Half Ironman Fueling: The Complete Nutrition Plan From Swim to Finish Line

You've done the training. The swim doesn't scare you anymore. You've practised transitions. But if you haven't nailed your nutrition for all 70.3 miles, none of that matters — because somewhere around mile 8 of the run, your legs will stop answering the phone.

Half Ironman nutrition is where most age-group triathletes get it wrong. Not because the science is complicated, but because a 70.3 sits in an awkward middle ground: long enough that fueling matters enormously, but short enough that most athletes wing it with whatever strategy they used in sprint races. That doesn't scale.

This guide breaks your 70.3 nutrition into four phases — pre-race, swim, bike, and run — with specific targets you can take straight into training.

Why 70.3 Nutrition Is Different From Sprint or Full Ironman

A sprint triathlon is short enough to race on breakfast alone. A full Ironman is long enough that you'll have time to eat solid food at aid stations, adjust, and recover from small mistakes.

A 70.3 gives you neither luxury. You're racing at a higher intensity than an Ironman, which means your gut is under more stress — but you're out there for 4.5 to 7 hours, which means you absolutely cannot rely on stored glycogen alone. Your body holds roughly 2,000 calories of glycogen. A 70.3 burns 3,500 to 5,500 calories depending on your size and pace. The maths doesn't lie: you need to eat while racing, and you need to eat the right things at the right times.

The other problem is that most 70.3 athletes underestimate how much the bike determines the run. The bike leg is where 80% of your race-day calories should go in. Get it wrong, and your marathon legs are done before you hit T2.

Phase 1: Pre-Race (3 Hours to Start)

Your race-morning nutrition has one job: top off glycogen and start the clock well-hydrated. Don't overthink it.

3 hours before swim start:

• Eat 1-2g of carbohydrate per kg of body weight. For a 75kg athlete, that's 75-150g of carbs — roughly a large bowl of porridge with banana and honey, or two slices of white toast with jam and a sports drink.
• Drink 500-750ml of water or electrolyte drink. Sip, don't chug.
• Keep fat and fibre low. This is not the morning for eggs and avocado toast.

60-90 minutes before swim start:

• Stop eating solid food. Your stomach needs time to empty before you're horizontal in open water.
• Small sips of electrolyte drink are fine.

15 minutes before swim start:

• Take one gel or a few swigs of concentrated sports drink. This tops off blood glucose without sitting in your stomach.
• 200-300ml of water to wash it down.

The biggest pre-race mistake is eating too much too close to the start. GI distress in the swim is almost always a breakfast-timing issue, not a food-choice issue.

Phase 2: The Swim (0g Carbs — Focus on What Comes Next)

The swim is the one leg where you don't eat. At race intensity, a 1.9km swim burns roughly 400-600 calories, but most of that comes from glycogen and fat stores you loaded this morning. You can't practically consume anything in the water, and you don't need to.

What matters during the swim is what happens in your head: mentally rehearse your T1 nutrition plan. Know exactly where your bottles and gels are on your bike. Know what you're grabbing first.

In T1:

• Take 3-5 big gulps from your bike bottle as you set up. You've just spent 25-40 minutes without drinking.
• Don't eat anything solid yet. Your heart rate is elevated and your gut is compressed from the swim position. Give it 10 minutes on the bike before taking your first gel.

Phase 3: The Bike (This Is Where You Win or Lose Your Race)

The bike leg of a 70.3 is your fueling window. It's where 80% of your race-day calories go in, and it's the only leg where you can eat comfortably — you're seated, your gut isn't bouncing, and you have storage for bottles and food.

Here's the target: 60-90g of carbohydrate per hour on the bike.

For a 75kg athlete riding a 2:30-3:00 bike split, that means consuming 150-270g of carbs over the entire bike leg. That's a lot more than most age-groupers realise.

How to Hit 60-90g Per Hour

The key is using a mix of glucose and fructose. Your gut can absorb roughly 60g of glucose per hour through one intestinal transporter, but adding fructose (which uses a separate transporter) pushes that ceiling to 90g or more. Most modern sports nutrition products use a 2:1 or 1:0.8 glucose-to-fructose ratio for exactly this reason.

A practical 70.3 bike fueling plan (per hour):

| Source | Carbs | Notes | |--------|-------|-------| | 750ml sports drink (6-8% carb solution) | 45-60g | Main hydration source — sip every 10-15 minutes | | 1 gel (or half a gel every 30 mins) | 20-25g | Take with a sip of water, not sports drink | | Total per hour | 65-85g | Adjust up in heat, down if GI stress appears |

Timing cues:

• First gel at 10-15 minutes into the bike (once your heart rate settles from the swim).
• Sip sports drink every 10-15 minutes — don't wait until you're thirsty.
• Take your last gel 15-20 minutes before T2. You want it absorbed before you start running.
• If the bike leg takes 3 hours, that's 3 gels and roughly 2-2.5 litres of sports drink.

The Under-Fueling Trap

Most age-group 70.3 athletes consume 30-45g of carbs per hour on the bike. That's half of what the science says they should. The reason is usually one of three things:

1. Fear of GI issues. They've had a bad experience with gels and now avoid eating enough. The fix isn't eating less — it's training your gut in training rides.
2. Didn't bring enough. Two gels and one bottle for a 3-hour ride is not enough. Do the maths before race day.
3. Forgot to eat. When you're focused on power output and drafting rules, eating drops off the priority list. Set a watch timer for every 20 minutes.

Under-fueling the bike doesn't hurt on the bike. It hurts on the run. You'll feel fine through T2 and the first 3-4km, then your glycogen debt catches up and your pace collapses. This is the number one reason for the "70.3 run death march" — and it's entirely preventable.

Phase 4: The Run (Shift to Maintenance Mode)

By the time you hit the run, your gut is under maximum stress. Heart rate is high, core temperature is up, and blood is being diverted away from your digestive system to your working muscles. This is not the time to try to eat a lot.

Your run fueling target: 30-60g of carbs per hour.

That's significantly less than the bike. The goal on the run is maintenance, not loading. You should have front-loaded your carbs on the bike so the run becomes about topping off, not catching up.

Practical run fueling:

• Gel or liquid carbs every 20-30 minutes (avoid solid food — your gut can't handle it at run intensity)
• Sip water at every aid station — small amounts, frequently
• If it's hot, add electrolyte drink or salt tabs. Sodium losses compound over 4-6 hours of racing.
• Cola at late aid stations is not a myth — the caffeine and simple sugar hit can rescue a fading run. But don't rely on it as your primary fuel.

What to do if your stomach turns: If you feel nauseous on the run, slow down slightly and switch to sipping flat cola or diluted sports drink. Don't stop eating entirely — that makes the bonk worse. Small, frequent sips of simple sugar are easier to absorb than gels when your gut is in distress.

If nausea is a recurring problem in your 70.3 races, the issue is almost always on the bike: either you ate too little (and now your body is panicking) or you ate too much in the final 10 minutes before T2 (and it's sitting undigested).

Hydration Across the Whole Race

Fluid needs vary massively between athletes. A 60kg woman racing in cool conditions might need 400ml per hour. A 90kg man in 30C heat might need 1,200ml per hour. Generic guidelines are next to useless here.

The only way to know your actual fluid needs is to know your sweat rate — and sweat rate changes with fitness, heat acclimatisation, humidity, and intensity.

General 70.3 hydration guidelines:

• Bike: 500-1,000ml per hour, adjusted for conditions. Most athletes should aim for 750ml per hour as a starting point.
• Run: 200-400ml every 20 minutes at aid stations. Don't skip stations, even if you don't feel thirsty — thirst lags behind actual dehydration by 20-30 minutes.
• Electrolytes: 500-1,000mg sodium per hour, depending on your sweat sodium concentration. Heavy, salty sweaters need more.

If you've raced before and noticed white salt stains on your kit afterward, you're a salty sweater and need to pay particular attention to sodium replacement. A standard sports drink provides roughly 300-500mg sodium per litre — which may not be enough for heavy sweaters.

Sweatr calculates your personal sweat rate and sodium needs from your Apple Watch or Garmin data, then builds a 70.3 hydration plan matched to your race-day conditions. No guesswork, no generic ranges.

Putting It All Together: A Sample 5-Hour 70.3 Plan

| Phase | Duration | Carbs | Fluid | Key Action | |-------|----------|-------|-------|------------| | Pre-race | 3 hrs before | 100-150g | 500-750ml | Porridge, toast, banana — low fat, low fibre | | Swim | ~35 min | 0g | 0ml | Mental rehearsal of T1 plan | | T1 | ~5 min | 0g | 200ml | Sip from bike bottle immediately | | Bike (hr 1) | 60 min | 70g | 750ml | First gel at 10 min, sip drink every 10 min | | Bike (hr 2) | 60 min | 70g | 750ml | Stay consistent — set a timer | | Bike (hr 3) | 60 min | 70g | 750ml | Last gel 15 min before T2 | | T2 | ~3 min | 0g | 100ml | Quick sip, nothing solid | | Run (hr 1) | 60 min | 45g | 400ml | Gel every 25 min, sip at every station | | Run (hr 2) | 60 min | 40g | 400ml | Maintain pace, add cola if needed | | Total | ~5 hrs | ~395g | ~3.8L | |

This is a template. Your plan should be adjusted for your body weight, sweat rate, and conditions. A 60kg athlete racing in cool weather needs less across the board. A 90kg athlete in the heat needs more.

How to Practise This in Training

Never race on a plan you haven't tested. In the 8-10 weeks before your 70.3, use your long bike sessions and brick workouts (bike-to-run) to rehearse:

1. Practise your exact products. Use the same gels, drink mix, and electrolytes you'll use on race day. Your gut needs to learn to absorb them at race intensity.
2. Simulate race timing. Set a timer to eat and drink on the same schedule you'll follow in the race. Build the habit so it's automatic.
3. Test your upper limit. Gradually push from 60g to 75g to 90g of carbs per hour on the bike. Some athletes can handle 90g comfortably; others max out at 60g. Find your ceiling in training, not on race day.
4. Do brick sessions fueled. Your bike-to-run transition is where nutrition plans fall apart. Run off the bike with food in your stomach and see how your gut responds.

If you feel bloated, nauseous, or crampy during a training session, that's useful data. Dial back the amount, try a different product, or adjust your timing. Every failed experiment in training saves you from a failed race.

The Mistake That Wrecks Most 70.3 Runs

It bears repeating: the run falls apart because of what happened — or didn't happen — on the bike.

If you consumed 30g of carbs per hour on the bike instead of 70g, you arrive at T2 with a 120g carbohydrate deficit. That's three gels you didn't eat. Your body has already started breaking down protein for fuel, your blood sugar is dropping, and your central nervous system is sending "slow down" signals.

No amount of run fueling can fix that deficit in real time. Your gut can only absorb 30-60g per hour on the run, and you needed that just for maintenance. You're now running to survive, not to race.

The athletes who run well off the bike in a 70.3 aren't tougher or fitter — they just ate enough in the first three hours.

Sweatr builds your complete 70.3 nutrition plan leg-by-leg, calculates your carbohydrate and fluid targets based on your actual training data, and adjusts for race-day conditions. Your watch already has the data — Sweatr turns it into a plan you can follow.