Loading, Bonking, Hi-Lo GI, the Wacky World of Carbs….

In any endurance sport, getting the right nutrition before and during is very important.  Depending on how hard you are pushing, there may only be a few hours worth of available energy stored as glycogen, so it is important to adopt approaches to maintain that available energy.  Glycogen is a complex form of glucose (essentially a polymer) that is stored in two main places in the body, the liver and in muscles.  There are a couple of approaches people adopt to help:

(1) Traditionally athletes have ‘carbo loaded’ in the days prior to an endurance event, to try and maximise the amount of stored glycogen in the body, and thus delay the onset of glycogen depletion.  Vast amounts of pasta are consumed.  It’s not something I have ever consciously tried to do, though I did eat quite a lot of flapjacks before last year’s Dart 10K 🙂

(2) Consuming carbs during the event.  The endurance swimmer’s method of choice for decades has been Maxim, which is basically a carbohydrate solution chugged every half an hour or so.  The carbs in Maxim (other carbohydrate drink products are available, but tend to be more or less the same) are present as maltodextrin, another glucose polymer.  A little aside for those who like chemistry:  maltodextrin is produced by the partial hydrolysis of corn or wheat starch, and comprises glucose units strung together end to end to make chains of up to 20 sugars long.  Glycogen also comprises glucose units strung together end to end, but in addition has the occasional branch, making it a more complex structure.  Both molecules serve the same purpose though,  acting as available sources of glucose to fuel the muscles, and importantly the brain.  I actually use the carb drink from SIS, for no better reason than the fact that it is sold in Tescos, and Maxim is not.  I also add Ribena for flavour.



Hitting the Wall

If you deplete the available glycogen, and cannot replace it quick enough with what you eat and drink, then bad things happen.  Physical exhaustion ensues, and your brain, which runs off glucose exclusively, goes to pot.  Hitting the wall is also known somewhat quaintly (and amusingly for Brits) as ‘bonking’.  What happens when you hit the wall?  Is all lost?  Well not necessarily.  The body can actually run off more than one sort of fuel, indeed routinely turning over a certain proportion of its stored reserves of fat and protein. But when the glycogen is exhausted, and the body is required to use those sorts of reserves to the exclusion of glycogen, then things become, well harder.  It should be remembered though that the provision of energy from feed before and during the event, and glycogen and other reserves stored in the body, is one big equilibrium.  It is an equilibrium that the body, under normal circumstances, is exquisitely good at maintaining.  Only when the system is put under a significant stress, 10 hours in to a marathon swim in cold water being one of them, do things typically go awry.

Keto Adaptation

Some endurance athletes employ a strategy to deal with hitting the wall called keto adaptation.  They cut carbs out of their diet completely or almost completely, following a protein and fat only regime.  The theory is that the body becomes used to metabolising those two energy sources preferentially, and also becomes more efficient at mobilising stored reserves of those energy sources.  Now most people, and definitely most cold water marathon swimmers, have fairly ample reserves of fat to draw upon, and becoming more efficient in mobilising those reserves should, in theory, avoid the situation where the wall is hit, and a slow and painful transition has to be endured.

A Diet Free of Carbs

So how easy is it in practice to maintain a diet low, or free, of carbs while maintaining a training regime commensurate with getting across a major channel in cold water?  This, I think, is the crux.  In the run-up to a major channel swim, the athlete will be spending many many hours, swimming in cold-to-cool water, using up massive amounts of energy.  There are the calories required to propel the athlete through the water, and the calories required to keep the athlete warm.  These need to be replenished, otherwise the athlete is likely to become more lean, as fat reserves are used up, and the valuable ‘bioprene’ that keeps him warm in the Channel will diminish.  I know of at least one swimmer who has suffered from this.

Alternative Sources of Carbs

Maxim, or one of the many equivalents (Maxi, Perpetuum, SIS etc etc) is the staple food of many marathon swimmers.  Loneswimmer has described it as ‘rocket fuel for swimmers’.  Can’t say I’ve ever felt that rocket-propelled myself, but there’s always hope.  The maltodextrin it contains is fairly rapidly processed in the gut to glucose that the muscles and other organs are craving.  There are places to go if you want carbs that are more slowly assimilated by the body, and later in the transit of food through the gut.

Low-GI Foods

GI, or glycaemic index, foods are those that are slowly processed by the body to sugar (glyc – glycosides i.e. sugars, haem -blood).  The lower the GI index, the slower the sugars it contains appear in the bloodstream, the higher the index, the faster the sugars appear in the blood.  Examples of GI indices in foods are:

HIGH:  White bread, white rice, potatoes, simple sugars, maltodextrin, glucose

MEDIUM:  Ice cream, banana, pitta bread, basmati rice, raisins, sucrose

LOW:  Beans, seeds, most vegetables, most sweet fruit, AND….. fructose

The sugars in this list make interesting reading.  White table sugar, aka sucrose, is actually medium GI, somewhat counter-intuitively.  And fructose (fruit sugar), a major component of high fructose corn syrup (HFCS), is actually low GI!  What’s that all about?  Even more surprising is that HFCS is used in many energy drinks in the US.  Why are the manufacturers using something that is slow to absorb is an interesting question.  I think the answer is twofold: firstly HFCS is cheap, secondly, it is about half and half fructose and…… glucose, which is high GI.

You might imagine that packing away a decent quantity of lower GI carbs in your feed might be a good idea, so that they are released slowly over a longer period of time.  I guess this is OK so long as there are not side effects. There is at least one manufacturer who markets just such a solution, known as superstarch from UCAN.  Superstarch is basically a more complex form of sugar polymer, in essence a low GI product, that releases glucose more slowly.  It is based on HMS, hydrothermally modified starch from corn.

Here are the marketing claims on the company’s website


They also conveniently provide a link to a journal article looking at the levels of (amongst other things) glucose and insulin in the blood of competitive cyclists over a 3 hour period of cycling at sub-maximal effort (measured as 70% of VO2peak).  One group were given maltodextrin feed, and the other Superstarch.  The trial was blinded, in that they didn’t know what they were getting, and a crossover study, in that they were invited to come back a week later and repeat, but this time they got the feed they DIDN’T get the first time.

Maltodextrin (open circles, HMS (closed circles)

Maltodextrin (open circles, HMS (closed circles)

Sure enough, you see a spike in glucose for maltodextrin in the first half hour after ingestion and the commencement of exercise.  This is accompanied by a spike in insulin that follows a similar profile.  No such spikes are observed for Superstarch.  ‘So far so good’, I hear you cry!  Well yes, I suppose.  I always ask myself at these moments the following question: ‘So what?’.  Why does this matter?  The line that UCAN offer is that insulin spikes harm the body’s ability to metabolise fat.  Maybe, but this effect only appears to be transient; for the remainder of the exercise period, both insulin and glucose map closely, before rising again once exercise ceases, and the glucose is no longer being consumed by the muscles.

Another proposed benefit I have heard is that the product allows you to feed less often.  Maybe again, maybe not.  The carb load you get from a gram of one will be very close to that you get from a gram of the other.  I can’t see that any magic happens to get round that fundamental, even if the profile of how that carb finds its way into your blood as useful glucose is modestly different.

Finally on this study, there is little discussion on the ‘So What’.  What is mentioned, however, is that there was no significant difference in performance after the monitoring period finished. “Upon completing the 150-min cycling bout, cyclists performed a time trial at 100% VO2peak to fatigue. Paired-samples t tests revealed that there was no difference between the HMS and MAL trials.”

I guess if people want to give this a try they can.  I know some distance swimmers who are doing precisely that, and one of them swears by it.  The unfortunate thing about Superstarch is that it doesn’t dissolve in the nice way the maltodextrin does, and has a fairly unpleasant mouthfeel.  It’s also not cheap.

Controlled experiments are always very difficult to conduct when it is just you as the subject of your own individual trial.  Confounding factors around performance, and the difficulty in obtaining objective data, cloud our ability to form judgements on what works best.  But at the end of the day, finding out what is best for us, while stress testing ourselves on long swims is the only way we have of coming up with the best thing for us.  I know that towards the end of my six hour Windermere swim in August, I was feeling a little peaky with all of the ribena-flavoured maltodextrin I chugged all of the way up the lake.  So I will make it less strong next time, or maybe drink less, or maybe both.

NOTE – I have no relationship, commercial or otherwise, with any vendors of products discussed today.


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