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Metabolism and Endurance Performance

Nutrition Articles

We've posted a lot of blogs about metabolism and endurance performance in the past so thought we'd kick the year off with a basic overview as most people are in the process of making resolutions! Over the past 20 years, there has been a shift away from low fat diets towards low sugar/carbohydrate. Sugar is now the enemy and not fat! When you're exercising you use both carbohydrate and fat as fuel sources, the way in which they interact is critical for your performance. Here's the low down on metabolism and performance...

What am I using and how much?

So you've entered a marathon, ultra or Ironman event in 2017 and need to better understand metabolism and fueling. Lets estimate that during your race, you will be using 700kcal per hour. If your metabolism is working correctly, it's likely that 50% of the 700kcal per hour (350kcal) will come from carbohydrate stores and the other 50% (350kcal) will come from fat stores.

So technically, you're using 700kcal per hour, but realistically, you only need to worry about the 350kcal of carbohydrate as that's the one which is critical. Even a very slim person has enough fat to last several days, but the carbohydrate stores are very limited and can run low very quickly.

So how much should I eat?

The 350kcal of carbohydrate equates to 88 grams of carbohydrate (there's 4 kcal per gram of carbohydrate, 88 X 4 = 352). So in theory, if the above scenario of 700kcal per hour with a 50/50 split is correct, then you just need to eat 88g of carbohydrate and all will be well.

The limitation of carbohydrate intake

Here's the big problem, you can only absorb approximately 60g of carbohydrate per hour. Eating more than 60g per hour will cause stomach issues and eating less means that you're not taking enough. If you're using 88 grams of carbohydrate per hour and you can only replace 60 grams per hour. That's a 28 gram / 112 kcal per hour deficit.

So I can't just eat more?

Unfortunately not. If you eat more, it's unlikely to be digested and will simply sit in your stomach or intestines without providing energy. There are a lot of people who suffer from gastric problems during long distance events and this is generally caused by eating too much food which they are unable to digest. It's really important that you understand, eating more food doesn't mean you'll have more energy and it may well mean that you'll face stomach upsets. I stress this point knowing how obsessed Ironman athletes in particular become with regards to feeding on the bike.

A deficit of 112 Kcal per hour doesn't sound too bad

No, it doesn't. But that is based on the presumption that you are only using 700kcal per hour, bigger people and less efficient people may be using more. It's also based on the assumption that 50% is coming from fat and that may not be the case at all, in fact, as much as 80-100% may be coming from carbohydrate. What makes this worse is that bigger people can't necessarily take on board more fuel, the 60g limit still pretty much applies. It's a gut issue, it's not about how big your muscles are and how much you can store in there.

So the 3 things you might want to know are:

1. How many calories do I burn per hour?

2. How many of them come from fat and carbohydrate?

3. How much should I be taking in as a consequence?

As a start point, you can probably work out your calorie usage by using a heart rate monitor or power meter. Run or ride at race pace and it'll do the calculation for you, although the power meter is a lot more accurate than the heart rate monitor, it's still a start point. Warm up, then do an hour at your 'race pace' and work out the figures.

There are many people who consider their training and racing to be 'serious', yet still don't know how many kcal they use when racing.

Once you've calculated how may kcal you use per hour, apply the following rule:

80/20: If you are struggling to ride 50 miles / run 15 miles even when fueling yourself throughout, then apply the 80/20 rule. That means 80% of your fuel is carbohydrate and 20% is fat.

65/35: If you can ride 50 miles / run 15 miles comfortably using fuel, then apply the 65/35 rule. That means 65% of your fuel is carbohydrate and 20% is fat.

50/50: If you can ride 50 miles / run 15 miles comfortably without using any fuel whatsoever, then apply the 50/50 rule. That means 50% of your fuel is carbohydrate and 50% is fat.

Are those figures accurate?

Absolutely not, I just made them up. They are by no means 100% accurate but they will give you a good start point and will allow you to calculate an approximate figure. The running figures are less 'straight forwards' than the cycling, as the impact of running can really fatigue your legs, so you may find 15 miles difficult, even if your fat burning and fuel economy is good. for cycling, the impact is low, so it's more likely governed by metabolism and fuel.

If you'd like to know exactly how many kcal you use when exercising and how much fat and carbohydrate you use, then contact us for a VO2 max test, which include metabolic analysis, we can give you accurate figures.

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