A new study has found that a ketogenic diet can restore a major training benefit that high blood sugar had shut down in mice, allowing exercise to raise oxygen use again.
That finding recasts the problem of exercise resistance, suggesting that metabolic control may decide whether endurance training can fully take hold.
Where benefits stall
In mice with high blood sugar, aerobic training no longer produced the usual rise in oxygen use described in the paper.
Tracking that failure, Sarah J. Lessard at Virginia Tech’s Fralin Biomedical Research Institute found that a ketogenic diet returned blood sugar to normal within one week.
Once glucose fell, the animals’ muscles again developed the endurance-oriented features that regular training had failed to build.
The result points to a limit of exercise itself and sets up the deeper question of why metabolism can determine whether training works as intended.
The role of oxygen use
In exercise science, VO2peak, the highest rate of oxygen use, shows how well the body sustains hard movement.
Across human studies, higher cardiorespiratory fitness consistently tracks with lower risks of early death and chronic disease.
For people with diabetes, a human review shows lower aerobic capacity also lines up with more cardiovascular problems and poorer physical function.
That helps explain why high blood sugar can quietly drain value from workouts, even when someone exercises regularly.
Fat becomes fuel
Once carbohydrate intake drops very low, ketosis, when the body mainly burns fat, starts taking over fuel use.
In the mice, that meant more fat burning during rest and exercise, while glucose use and storage fell.
Because fat needs more oxygen than carbohydrate to make energy, oxygen consumption rose even before fitness improved.
That detail matters, because higher oxygen use alone did not mean the diet automatically improved performance.
Muscles adapt to keto
Inside muscle, the ketogenic diet lowered proteins that pull glucose into cells and raised ones that move fatty acids.
At the same time, mitochondria – the structures that turn fuel into usable energy– became larger and more abundant.
Those changes help explain why the animals relied on fat and looked ready for endurance work.
Still, the rewiring mattered most when exercise supplied the signal telling muscle what to build next.
Vessels grow back
During training, capillaries, the tiniest blood vessels in muscle, increased in ketogenic mice but barely changed with regular chow.
More capillaries create more routes for oxygen to reach muscle, raising aerobic capacity as exercise gets harder.
“Over time, the diet caused remodeling of the mice’s muscles, making them more oxidative and making them react better to aerobic exercise,” said Lessard.
That combination restored the muscle changes high blood sugar had blocked, linking better glucose control to better oxygen delivery.
Performance did not rise
Even with better oxygen use, ketogenic mice did not last longer on maximal treadmill tests than chow-fed mice.
“Their bodies were more efficiently using oxygen, which is a sign of higher aerobic capacity,” said Lessard.
One likely reason was low glycogen, stored carbohydrate in muscle and liver, which fell to half the chow level.
When researchers restored carbohydrates for one week after long-term keto feeding, treadmill performance improved even as ketones fell.
Normal sugar differs
In mice whose blood sugar started normal, the ketogenic diet did not improve the training response the same way.
Sedentary animals showed some rise in aerobic capacity, but exercise-trained animals gained no extra training edge from keto.
That contrast suggests glucose lowering did most of the work in the high-sugar mice, not fat burning alone.
It also warns against assuming a diet that helps one metabolic state will help every state equally.
What humans face
Because the experiment used male mice with induced diabetes, nobody should read this as a ready-made human plan.
Outside disease, sports nutrition experts claim that ketogenic diets are usually neutral or worse for athletic performance.
“Our previous studies have shown that any strategy you and your doctor have arrived at to reduce your blood sugar could work,” said Lessard.
That leaves room for Mediterranean-style eating backed by a guideline, but any real plan still needs medical guidance.
Next phase of study
Human trials now need to test whether lowering blood sugar before and during training restores this lost exercise benefit in real people.
Researchers will also need to separate two possible drivers, lower glucose itself and higher ketones during exercise.
Earlier human evidence shows the problem is clinically plausible, not just a quirk of mice.
That makes the mouse result more than a curiosity, because it offers a mechanism worth testing in clinical studies.
What this changes
Exercise still has value, but this work argues muscles respond best when training and metabolism stop pulling opposite ways.
For people with chronically high blood sugar, the next useful question is which diet makes exercise work again.
The study is published in Nature Communications.
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