How to Fuel a 70.3 or Ironman: A Sports Dietitian’s Triathlon Nutrition Guide

Spring triathlon season is officially here, and for those who compete, everything is starting to feel real again. The long rides are getting longer, brick workouts are back on the calendar, and race day is inching closer.

But while many athletes focus on swim splits, bike watts, and run pacing, triathlon nutrition is often what determines whether your day comes together or falls apart.

Unlike a standalone run, swim, or cycling event, long-course triathlon fueling is a three-sport logistical challenge. You can’t fuel during the swim, transitions can be chaotic, your gut tolerance changes from bike to run, and weather conditions can shift dramatically over the course of the day.

That’s why the best 70.3 and Ironman nutrition plans go far beyond simply aiming for hourly carb, fluid, and sodium goals. They account for triathlon-specific logistics and race-day problem solving under fatigue.

I’m Claire, a Board-Certified Sports Dietitian (MS, RD, CSSD) with over 11 years of experience helping recreational through elite endurance athletes fuel for health and performance. As a parent and longtime distance runner, I understand how hard it can be to stay on top of your nutrition while balancing training with work, family life, household responsibilities, and everything else life throws your way.

That’s why my approach to triathlon nutrition is always rooted in real-life execution. Rather than following an overly rigid plan, we build a flexible strategy that works for your body, race, and current season of life…one that helps you stay consistent in training and confident on race day.

In this triathlon nutrition guide, I'll cover:

• What makes triathlon nutrition unique

• Common fueling mistakes triathletes make

• How to fuel the morning of your race

• Nutrition strategies for T1, on the bike, T2, and during the run

• Troubleshooting if the wheels start to come off on race day

• How to handle heat, humidity and extreme climate swings

• How to practice your fueling plan in training

• Differences between 70.3 and Ironman fueling plans

What Makes Triathlon Nutrition Unique

Long-course triathlon introduces challenges that standalone runners, swimmers, and cyclists don’t face in the same way.

Triathlon Challenge #1: Fueling Gap During the Swim

First, most triathletes don't fuel or hydrate during the swim. (I'm obviously not counting those unintentional sips of water...).

I have some clients who tuck a gel into the pocket of their kit for right after the swim, once they take their wetsuits off. But fueling while swimming? It's just not realistic, logistically speaking.

Depending on your pace and race distance, that means you may go 30–90+ minutes without carbs or fluid, which makes your carb load, pre-race breakfast, and pre-swim top-off of fuel even more important.

Triathlon Challenge #2: Different Conditions for the Gut

Second, your gut experiences different conditions across each leg of the race:

• Swim: horizontal positioning, no fueling

• Bike: lower impact, easier to eat and drink

• Run: higher heart rate, more jostling, often higher temps, reduced GI tolerance

In other words, the bike is where you set yourself up for a strong run, because once you’re on the run, it's much harder to fully meet carb and fluid needs.

Triathlon Challenge #3: It’s a Logistics Sport

Third, triathlon is a logistics sport. Your fueling success depends on things like:

• Bottle placement

• Bento box access

• Learning to eat and drink while riding

• Aid station timing

• Special needs bags

A triathlon nutrition plan that ignores logistics is rarely a plan that survives race day.

Key takeaway: Triathlon nutrition success depends on accounting for the swim fueling gap, respecting bike vs. run gut differences, having the right gear and nutrition products in transition, and planning for other logistics ahead of time, especially if you’re traveling to a race.

Pre-Race Fueling: Breakfast + Pre-Swim Top-Off

Your pre-race meal and snack matter even more in triathlon because of the fueling gap during the swim. So does your carb load and pre-race dinner, but you can read about that separately.

Timing will vary depending on race logistics (arrival time in transition, race start etc) and individual tolerance, but most 70.3 and Ironman athletes should aim to eat their main breakfast 3–3.5 hours before the swim begins.

Yes, that may mean eating at 3:00 or 3:30 AM. Trust me…it’ll be worth it!

A solid pre-race meal should primarily focus on lower fiber, easy-to-digest carbs, like:

• Oatmeal

• Rice

• Banana

• Bagel

• Pancakes/waffles with syrup

• Sports drink

• Applesauce

• Pop Tarts

The general guideline is to have 1-4g/kg of carbs 1-4 hours before your event, and you can spread these carbs out across your pre-race breakfast and snacks.

Because race day is so long, many athletes do well with a small amount of protein at breakfast (and some fat, as tolerated). This may look like:

• Peanut butter on your bagel or in your oatmeal

• Oatmeal made with milk or soy milk instead of water

• Egg(s) with your rice

• Sweetened yogurt or kefir

But remember, carbs are most important, and the other macros shouldn't crowd them out.

For more recommendations and pre-race meal/snack ideas, check out my blog post on what to eat before a long run or other endurance activity. And don't forget - you must practice all of this in training!

Pre-Swim Carb Top-Off

This part will vary depending on breakfast timing (and how much you ate), swim start time, and individual tolerance and preference. Some athletes get hungry during their event and need a bit more solid food leading up to the start, while others prefer sports products.

For example, you can have a snack with ~40-60g+ of easy-to-digest carbs (e.g. solid foods and/or sports products) ~1-2 hours before the swim start, followed by 20-30g carbs (e.g. a gel) 5-10 minutes before you hop in the water.

Or, you may opt to take in ~40g of carbs ~30-45 minutes before the swim start. Examples include:

• a full pack of chews

• a gel + sports drink

• a banana + sports drink

However you do this, your pre-swim snacks will help you bridge the swim fueling gap and start the bike leg feeling strong.

Key takeaway: A well-timed breakfast plus a pre-swim snack and/or carb-top off help offset the unavoidable fueling gap created by the swim.

How to Fuel in T1

Some athletes try to force a gel or other products immediately after they get out of the water. This may work for some (and if so, awesome), but it can lead to GI distress in others. This is because your heart rate is elevated, you may have swallowed water, and your body is adjusting from horizontal to upright positioning as you run into T1.

I often recommend starting with small sips of a sports drink once you're in T1, and if your transition time is short, you may even want to wait until you're on the bike before your first true feeding. Either way, allow your heart rate to chill out a bit before slamming your fuel to help reduce:

• sloshing

• reflux

• early nausea

• the risk of overloading your gut

This has to be individualized, and the amount of time you spend in T1 needs to be taken into account.

Key takeaway: Don’t rush the first major carb hit. Check in with your body and let your heart rate and stomach settle before feeding aggressively in T1 or during the first 15 minutes of the bike.

Bike Nutrition: Where You Do the Heavy Lifting

The bike is where you need to lay a strong foundation of fueling and hydration for your race.

It's your best chance to:

• Maximize carb intake

• Replace a higher percent of sweat losses

• Keep sodium intake consistent and in line with fluids

• Arrive at T2 not feeling like a hot mess

  
  

If you under-fuel on the bike, the half marathon or marathon becomes a damage-control situation. Ironman marathons are often total carnage, with athletes in pure survival mode, but it doesn't have to be this way!

Carb Intake on the Bike

For most long-course athletes, a strong baseline target for carbs is 60–90g per hour on the bike. I encourage my athletes to aim for the higher end of this range, as tolerated. This is usually achieved through a mixture of:

• Sports drinks (i.e. with carbs)

• Gels

• Chews

• Bars (e.g. Clif, Bobo, Nature's Bakery, etc)

• PB&J or other simple solids

A smart progression is to use slightly more substantial foods early, then shift toward simpler carbs like gels and drink mixes later in the ride. You can also slightly reduce your carb targets in the last hour of your ride. This will help prepare your stomach for the run.

And don't forget to think about logistics. Fueling on the bike can be tricky, especially if you're newer to cycling, and requires planning and practice. Cut your PB&J into quarters and bars in half, unwrap bars and chews, etc. Try to make it as easy as possible to grab and eat what you need, safely!

Triathlon Hydration on the Bike

The bike also is your best chance to stay on top of your fluid and sodium losses. This is especially important if it's a warm day, as it will be a lot harder to get adequate fluids in on the run.

While it's not recommended (or realistic) to fully replace what you lose through sweat, you should aim to replace ~75-90% of your fluid losses. This can be challenging, especially if it's warm and sweat rate is high. Practice and planning are key!

If you know you need a lot of fluids, make sure to maximize what you carry on the bike (e.g. use bigger bottles, add another cage). You will also need to use aid stations for bottle swaps or refills.

The other thing that's important to note about hydration on the bike is that most triathletes depend on their bottles for fluids, salts, AND carbs. So keep that in mind when putting together your entire fueling plan (i.e. how much carbs are you getting from fluids per hour?), and be sure to carry a backup gel or emergency carb source in case a bottle launches.

Key takeaway: The bike is your best opportunity to stay ahead on fueling and hydration before GI tolerance drops on the run.

Not sure what your sweat rate is? Want to learn about your fluid and sodium needs as an endurance athlete? Check out my Hydration for Endurance Athletes blog.

How to Fuel in T2

How you fuel during T2 will depend on what you just did on the bike.

For instance, if you just finished a bottle and took a gel on the bike 5 minutes ago, and you don't take too long in transition, then maybe you don't need to fuel during T2. But if it's been 20min since your last gel, and/or your transition is taking you 15-20 minutes, you likely will need to drink and eat something as you prep for the run.

Run Nutrition: Shift From Optimization to Tolerance

Once you hit the run, fueling becomes less about maximizing intake and more about preventing the wheels from coming off. IYKYK.

A realistic target is at least ~60g carbs per hour on the run. To keep things simple, try to take something at least every 30 minutes. Again, don't forget to account for any carbs coming from fluids when selecting which gels or other products to use.

That may look like:

• A gel every 25-30 minutes

• A sports drink every aid station (or in your handheld) + gel every 30-45 minutes

• Coke later in the race from the aid station if flavor fatigue sets in or if stomach feels off

For triathlon hydration on the run, aim to replace roughly 75% of sweat losses per hour, as tolerated. (Learn how to test your sweat rate here). 

You can't really "catch up" with fueling and hydration on the run. It's more about doing whatever you can tolerate until the finish. That's why your fueling and hydration strategy leading up to the race start and on the bike matter so much.

Use Every Aid Station as Home Base

Every aid station is a chance to assess:

• Do I need carbs?

• Am I drinking enough?

• Is my stomach turning?

• Do I need water instead of a sports drink?

• Would a short walk or reduction in effort level reset things?

This simple checkpoint mindset helps prevent small issues from becoming major problems.

How to Troubleshoot GI Issues On The Run

GI issues are very common on the run during 70.3 and Ironmans. Practicing your fueling and hydration strategies in training (i.e. training the gut) and not over-doing your pace relative to your fitness level are your best bets when it comes to preventing GI issues on race day.

But sometimes sh*t (and other lovely GI issues) happens, quite literally. If a GI problem occurs, here are a few tips:

• Slow your pace, or take a walking break as needed

• Slow your fueling and hydration (i.e. small sips and bites) but try not to stop

• Sip a little coke at an aid station (this helps to revive many athletes)

• See if anything else at an aid station appeals (e.g. salty/savory food)

• Suck on a ginger candy/chew

• If it's hot out, cool your body with water, ice, cold drinks

Upper GI issues (e.g. nausea, vomiting) often is linked to hydration, whereas lower GI issues (e.g. diarrhea) may be linked to your fueling. But GI issues can be complex, and are not always linked to your fueling and hydration plan.

If you frequently have GI issues, this is a great reason to work with a sports dietitian!

Key takeaway: On the run, consistency beats perfection. Something every 30 minutes, consistent hydration, and smart aid station decisions go a long way.

Heat, Humidity, and Extreme Climate Swings

Many Ironman and 70.3 races start cool and finish hot. You might begin in chilly water, ride as it starts to warm up, then hit an exposed run course in the heat. If you didn't stay on top of your fueling and hydration early on, by the time you hit the run, it's too late.

Heat has many impacts on fueling and hydration. First, your sweat rate increases, which means you need more fluids and salts. Heat also can lead to higher risk of GI issues, as less blood is flowing the to gut to prioritize skin cooling. Additionally, dehydration (which is incredibly commonly in long-course triathletes) further worsens GI issues.

Humidity also can lead to GI and performance issues, as high humidity hinders sweat evaporation, the body's main cooling mechanism. This can lead to increased core temperatures, higher heart rates, and faster dehydration.

In hot races, it can help to front-load an extra ½ to 1 bottle in the first hour of the bike if tolerated, adjusting gels and chews downward so the sports drink provides more of the carbs. This helps you get ahead before the hardest hydration section of the day…the run.

This is also where it's helpful to acclimatize to heat in training, if possible (e.g. sauna, training later in the day, etc), and choosing appropriate gear so you don't overheat.

Key takeaway: In hot races, prioritize early bike fluids so you don’t start the run already behind on hydration. Test your sweat rate in different temperatures to be sure you're adequately replacing your losses.

The Biggest Long-Course Triathlon Nutrition Mistakes

The most common mistakes I see with triathlon nutrition include:

• Not having a plan (duh)

• Not doing a carb load (or not doing it correctly)

• Under-fueling before the race starts

• Not practicing fueling before early swim practices

• Waiting too long to eat after the swim

• Treating bike and run fueling the same (i.e. not fueling enough on the bike)

• Trying to catch up with fueling on the run

• Failing to adjust for heat

• Relying on only one flavor/product all day

• Not practicing transitions

• No lost bottle backup plan

• Never testing race pace fueling in bricks

I cannot emphasize enough how important it is not only to educate yourself and make a plan, but also to PRACTICE your planned race fueling strategies in training. There will always be things that happen that are out of your control, but the more you practice and figure out what works best for you, the more likely it will be that you'll have a good day.  

Key takeaway: Many triathlon nutrition mistakes are planning and execution errors, which means this can be addressed in training.

How to Practice This in Training

Don’t wait until race day to figure out your triathlon nutrition and hydration strategy. Use long bricks and race-specific workouts to practice:

• Breakfast timing

• Pre-swim carb timing

• T1 and T2 fueling

• Bottle grabs

• Bento box access and other on-the-bike fueling strategies

• Run aid station simulation (practice races are helpful here)

• Fueling at race pace

• Heat hydration adjustments

Key takeaway: The more race-day logistics you rehearse in training, the more automatic your fueling becomes under stress.

70.3 vs. Ironman: How Your Fueling Strategy Evolves

While the principles stay similar, execution changes between a 70.3 (½ Ironman) and a full Ironman.

70.3

• Higher intensity (higher sweat rate and carb needs, potentially higher risk of GI issues)

• Simpler run fueling

• Less flavor fatigue

• Less time for things to go wrong, GI wise

  
  

Ironman

• Longer swim fueling gap

• More flavor fatigue

• Greater climate variation

• More GI troubleshooting

• Stronger need for contingencies

The longer the race, the more your plan must account for palate fatigue, mental fatigue, potential GI issues, and problem solving.

Key takeaway: The longer the race, the more important flexibility, flavor variety, and backup plans become.

Learn More About Specific Fueling Strategies

If you’re looking for much more in-depth guidance on intraworkout nutrition, hydration, and carb loading for endurance athletes, check out these resources: 

• What to Eat Before a Long Run (or other endurance activity)

• What to Eat During Long Runs (or other endurance activities)

• What to Eat After a Long Run (or other endurance activity)

• Hydration for Endurance Athletes

• How to Carb Load for Your Next Race

• What to Eat Before Race Day

Need Help Fueling Your Next 70.3 or Ironman?

The best triathlon nutrition plans aren’t the most complicated. They’re the ones you can execute when you’re tired, hot, stressed, and making decisions under pressure. A flexible plan beats a perfect plan any day… because let’s be honest, no triathlon is perfect. 

For long-course racing, success comes from:

• fueling the bike well

• simplifying the run

• staying proactive with triathlon hydration

• practicing transitions

• adapting to conditions

When done right, your fueling supports not just survival, but your strongest possible finish.

If you’re looking for support in fueling your next 70.3 or Ironman, I’d love to help!

What Triathletes Are Saying 

"I have had the pleasure of observing Claire run coach as well as nutrition coach. As a running and cycling coach with 10+ years of experience, a former Division I runner, an Ironman, and an avid cyclist, I have a great appreciation for nutrition and run coaching. With that said, it is one thing to understand the science behind nutrition and running, but it is very much another to convey it to people in a way that they understand and can internalize. Claire does an excellent job of both and as a result, her clients see great results." - Matt Wilpers, Peloton Instructor & Coach

"I had the pleasure of working with Claire to prepare for my first Ironman. Claire really helped me focus on my daily meal plan during the months of training leading up to the race and then to dial-into my nutrition plan for race day. We experimented with different options during my long training days until we came up with a plan that we thought would work best for me. I kept to the plan on race day and was able to maintain the necessary energy throughout. Thanks to Claire I had a great race and exceeded my expectations." - Jennifer F. 

“Thank you for the guidance you provided me over the last few months of Ironman prep. The information you gave me was so helpful on race day. I stuck to the plan, which led me to feel so good on the run, and finished much faster than I anticipated. You're awesome and I'm so grateful you were on my team :)” - Sarah B. 

Ways to Work With Me

• Join my low-cost, low-commitment membership service, Fuel for Life Crew, for $39/month (cancel anytime)

• Check out my free nutrition downloads, including fueling plan templates

• Browse my library of nutrition masterclasses (all are included inside the Fuel for Life Crew membership)

• Fill out a new client inquiry form if you’re interested in 1:1 nutrition coaching