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Metabolism and Endurance Performance (Part 2)

Nutrition Articles

So last week we talked about how many calories you use during an event and how to make a quick calculation of fat and carbohydrate contributions. Read last week's blog by GOING HERE

Let's give ourselves a simple scenario as a recap. Tom is 43, weighs 82 kg and is racing Ironman triathlon. Tom has a good aerobic base and when he rides / runs, 50% of his energy comes from fat and 50% of his energy comes from carbohydrate... well done Tom, nice work!

When Tom is riding his bike at his Ironman pace, he is using 820 Kcal per hour, so the calculation works like this: Fat contribution:  820 Kcal x 50% = 410 Kcal Carbohydrate contribution: 820 Kcal x 50% =410 Kcal.

Step 1: Discard the Fat

The calories which come from fat do not need to be replaced, even the leanest athlete has ample fat stores for the longest endurance events. Step 1 is therefore to discard the Kcal from fat and focus on the carbohydrate contribution. Carbohydrate is the fuel which must be replaced!

Step 2: Focus on the carbohydrate

For Tom, our calculated figure is 410 Kcal of carbohydrate per hour, so this is our target to replace during the ride. It's often easier to work in grams as most foods are also measured in grams. Each gram of carbohydrate contains 4 Kcal, so we calculate grams of carbohydrate as follows: 410 Kcal per hour / 4 = 102.5 grams per hour

Step 3: Apply the 60-80g intake rule

The maximum amount of carbohydrate you can take during exercise is 60-80g per hour. This is not determined by your size, it's the same for everyone. There is an issue with taking 80g, which is the possibility of stomach upsets. The more carbohydrate you take, the higher the risk of stomach issues, but for Tom we know that he's using 102.5g per hour, so we need to push the limit and give him 80g per hour. That's still 22.5g per hour short of what he's using.

Step 4: Work out the race total

Tom's bike time is estimated to be 6.5 hours. If he's losing 22.5 grams of carbohydrate per hour which can't be replaced, what does that add up to over the total bike ride? Well, the calculation is simple: 6.5 x 22.5 = 146.25 grams. That means that Tom will lose 146.25 grams of carbohydrate, which he can't replace, by the end of the 6.5 hour bike ride. The equivalent of about 7 extra energy gels.

Step 5: Is that a big deal?

It's very rare that an athletes can replace all of the carbohydrate they are using during an endurance event and to compensate, they'll slow down slightly and their body will switch to a higher fat use and lower carbohydrate use.

Step 6: So all is good right?

We should start by saying that Tom's figures are actually pretty good. A lot of athletes don't get 50% of their energy from fat. Take a look at the picture below. This is some data for an Ironman athlete taken in our lab, male veteran, approx 68 kg with a long history of endurance competition. There's 12 minutes of data on the screen, the first column shows the power output (watts) and the third column shows time in minutes. Prior to this the rider warmed up for 10 minutes at 100-120 watts. Now look at columns 11, 13 and 14 on the far right hand side, they show Kcal per hour, fat% and carbohydrate%. Consider that 120/150/170 watts is not high intensity, despite that and the previous warm up, you can see that the carbohydrate use is very high. Take into account that our athlete is only 68 kg and that Kcal per hour will be greater in larger athletes.

Looking at the data, can that person finish Ironman?

Your body is pretty clever so it will make some changes along the way to help you out. Throughout the event, your metabolism will switch, so it's reasonable to suggest that by the time the bike has ended, a much bigger percentage of your energy will be coming from fat than it was at the start. That means you may only be using half the amount of carbohydrate every hour, compared to when you started.

That's good right?

In some ways yes it is, it's saving your carbohydrate stores by halving the amount used every hour. But you need to consider why this change occurs. Your body switches to use a larger amount of fat because it's 'RUNNING OUT OF CARBOHYDRATE' so whilst every cloud does have a silver lining, let's not look too positively on this change.

As most people struggle to metabolise fat, having to rely upon it will lead to a drop in pace and performance. If we continue our theme of 'clouds and silver linings', at least the slower pace means you will be using less Kcal per hour (slower pace = less energy required) so that also helps to further reduce the amount of carbohydrate required.

So what do I need to do to perform at my best?

This is really simple. You need to train your body to use as much fast as possible so you're using as little carbohydrate as possible. You then need to replace the carbohydrate you're losing with a structured nutrition plan. You can't simply eat more as the rate of absorption is limited and you'll just get stomach issues. You may need to adjust your pace and ride/run at a slower speed to use less Kcal and utilise more fat (the higher the pace, the more carbohydrate you use)

How can I find out if I'm a fat burner?

To give you the background, we've been coaching athletes for 15 years and in that time have completed over 1500 metabolic assessments. The blog above is based on any specific research, it's based on our personal experience. We offer a simple metabolic assessment which involves running on the treadmill or cycling on the watt bike, whilst wearing a mask. By measuring inspired and expired gases, we can calculate kcal usage and fat / carbohydrate usage. The test is only £60 and takes approx 1 hour. If you're training for Ironman or ultra events, it's probably something that you should consider before planning your training and nutrition. Email to book an appointment.

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