Eating for a Marathon: Carb Loading

Part of marathon preparation is doing some shorter races in the lead up to the big day. Shorter races can be a good gauge of how your training is progressing – many people set new personal best times at shorter distances when training for a marathon. Shorter events also give you the chance to practice all your routines around racing – training the week before, your clothing choices, pacing during the race, and of course, your nutrition.

I had looked to do at least one 10K and a half marathon before my full marathon in October, but those races were cancelled. So, with the help of my husband, I did a couple of faux races. This involved getting up bright and early, going through my pre-race routines (same breakfast, warm up, etc.), driving to a park 20 minutes away (so I was less familiar with the route), and starting at exactly 9am. For the half marathon I even did a full, three day, carb loading diet – and boy, was that an eye opener!

Carbohydrates are stored in the liver and muscles as glycogen. When we exercise, that glycogen is broken down for energy. The higher the intensity of the exercise, the faster you burn through your glycogen stores. In training, having slightly depleted glycogen stores is normal, and can even encourage the body to use fats as fuel (see the previous post on Using Fats as Fuel). However, in a race situation, you want all the carbs you can get to maximise performance.

Carb loading is when an athlete eats a lot of carbohydrates in the days before a race, to maximise glycogen stores. Generally speaking, you only need to do this if the race will last more than 90 minutes. There are various ways to carb load, but recent research has shown that eating at least 6g/Kg of carbohydrate, and up to 12g/Kg, in the 2-3 days leading up to the race is enough to fully top up glycogen stores1. Especially as most people are resting or training very lightly in the days leading up to their race.

This is well known and I have dispensed this advice to my athletes for years. What I didn’t realise is just how much carbohydrate that is, and how hard it is to eat that much! Especially if you are trying to keep the calorie count within reason. For example, I weigh 62 Kg so 6g/Kg is 372g of carbohydrate, so that’s my minimum target. The day before my half marathon I ate:

Breakfast: Two boiled eggs with two gluten free crumpets topped with butter and jam, a 200ml glass of pineapple juice, and a large mug of tea with milk and sugar.

Morning Snack: A mug of tea with milk and sugar and two small, gluten-free custard creams.

Lunch: Small grilled mackerel fillet with plenty of rice, steamed broccoli and carrots. Watermelon for dessert followed by a mug of tea with milk and sugar and two small, gluten-free custard creams.

Afternoon Snack: Three slices of toasted gluten free bread topped with reduced fat Brussels pate and caramelised red onion chutney, with 36g of dried apricots on the side. Two satsumas (small oranges) shortly afterwards.

Dinner: Free from spaghetti with homemade tomato sauce and a beef and lentil meatball, topped with a small amount of Parmesan cheese. Frozen yoghurt ice lolly.

In total that was 2660 kcal – at least 500kcal more than I would normally eat on a rest day. Of that, 411g was carbohydrates, including 146g of sugar – well above the recommended. That’s only 6.6g/Kg of carbs and I have to say I felt stuffed! I can’t imagine eating the amount of food an 80 Kg runner would have to take in to get above 7 or 8g/Kg of carbs. Even though, as a nutritionist, I have created menu plans that do exactly that!

You’ll notice my intake of processed foods was well above normal – I am not in the habit of having crumpets, bread and biscuits all in the same day! Part of that was trying to avoid too much fibre. We get most of our fibre from carbohydrates (wholegrains, vegetables, etc.), so carb loading can mean a big increase in fibre. Normally a high fibre diet is good, but too much fibre too close to a race can lead to gasto distress during the race. So many athletes switch to lower fibre, “white” carbs when carb loading. Hence my breakfast of crumpets rather than the usual porridge.

There was also 88g of protein (1.4g/Kg) and 73g was fats. This is a perfectly healthy amount of both but probably too much when carb loading. At the time I chose mackerel because it’s an oily fish high in Omega 3 fatty acids, which reduce inflammation and help with recovery. And I had pate on my toast because it’s high in iron (often low in runners) and vitamin A. However, both are high in fat and in hindsight, I’m not sure I need. I could have kept the calorie count down by choosing lower fat white fish and a lower fat, high carb toast topping (like honey or jam).

Is carb loading worth it? Yes! I had a really good half marathon, setting a new PB. But if I’ve learned anything, it’s that carb loading is more complicated than even I thought (as demonstrated by the length of this post)!



1. Vitale, K. and Getzin, A. 2019. Nutrition and Supplement Update for the Endurance Athlete: Review and Recommendations. Nutrients. 2019 Jun; 11(6): 1289. doi: 10.3390/nu11061289

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