When should you start carb-loading for a triathlon?

Carb loading's been hugely popular for years, but what is the proven way to carb-load before a triathlon? We asked James Witts to explain

We explain how to carb load for triathlons up to Ironmans. Credit: Remy Whiting

Carb-loading has long been a popular practice among triathletes in the lead-up to their goal event for years. It’s where you increase the proportion of calories from carbohydrates to pack your muscles and livers with glycogen. It ties in with tapering where you reduce training duration but maintain intensity, both the feeding and easing off of volume designed to have you at your optimum come race day.


While sprint-distance triathletes can get away with maintaining a healthy diet, upwards of this and you’re looking to increase carbohydrate content for at least two days prior to your race. “For events over 90mins, carb-loading is relevant,” explains noted exercise physiologist John Hawley. “You do put on a bit of weight when you carb-load because, when you store carbohydrate in the form of glycogen, you also store water with it. You could put on 1-1.5kg in weight when carb loading – common for a 70kg triathlete. You might ask yourself is that beneficial? Well, for the first part of the race it’s not. But as it rolls on and you sweat out water, you’ll enjoy the benefits of those raised glycogen levels.”

How carb-loading works

Carb-loading’s importance is highlighted by studies that show the rate of glycogen breakdown during exercise is directly proportional to the amount of glycogen present in the muscle. So if you’ve very high muscle glycogen stores, you’ll break them down faster than when you have normal or high glycogen stores.

Key is that you shouldn’t just eat more. Both training and energy expenditure are reduced anyway. So you must emphasise increasing carb content (up to 70%) and reducing fat intake, aiming for around 5-7g/kg bodyweight each day.

Also of note is that the type of carbohydrate has little or no effect, so both liquid and solid carbs are fine. Finally, if you’re susceptible to gastrointestinal issues, choose carbohydrates with lower fibre intake like white rice over brown.

Carb-loading top tip

Many triathletes eat only carbs and avoid protein-rich foods the days before their event. Avoid this mistake. Your body demands protein on a daily basis. Hence, you should eat a small serving of low-fat proteins such as poached eggs, yoghurt, turkey or chicken as the accompaniment to your carb-loading meals. Or go for plant proteins such as beans and lentils. Around 1g protein per KG of bodyweight should be sufficient for muscle repair.


Here’s a day of carb-loading for a 77kg Ironman triathlete based on an extreme 10g/kg bodyweight. Total calorie count comes in at around 4,500 with 70% derived from carbs.


Glass of milk; one plain bagel with two tablespoons of peanut butter, honey and chopped banana. 809cals


Raisin and almond cereal bar washed down with glass of grape juice. 585cals


Chicken breast (skin removed) sandwich. Use four slices of white bread, Romaine lettuce, tomato slices and light mayonnaise. Nibble at baby carrots and some low-fat tortilla chips. Drink reduced-fat chocolate milk. 982cals


Low-fat yoghurt with low-fat granola and blueberries; glass of cranberry juice. 647cals


Wild salmon with two bread rolls and salad containing lettuce, red pepper, chopped apple, chopped walnuts, dried cranberries, asiago cheese and reduced-fat salad dressing. 1,059cals



Granola bar and glass of apple juice. 450cals

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