Energy systems: how you produce energy (and why lactate is not the enemy)

“That climb is aerobic, that effort is anaerobic.” I have heard it said a thousand times, and it is almost always a simplification that leads you astray. Energy systems are not three switches that turn on one at a time: they always work all three together, and what changes is the proportion. Understanding that proportion is what lets you choose what to train and what to eat.

Three systems, no switches

Everything you do — a jump, a two-hour climb — is paid for with the same currency: ATP. The body keeps very little of it in the till, enough for a couple of seconds. The rest has to be made again continuously, and there are three ways of doing it.

Phosphagens. Creatine phosphate recharges ATP almost instantly, without oxygen and without waste. It is the system of pure power: a sprint, a maximal lift, an explosive move on a steep section. It does not last long — eight, ten seconds — and then the till is empty. It refills in a few minutes of rest, which is why long recoveries in strength work are not wasted time: they are part of the stimulus.

Glycolysis. Glucose is broken down to pyruvate and produces ATP quickly, without needing oxygen to do so. It is the system that carries you from ten seconds to a couple of minutes of intense effort: the ramp that never ends, the change of pace to drop someone. It is fast but expensive — it yields little ATP per molecule of glucose — and it produces hydrogen ions, that is, acidity.

Oxidation. In the mitochondria, glucose and fat are oxidised with oxygen and produce an enormous amount of ATP per unit of fuel. It is slow to start and limited in power, but practically inexhaustible: it is the system every endurance sport lives on, and the one you build with hours of base training.

Aerobic and anaerobic are not two worlds

The point where they meet is pyruvate. Glycolysis always produces it. If the rate at which you produce it stays within your mitochondria’s capacity to clear it, pyruvate enters the mitochondrion and is oxidised — and you call that effort “aerobic”. If you produce it faster than you can clear it, the excess is converted into lactate — and you call that effort “anaerobic”.

But it is exactly the same process, at a different flow rate. There is no threshold beyond which “the anaerobic switches on”: there is a flow that at some point exceeds the clearing capacity. Training your aerobic base means, in practice, increasing that clearing capacity — more mitochondria, more capillaries, more transporters — so that you are more comfortable at every pace.

Lactate is not what burns your legs

This is the part where exercise physiology changed its mind, and common sense has not caught up yet.

For decades lactate was described as the toxic waste that causes the burn, the acidity and the crash. It is not. The burn depends on the build-up of hydrogen ions, which lower pH; if anything, producing lactate is a reaction that consumes a hydrogen ion, so it slows acidification rather than causing it. Lactate is the marker you measure, not the culprit.

There is more: lactate is a fuel. It is produced by fast-twitch fibres, leaves the cell, travels in the blood and is taken up and oxidised by slow-twitch fibres, the heart, the kidneys and the brain — this is what is called the lactate shuttle. Under effort, the heart is a greedy consumer of it. It is also a signalling molecule: it helps trigger the mitochondrial adaptations you are after when you do intense work.

So no: lactate is not the enemy to avoid. It is a fuel, a messenger and a thermometer of intensity, all at once.

The real ceiling is the gut: 60 g + 60 g

Here we move from theory to what you decide on race morning.

During a long effort the limiting factor is not how much sugar your muscles can burn: it is how much your gut can let through. Glucose crosses the intestinal wall thanks to a transporter called SGLT1, and that transporter saturates at around 60 grams per hour. You can swallow ninety: sixty get through, the rest stays there fermenting, and you end up bloated and nauseous halfway through the race.

The key is to use a second door. Fructose does not use SGLT1: it enters through a different transporter, GLUT5. Adding the two routes together raises the ceiling considerably: we have gone from the 90 g/h of the early research to the 120 g/h that is standard practice in professional cycling today — in practice 60 grams of glucose plus 60 of fructose, or ratios close to that (1:0.8 is the most used today).

Three caveats that matter as much as the numbers:

• The gut can be trained. The number of transporters increases with exposure: going from 40 to 120 g/h overnight will only hurt you. You build up gradually, in training, never for the first time in a race.
• Not everyone needs 120 g/h. It is a figure for many hours of high-intensity effort. For an amateur gran fondo, 60-90 g/h that you tolerate well are worth more than 120 that sit in your stomach.
• Sugar and fluid travel together. Concentrations that are too high without enough water slow gastric emptying and make everything worse.

And the lactate they drink at the Tour

For a couple of seasons there has been talk of Tour de France teams using exogenous lactate supplements. It sounds like a contradiction — drinking what everyone considers waste — but in light of the above it has its own logic.

First: if lactate is an oxidisable fuel, drinking it means supplying ready-to-use energy that the heart and slow-twitch fibres can burn while sparing glycogen.

Second, and this is the most interesting point: lactate does not use the same transporters as sugars. It is absorbed through the monocarboxylate transporters (MCTs), so it competes with neither SGLT1 nor GLUT5. In theory it allows energy to be added beyond the 120 g/h carbohydrate ceiling, which is exactly the wall professional cycling is now running into.

Third: as a signalling molecule, lactate has effects on metabolism that are still being studied.

All that said, the rest must be said too: the evidence is still thin. We are in the territory where top teams experiment before research has produced solid answers, and it would not be the first time something promising in that context deflates once it is measured properly. I mention it because it explains well how the physiology works, not because it is something to put in an amateur’s bottle.

What I do with this when I programme

Knowing this changes three concrete things in the way I work:

1. Easy sessions are not wasted time. They build the capacity to clear pyruvate and lactate, which raises the pace at which you stay comfortable. It is the piece amateurs cut first and the one that weighs the most.
2. Intense work has a dose. It is there to trigger mitochondrial adaptations, but it is costly to recover from: it should go where the body can absorb it, not wherever you feel like pushing.
3. Race fuelling is trained like a session. The grams-per-hour strategy is tested, adjusted and made automatic in training — otherwise the best engine in the world stops because of a gut problem.

Physiology is not there to impress with technical terms. It is there to understand why you are doing what you are doing — and to stop fearing something that, all this time, was on your side.