So you’re at your local pub when “I’m Carbo-loading” is slurred at a particularly loud volume straight in your face, complete with some complimentary chicken salted spittle to your eyeball.
Despite being epically un-original, this one liner reliably earns a few giggles due to the irony of generally being stated so proudly by those least likely to have any plans to burn off their enhanced muscle glycogen tomorrow – or indeed at any stage in the short, medium or long term future! But taking this term outside of the pub context – what exactly does Carbo-loading mean and if you are preparing for an endurance race should you embrace this beast?
It just so happens that I’m pondering this very question right now as I prepare for my first ever 50km trail run next week in the Blue Mountains. Let’s be honest: I’m a complete novice, as green as they come and I use the words ‘prepare’ and ‘run’ very generously given that my preparation has been woeful (injury after injury) and therefore very little running is actually forecast for the big day.
So, in my desperate search for anything that is going to increase my likelihood of survival, let alone avoiding a ‘DNF’ being written beside my name – reading that research has found that carbo-loading can delay fatigue by 20% in races longer than 90 minutes is naturally piquing my interest somewhat!(1)
Basically, ‘carbo-loading’ is the process of elevating your muscle glycogen stores in the hope that you will be able to go faster and/or longer and is generally of particular interest to those competing in endurance events (those races longer than 60-90 minutes).
For those – not quite in ‘the know’ – glycogen is basically your body’s way of storing carbohydrates as fuel for movement or activity, and the average humanoid has around 500g of it distributed around their body, mostly in the muscular tissue (with an additional 90 odd grams in the liver). Therefore, the amount of glycogen which is stored in the body is logically dependent on two main factors – how much muscular tissue you have and whether your diet is high or low in carbohydrate. For women, where you are with your menstrual cycle will also be a factor: it’s been shown that in the luteal phase you’ll store more.(2, 3) When combined with the glucose in your blood, this means that for most – generally, the body will store around 2000 Calories worth of carbohydrate fuel.
Side note: for every 1g of glycogen, your body will hold around 3ml of water – hence why when you go on a really low energy/carbohydrate diet you’ll miraculously lose 2-3kg in the first week….but then why of course that just comes straight back when you eat normally.(4)
Bodily fat is also an important fuel for endurance events, especially for ultra-endurance events that are conducted at a lower intensity (lower heart rate), but fat is generally not as easily broken down for energy.(5, 6) There are many practices that athletes employ to try to maximise training adaptations or fat-burning efficiency to improve this rate and/or reduce their reliance on consuming carbohydrates during their races (who doesn’t want less weight to carry, less chance of gastrointestinal upset and to save time and effort in not having to physically consume as much carbohydrate during the race?!)(7, 8)
These are summarised in the figure below but in it’s most basic form, this can involve training in the morning before breakfast to more elaborate training/nutrition sequencing and with the extreme end of the spectrum being to train and live in a ketogenic state.
Source: From Asker Jeukendrup at http://www.mysportscience.com/single-post/2015/07/13/6-Ways-to-trainlow
However, given that the vast majority of research still supports that fatigue will be reduced and performance maximised with initially high glycogen levels and continued carbohydrate absorption during the event, maximising glycogen stores prior to a big event is still a highly recommended practice for endurance athletes.
But what does carbohydrate loading actually entail?
Back in the 60’s, a week prior to competition, a final training session would be undertaken to severely deplete the body of all of it’s glycogen stores. The athlete would then cease training and eat very little carbohydrate for 2-3 days before then eating a very high amount of carbohydrate in the 3 days prior whilst still not training.(9-11) This was very effective in maximising the glycogen stores with a ‘supercompensation’ effect – but it was seen to have too many negative side effects. This included that for the 2-3 days of carbohydrate restriction, athletes who were not used to having a high-fat diet experienced gastrointestinal upset and there were questions about the impact on recovery and the immune system by restricting carbohydrates after an intense final session. Many athletes also reported feeling mentally frustrated or anxious about not being able to train at all for the full week prior to the event.
This protocol has become increasingly moderate in nature over the last 50 years or so where a taper in training for the week leading up the event with a gradually increasing carbohydrate intake has been found to increase the glycogen sufficiently (though not to the same super-compensatory effects of the original protocol).(12) However, athletes report feeling happier with the process as it’s less arduous and restrictive, they feel less mentally stressed and research now supports a protocol of having 1-2 days of increased carbohydrate and that this is sufficient to achieve a good glycogen level – especially in the more trained individuals (unfortunately, lesser so for schleppers like me!).(12, 13)
Interestingly though, these carbo-loading practices are believed to be less effective for increasing glycogen and improving performance in women as compared to men though debate among researchers continues.(3, 14) Notably for both genders, the more elevated your initial glycogen level is, the quicker the rate that your glycogen will be used with little difference in glycogen levels being observed by the 60-90minute mark in tests where subjects had either high or very high glycogen levels to start with.
So really, how relevant is undertaking a ‘super-carbo load’ for events of 4, 8 or 12+hours if there’s little difference in glycogen levels after 1-2hours? Ie – is it worth undergoing full ‘carbo-loading’ to achieve the very maximum glycogen level that you can, or is a moderate increase good enough with the difference being made up by consuming carbos during the race?
In answering this – lets see what you’re getting yourself into if you undertake a proper carbo-load. ‘High intake’ of carbohydrate is in the context of this process is 9-12g of carbohydrate per kg of body mass per day. So consider a 65kg person and using 10g/kg of carbohydrate each day, the following meal plan example was written keeping the other macronutrients relatively low so that total calorie intake is not blown up to astronomical proportions:
|Carbo loading meal plan example*|
|Notes – based on a 65kg person and 10g carbo/kg of BM per day
This meal example shows what a very high carbohydrate intake looks like with a lowered % of fat/protein and especially in the last day or so, researchers also recommend to lower fibre further also* Not to be used outside this endurance carbo-loading context as not balanced or designed as a diet for long term health
|Café Latte with 1 sugar and Fat-free (cows) milk – 250ml||17||439||105|
|2 tip top white english muffins||55||1289||307|
|Peanut butter (1 tablespoon)||4||400||95|
|Honey (2 tablespoons)||34||502||120|
|Mixed berries (1 cup)||17||293||70|
|2 uncle toby’s muesli bars||40||920||219|
|V8 juice, tropical blend (400ml)||30||500||119|
|Breaka choc milk (250)||27.8||833||198|
|2 Pieces of bread (white in last 2 days)||44||1004||239|
|shredded chicken (100g)||0||554||132|
|Cup of salad||3||60||14|
|Cup of mashed potato, no butter (a little milk)||36||750||179|
|Yoghurt, yoplait – low fat, flavoured (100ml)||14||400||95|
|Low-fat fruit granola (1 cup)||80||160||38|
|Tuna in springwater (150)||0||686||163|
|2 Helgas white wraps||70||1699||405|
|2 Cups of salad||6||120||29|
|Mayonnaise, light (2 tablespoons)||2||385||92|
|Honey (2 tablespoons)||34||502||120|
|Mixed berries (1 cup)||17||293||70|
|Low-fat fruit granola (1/2 cup)||40||795||189|
|Yoghurt, yoplait – low fat, flavoured (150ml)||14||400||95|
Whhaaa……!? This is an insane amount of food!
I don’t know about the average 65kg person out there – but even if I lowered the intake of fat, protein and fibre further – on a personal note – I’d totally struggle to consume that volume of food and it’d most likely make me feel awfully heavy, bloated and blergh – not to mention having to ride that blood glucose rollercoaster that comes with such quickly absorbed carbohydrate sources! If you decide nonetheless to carbo-load and also doubt whether you could comfortably eat that much, it’s worth noting that these days there is a much wider and more readily available range of proprietary, specialised products that can deliver a high amount of carbohydrate in a lower bulk package than ‘standard food’ does.
Of likely concern for many healthy athletes out there, is this sort of an eating plan is likely to be such an astronomical change from their regular diets and this comes with risk of serious gastrointestinal upset – notably, a lowered fibre intake could spell huge issues in the #2 department! Who wants to contend with that in the lead up to and during a race?!
So, despite being desperate for anything to help me to get to the finish line I found myself questioning how well carbo-loading works in reality and whether it’s commonly undertaken. I reached out to some of my more trained up running friends, including many who are heading for the UTA next week. Though I’m usually the first to point out that the plural of anecdotes is not data – it was interesting to see that very few consider themselves to carbo load and who only a token pasta added here or there in the days prior. Far more importance for the regular athlete out there was placed on not changing up your regular eating habits too much so that you don’t experience unusual GI reactions but certainly to ensure that you eat a good lunch and dinner the day prior and on making sure some decent breakfast is consumed before to the race.
As Sam Steadman who heads up Outer Limits here in Townsville and is incredibly experienced in a range of endurance events indicated “if you expect to race off the start line, push your body to the limits and run on minimal intake of carb to save weight and time, you might need to increase your glycogen stores for up to 3 days before” – otherwise, don’t really change your diet or nutrition or clothing or anything really before the race.” In other words – for the vast majority of us out there it’s probably best to stick with what you know and what you’re used to!
The other key focus from my unofficial focus group, especially from those in the longer distance events (where already by half way through, without replenishment their original muscular glycogen stash would have burnt out, regardless of any carbo-loading) – was put on figuring out what carbohydrate intake/hr is appropriate to support their exertion over such a long time. It was stressed that it must be palatable, easily eaten and well-absorbed on the run. Too much intake could mean GI upset, not enough would mean that your energy levels are low and you fatigue or ‘bonk out’.
Certainly research, including a recent systematic review confirms the importance of consuming carbohydrates during endurance events.(15) For those events longer than 2 hours, most studies used a rate of 1-1.5g/kg of ingested carbohydrate per body weight/hr or at or above 90g of carbohydrate per hour.(10, 15) Additionally, there is evidence to support using multiple transporters by adding fructose to the mix rather than having just a glucose-based blend which will enhance absorption but that this may be athlete-dependent as fructose is not always well tolerated.(15)
So, in conclusion – taking on board all this theoretical and practical advice: carbo-loading for 1-3 days in accordance with the recommendation of consuming 9-12g of carbohydrate per kg of body mass per day may give you that edge if you’re an elite, professional racer looking to make the podium and if (and only if!) you’ve actively practiced your carbo-loading during training. Otherwise it seems to offer more risk than reward.
Personally, for me, next week will be a ‘normal’ diet, plus a few extra carbo serves in the days prior to the race and making sure there’s plenty of room in my pack for snacks and magical unicorn tear essence-infused carbohydrate/electrolyte containing powders……then I’m going to pray with all my might that there are still hot chips at the van at checkpoint two by the time I get there and mix that fantasy with a giant dose of naivety, stubbornness, bluster and cross fingers that I can faff my way to the end somehow!
Wish me luck!
- Hawley JA, Schabort EJ, Noakes TD, Dennis SC. Carbohydrate-loading and exercise performance. An update. Sports Med. 1997;24(2):73-81.
- Hackney AC. Effects of the menstrual cycle on resting muscle glycogen content. Horm Metab Res. 1990;22(12):647.
- Walker JL, Heigenhauser GJ, Hultman E, Spriet LL. Dietary carbohydrate, muscle glycogen content, and endurance performance in well-trained women. J Appl Physiol (1985). 2000;88(6):2151-8.
- Astrup A, Meinert Larsen T, Harper A. Atkins and other low-carbohydrate diets: hoax or an effective tool for weight loss? Lancet. 2004;364(9437):897-9.
- Achten J, Venables MC, Jeukendrup AE. Fat oxidation rates are higher during running compared with cycling over a wide range of intensities. Metabolism. 2003;52(6):747-52.
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- Sherman WM, Costill DL, Fink WJ, Miller JM. Effect of exercise-diet manipulation on muscle glycogen and its subsequent utilization during performance. Int J Sports Med. 1981;2(2):114-8.
- James AP, Lorraine M, Cullen D, Goodman C, Dawson B, Palmer TN, et al. Muscle glycogen supercompensation: absence of a gender-related difference. Eur J Appl Physiol. 2001;85(6):533-8.
- Stellingwerff T, Cox GR. Systematic review: Carbohydrate supplementation on exercise performance or capacity of varying durations. Appl Physiol Nutr Metab. 2014;39(9):998-1011.