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Unlike the other components found in food such as fats and proteins, carbohydrate has one massive advantage for those seeking to maximise sports performance – namely that it can be broken down to produce energy very rapidly when needed, for example during high-intensity exercise. Carbohydrate then can be considered as the body’s true ‘high-octane’ fuel, which is why getting your carbohydrate nutrition right is so vital if you’re serious about reaching your sporting
potential.

Carbohydrate loading is all about maximising your body’s storage of this vital fuel in order to ensure that you have maximum energy to power you through long, hard workouts. And contrary to popular belief, carbohydrate loading to keep your muscle stores topped up with high octane fuel is something you need to think about day in, day out - not just as you wind down for a big event such as a marathon.


Carbohydrate terminology

The term ‘carbohydrate’ includes:

• All single and short chain sugars such as glucose, fructose and sucrose;
• All medium chain sugars such as dextrins and glucose polymers;
• All starches (long chains of sugars) including the muscle storage carbohydrate glycogen;
• Glycogen – a highly branched chain type of carbohydrate (synthesised from dietary glucose) and stored in muscles, which forms the principle fuel for high-intensity exercise.

The term ‘simple carbohydrate’ generally refers to sugars (i.e. simple sugars and very short chains of linked glucose molecules). The term ‘complex carbohydrate’ generally refers to starches (i.e. long chains of linked glucose molecules.

Additionally, fibres such as cellulose are actually indigestible starch chains. These chains of glucose molecules cannot be broken down in the body because of the way the individual glucose molecules are chemically linked. Like digestible starches, there are a large number of fibre types, which depend on the length and type of branching present in the chain.

Glycogen – your five star fuel

When it comes to understanding carbohydrate loading, glycogen is especially important to consider. This is because carbohydrate loading is all about optimising your intake of dietary carbohydrate so that it can then be broken down and converted into muscle carbohydrate - in very simple terms, fuelling your body in order to maximally load up the ‘fuel tanks’ in your muscles with glycogen!

When you exercise vigorously, some of the energy you need comes from fat and protein oxidation, and some comes from circulating blood sugar, which can be transported to the working muscles as and when needed. However, the bulk of the energy during high-intensity exercise comes from stored muscle carbohydrate – glycogen.

Glycogen is body’s premium grade of fuel for exercise because it can be stored where it’s needed, and then rapidly converted to energy in the muscles, enabling you (providing you’re fit enough) to exercise at high intensity for sustained periods. While fat and protein can also provide fuel for muscular contraction, they are not converted to energy so readily and are therefore unable to fuel the body through high intensity workouts. Indeed, even if you’re working at lower intensities, some of that energy still comes from muscle glycogen and even a mild shortfall in muscle glycogen can leave you feeling tired and leaden. High levels of muscle glycogen on the other hand can help power you through even very long workouts, gentle or intense.

However, the caveat is that muscle glycogen storage is comparatively limited; fully loaded, muscles can only store around 400g or so of glycogen - enough to supply around two hours’ worth or so of intense exercise. After this point, increasing amounts of energy have to be derived from fat, which is a less efficient process and results in lower energy levels and greater perceived effort. It follows therefore that all other things being equal, the more muscle glycogen you have successfully stored by ‘carbohydrate loading’ before training or a long event, the longer you can maintain your exercise intensity before fatigue sets in.

During exercise, your stores of muscle glycogen in those muscles that are contracting steadily drops, and this is one reason why fatigue begins to set in during longer workouts (although the central nervous system also plays a role – see section 2). After exercise, when your muscle glycogen is low, one of your key priorities therefore is to rapidly replenish that lost muscle glycogen.


Post-exercise nutrition

This is where post-exercise nutrition becomes extremely important. Study after study has shown that muscles are best able to rapidly absorb dietary carbohydrate for the re-synthesis of muscle glycogen in the period immediately following and for up to about 90-120 minutes after training. This period can be thought of as a carbohydrate loading ‘window of opportunity’, during which your muscles behave like sponge, soaking up what they need to recover and power you through your next workout. Of course, there’s more to carbohydrate loading than just consuming carbohydrate immediately following your workout, but the importance of this period can’t be over-emphasised.

In the longer term, consuming carbohydrate after this initial period is also important and indeed, if you’re really looking to maximise your carbohydrate loading, you need to think about your carbohydrate intake not just for the rest of the day following training/competition, but also in the days that follow. This is especially important when you’re tapering down for a big event such as a marathon.

Successful carbohydrate loading also involves maintaining maximum levels of muscle glycogen wherever possible, and that means thinking carefully about the carbohydrate content of your day-today diet and even topping up carbohydrate intake during exercise itself; after all, the less you empty those muscle glycogen stores, the easier it is to top them up!


Practicalities of carbohydrate loading

There are four main questions to answer here: When, how much, how often should you carbohydrate load?


When

There’s absolutely no doubt that when carbohydrate is given, the highest rates of muscle glycogen storage following exercise occur during the first hour following exercise. This because the activity of the enzyme that promotes glycogen storage is boosted and so is the insulin response. The golden rule therefore is to consume carbohydrate immediately after exercise, because this is a sure-fire way to speedy recovery.

Although some studies have suggested that delaying your first carbohydrate meal/drink for a couple of hours or more after exercise doesn’t affect your overall ability to top up glycogen in the longer-term (over 1-2 days), if you have to train again later the same day, or even the next day, the chances you’ll struggle to optimally load those muscles!

It’s also worth adding that if you’ve undertaken a long bout of hard exercise, you’ll need more carbohydrate to replenish and reload your muscles than a shorter, less intense session (see paragraph ‘how much?’). The more carbohydrate you need, the harder it will be to replace in ‘one hit’. 

Replenishment after long and hard training sessions therefore should begin immediately, but can continue for more than an hour after training; there’s less potential for tummy upsets this way and your body will still be able to make good use of any carbohydrate you consume after the first hour. A useful tip is to consume a ready-mixed carbohydrate drink immediately after training and then to follow this up with a carbohydrate-rich meal as soon as you get home.

If you’re unable to carbohydrate load immediately after training for any reason, don’t despair! Although it will take longer, and be a less efficient process, all the evidence suggests that provided you subsequently consume the required amount of carbohydrate, you will achieve good levels of muscle glycogen. The downside of course is that if you’re training again within 24 hours, this may not be soon enough to fully reload those muscles!

Take home message # 1:

For rapid and optimal recovery, begin consuming carbohydrate immediately after training and no later than within an hour of training cessation. When replenishing large amounts, continue to consume carbohydrate after the first hour.

How much?

Research suggests that when it comes to replenishing muscle glycogen after training, the key to is to consume enough carbohydrate following exercise. Evidence shows that until the point where the muscles are saturated (fully loaded) with glycogen, the more carbohydrate you consume, the more glycogen is synthesised in the muscles . The $64,000 question of course is just how much is enough?

Some studies have shown that muscles can be fully saturated when you consume 7-10 grams of carbohydrate per kilo of bodyweight following training 6, 7 (eg if you weigh around 70kg, this would equate to 490-700g of carbohydrate) and this has led to a generalised recommendation among some sports scientists.

However, there are two problems with this global recommendation; the first is that how much carbohydrate you need after training will depend on how active you are during this period. For example, if you train and then go to work and your job involves manual labour, you’ll need more carbohydrate than if you train then put your feet up and watch TV for the next four hours! It also depends on how much you’ve ‘emptied’ your glycogen stores; a hard 5-hour bike session will empty those muscle stores far more than an easy 30-minute recovery run. Given the above, it’s easy to see why Tour de France cyclists voluntarily consume 12-13g of carbohydrate per kilo per day and still struggle to maintain their stores of muscle glycogen!

There’s also another potential problem with a blanket recommendation of 7-10g of carbohydrate per kilo as a loading regime - weight gain. Using the 70kg bodyweight example again, even at the lower end of the 7-10g range, 500g of carbohydrate at four calories per gram contains 2,000 calories. By the time you’ve consumed the required 100g or protein or so and some essential dietary fat, your daily calorie intake in going to be pushing 3,000kcals - and that’s assuming you’re quite disciplined with your diet!

If you’re training hard for a couple of hours each day, that kind of calorie intake is not a problem, but if you have a sedentary job and your typical daily training consists of 30 minutes to an hour of moderate intensity training, you could end up consuming more calories than you need, leading to performance-blunting weight gain.

For these reasons, a blanket ‘how much’ figure is hard to give. A much better way may be a calculation based on calorie burn; if you know roughly the energy expenditure of your training session in calories (many heart rate monitors such as Polar and Suunto will now calculate this for you reasonably accurately), you can simply divide this calorie burn figure by four to give an approximate gram requirement for post-exercise carbohydrate. 

For example, suppose you’ve cycled for 90 minutes and your calculated calorie burn is 1,200kcals, you would need to consume in the region of 300g of carbohydrate (1200 divided by 4) to replenish the muscle glycogen used (actually this slightly overestimates glycogen usage because some of that energy will have come from fat oxidation, but it’s a near enough approximation and provides an additional safety margin). 

The first four hours: A number of studies have found that regardless of the quantity of post-exercise carbohydrate consumed, the first four hours after training are particularly important9-11. The consensus was that four hours of repeated small feedings of carbohydrate of 1.0-1.8g per kilo of bodyweight per hour every 15-60 minutes seems to produce the highest muscle glycogen synthesis rates. In 2003, the International Olympic Committee (IOC) used these data (slightly modified) to recommend repeated feedings of 1.2g per kilo per hour of carbohydrate for the first few hours post-exercise in its consensus guidelines.


Take home message # 2:

For optimum post-exercise glycogen replenishment, consume approximately a gram of carbohydrate for every 4 calories of expended energy and more if your recovery period is physically active. Alternatively, consume 1.2g of carbohydrate per kilo of your bodyweight per hour for the first four hours post-exercise.

How often

So far, we’ve seen that for effective post-exercise carbohydrate loading, the key is to consume carbohydrate as soon as possible after training and to consume ample to cover your energy expenditure and post-exercise activity needs. The research from these areas also suggests that a higher frequency of feeding produces higher rates of glycogen synthesis than taking all the required carbohydrate in one big slug, especially during the first four hours-9-11.

There’s also another issue – gastric distress. If you’ve done a very energy-intensive training session – for example a 3-hour run - you’ll probably need in excess in 2,000kcals of post-exercise carbohydrate. Taking this all in one go immediately following your workout maybe unpleasant, particularly for those with sensitive tummies. A better option would be to start feeding immediately, taking on board what you can during the first hour then following up with repeat feedings at 30-minute intervals for the next few hours.

This concept can be extended; during longer workouts you can begin reloading muscle glycogen by consuming carbohydrate drinks before you complete your training session – ie taking on fuel on the move. Apart from the proven performance benefits of consuming carbohydrate during longer workouts12-17, this practice can help make the process of post-exercise muscle glycogen reloading easier and more efficient.


Take home message # 3:

For shorter, less-energy-intensive workouts requiring modest carbohydrate replacement, a single carbohydrate drink/meal taken in the first hour after training will suffice;

For longer, more energy intensive workouts requiring more carbohydrate replacement, start immediately with a generous helping but then split subsequent intake over several feedings for the next four hours to avoid gastric distress;

For longer workouts, consider taking carbohydrate drinks during training as well as after to help make post-exercise refuelling easier.

In the next article, we’ll look at which types of carbohydrate are best for fuelling the muscles and why!


References

1. J Appl Physiol,: 64:1480-5, 1988
2. Diabetes; 50:265-9, 2000
3. J Appl Physiol,: 66:876-85, 1989
4. Med Sci Sports Ex; 29:220-4, 1997
5. J Sports Sci; 22:15-30, 2004
6. J Appl Physiol; 263:98, 1995
7. Am J Clin Nutr; 34:1831-6, 1981
8. Int J Sports Med; 10(suppl):S26-31, 1989
9. J Appl Physiol; 74:1848-55, 1993
10. J Appl Physiol; 88:1631-6, 2000
11. J Appl Physiol; 81:346-51, 2000
12. Med Sci Sports Ex; 25:42-51 1993
13. Int J Sports Med; 15:122-125 1994
14. Med Sci Sports Ex; 28:i-vii 1996;
15. J Athletic Training; 35:212-214 2000
16. Int J Sports Nutr; 7:26-38 1997
17. Nutrition Reviews; 54:S136-S139 1996

 


Article Contributed by:

Ultra Fit Magazine
www.ultrafit.com.au