Carbs, Fats, and Exercise
Why Your Muscles Reach for Carbohydrates When Work Gets Hard
If you’ve ever felt like your legs “hit a wall” during a tough workout, a long walk, or even shovelling snow, you’ve experienced your muscles asking for faster fuel. Many people are told that burning fat is the ultimate goal of exercise, but the reality inside your muscles is more nuanced.
Newer research is helping us better understand why your body relies more on carbohydrates than fats during intense effort, and how this matters not just for athletes, but for everyday people trying to move better, exercise safely, and manage pain or fatigue.
How your muscles make energy
Inside every muscle cell are structures called mitochondria, often described as the “powerhouses” of the cell. Their job is to convert the food you eat into usable energy so your muscles can contract.
Mitochondria can use both fats and carbohydrates as fuel. At rest or during low-intensity activity, your body can rely heavily on fat. But as exercise intensity increases, whether that’s climbing stairs, lifting weights, or clearing heavy snow, your muscles demand energy faster.
Carbohydrates can be broken down and used more quickly than fats, which is why your muscles increasingly depend on carbs as effort rises.
Why intensity changes fuel choice
During harder exercise, the environment inside your working muscles changes. One major change is acidity. As muscles work harder, they produce by-products that make the muscle environment more acidic.
Research from the University of Guelph has shown that this acidic environment makes it harder for mitochondria to use fat efficiently. Key pathways that transport fat into the mitochondria become less effective, while signals that block fat use become stronger.
In simple terms: when things get intense, your muscles temporarily lose easy access to fat and switch to carbs instead.
But don’t carbs also struggle in acidity?
Interestingly, acidity can reduce carbohydrate use as well, at least in theory. Yet we know from decades of exercise research that carbohydrate use actually increases during intense activity.
Why? Because during exercise, your body releases more carbohydrate breakdown products from stored glycogen in your muscles. When carbohydrate supply is high enough, mitochondria can continue producing energy efficiently, even in acidic conditions.
This helps explain why people who are poorly fueled may feel unusually fatigued, weak, or shaky during exercise or physically demanding tasks.
What this means for everyday movement
You don’t need to be a competitive athlete for this science to matter. Many everyday activities briefly push your muscles into higher-intensity zones:
Shovelling snow
Carrying groceries
Lifting children
Walking uphill
Strength training
Long workdays on your feet
If your muscles don’t have adequate fuel, or if your body is already dealing with pain, stiffness, or poor movement patterns, fatigue and strain can appear quickly.
This can increase the risk of:
Muscle strains
Joint overload
Poor exercise tolerance
Slower recovery
Carbs vs. fats: it’s not a rivalry
There’s a lot of noise online about low-carb versus low-fat diets. Research from the National Institutes of Health and others shows that both approaches can lead to fat loss, largely depending on total calorie intake and sustainability.
What’s important for movement and rehabilitation is understanding that:
Fat is a great fuel at rest and low intensity
Carbohydrates are essential when effort increases
Total energy intake and food quality matter more than rigid rules
There is no single “best” diet for everyone, especially when pain, injury, or health conditions are involved.
Why fueling matters for pain and rehab
When people are under-fueled, fatigued muscles lose their ability to protect joints effectively. This can lead to increased stress on the knees, hips, back, and shoulders.
Adequate fuelling, combined with appropriate strength and mobility work, helps:
Maintain muscle function
Improve exercise tolerance
Support joint stability
Reduce injury risk
This is especially important for people returning to activity after injury, managing chronic pain, or starting exercise later in life.
How Muscle & Joint Clinic can help
Understanding how your body uses fuel is just one piece of the puzzle. How you move, how strong you are, and how well your joints function all play a role.
At Muscle & Joint Clinic, our team takes a collaborative approach:
Physiotherapy helps identify movement inefficiencies, restore strength and endurance, and design exercise programs that match your current capacity, without overloading your system.
Chiropractic care supports joint mobility, posture, and movement mechanics, helping muscles work more efficiently and reducing unnecessary strain during activity.
Naturopathic care can provide individualized guidance around nutrition, energy balance, inflammation, and recovery, especially helpful for those navigating fatigue, metabolic concerns, or dietary confusion.
Together, we help patients move better, recover faster, and build confidence in their bodies.
The takeaway
Your body is smart. When effort increases, it chooses the fuel that can keep you moving safely and effectively. Supporting that system, with proper movement, strength, and nutrition, can make everyday activities feel easier and reduce the risk of injury.
If exercise or physical tasks leave you feeling drained, sore, or stuck, a personalized approach can make all the difference.
Sources & Further Reading
University of Guelph, College of Biological Science – Burning Questions: Why Your Muscles Prefer Carbs Over Fats During Intense Exercise
Hall et al., Nature Medicine (2021): Low-fat vs. low-carb diets and energy intake
NIH Research Matters – Dietary fat vs. carbohydrate and body fat loss
NPR – You Don’t Need to Go Low-Carb to Burn Body Fat
Journal of Biological Chemistry – Mitochondrial fuel utilization research
Disclaimer:
This article is for educational purposes only and does not replace individualized medical or nutritional advice. Always consult a qualified healthcare professional before making changes to exercise or diet, particularly if you have medical conditions or injuries.