Carb loading - what is it, when should you do it and why it works

Carb loading is the process of maximising your glycogen stores in preparation for a long endurance event, usually something longer than 90 minutes in duration. You can only store so much carbohydrate in the body, so it makes sense to maximise those stores to delay fatigue and optimise your performance.During your event, you will be able to maximally uptake 60g of carbohydrate per hour. This only equates to 240kcal, which will not meet the amount of calories you will burn as you run, which is likely to be at least 400kcal per hour. Therefore you will need to be drawing on your muscle glycogen stores to keep you running.

When should runners start carb loading?

Carb loading is more than just having a bowl of pasta the night before. It needs to be something that you focus on for the 48 hours prior to your event starting. It also isn’t eating as much as possible for the two days before your event which will only weigh you down and leave you feeling heavy and bloated.

Carb-loading research

The technique was originally developed in the late 1960s and typically involved a 3-4 day 'depletion phase' involving 3-4 days of hard training plus a low carbohydrate diet. This was a very unpleasant phase to do and was not always proven to help performance as it was damaging psychologically.More modern research, led mainly by John Hawley et al (1997), has found that two days of carb loading with a tapered training program, particularly in the last 1-4 days beforehand, is sufficient to boost muscle glycogen levels.

Does carbohydrate loading improve performance?

Muscle glycogen levels are normally in the range of 100-120 mmol/kg wet weight. Carbohydrate loading enables muscle glycogen levels to be increased to around 150-200 mmol/kg wet weight. It is estimated that this additional amount of muscle glycogen can improve endurance performance by 2-3%.

The following diet is a meal plan for a 70kg athlete aiming to carbohydrate load:

Breakfast: 3 cups of low-fibre breakfast cereal with milk, 1 medium banana and 250ml orange juice.

Snack: 1 x Toasted muffin with honey and 500ml sports drink

Lunch: 2 sandwiches (4 slices of bread) with filling as desired, 200g tub of low-fat fruit yoghurt

Snack: Banana smoothie made with low-fat milk, banana and honey
cereal bar

Dinner: 1 cup of pasta sauce with 2 cups of cooked pasta,
 3 slices of garlic bread, 2 glasses of cordial

Late Snack: Toasted muffin and jam,
 500ml sports drink

This sample plan provides ~ 3500kcal, 600 g carbohydrate, 125 g protein and 60 g fat.

The fat content should be kept low to keep gut residue low and ease the transit of food through the gut. Low fibre is also desirable so that you can be happy to start your race without concerns about needing the toilet later into the day.

If the food volume seems like a lot, you can use energy drinks to top up your carbohydrate intake without having a lot of food bulk.

And now for an interesting fact...

Carbohydrate loading will most likely cause body mass to increase by approximately 2kg. For every extra gram of glycogen stored you will also store 2g of water. This can be a concern for many runners, but the potential negatives of setting off slightly heavier are far outweighed by the potential performance benefits.