Maintaining energy during a workout comes down to three things: what you eat before and during exercise, how you hydrate, and how you structure your rest. Your body stores a limited supply of fuel in the muscles and liver, and once that runs low, performance drops noticeably. The good news is that each of these factors is easy to control once you know the specifics.
How Your Body Fuels Exercise
Your muscles run primarily on glycogen, a stored form of carbohydrate packed into muscle fibers and the liver. How fast you burn through it depends on intensity. At around 75% of your maximum effort (a hard but sustainable pace), your muscles chew through glycogen steadily and hit near-depletion at the point of exhaustion. At maximum intensity, the depletion rate jumps to roughly 11 units per kilogram of muscle per minute, which is why all-out efforts feel unsustainable so quickly.
For short, explosive movements like sprints or heavy lifts, your body also relies on a rapid-fire energy system that recycles a molecule called phosphocreatine. This system is powerful but burns out in seconds and needs rest to recharge. Research on high-intensity plyometric exercise found that a 1:2 work-to-rest ratio (for example, 45 seconds of work followed by 90 seconds of rest) was the minimum needed to repeat the next set at similar power. If you’re cutting rest periods short and wondering why your last few sets feel terrible, this is why.
What to Eat Before You Train
A pre-workout meal tops off those glycogen stores so you start with a full tank. The general recommendation from the International Society of Sports Nutrition is 1 to 2 grams of carbohydrate per kilogram of body weight, eaten 3 to 4 hours before exercise. For a 70 kg (154 lb) person, that’s 70 to 140 grams of carbs, roughly equivalent to a large bowl of oatmeal with a banana and some toast.
If you’re eating closer to your workout, keep the meal smaller and easier to digest. Eating within an hour of exercise shows mixed results in studies. Some people feel fine; others get sluggish or nauseous. A good rule of thumb: the closer to your session, the simpler and smaller the carbs should be. A piece of fruit or a handful of pretzels 30 minutes out works for many people without causing stomach issues.
Fueling During Your Workout
For workouts under an hour, you generally don’t need to eat anything mid-session. Your existing glycogen stores can handle it. Interestingly, even just swishing a carbohydrate drink in your mouth and spitting it out has been shown to improve performance by 1.5% to nearly 12% during moderate-to-high intensity exercise lasting about an hour. The mechanism isn’t metabolic at all. Carbohydrate sensors in your mouth activate reward centers in the brain, essentially tricking your nervous system into feeling less fatigued.
Once you push past 60 to 90 minutes, mid-workout carbs become genuinely important. The American College of Sports Medicine recommends 30 to 60 grams of carbohydrate per hour during endurance exercise. Research shows performance improvements are dose-dependent, with the greatest benefit appearing between 60 and 80 grams per hour. A single carbohydrate source (like glucose alone) tops out at about 60 grams per hour in terms of absorption, so if you need more than that, combining glucose and fructose in a 2:1 ratio allows your gut to use multiple absorption pathways. This combination increases the total amount of fuel your body can actually use and reduces the bloating and cramping that come from overloading one pathway.
Practical sources during a workout include sports drinks, energy gels, bananas, or dried fruit. The key is finding something your stomach tolerates at intensity, which varies from person to person. Practice during training, not on race day.
Stay Ahead of Dehydration
Losing just 2% of your body weight through sweat (about 1.4 kg or 3 lbs for a 70 kg person) is enough to impair endurance performance, especially in warm conditions. At 3% or more, the performance decline becomes consistent regardless of whether you’re aware of it. Some individuals are more sensitive than others. In blinded studies where participants didn’t know their hydration status, the range of performance impairment varied enormously, from 1.5% to over 19%.
Sweat doesn’t just carry water. You lose sodium at a rate of roughly 45 to 64 millimoles per liter of sweat, depending on how acclimatized you are to the heat. People working in moderately hot conditions for extended periods can lose 5 to 6 grams of sodium in a day, equivalent to 12 to 15 grams of table salt. For a typical one-hour gym session in air conditioning, water alone is usually sufficient. But if you’re training for over an hour, sweating heavily, or exercising in heat, adding electrolytes (particularly sodium) to your fluid helps maintain blood volume and keeps your muscles contracting properly.
A simple hydration strategy: weigh yourself before and after a few workouts to gauge your sweat rate. Aim to replace most of what you lose, sipping throughout rather than chugging large amounts at once.
Caffeine as an Energy Tool
Caffeine is one of the most consistently supported performance boosters in sports nutrition research. A dose of 3 to 6 mg per kilogram of body weight, taken 30 to 90 minutes before exercise, improves endurance, strength, reaction time, and subjective feelings of energy and focus. For a 70 kg person, that works out to roughly 210 to 420 mg, or about two to four cups of coffee.
Even lower doses show benefits. One study found that an energy drink providing just 1.8 mg/kg of caffeine improved reaction time and feelings of energy in strength and power athletes. If you’re sensitive to caffeine or train in the evening, starting at the lower end of the range is reasonable. The performance benefit is real at moderate doses, and going higher increases the risk of jitteriness, elevated heart rate, and disrupted sleep without proportional gains.
Why You Feel Exhausted Mid-Workout
That heavy, “I can’t push anymore” feeling during a workout isn’t always about your muscles running out of fuel. Fatigue has two distinct sources. Peripheral fatigue happens at the muscle level, driven by the buildup of metabolic byproducts like hydrogen ions, inorganic phosphates, and potassium leaking out of muscle cells. This is the burn you feel during a hard set, and it resolves relatively quickly with rest.
Central fatigue is different. It originates in the brain and spinal cord, where changes in neurotransmitters like serotonin and dopamine reduce the signal your motor cortex sends to your muscles. Your muscles may still have capacity, but your brain dials down the drive. This is why caffeine helps: it counteracts some of these central fatigue signals. It’s also why the carbohydrate mouth rinse trick works. Your brain responds to the taste of sugar as a reward signal even before any calories are absorbed.
Both types of fatigue accelerate once you exceed a critical intensity threshold where lactate, hydrogen ions, and ammonia begin accumulating in the bloodstream faster than your body can clear them. Staying just below that threshold, or structuring intervals with adequate rest, lets you sustain higher total work output across an entire session.
Putting It All Together
For a typical strength or conditioning session under an hour, the formula is straightforward: eat a carb-rich meal a few hours beforehand, stay hydrated with water, consider caffeine if you train in the morning or early afternoon, and respect your rest intervals. For longer endurance sessions, add 30 to 60 grams of carbs per hour from easily digested sources and include sodium in your fluids.
The biggest mistake most people make isn’t a lack of supplements or special drinks. It’s skipping meals before training, cutting rest periods too short, or showing up already mildly dehydrated from a long day of not drinking enough. Fix those three basics and the difference in how you feel at minute 45 versus minute 5 will shrink dramatically.