin signaling
Muscle contraction
Energy utilization
Stress responses
Growth and repair

In this study, calorie restriction and insulin exposure altered phosphorylation on hundreds of sites across many proteins, especially those involved in muscle structure and contraction. Because skeletal muscle makes up such a large portion of body mass, even subtle biochemical improvements can significantly influence whole-body metabolic health.

Broader Context: Calorie Restriction and Insulin Sensitivity

Calorie restriction has been widely studied for decades, known to:

Extend lifespan in many species
Improve metabolic health
Reduce inflammation
Enhance insulin sensitivity

However, the degree of restriction and the way different individuals respond can vary. This research adds a valuable layer by showing that aging male and female bodies don’t just react differently externally — they also diverge internally at the biochemical level.

Potential Future Directions

Researchers noted several follow-up possibilities:

Testing whether modifying phosphorylation on Lmod1 or Ehbp1l1 directly affects muscle glucose uptake
Examining whether similar phosphorylation patterns occur in human skeletal muscle
Exploring whether calorie restriction mimetics — drugs that mimic CR’s molecular effects — should be sex-specific
Integrating proteomics with metabolomics, transcriptomics, and genetic analysis for a more complete understanding

This research represents a step toward personalized metabolic medicine, where treatments for insulin resistance or type 2 diabetes might one day be based on a person’s sex and their molecular profile.