When to Eat
This will vary from athlete to athlete, so a bit of experimentation will be needed, although most of the rules below will make sense to most athletes. It is particularly in the pre-exercise eating that some experimentation might be needed, in order to check that you don't either feel lacking in energy when running, or to ensure you don't get stomach cramps.
A lot of this will depend on the time of day you are running. If it is early in the morning you may not be able to consume much before you run, as your glycogen stores will be low.
Ideally a medium-sized breakfast that is low in fat (a large bowl of cereal or the like) should be eaten. If you can't digest this before your run, then a smaller meal - or at least a drink with carbohydrates (either a sports drink, or a smoothie) - should be taken on board.
If you are running later in the day then a snack (200-400 calories) about two hours before you run is a good idea, in addition to your other meals. This snack should not have too much sugar in it to avoid big changes in body sugar levels.
Generally, people eat their largest meal of the day in the evening - however, if you are running in the evening it is a better idea to make the main meal a lunchtime one.
Your body will be depleted after a run, so it is important to re-fuel. Judy Ridgway, a renowned name in the field of nutrition, says in her book "Food for Sport" that:
"It is crucial to consume 50g/2oz carbohydrate in the first two hours after each training session and another 50g/2oz within the next two hours."
In practice, a good idea is to have a banana or other fruit, fruit smoothies, sweets or low-fat biscuits as soon after training as you can stomach, and then try to eat a main meal (with plenty of complex carbohydrates - potatoes / rice / pasta) within a couple of hours.
Throughout the Day
It is good for runners to eat little and often throughout the day (making sure you don't just add vast numbers of calories), but reduce the size of the large meals (principally the evening one). In addition it is worth keeping a drink on the go throughout the day, as in total you'll need a total of 2-3 litres of water to perform at your best.
Note: The information here is written by an athletics coach who has read widely into the subject, and not a sports nutritionist, so is about gearing your food and drink to the practicalities of running as opposed to expert dietary advice.
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