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What should I eat to prepare for a marathon?

Are you preparing to run a marathon? With any form of endurance exercise, the right diet can mean the difference between hitting your personal best or hitting the wall.

BMI Healthcare’s Clinical Lead Physiologist, competitive runner and Sports Scientist, Colin Thomas, offers his advice on the best foods to fuel you before the big race.

What to eat when you're training for a marathon

When you’re fuelling for exercise and increased performance, there are a number of things you should be aware of.

1. Controlling blood sugar

If your blood sugar level is unstable, (e.g. from eating too many simple carbohydrates and high sugar foods), the body will store more fat. This can lead to weight gain, which can impair performance.

If blood sugar levels drop during exercise, an athlete can experience what is commonly known as ‘hitting the wall’. This is when your body suddenly runs out of energy. Maintaining a balanced diet is crucial to controlling blood sugar.

What should I eat to control my blood sugar?
A healthy, balanced diet including complex carbohydrates will help prepare you for endurance running. Foods containing good, complex carbohydrates include:

  • Potatoes and sweet potatoes
  • Brown rice
  • Wholegrain pasta
  • Oats
  • Wholemeal breads1

2. Maintaining hydration levels

Whether you’re exercising or not, it’s important to maintain a good level of hydration at all times.

If you’re losing water to sweat evaporation without replacing it, the chances are you’re reducing your blood volume and decreasing performance.

What should I drink to stay hydrated?

It’s recommended to drink 6-8 glasses of fluids a day to maintain hydration levels.2 If you are exercising, this should be increased in order to replace the fluids lost through activity.3

Athletes need to be very aware of hydration. An athlete working intensely in the heat can lose up to 2.5 litres of sweat per hour.4

In general, water is a great way to stay hydrated. However, for any exercise over one hour in duration, you can also stay hydrated with an isotonic drink that supplies the body with electrolytes.5

3. Eating more fruit and vegetables

Fruit and vegetables are an essential part of a healthy diet.6 They contain an abundance of vitamins and minerals that are responsible for energy metabolism, carrying oxygen and removing waste products.

There are many vital nutrients required in a healthy diet and some of these are especially important for an endurance athlete. You should aim for at least five portions of fruit and vegetables a day, and ideally more than five.7

What counts as a portion of fruit or veg?

There can be some confusion around what counts as a portion when it comes to getting your 5 A Day.

Foods that count as a portion are:

  • 80g of fruit or vegetables (fresh, canned and frozen all count)
  • 30g of dried fruit
  • 150ml fruit or vegetable juice
  • 80g beans or pulses
  • You should limit fruit or vegetable juice to one 150ml portion a day due to the high sugar content.

Beans and pulses only count as one portion a day however many you eat, because they are lower in nutrients than other fruits and vegetables.7

4. The importance of carbohydrates

Endurance athletes should be careful to consume adequate carbohydrates.8 Overlooking their value can lead to an over reliance on protein and fats.

Carbohydrates give us a fast supply of energy, whereas fat and protein release energy much more slowly. Because of this, carbohydrates are generally the preferred energy source for performance.9

What is carbohydrate loading?

Carb loading is common practice among athletes preparing for an endurance event.

As most people do not generally store enough glycogen in their body to run all 26.2 miles of a marathon, a carb-loading program can help ensure you have a ‘full tank’ before you cross the start line.

How many carbs should I be eating?

You should be aiming for 10 grams of carbohydrates per kilogram of body weight per day. So for example, if you weigh 70kg you need about 700 grams, which is a lot of carbohydrates.

Your body needs three grams of water to absorb and store every one gram of carbohydrates,10 so ensuring that you also have an adequate water intake is vital.

What to eat the week before a marathon

A carbohydrate load should start seven days before your marathon with a glycogen depletion phase. This starts with a hard ten-mile run. You are essentially emptying your fuel tank to make space for fresh fuel.

Follow this run with 2-3 days of low-carb, high-protein intake. The protein helps any damaged muscles to repair, while the lack of carbohydrates causes your body to start to crave them.

Stage 1: What to eat at the glycogen depletion stage:

During the glycogen depletion phase, recommended foods include:

  • Lots of: nuts, fish, meat, lentils, beans, eggs, seeds, cheese, water, squash/cordial
  • Small amounts of: fruit, vegetables, milk, tea, coffee
  • Absolutely NO: pasta, rice, cereals, bread, sweets, chocolate, fizzy drinks, potatoes, sweet potatoes, cakes, bakery products, oats, biscuits, alcohol

Five days before your marathon, the loading phase begins. Load up on healthy carbs such as sweet potatoes, rice and pasta. There is nothing wrong with topping this off with small portions of less healthy carbs such as ice cream, chocolate or sweets.

Stage 2: Carb loading

After the glycogen depletion stage, it’s time to start refuelling your stores. Recommended foods include:

  • Lots of: brown rice, oats, cereals, potatoes, sweet potatoes, wholegrain pasta, water, juice, vegetables, fruit, wholegrain bread
  • Small amounts of: meat, fish, nuts, seeds, sweets, chocolate, bakery products
  • Absolutely NO: alcohol, fizzy drinks, tea, coffee

What to eat the day before a marathon

On the day before the marathon you should start to reduce your food intake back to normal from lunch time onwards.

A normal evening meal and an early breakfast should see you through to the start line. Remember, the majority of this food will be sitting in your digestive tract the next day as you run the marathon.

Getting your seven-day carb load right can really get you on the right track and across the finish line.



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