Last week we showed you how to calculate the amount of carbohydrate used during cycling and running, in order for you to produce a structured plan for race nutrition. This week, we begin the process of looking at what and when to eat on race day and the first step, is to explain the basic physiology.
Carbohydrates or ‘sugars’ are a prime source of fuel when exercising and unfortunately, as explain in previous blog posts, they are relatively limited. The term for stored carbohydrate is ‘glycogen’ and we store glycogen in the muscles (to use for movement) and in the liver (to supply sugar to the brain and maintain blood sugar levels)
There are different types of carbohydrates, but ultimately, they are all broken down to glucose as this is what we use as our prime fuel. When you eat or drink carbohydrate, it first goes into your stomach. We can’t absorb carbohydrates through the stomach wall, so they progress from there into the intestines and are absorbed through the intestine wall
The rate of absorption
The carbohydrates you eat are broken into glucose and pass through the intestine wall into the blood stream. Remember that in recent blogs, we’ve stated that the limit for this seems to be around 60g per hour of glucose. Depending upon the type of carbohydrate and how it is consumed (drink or solid food), the time taken for the carbohydrate to be broken down and absorbed will vary.
Many of you will have heard of the ‘Glycemic Index’. This scale was designed with diabetics in mind and it dictates how quickly foods are broken down and absorbed through the intestine wall. To measure the GI of a food is relatively simple (but complex at the same time). The process is to give someone a specific food, then take blood samples at regular intervals for the next hour to see how rapidly the blood sugar (glucose) levels rise.
The ‘GI’ scale runs from 1-100. Foods with a low score will take longer to reach the blood and give a more consistent supply (slow drip feed). Foods which enters the blood stream quickly, will give a more immediate spike in the blood sugar levels. Pure glucose has a score of 100 as that will lead to a rise in blood glucose more quickly than anything else.
The role of insulin
The reason why the GI scale is so important for diabetics, is the ‘insulin response’. A rise in blood glucose will lead to a rise in blood insulin, which is the hormone responsible for removing glucose from the blood and pushing it into the liver and the muscles.
Your daily diet should be made up of foods low on the GI scale. It you eat foods which are high on the GI scale throughout the day, this results in repeated sugar spikes and subsequent insulin spikes. Over time, your insulin will become less effective (overuse can lead to it becoming less sensitive to glucose). The reduced sensitivity can eventually lead to type 2 diabetes (insulin doesn’t work correctly). You may think, as an athlete, you are not susceptible to type 2 diabetes, but you’d be wrong. It is critical for you that your insulin works correctly, so you should be doing everything in your powers to ensure it does.
The secondary issue relating to insulin, is the impact upon fat metabolism. It reduces the amount of circulating fat by encouraging storage in fat tissue. In terms of your daily diet and metabolism, this has significant consequences upon weight loss and fat use. Constant spikes in blood glucose and insulin will reduce the amount of fat you metabolise throughout the day.
Are we discussing daily diet in this blog or race day nutrition?
Okay, I am going off track a bit, but I think it’s important to understand the foundations. As an athlete, you need a constant and balanced supply of energy to complete your training sessions and to recover quickly. Therefore, you should be eating to encourage a more stable metabolism and that is derived from low to moderate GI foods, NOT high GI foods.
So how does this work during racing?
- If you are racing an endurance event, you really need to ensure that you have a steady supply of glucose, without disrupting fat metabolism. A large spike in glucose and insulin, could inhibit fat metabolism, which means you’ll be forced to use more glycogen and run out more quickly.
- There are times when you may well need a rapid rise in blood glucose. If you have an extreme low point in the race and find yourself sitting on the road side, you may need an ‘instant hit’. Under such circumstances, you need the quickest glucose spike possible to get you back on your feet. At times like this, who cares about insulin!!
Here’s the thing, if you do scenario number 1 correctly, then you shouldn’t experience scenario number 2. The other issue is that products that are sold to deal with scenario number 2 (high energy, quick acting gels) are being used for scenario number 1. If you are half way through a 12 hour event and your energy levels are feeling relatively good, should you take a product which gives you an instant spike in glucose and insulin? Remember, we are aiming for a ‘constant’ and ‘balanced’ blood glucose level, we are trying to avoid blood glucose ‘fluctuations’.
So this blog is not what I intended when I started typing and none of the original planned information has made the page. Needless to say, this 4 part ‘race nutrition’ series is not going to be 4 parts.
A critical point to take away is that over the years of giving advice, we’ve found that the daily diet can have a huge impact upon training and racing performance. In particular, cutting out high GI foods can lead to a dramatic change in metabolism and enhance your fat utilisation.
Your homework for this blog is to take a look at the glycemic index. You’ll find this very interesting and you’ll probably find that many of the foods you presumed to be ‘sugar spiking’ are not and vice versa.
What if I said that Coke had a score of 60, whilst mashed potato AND jacket spuds had a score of 90 (that means mashed potato and jacket spuds can give more of a sugar spike than Coke!!)
You’ll notice that there is also something called ‘glycemic load’. This takes the volume of food into account e.g. you can’t compare a full jacket spud with a teaspoon of glucose as the volume is very different. Don’t worry about that for now, it’ll over-complicate the matter!
Take a look for yourself, and see what’s low and high. There’s loads of them on the internet, Google glycemic index table. Next week we’ll look at the foods you eat during the event. How can you manipulate the use of drinks, bars and gels to maintain a steady blood glucose supply and avoid the fluctuations that we have discussed above. See… I told you there was a point to this blog.
Until then, stay healthy.
– Marc Laithwaite
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