So last week (Part One Here)we finished by talking about how many calories you use during an event and how to make a quick calculation of fat and carbohydrate contributions. To recap, we said:

80/20: If you are struggling to ride 50 miles / run 15 miles even when fuelling 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.

Let’s give ourselves a simple scenario. Tom is 43, weighs 82 kg and is racing Ironman triathlon, he falls into the 65/35 category and his main objective is to complete the event without major disaster and to run as much of the marathon as possible. When Tom is riding at his Ironman pace, he is using 820 Kcal per hour, so the calculation works like this:

Fat contribution:  820 Kcal x 35% = 287 Kcal

Carbohydrate contribution: 820 Kcal x 65% = 533 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. Carbohydrates is the fuel which must be replaced!

Step 2: Focus on the carbohydrate

For Tom, our calculated figure is 533 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:

533 Kcal per hour / 4 = 133 grams per hour

Step 3: Apply the maximal intake rule

You may remember from the last couple of weeks, we discussed that the maximum amount of carbohydrate you can take during exercise is 60g per hour. Tom is using 133 grams per hour (just to clarify, that’s not excessive and is realistic). If the maximum Tom can take is 60g per hour, that means there’s 73 grams (133-60) that he’s losing and can’t be replaced every hour.

Step 4: Work out the race total

Tom’s bike time is estimated to be 6.5 hours. If he’s losing 73 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 73 = 474.5 grams. That means that Tom will lose 474.5 grams of carbohydrate, which he can’t replace, by the end of the 6.5 hour bike ride.

Step 5: Work out your time to collapse

The big figure missing here is the actual amount you have got stored in your body, is losing 474.5 grams a big problem? The average human stores 400 grams of carbohydrate stored in the muscles,  and 100g is stored in the liver. There’s also approx 25g circulating in your blood at any given time. For the astute amongst you, the problem has already struck you squarely between the eyes. Tom, sadly, will not be running the majority of the Ironman marathon.

Does this happen in the real world?

Definitely, take a look at the photo below. This is some data for an Ironman athlete taken this week, 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.

Would these fugures be similar for running?

Yes, pretty much. The Kcal usage is slightly higher when running at a similar intensity, but the fat usage tends to be a little higher also. I’d suggest that the fat usage is slightly higher as running requires less ‘fast twitch’ fibre contribution, cycling requires a cretain amount of ‘stregth’. Running intensity also tends to be a bit more consistent. Cycling can be hard on the uphill and then rest and freewheel on the downhill, but running is less so.

Should Tom withdraw his entry right now?

Hang on… we know that people can ride the full Ironman bike and then run the marathon. We also know that people run 100 miles, so there’s got to be a catch, these calculations can’t be correct. Will Tom be completely depleted of all carbohydrate even when taking in the recommended 60g every hour?

No, indeed he won’t and the calculations are not so clear cut as above. 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, 50-60% of Tom’s energy will come from fat, rather than the 35% contribution at the start point. That means he’ll only be using half the amount of carbohydrate every hour, compared to when he 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 reduce the amount of carbohydrate required.

Is anyone else getting concerned here or is it just me?

It’s ok, there is an answer. The 2 key areas for improvement are economy (Kcal per hour) and substrate ulilisation (fat or carbohydrate). If you are aerobically fit, you will be more economical than most people. In fact, for endurance performance, economy is perhaps the most important thing. We can define economy very simply as ‘how much energy do you need to ride or run at any given speed?’

If you take your unfit pub mates for a run, you may well trot along at 8 minutes per mile and hold a comfortable conversation. Your mate on the other hand, may be breathing like a bulldog in a hot car, blowing out of most parts of his body. He will be using far more energy, require far more oxygen and use far more calories. People are like cars, some can go a long way using only a small amount of fuel and some require a regular filling due to their poor economy.

The second thing to consider is substrate utilisation. This simply refers to the relative contributions of fat and carbohydrate towards your total energy need. We’ve discussed this above and in pretty much every blog in the last 6 weeks, so hopefully you’re already familiar with this concept. If more of your energy comes from fat, you’re less likely to run out of carbohydrate. The best athletes in the world require a small amount of energy (Kcal) to ride or run at race speed. If a large chunk of that energy requirement comes from fat, their total carbohydrate use is very small indeed.

The new Tom… we can rebuild him

By making changes to Tom’s training and diet, the new version arrives for the Ironman triathlon using only 700Kcal per hour and 55% is being provided by fat. A quick maths calculation reveals the following:

1. He’s using 315 Kcal of carbohydrate per hour on the bike, compared to the previous figure of 533

2. With his intake of 60 grams per hour (240 Kcal), he now only has a deficit of 75 Kcal per hour compared previously with 292 Kcal (73 grams)

3. As a consequence, Tom runs the whole marathon and Tom becomes a LEGEND…..

Do you want to become a legend? If so, do the calculations and work it out for yourself, then let’s go forwards from here.

– Marc Laithwaite

About Marc:

Sports Science lecturer for 10 years at St Helens HE College.

2004 established The Endurance Coach LTD sports science and coaching business. Worked with British Cycling as physiology support 2008-2008. Previous Triathlon England Regional Academy Head Coach, North West.

In 2006 established Epic Events Management LTD. Now one of the largest event companies in the NW, organising a range of triathlon, swimming and cycling events. EPIC EVENTS also encompasses Montane Trail 26 and Petzl Night Runner events.

In 2010 established Montane Lakeland 50 & 100 LTD. This has now become the UKs leading ultra distance trail running event.

In 2010 established The Endurance Store triathlon, trail running and open water swimming store. Based in Appley Bridge, Wigan, we are the North West’s community store, organising and supporting local athletes and local events.

Check out the endurance store HERE