IRONMAN 70.3 Nutrition Plan: How to Fuel Without Bonking or GI Distress

Your muscles will run out of glycogen in roughly 90 minutes. Your 70.3 will take you four to seven hours. That math problem has exactly one solution: you have to fuel, deliberately and on a schedule, from the moment you exit the water — not when you feel like it, not when you're hungry, and definitely not after you start to fade. The athletes who get this wrong don't bonk dramatically. They just gradually slow down and wonder why their legs stopped working ten miles into the run. The athletes who get it right build a protocol and execute it without negotiation.

The Physiology Problem You're Actually Solving

Your body stores carbohydrate as glycogen — primarily in your muscles (roughly 300–500g) and your liver (roughly 80–100g). At race-pace intensity, you're burning through both stores at a rate that outpaces what fat oxidation can cover. By the time you hit the halfway point of the bike leg, you're already drawing down reserves that have to last you another 13.1 miles on foot.

The practical ceiling for carbohydrate absorption from a single source — glucose — is approximately 60 grams per hour. That's because glucose relies on a specific intestinal transport protein (SGLT1), and that transporter saturates. Push past 60g/hr of glucose-only fuel and you're not absorbing the excess — you're feeding gut bacteria, generating gas, and creating the conditions for the GI spiral that ends so many 70.3 races.

The breakthrough in sports nutrition research came when Asker Jeukendrup and colleagues demonstrated that fructose uses an entirely different transporter — GLUT5 — which operates independently of SGLT1. That means glucose and fructose can be absorbed simultaneously, through parallel pathways, effectively doubling your absorption ceiling. A 2:1 glucose-to-fructose ratio can push oxidation rates to 90 grams of carbohydrate per hour — and more recent research by Podlogar and colleagues suggests ratios closer to 1:0.8 may drive even higher oxidation with equivalent GI tolerance. The practical upshot: the composition of your fuel matters as much as the quantity.

Why does this matter for your race? Because 43% of IRONMAN-distance athletes report significant GI problems during racing, and GI distress is one of the leading causes of DNFs. The majority of those problems are preventable. They come from the wrong fuel composition, the wrong concentration, fueling too late, or going too hard before the gut is ready to process what you're giving it.

Athlete Takeaway: Your fueling problem is a biochemistry problem. Understand the transporter limits and you understand why multi-source carbohydrates — not just "gels" — are non-negotiable for a 70.3.

Your 70.3 Nutrition Protocol: The Numbers That Actually Matter

Carbohydrate Targets by Segment

A 70.3 moves through three distinct physiological windows, and your fueling should reflect that. The bike offers your best opportunity to take in fuel — heart rate is lower, you're not impact-loading your gut, and gastric emptying is faster. The run is where the wheels come off if you got the bike wrong, and where you need to shift to smaller, more frequent doses.

Target range: 80–120+g of carbohydrate per hour on the bike, from a multi-transporter source (glucose + fructose or maltodextrin + fructose) — more on product selection below. On the run, drop to 40–60g per hour, primarily from liquids and gels. Solid foods become progressively harder to process as run intensity and core temp rise.

Reference Fueling Table (adjust by body weight and race conditions)

Segment	Carbs (g/hr)	Sodium (mg/hr)	Source Examples
Bike hrs 1–2	80–100g	800–1,000mg	Raw Fuel (4 scoops) + RAW Replenish
Bike hr 3	60–75g	900–1,100mg	Raw Fuel + RAW Replenish or gels
Run mile 1–4	40–55g
Run mile 5–9	40–55g	500–700mg	Gel + RAW Replenish + cola if offered
Run mile 9–13	30–45g	300–500mg	Gel or chews + water

Sodium targets assume moderate heat and average sweat sodium. High sweaters should push toward the upper end.

NVDM Coaching Sponsor: NVDM Coaching uses and recommends RAW Fuel and RAW Replenish from the RAW Endurance line. Raw Fuel’s 1:0.8 glucose-to-fructose formula (maltodextrin + cane sugar + fructose) and 300mg sodium per serving are built for exactly the 80–120g/hr targets in this article — the numbers align because Natasha van der Merwe co-developed the product and trains and races on it herself.

Sodium: The Variable Most Athletes Underdose

Sodium does two critical jobs: it facilitates fluid absorption across the gut wall, and it maintains plasma osmolality — keeping fluid where it belongs, in your blood, rather than leaking into surrounding tissue. Athletes who drink large volumes of low-sodium or plain water during a 70.3 often end up heavier at the finish than at the start — fluid is accumulating in the wrong compartment. This is early hyponatremia, and it feels like dehydration even when it isn't.

The general guideline is 500–1,000mg of sodium per hour, but high-sodium sweaters — identifiable by white salt residue on skin and kit, or by sweat testing — can need 1,200mg/hr or more in hot conditions. Your sports drink is your primary sodium delivery vehicle on the bike — Raw Fuel delivers 300mg per serving via a sea salt/sodium citrate blend that’s designed for gut tolerance at high intake rates. On the run, supplement with sodium-containing gels and RAW Replenish if needed, particularly on hot-weather courses.

Caffeine: Timing Is Everything

Caffeine is a legitimate ergogenic aid at 2–3mg per kilogram of body weight — for a 70kg athlete, that's 140–210mg. The mechanism is central: caffeine blocks adenosine receptors, lowering perceived effort and raising pain threshold. In a long-course race, that effect is most valuable when fatigue is accumulating, not when you're fresh out of T1.

The recommended timing for a 70.3: take your first caffeine dose 60–90 minutes into the bike leg, with a second dose at the bike-run transition or early in the run. A caffeinated gel (typically 50–75mg caffeine) at mile 2 of the run is a common race-day tool that most athletes find practical and well-tolerated.

Critical caveat: Caffeine accelerates gastric motility. In athletes who are already running at high intensity with a warm gut, this can tip a manageable GI situation into an urgent one. If you have any history of run-leg GI distress, trial caffeine in training before committing it to your race plan.

Athlete Takeaway: Build your protocol around the numbers: 80–120g carbs/hr on the bike, 40–60g/hr on the run, 800–1,200mg sodium/hr, and caffeine timed for the second half of the bike and early run.

Gut Training: The Step Every Protocol Guide Leaves Out

Here’s the inconvenient truth about 70.3 nutrition plans: knowing the right targets and executing them on race day are different skills. If you’ve been training for months without practicing your fueling at race-like rates and intensities, your gut is not prepared to absorb 100+g of carbohydrate per hour at Zone 3 bike effort. It will object. Loudly.

The gut is highly trainable. Research published in the Gatorade Sports Science Institute review (Jeukendrup, 2014) established that regular carbohydrate ingestion during training increases intestinal carbohydrate absorption capacity, reduces GI distress, and improves exogenous carbohydrate oxidation — meaning your body gets better at actually using the fuel you give it. A 2025 systematic review published in the Journal of the International Society of Sports Nutrition confirmed that gut training protocols show promise in improving GI symptoms over time in endurance athletes.

A Practical Gut Training Protocol

Begin structured gut training 6–8 weeks before your race. The goal is progressive: start at the lower end of your carbohydrate target and work upward over successive long sessions.

• Weeks 8–6 out: 60–70g carbs/hr on all long bike sessions and bricks. Use your race-day fuel products — not training convenience products. This is when you introduce Raw Fuel if it’s new to you.

• Weeks 5–4 out: 80–90g carbs/hr on long bike sessions. Introduce your run fueling protocol (gel every 20–25 minutes) in your brick runs.

• Weeks 3–2 out: 90–110g carbs/hr on key long rides. Execute your full race-day nutrition plan on your 2–3 hour simulation sessions.

• Week 1 out: Race-simulation fueling on your final race-prep sessions. Nothing new.

One non-negotiable: practice your nutrition at race intensity, not easy training pace. At easy effort, your gut tolerates almost anything. At 85% FTP on the bike, the same fuel that felt fine at recovery pace can cause bloating and nausea. Train the gut under the conditions it'll face on race day.

Athlete Takeaway: Gut training is not optional. If your only nutrition practice is during easy training rides, you haven't trained your gut for a race. Schedule at least 4–6 race-intensity fueling sessions in the 8 weeks before your event.

The GI Distress Triggers You Can Actually Control

GI problems in a 70.3 feel like bad luck. They're usually bad planning. Here are the most common controllable triggers:

Single-Source Carbohydrates at High Doses

Fueling on glucose-only products (certain brands of gel use maltodextrin exclusively, which converts to glucose) at 80+g/hr saturates your SGLT1 transporters. The unabsorbed carbohydrate sits in your intestine and ferments. This is the physiological cause of the bloating and cramping that appears at mile 50 on the bike and ruins your run. Check your products' ingredient labels — you want glucose (or maltodextrin) combined with fructose or sucrose.

NSAIDs Before or During Racing

Research has shown that ibuprofen use increases GI complication risk by 3–5 times and aggravates exercise-induced intestinal injury. The "preventative" ibuprofen that some athletes take before a long race is a significant GI risk factor. If you're managing a genuine injury in the lead-up to race day, discuss pain management with your physician — but leave NSAIDs out of your race-day plan.

Going Out Too Hard

High-intensity exercise diverts blood from the gut to working muscles — splanchnic blood flow can decrease by up to 80% during maximal efforts. An athlete who rides the first 20 miles of the bike above threshold doesn't just deplete glycogen faster — they've compromised the gut's ability to absorb the fuel they're trying to take in. Pacing isn't just a performance strategy. It's a nutrition delivery strategy.

Athlete Takeaway: The most GI-protective thing you can do is ride the first 30 minutes of the bike conservatively. A well-paced gut that's absorbing fuel beats a hard-charged athlete who can't process anything from mile 40 onward.

Race Morning and Pre-Race Fueling

What you do the morning of the race sets the table for everything that follows.

The Pre-Race Meal

3–4 hours before race start: 1–4g of carbohydrate per kilogram of body weight, from easily digestible sources — oatmeal, rice, white bread, banana. Low fat, low fiber, moderate protein. The goal is arriving at the start line with full glycogen stores but an empty enough GI tract that you're not fighting your breakfast on the first loop of the bike.

The Start-Line Window

If race start is more than 60 minutes away after your pre-race meal, a small carbohydrate top-up (20–30g, such as a gel or sports drink) 5–10 minutes before the gun can be useful. Avoid carbohydrates in the 30–60 minute window before start — insulin response can cause transient hypoglycemia at effort onset in some athletes.

The Swim

You won't fuel during the swim, but the swim isn't metabolically neutral. A 25–40 minute open water swim at race effort does consume glycogen. Don't delay your first bike nutrition past the first 10–15 minutes because "you just got out of the water and aren't hungry yet." Start fueling on the schedule. Hunger is a lagging indicator — by the time you feel it, you're already behind.

Athlete Takeaway: Eat your pre-race meal, follow it precisely as you've practiced in training, and take your first nutrition on the bike within 10–15 minutes of mounting. Not when you're hungry. On schedule.

Bottom Line — What This Means For You

IRONMAN 70.3 nutrition is not complicated, but it requires specificity. The athletes who nail it aren’t smarter — they’re more deliberate. They know their per-hour carbohydrate target (80–120+g on the bike, 40–60g on the run). They’re using a multi-transporter product built for those targets. They’ve trained their gut at race intensity, on race products, under race conditions. They execute the same plan they practiced — without improvising.

"Eat some gels and drink when thirsty" is not a nutrition plan. It's an approach that gets athletes to mile 9 of the run wondering why they can't hold pace. Know your numbers, train your gut, and execute without negotiation. That's how you fuel a 70.3.

Want a nutrition plan built specifically for your race?

NVDM Coaching's triathlon plans include periodized nutrition guidance — not generic targets, but specifics matched to your race distance, sweat rate, and training load. If you're 4–8 weeks out from a 70.3 and want to get your fueling dialed before race day, explore NVDM coaching plans at nvdmcoaching.com.