70.3 Triathlon Nutrition Plan: How Many Carbs Per Hour to Fuel a Half Ironman

A proper half ironman nutrition plan is the difference between bonking on the run with GI problems and crossing the finish line strong. This article tells you exactly how many carbs you need per hour, what to eat, and when. All based on peer-reviewed research.

How many calories does a half IRONMAN (70.3) burn

Before we build a half triathlon nutrition plan, it’s interesting to understand what’s at stake energetically. A 70.3 is a serious metabolic event.

Research on full Ironman triathletes gives us a useful reference point. A study published in the International Journal of Sport Nutrition and Exercise Metabolism found total energy expenditure of approximately 10,036 kcal in male and 8,570 kcal in female Ironman finishers.

While no large-scale study has measured energy expenditure specifically in 70.3 racing, extrapolating from full Ironman data gives us a solid estimate:

A half Ironman triathlete would burn roughly 4000 kcal (female) to 5000 kcal (male). This is based on a finish time of roughly 6 hours. Interestingly, triathletes burn far fewer calories in the swim portion of the race, compared to the bike or run.

If you’re expecting to take longer, you’re likely to burn more calories. The same is true for body weight: heavier athletes burn more calories.

Here’s the critical insight though: you cannot — and should not — try to replace all of those calories during the race. For example, the triathletes in the study only ate 35-40% of the calories burned. For 70.3 triathletes, that would equal 1558 — 1970 kcal.

Which brings us to the key point: calories are not the right metric to guide your race-day nutrition. Half Ironman fueling is about something else.

IRONMAN 70.3 fueling

Theoretically all people have enough energy stored in the body to complete a 70.3 triathlon, without nutrition. An 80kg person with 20% body fat has 16kg of fat stored in the body. That equals 144,000 kcal, almost 30x the energy required to finish a half triathlon. Even a 60kg person with 5% body fat still has 5x the energy required.

So why even bother talking about fueling for a half Ironman?

Because your goal is not only to finish a 70.3, you want to race it.

As soon as you increase intensity slightly — e.g. from walking to jogging — your body increasingly relies on carbohydrates as fuel. You cannot only burn fat. In fact, if you would race your 70.3 at lactate threshold intensity, fat contribution becomes negligible.

So even though training the body to burn more fat can be beneficial, it’s clear that from a fueling perspective, we do not need to worry about fat intake.

What about carbohydrates? An interesting scientific case study found that during a half Ironman, carbohydrates accounted for 80-90% of total energy expenditure. Unfortunately — unlike fat — your carbohydrate stores are limited. Depending on your pre-race nutrition and your race intensity, you could empty your carb stores within an hour. Which would result in a DNF.

This is why carbohydrates — not total calories or fat — are the central focus of 70.3 race nutrition. Getting your carbohydrate intake right is what keeps you at race pace and prevents bonking during the run. So how many carbohydrates do you need during a half triathlon?

How much carbohydrates do you need during the 70.3 race

The half Ironman case study shows that a triathlete burns roughly 640 gram carbs during the bike segment and 370 gram carbs during the run. That’s well over 1000 gram carbs for a half triathlon, especially because it does not include the swim yet.

This highly trained triathlete finished the 70.3 in less than 5 hours. The bike and run segment took a little over 4 hours. Which results in a carbohydrate oxidation rate of about 240 grams per hour. That equals 10 bananas… per hour.

Clearly, no athlete can consume 240 grams of carbohydrates per hour during a race. But luckily, you don’t need to. That is because a significant part of those carbohydrates can come from your carbohydrate storage in the muscles, also known as glycogen. You can boost glycogen levels with the right pre-race nutrition and a pre-race taper week.

So how many carbohydrates you need to consume during your 70.3 depends on how much carbohydrates you burn and how much carbohydrates you have stored in the muscles:

Required carb intake = Carbs burned — Carbs stored (glycogen)

Numbers from the use case:

• Carbs burned during bike and run: ~1010 gram

• Carbs coming from carb storage: ~700 gram

• Carbs consumed during bike and run: 308 gram (74g per hour)

There are two important things we can learn from this:

1. If you race faster (= burn more carbs), you need more glycogen or carb intake

2. If you increase carb intake or glycogen, you can theoretically race faster

Really? Race faster by consuming more carbohydrates?

This study shows that during IRONMAN Kona, Hawaii, faster finish times are correlated with higher carbohydrate intake rates.

So should you simply copy the >100 grams of carbohydrate per hour for your 70.3? Probably not. Here’s what to do instead.

How many grams of carbs per hour during half triathlon

A 70.3 triathlon typically takes between 4 and 7 hours to complete. That puts it firmly in the category of ultra-endurance exercise. Science gives us a clear target. If you want to race a half Ironman as fast as possible, you should consume up to 90 grams of carbohydrate per hour.

Depending on your preferred foods — which we talk about in a minute — that equals roughly:

• 3-4 gels, OR;

• 1-2 gels and one 0.5L sports drink, OR;

• 1 energy bar and 2 gels

Even though some professionals consume up to 120 grams per hour on the bike, recreational triathletes should be cautious. Consuming more than 50-60g carbs per hour requires a well thought out strategy. So before you implement this, continue reading about the importance of timing, intake vs uptake and carbohydrate types.

How many carbs per hour during the swim (1.9 km)

You cannot fuel during the swim. The swim leg of a 70.3 typically takes 30-60 minutes, depending on your level. During that time your body is drawing entirely on the energy storage you built through pre-race nutrition.

When the swim is over, it’s time to start fueling as soon as possible.

How many carbs per hour cycling (90 km)

Cycling provides the best opportunity to ingest carbohydrates during a triathlon. Most of the carbohydrate intake during a 70.3 triathlon race occurs during the cycling leg, with intake during the bike 2-3x higher than during the run.

The reason is straightforward: you can easily carry bottles, gels and other foods + you’re in a stable position, making food intake and gastric emptying easier compared to the run. It should be clear that missing your carb targets on the bike cannot be compensated for during the run.

Aim to take 90 grams of carbohydrates per hour when cycling. But only if:

• You’ve experimented with this high intake during training

• Your nutrition is a mix of glucose and fructose (2:1)

• You’re racing at a high intensity

(More about these requirements in a bit)

If you don’t check all 3 boxes, 50-60 grams per hour is much more realistic.

So what do others do? Precision hydration looked at 137 case studies and found that on average, 70.3 Ironman athletes consume 91 g of carbohydrate per hour during the cycling leg.

“On average, Ironman athletes consume 91 g of carbohydrate per hour during the cycling leg.”

They also saw a wide range of carb numbers, from as little as 28g/h to 126g/h during the entire 70.3 Ironman.

How many carbs per hour running (half marathon)

This is where many 70.3 triathletes go wrong. They either reduce carb intake to a minimum or push the same quantities they managed on the bike.

Experienced triathletes continue to consume carbohydrates at a fairly high rate, but go well below 90g per hour during the run.

Again, the reason is pretty straightforward: it’s practically harder to drink and eat + running creates far more stress on the gut than cycling.

Aim to take 30-60 grams of carbohydrates per hour when running.

Precision hydration shows that the half Ironman participants in their research averaged 57g/h on the run. Which is only 37% less than during the bike segment.

Important note: your intake does not necessarily equal your uptake. Just because you can eat (swallow) 3 gels per hour doesn’t mean those carbohydrates actually enter your bloodstream. This is where choosing the right type of food comes into play. And equally important: training your gut. Let’s cover those topics next.

Half triathlon nutrition plan

Now that we know how many carbohydrates we need to consume, it’s time to talk about what exactly to eat and drink during your 70.3. We’ll cover:

• What to eat

• What to drink

• Glucose: Fructose

• Caffeine

Here’s your nutrition plan for a half Ironman:

What to eat during a 70.3 triathlon

Should you eat gels, bars or real food? Each carbohydrate source has its place in a 70.3, and the right choice depends on where you are in the race.

Gels are compact, predictable, and easy to carry, making them the go-to choice for the run when solid food becomes difficult to tolerate.

Bars and solid (real) foods are harder to digest at race intensity and carry a higher risk of gastrointestinal problems, particularly because fat, protein and fibre slow gastric emptying. If you struggle with the sweetness of gels over a long duration, you can use them in the first half of the bike segment. However, avoid them when you’re within an hour of T2.

The general rule: start with more variety on the bike, simplify aggressively on the run. And equally important: experiment in training, not on race day.

How often to take gel when running

A standard gel contains 20-25 grams of carbohydrates. To reach 30-60 grams of carbohydrates per hour, you need 2-3 gels per hour. In other words: take a gel every 20-30 minutes when running.

Most age-groupers run 1.5 to 2.5 hours during a 70.3 triathlon. That means you need to take about 3-7 gels in total. Practice this in training, so your gut is accustomed to it.

Also important: try to take gels with water, rather than a sports drink. Otherwise you’re increasing the carbohydrate concentration even further, which can slow absorption. If you are planning on taking sports drinks, adjust the number of gels and consider a less concentrated gel.

That brings us to the next topic.

What to drink during a 70.3 triathlon

Sports drinks kill two birds with one stone. They deliver carbohydrates and fluid simultaneously, making them ideal throughout the entire 70.3 triathlon.

But how much should you drink?

Precision hydration looked at the hydration plan of 137 triathletes and found 70.3 athletes drank 706ml per hour during the cycling leg and 502ml per hour during the run.

Isotonic vs Hypertonic vs Hypotonic

The concentration of carbohydrates in your sports drinks affects how quickly fluid is absorbed.

Isotonic drinks contain a carbohydrate concentration that is similar to the concentration of fluids in your body. This allows for a fairly quick uptake. Isotonic drinks are your go-to during a 70.3, balancing carbohydrate delivery and hydration.

Hypotonic drinks contain a lower carbohydrate concentration. They are absorbed even faster, but deliver fewer carbohydrates. They are useful when hydration is the priority, particularly in hot conditions.

Hypertonic drinks contain a higher carbohydrate concentration. Before they are absorbed, they need to draw water into the intestine to dilute the solution. This slows down the energy delivery and increases GI distress risk. Avoid relying on hypertonic drinks as your main fluid source during the race.

Gels are often hypertonic though. That’s actually practical: it allows you to carry a lot of carbohydrate in very little space. Always take them with water to aid absorption and reduce GI distress risk.

Electrolytes

When you sweat, you lose sodium. Replacing it helps your intestine absorb both water and carbohydrates more efficiently. Most isotonic sports drinks already contain sodium, so if you’re relying on these throughout the race you’re likely covered.

However, if you’re a relatively slow triathlete who drinks large amounts of water, you should consider adding electrolyte tablets to prevent sodium from dropping too low (hyponatraemia).

For hydration, aim to limit body weight loss through sweat to no more than 2-3% during the race. You can test this in training.

Glucose: Fructose

Not all carbohydrates are equal during exercise.

Glucose is absorbed via a transporter that maxes out at around 60 grams per hour. Once that transporter is saturated, extra glucose simply isn’t absorbed. It sits in your gut and increases the risk of gastrointestinal problems.

But we are aiming for 90 grams per hour when cycling.

That’s where fructose comes in. Fructose uses a completely different transporter. As a result, combining glucose and fructose allows you to increase carb intake from 60g (only glucose) to 90g per hour.

In practice, check the label on your gels and sports drinks to see whether they only contain glucose, or also fructose. A glucose-to-fructose ratio of 2:1 is recommended.

Caffeine

Looking for the most effective legal performance enhancer? Search no further. Caffeine is a well-researched and legal performance enhancer. It reduces your perception of effort, which is especially useful in the latter stages of a 70.3.

Timing wise it’s probably best to take caffeine during (the end of) the bike segment, since it takes 45 minutes to peak in the bloodstream. Caffeine intake during your half marathon might be a bit too late.

Many sports nutrition products contain caffeine. As with all nutrition recommendations: test it in training before implementing it on race day.

Making nutrition part of your half IRONMAN training

In a way, your gut is like your muscles: it is highly adaptable. Research shows that both gastric emptying as well as stomach comfort can be trained. Adaptations that occur appear to be nutrient specific. Regularly consuming carbohydrates during training increases the capacity of intestinal transporters.

In practice: don’t save your race nutrition for race day. Use your long training sessions to systematically increase your carbohydrate intake, starting at 40-50g per hour and building toward your race target over several weeks.

FAQ: How to avoid GI problems during your 70.3 triathlon

Roughly 30 to 70 percent of the participants experience gastrointestinal problems during a 70.3. Research shows that 14% of half Ironman athletes have serious GI symptoms. The most important risk factors are ones you can control. Avoid GI problems by avoiding high carbohydrate intake without gut training, hypertonic drinks, and consuming fibre, fat or protein during the race.

Minimize the risk of experiencing GI problems by keeping race nutrition simple: isotonic drinks, gels with water and no solid foods in the final hour before T2. Equally important: train your gut systematically in the weeks before the race. Never try anything new on race day.

Literature

Total energy expenditure is approximately 10,036 kcal in male and 8,570 kcal in female Ironman finishers.

• Kimber, Nicholas E et al. “Energy balance during an ironman triathlon in male and female triathletes.” International journal of sport nutrition and exercise metabolism vol. 12,1 (2002): 47-62. doi:10.1123/ijsnem.12.1.47 https://pubmed.ncbi.nlm.nih.gov/11993622/

During a half Ironman, carbohydrates account for 80-90% of total energy expenditure.

• Gillum, Trevor L et al. “Muscle glycogenolysis and resynthesis in response to a half Ironman triathlon: a case study.” International journal of sports physiology and performance vol. 1,4 (2006): 408-13. doi:10.1123/ijspp.1.4.408 https://pubmed.ncbi.nlm.nih.gov/19124898/

A triathlete burns roughly 640 gram carbs during the bike segment and 370 gram carbs during the run.

• Gillum, Trevor L et al. “Muscle glycogenolysis and resynthesis in response to a half Ironman triathlon: a case study.” International journal of sports physiology and performance vol. 1,4 (2006): 408-13. doi:10.1123/ijspp.1.4.408 https://pubmed.ncbi.nlm.nih.gov/19124898/

During IRONMAN Kona, Hawaii, faster finish times are correlated with higher carbohydrate intake rates.

• Pfeiffer, Beate et al. “Nutritional intake and gastrointestinal problems during competitive endurance events.” Medicine and science in sports and exercise vol. 44,2 (2012): 344-51. doi:10.1249/MSS.0b013e31822dc809 https://pubmed.ncbi.nlm.nih.gov/21775906/

If you want to race a half Ironman as fast as possible, you should consume up to 90 grams of carbohydrate per hour.

• Jeukendrup, Asker E. “Nutrition for endurance sports: marathon, triathlon, and road cycling.” Journal of sports sciences vol. 29 Suppl 1 (2011): S91-9. doi:10.1080/02640414.2011.610348 https://pubmed.ncbi.nlm.nih.gov/21916794/

Most of the carbohydrate intake during a 70.3 triathlon race occurs during the cycling leg, with intake during the bike 2-3x higher than during the run.

• Jeukendrup, Asker E et al. “Nutritional considerations in triathlon.” Sports medicine (Auckland, N.Z.) vol. 35,2 (2005): 163-81. doi:10.2165/00007256-200535020-00005 https://pubmed.ncbi.nlm.nih.gov/15707379/

Both gastric emptying as well as stomach comfort can be trained.

• Jeukendrup, Asker E. “Training the Gut for Athletes.” Sports medicine (Auckland, N.Z.) vol. 47, Suppl 1 (2017): 101-110. doi:10.1007/s40279-017-0690-6 https://pubmed.ncbi.nlm.nih.gov/28332114/

14% of half Ironman athletes have serious GI symptoms.

• Pfeiffer, Beate et al. “Nutritional intake and gastrointestinal problems during competitive endurance events.” Medicine and science in sports and exercise vol. 44,2 (2012): 344-51. doi:10.1249/MSS.0b013e31822dc809 https://pubmed.ncbi.nlm.nih.gov/21775906/