An hour of moderate exercise barely touches your glycogen stores. That’s the fact that changed how I approach my daily gym sessions, and it came from a conversation with a sports scientist who looked genuinely puzzled when I mentioned my habit of sipping isotonic drinks through every single workout, regardless of intensity or duration.
Here’s what he explained: your muscles and liver hold somewhere between 400 and 500 grams of glycogen when you’re well-fed, which translates to roughly 1,600 to 2,000 calories of stored energy. A typical hour-long workout, unless you’re doing something genuinely brutal like competitive cycling or a marathon-pace run, burns through a fraction of that. For most gym-goers doing weights, moderate cardio, or a fitness class, you’re dipping into maybe 20-30% of those reserves. Your body isn’t running on empty. It’s running on a nearly full tank, and I’d been topping it up anyway, one sugary sip at a time.
Key takeaways
- Your muscles store 1,600-2,000 calories of glycogen, but a typical one-hour workout only burns through 20-30% of that reserve
- Isotonic drinks were designed for endurance athletes doing 90+ minutes of intense exercise, not casual gym sessions
- A standard sports drink contains as much sugar as a can of cola—but most moderate workouts don’t require mid-exercise carbohydrates at all
Why isotonic drinks exist in the first place
Isotonic drinks were designed for endurance athletes: think triathletes, marathon runners, footballers playing 90 minutes at competitive intensity. The formula makes sense in that context. These drinks typically contain a 6-8% carbohydrate solution, matched to blood osmolality so the body absorbs both fluid and sugar quickly, alongside electrolytes like sodium to replace what’s lost through heavy, prolonged sweating.
The research supporting their use comes from studies on exercise lasting 90 minutes or longer, where glycogen depletion genuinely becomes a limiting factor. The American College of Sports Medicine has long recommended carbohydrate intake during exercise specifically for sessions exceeding an hour at high intensity, or significantly longer at moderate intensity. Below that threshold, the physiological need for mid-workout sugar simply isn’t there for most people.
What struck me most was learning that a standard 500ml isotonic drink often contains around 30 grams of sugar. That’s not far off a can of cola. I’d been treating it as sports fuel, when for my one-hour gym sessions, it functioned more like a dessert I was drinking while doing squats.
What your body actually does during a shorter workout
During moderate exercise under an hour, your body draws primarily on glycogen and fat stores already sitting in your muscles, plus whatever glucose is circulating in your bloodstream from your last meal. This system is remarkably efficient. It’s been fine-tuning itself for the entire history of human physical activity, long before sports drinks existed.
The sports scientist made a comparison I still think about: adding sugar mid-workout for a typical hour session is a bit like refuelling a car that’s already three-quarters full before a short drive to the shops. Technically you’re adding fuel. Practically, it changes nothing about whether you’ll get there.
Where things shift is with intensity and heat. A punishing HIIT session, hot yoga in a heated studio, or training in genuinely warm conditions increases sweat rate and fluid loss substantially, and that’s where electrolyte replacement starts to matter more than the sugar itself. Plain water handles hydration for most moderate one-hour sessions perfectly well. If you’re sweating heavily and want extra reassurance, a pinch of salt or an electrolyte tablet without the added sugar load does the job without the extra calories.
The habit I’ve actually changed
I still drink something during my workouts, water mostly, sometimes water with a squeeze of lemon. What changed is when I reach for something with actual carbohydrate content: longer sessions, back-to-back training days, or genuinely high-intensity work that leaves me depleted. On an ordinary hour at the gym doing strength training or a moderate cardio class, my body has what it needs already stored, and the extra sugar was doing very little beyond adding calories I didn’t need and hadn’t accounted for.
This isn’t an argument against isotonic drinks generally. For someone training for a half marathon, cycling for two hours, or playing a full match of football in summer heat, they remain a sensible, evidence-backed tool. The distinction that matters is duration and intensity, not blanket rules about sports drinks being good or bad.
If you’re unsure where your own workouts fall, a rough guide helps: sessions under an hour at moderate intensity rarely need carbohydrate replacement mid-exercise; sessions over 90 minutes, or anything at high intensity in heat, likely benefit from it. Anyone managing a health condition like diabetes, or with specific dietary concerns, should check with their GP or a registered dietitian before changing their routine, since individual needs vary more than generic guidelines suggest.
What I didn’t expect from this whole conversation was learning that plain milk, oddly enough, performs comparably to commercial recovery drinks after moderate exercise, thanks to its natural mix of carbohydrate, protein and electrolytes. It’s not glamorous, there’s no clever marketing behind a pint of semi-skimmed, but it turns out your fridge already stocks better recovery fuel than half the sports aisle.