Fuel for the work.
Zones, fat-burning, and the one change that makes most of us faster.
The 60-second
version.
What your body burns depends on how hard you go. Go easy and you run mostly on fat. Go hard and you run mostly on carbs. That's a hard physiological rule, and no diet trick changes it.
You can nudge the mix toward fat by training more, training fasted, or eating low-carb. But shifting your fuel mix doesn't by nature make you any faster.
But most of these studies are done on professional or elite athletes, already with extremely low body fat. For any normal human the prize isn't the performance of the engine, it's power-to-weight. Lose a few kilos of spare fat and every hill shrinks.
So how do we lose fat while keeping the energy to train? The two trendy terms in recent research are metabolic flexibility and carb periodisation. In plain English: train your body to be good at burning both fat and carbs, and plan your carbs around your training.
The amount of carbs eaten during long races keeps climbing, and the guidelines have moved to meet it: 90 to 120g an hour. Recent research is showing positive signals for up to 150g/h, which for most people requires a lot of gut training.
And protein? A rare case where the marketing hype stands up under the microscope, one of the most widely evidenced areas of nutrition. 1.6 to 2.2g per kg of bodyweight a day covers most of us, and the returns flatten above that.
One of the best techniques for improving body composition (reducing fat while protecting and growing muscle) is resistance training. Two or three strength sessions a week sends a powerful signal to your body.
In many ways the conclusion is unglamorous: match your food to your training, day by day. Keep the carbs light on easy days, fuel the hard sessions properly, treat protein as non-negotiable.
It's worth caveating all of this. The majority of these studies are done on men, and on groups of highly trained athletes. Current work is correcting this, but for now these are known limitations in modern sports science.
The terms,
in plain
English.
The chemist's shorthand (carbon-hydrogen-oxygen), used everywhere in papers. "60g CHO/h" just means 60 grams of carbs an hour.
Carbs packed away in muscle and liver, ready to burn. Around 400–500g total, roughly 2,000 kcal. Empty it and you bonk.
The two dividers in your effort range. Below LT1 is genuinely easy; above LT2 is genuinely hard. Everything between is the grey middle.
The intensity where carbs overtake fat as your main fuel. Trained athletes push it later; that's what "a good aerobic engine" means.
The effort at which you burn fat fastest. Marked on the crossover curve, and it sits well below the crossover.
The rebuild process: your body assembling new muscle protein after a session. It stays elevated for 24–48 hours, which is why the 30-minute window is a myth.
Intermittent fasting by the clock: all your food inside a set window. The common version is 16:8: fast for 16 hours, eat within 8.
Doing a session on purposely empty carb stores (fasted, or after depleting them) to push adaptation.
Matching carbs to the day's demand instead of eating the same every day. Big on hard days, lean on easy ones.
Being good at burning both fuels and switching between them cleanly, rather than being locked into one.
What happens when you train hard on too little food for too long: hormones, bone and performance all suffer. The failure mode to avoid.
Running the carb tank dry mid-effort. The legs go, the pace collapses. You never bonk from running out of fat.
The two "doors" carbs enter through: glucose uses one, fructose the other. Using both is how you absorb more than 60g an hour.
Watts you produce divided by the kilos you carry. The number that decides how fast you climb, and the one most amateurs can move.
The rules of
the game.
The physiology you can't break.
Where fat
hands over
to carbs.
Push the effort up and your fuel gauge slides from fat to carbs. The handover has a name: the crossover.
Below it you're fat-fuelled and can go for hours. Above it you're burning your small, precious carb tank, and the clock starts.
Even at FatMax (the effort at which you are burning the most fat), a trained person on a normal diet burns about 0.5g of fat per minute, roughly 4.5 kcal.[Achten]
LT1 is where lactate first starts to rise above resting: the ceiling of genuinely easy, fat-fuelled work.
LT2 is where it starts accumulating faster than you can clear it, where carbs become the dominant energy source.
The seven-zone model is the modern coaching standard. A finer grid that is more useful for prescribing work, as used for all sessions in Beina. It's the same axis, cut differently.
A fitter engine
moves the
whole curve.
Get fitter and the whole handover slides right: more fat burned, at higher intensities, before carbs take over.
The gap is wide. Well-trained athletes don't cross over until around 75% of VO₂max; moderately active ones cross nearer 55%. This is what "a good aerobic engine" physically is.
This varies hugely from person to person, highlighting the fact that nutrition, and specifically metabolism, are hugely personal. In one study of trained men, individuals ranged from 0.24g/min to 0.89g/min. Nearly fourfold, in the same training state.[Achten]
Diet moves the curve too. Train fasted or low-carb and you'll burn more fat at any intensity within days. More on that later…
One engine,
two fuel
supplies.
You can think of your muscle as a single combustion engine with two distinct sources of fuel.
You never bonk from running out of fat, you bonk from running out of carbs. That tiny tank is the limiter, which is exactly why the vast majority of athletes fuel hard races with carbs. "Better fat-burning" stretches the carb tank: a fuel-economy gain, not a bigger engine.
The moves
you can make.
What, and when, you eat.
Top up the
tank while
you go.
For long or hard sessions, the widely accepted move is to feed the muscle directly, sparing your own glycogen so you bonk far later. See some specific guidelines below.
| Session | Carbs / hour | How |
|---|---|---|
| Up to ~75 min | ~0 | Your tank covers it. A carb mouth-rinse still buys ~2–3% via a brain effect. |
| 1–2.5 h, moderate–hard | 30–60g | A gel or two, or a bottle of mix. Plain glucose is fine here. |
| 2.5 h+ or racing | 60–90g | Now you need a glucose + fructose blend to absorb this much. |
| Ultra / racing hard | 90–120g | The current guideline ceiling, and where most pros are now racing. |
| The live frontier | 120–150g+ | Tolerance and oxidation at these rates are being demonstrated, and some riders gut-train to 200g+. What isn't settled is whether it beats 120 on the clock. Months of gut training, and highly individual. |
SGLT1
GLUT5
MUSCLE
Glucose saturates its intestinal transporter near ~60g/h. Fructose goes through a different door and adds roughly 30 to 45g/h on top, which is how a trained gut reaches 90 to 120g/h. That's why the best fuels use a glucose-to-fructose blend (~1:0.8).[Jeukendrup · 2026 revision]
Feed the
adaptations.
An easy thing to forget: you don't actually get any fitter during a training session. Technically, you get weaker.
It is in the 24 to 48 hours after the work, when your body adapts to the stress you put it under, that fitness is gained. Which is why what you do in that window matters so much.
It is also the slightly upsetting reason why having a few beers after that big mission is probably the worst time to have a few beers :(
Before any ice bath, massage gun or supplement, cover these four. They're the whole of recovery nutrition, in order of how fast they matter.[Bonilla]
Weigh yourself before and after. Drink one-and-a-half times the deficit, with sodium so it stays in you.
Carbs back into the tank. Only urgent if you train again within ~8 hours. Otherwise the daily total is what counts.
Quality protein across the day. This is the part that turns the session into fitness.
Where the rebuild actually happens. A slow protein before bed helps; alcohol can undo all of your hard work.
Protein is
the rebuild.
Carbs fuel the session. Protein is what turns it into fitness.
Protein isn't just raw material, it's a signal. Leucine is the amino acid your muscle reads, and about 2.5g of it in one sitting is what flips the switch. That's the real reason whey wins: it's roughly 13% leucine against casein's 10% and soy's 8%.
The fascinating part is that protein raises muscle protein synthesis on its own, at rest, with no training at all.[Tang] Training amplifies it. The food alone starts it.
Our meta-analysis puts the daily protein target at 1.6g/kg, with the peak around 2.2.[Morton]
The 30-minute panic is finished. Muscle stays sensitised to protein for 24 hours or more after a hard session: the rebuild is a day-long window, not a half-hour one. What matters is your daily total, spread across the day. The only thing that's genuinely urgent post-session is carbs, and only if you train again within ~8 hours.
Burning fat:
the amateur's
superpower.
Those fuel-mix trials kept finding nothing on performance. Not because they studied the wrong people, but because they measured the wrong outcome for you.
They tested time-trial power at a fixed bodyweight. That is a fair test of whether shifting your fuel mix makes the engine stronger, and the answer is no.
But power is only half of how fast you go. The other half is what you carry up the hill, and those trials were never designed to move it. That's the lever they left on the table, and for most of us it's the bigger one.
W/kg is bodyweight, as it is normally quoted. The climb estimate is modelled from cycling power physics on a sustained climb, and does include the weight of a bike and kit, which is why it is not simply 6.7% faster. Treat it as an illustration of the direction, not a prediction of your time.
Power
moves.
The rider above got faster by subtracting. The real magic is doing both at once: increasing muscle, while reducing fat. Body composition is the name of the game.
Lift weights 2–3× a week, and eat the protein to go with it.
In a deficit your body is deciding what to burn. Resistance training plus high protein is the instruction: burn the fat, keep the machine.
In a four-week trial of 40 overweight young men, everyone lifted six days a week in a 40% calorie deficit. The only thing that differed between the groups was how much protein they ate.[Longland]
Same training, same deficit, double the protein: the difference is whether you hold the muscle or add to it. The direction holds for you, the magnitude almost certainly won't, because they were previously untrained and had far more spare adaptation to find.
Lose it slower than you want to: 0.5–0.7% of bodyweight a week.
Two groups of elite athletes reached a similar scale weight at different speeds, and the bodies underneath were not alike. The slow group added lean mass. The fast group held theirs flat, and gave up the gain.[Garthe]
n=24 elite athletes, 13 slow and 11 fast. Losing fast didn't make them weaker. It cost them the muscle the slow group gained.
Keto &
fasting.
These two get bundled together, but they pull different levers.
Keto is what you eat: usually defined as under 50g of carbs a day, with the rest of your calories coming mostly from fat and protein. Fasting is when you eat: often practised as something like 16:8, fasting for 16 hours each day and eating within 8.
Very low carb, high fat, all the time. Within weeks it shifts your fuel mix hard toward fat, and that part is real and uncontested. It's also an established therapy for epilepsy and type-2 diabetes, so none of it is woo.
The catch is the top end. The carb system you've spent months down-regulating is exactly the one hard racing runs on, and the tank you're leaning on delivers 0.5g a minute.
Time-restricted eating and train-low sessions change when fuel is available, not what's on the plate the rest of the time. Going without switches on autophagy, the cellular clean-up.
Train fasted and you'll build better fat-burning machinery. Whether that machinery actually makes you faster has been tested directly, and the answer is below.
In a 6-week trial, fasted training built better fat-burning machinery and delivered no detectable extra speed. Fasted and fed groups both improved their 60-minute time-trial by ~8%, and VO₂max by ~9%, with no difference between them.[Van Proeyen]
n=20 young men, 10 per group, 6 weeks. Small enough that a real difference could hide in it.
autophagy off
AGAINST
EACH OTHER
building off
Both are useful tools, they just answer different questions. Fasting is about timing, and it earns its place on easy sessions and in off-season blocks, where teaching your body to work on less costs you nothing. Keep it away from the days you need to go hard and it's a free adaptation.
Keto is about composition, and it does what it says: a genuine shift in how you fuel, with real uses in an off-season block or a fat-loss phase. The trade is at the top end, so it suits periods where the top end isn't what you're chasing. Use both in a window, and come back out for race season.
Where the
game bites
back.
Heavy alcohol after training
Your body treats alcohol as a toxin and prioritises clearing it, blunting the rebuild clock. After a hard session, a heavy dose cut muscle protein synthesis 37% when taken with carbs, and still 24% when taken with protein. Protein blunts the damage. It doesn't stop it.[Parr]
n=8 physically active men. The dose was 1.5g/kg, about twelve standard drinks for an 80kg athlete. Two beers is not this.
Under-fuelling into a hole
Starve your body of energy and it starts breaking down its own muscle for fuel. Train hard and under-eat for too long and you finish fitter on paper but weaker in reality: RED-S.[Mountjoy]
Not to be
forgotten.
Three practical things that are worth leaning into.
The best-evidenced legal aid in endurance. 3 to 6mg/kg, about an hour before, worth roughly 2 to 3% on power and time-trial time.
3mg/kg performs about as well as 6, so start low. For a 75kg rider that's ~225mg, roughly two coffees. Practise it in training: it wrecks some people's sleep and some people's stomachs.
Told you to do it repeatedly and never said how. Here it is: take carbs on your long sessions, start at 60g/h, and add 10g/h every couple of weeks.
Use race products at race concentration, not something you'll never see again on the day. If your gut complains, hold at the last comfortable rate for another fortnight rather than pushing through.
Sweat rates vary enormously, so weigh yourself before and after a long session and replace what you lost. In the heat, 500 to 800mg of sodium an hour keeps it in you.
You can also drink too much. Taking on more fluid than you sweat dilutes your blood sodium, which is hyponatraemia, and in a long event it is more dangerous than being a little dry.
Taboo
moves.
Two that get whispered about rather than written down. One is unglamorous but real; the other is everywhere in the peloton and barely stands up.
Baking soda buffers the acid that floods a hard effort, so it helps the short, sharp stuff — 30 seconds to 12 minutes, repeated sprints, the final climb — not steady endurance. 0.2 to 0.3g/kg, 60 to 180 minutes before, worth around 2%.[Grgic]
The catch is your gut: neat bicarb means bloating and cramps, which is why it fell from fashion. It's back because hydrogel versions (Maurten's, used at WorldTour level) get it past the stomach. Legal, and worth practising before you race it.
Pouches and snus are rife in pro cycling and team sport — used by roughly a quarter to a third of athletes. The pitch is focus, blunted effort and appetite control.[Mündel]
The evidence doesn't back it: of ten performance studies, two helped, one hurt, seven did nothing — no reliable edge, and it's addictive.[Mündel] WADA watches it rather than banning it, for now. Use has run well ahead of the science; "everyone's on it" is the reason, not proof.
Remember
one thing.
Fuel the work, and fuel the adaptation. Eat for the session in front of you, protect your muscle, and lose the spare fat slowly.
Boring. Patient. And more than any shortcut on the internet can say. That's how most of us get genuinely faster.
Putting it into
practice with Beina.
The science in this report sits in your coach's context on every message you send it. Ask about the grey middle, the 4Rs, what to eat before Saturday, and you're talking to something that has already read it.
Type /fuel and it stops being theory. Beina reads your actual planned sessions for the week ahead and your bodyweight, then puts the carbs where the work is: what to eat before each session and how many hours before, what to take during, whether the refuel after genuinely matters, plus a daily protein target.
Then close the loop. Ask your coach to put two strength sessions a week into your plan and it writes them in, exercises and all, right alongside the rest of your training.
Start your 7-day trial →
love from Scotland