I swallowed an energy gel 40 minutes before every session just to be sure I had fuel: from the very first minutes, I understood what my heavy legs were really reacting to

Heavy legs within the first ten minutes of a session, every single time, despite a gel sitting in my stomach for forty minutes beforehand. I assumed I simply needed more fuel, so I added another sachet. The legs stayed heavy. It took a sports dietitian pointing out the timing of my ritual, not the quantity, for me to realise the gel itself was the problem.

The pattern was suspiciously consistent. Warm up, feel sluggish, push through, and by minute fifteen things would settle and the session would feel normal again. That dip always landed at almost exactly the same point after I’d swallowed the gel, which is what eventually made me question the fuelling strategy rather than my fitness.

What was actually happening in my bloodstream

When you eat fast-digesting carbohydrate, your pancreas releases insulin to shuttle that glucose into cells and bring blood sugar back to a stable range. When you consume carbohydrates, your body releases insulin to help process it and keep your blood sugar stable. The trouble starts when exercise begins right as that insulin surge peaks. However, when you begin exercising, your body starts using those carbohydrates and a further decrease in blood glucose occurs. The result is a temporary dip called reactive, or rebound, hypoglycaemia, and it can feel remarkably like the “heavy legs, foggy brain” sensation runners associate with running out of fuel entirely, even though the actual cause is a glucose trough rather than empty glycogen stores.

This isn’t a rare quirk. Studies show that up to 30% of athletes experience reactive hypoglycemia. A 2023 study by Zignoli and colleagues, cited by sports nutrition specialists, looked directly at when these glucose crashes tend to strike. The majority of reactive hypoglycemia events, defined as blood glucose concentrations below 70 mg/dL in the first half hour of exercise, occurred from meals eaten 30-90 minutes pre-exercise. The worst point on that curve wasn’t 30 minutes or 90 minutes. The peak was in fact at about the 60-minute mark. Forty minutes sits squarely inside that danger window, which explains why my “just to be sure” habit was quietly working against me every single time.

Why the exact timing matters so much

The physiology behind this is almost counterintuitive once you see it laid out. Carbohydrate eaten at rest gets treated by the body as a meal, triggering a full insulin response designed to store energy, not release it for immediate use. If you are not exercising, the body will release insulin and convert into long term stores (glycogen) actually leading to lower blood sugar levels. Move that same carbohydrate to right before or during activity and the picture changes completely, because working muscles start pulling glucose out of the blood immediately and adrenaline dampens the insulin response. During exercise insulin levels will not rise (or not rise as much) because adrenaline will reduce insulin.

Research into exactly how close to “go time” you need to be is fairly specific. When taken just 15 minutes before, few people were affected, and no athletes were affected when taking carbohydrates five minutes before exercise or during warm up. One nutrition guide phrases the practical takeaway bluntly: avoid taking a gel within 15 minutes of starting your event unless you are already exercising. That last clause matters. Once you’re actually moving, the equation flips again. Taking a gel within 15 minutes of starting is fine, while consuming a gel 30-60+ minutes before exercise can trigger an insulin response that lowers blood sugar and leaves you feeling tired at the start.

What changed once I moved the goalposts

I stopped eating the gel as a pre-session ritual and started treating it as either a five-minute-before top-up or, more often, something I take once I’m already ten minutes into the warm-up. The heavy-legs sensation didn’t vanish overnight, training fatigue is still training fatigue, but that specific early crash disappeared within a fortnight. A specialist I read on this described working with an athlete who trained after a full dinner and still hit an unexpected wall, only for a small carbohydrate snack during her warm-up to solve it. Adding a quick carb intake during her warm-up made a noticeable difference. My own fix followed the same logic, just applied in reverse: less pre-loading, better timing.

Not everyone needs to overhaul their fuelling at all. Give it a try with no food before a workout lasting under 75 minutes and you’ll likely do just fine, since most of us are carrying several hours of stored glycogen regardless of what we ate that morning. For sessions where fuelling genuinely helps, the safest windows appear to bookend the risky middle ground: either grab your carbohydrate in the final minutes before you start moving, or wait and take it once you’re rolling. Some athletes prefer a slower-release option earlier in the day precisely to sidestep this insulin swing altogether, pairing that with a fast carbohydrate closer to the start.

If heaviness, dizziness or shakiness keep showing up early in your sessions no matter how you adjust your fuelling, it’s worth mentioning to your GP, particularly if symptoms are severe or don’t settle with these timing tweaks, since genuine blood sugar disorders need proper assessment rather than trial and error with sachets. For most of us, though, the fix sits somewhere surprisingly simple: not more fuel, just fuel at the right minute rather than the wrong one.

Sources : ncbi.nlm.nih.gov | highfive.co.uk