Muscle glycogen is replenished most rapidly when carbohydrates are consumed

Numerous studies have shown that consuming a carbohydrate-rich diet is a pre-requisite for optimum performance in endurance sports such as cycling, running, triathlon, swimming etc. This is because the bulk of the energy during high-intensity exercise (and a significant proportion during moderate intensity exercise) comes from muscle glycogen. You can think of glycogen as your body’s premium grade of fuel for exercise because it can be stored where it’s needed (the working muscles), and then rapidly converted to energy, enabling you (providing you’re fit enough) to exercise hard for long periods of time. 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.

Índice Show

The science of carbohydrate replenishment
Figure 1: Progressive glycogen depletion over 3 days of exercise(1)*
Practicalities of carbohydrate loading
*What carbohydrate type(s)?

However, muscle glycogen storage is comparatively limited; fully loaded, your muscles can only store around 400g or so of glycogen – enough to supply around 2-3 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 requiring more oxygen to liberate muscular energy. This inevitably results in greater perceived effort for at sub-maximal pace and a slower maximum pace.

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’ll be able to maintain your exercise intensity before fatigue sets in. It also follows that after exercise, one of your key priorities is to rapidly replenish that lost muscle glycogen.

The science of carbohydrate replenishment

One of the earliest landmark studies in carbohydrate nutrition demonstrated that a typical Western diet (with about 45% of calories derived from carbohydrate) produced a steady depletion in muscle glycogen during three successive days of running training (10 miles per day)1. However, when runners were provided with additional carbohydrate supplementation, they achieved near maximal repletion of muscle glycogen within 24 hours (see figure 1).

Figure 1: Progressive glycogen depletion over 3 days of exercise(1)*

The dotted line shows how glycogen levels are quite well maintained over three days of 2-hour training bouts when a typical moderate carbohydrate diet is supplemented with additional dietary carbohydrate. The solid line shows a progressive drop in muscle glycogen when the moderate carbohydrate diet is not supplemented, leading to significantly reduced performance. *Adapted from Int J Sports Med 1980; 1:2-14

Subsequent studies showed that to maximise glycogen replenishment, carbohydrate consumption should be a priority after exercise. In fact a literature review of a large number of studies concluded that the highest muscle glycogen synthesis rates occur when large amounts of carbohydrate (1.0–1.85g per kg of body weight per hour) are consumed immediately after exercise and at 15–60 minute intervals thereafter, for up to five hours2.

As a corollary to this, delaying your post-exercise carbohydrate ingestion by several hours may lead to lower rates of muscle glycogen synthesis. Of course, if you’re not training or competing again for several days, this doesn’t matter, but if you’re engaging in daily (or even more frequent) training, post-exercise carbohydrate consumption is critical. It’s true that some animal studies have shown that there’s a degree of enhanced glycogen replenishment for up to 72 hours post-exercise3,4, but of course the training schedules of most athletes simply don’t allow for up to 72 hours of recovery time between sessions!

Practicalities of carbohydrate loading

When it comes to understanding the practicalities of carbohydrate loading, four main questions need answering: When? How much? How often? What types(s)?

*When

When carbohydrate is given, the highest rates of muscle glycogen storage following exercise occur during the first hour following exercise5. 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)6, 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!

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. However, the more carbohydrate you need, the harder it is to replenish in ‘one hit’. Replenishment after long and hard training sessions therefore should begin immediately, but should also 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, followed by 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 can still 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 messages:

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?

Even if you can’t manipulate your timing to maximise the window of opportunity, research suggests that when it comes to replenishing muscle glycogen after training, the key to is to consume enough carbohydrate following exercise. The $64,000 question of course is just how much is enough?

Some studies have shown that muscles can be fully replenished with glycogen by consuming 7-10 grams of carbohydrate per kilo of bodyweight following training7,8 (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.

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).

Take home message:

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.

*How often

Research suggests that a higher frequency of small feedings produces higher rates of glycogen synthesis than taking all your required carbohydrate in one big slug, especially during the first four hours9-11. Smaller, more frequent feedings also help to reduce the risk of tummy upsets. 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 workouts, this practice can help make the process of post-exercise muscle glycogen reloading easier and more efficient.

Take home messages:

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.

*What carbohydrate type(s)?

Because your muscles need rapid replenishment during the post-exercise ‘window of opportunity’, relatively quick-releasing carbohydrates (eg from sugars) are particularly effective, at least in the first instance. Indeed, the immediate post-exercise period is one of the few instances in nutrition where quick-releasing carbohydrates can preferable to the slower releasing starchy carbohydrates such as bread, cereals, potatoes, pasta etc normally recommended by nutritionists.

However, after longer training sessions/events, the process of carbohydrate loading takes longer – often several hours. After the initial couple of hours, slower-releasing carbohydrates such as breads, cereals, pasta rice etc become increasingly important in terms of providing glucose for muscle glycogen re-synthesis.

As a rule of thumb, the more time has elapsed after exercise, the less important it becomes to consume quick-releasing carbohydrates; indeed, if you’re undergoing an extended period of tapering, for example taking 3 or 4 days’ complete rest before a marathon, once the initial period following your last workout has elapsed, you’re better off sticking slow-releasing carbohydrate foods.

Take home messages:

For glycogen replenishment after shorter training sessions (1-2 hour’s duration), consume some quick-releasing carbohydrate immediately after training and again after 60 and 120 minutes. Follow up with some slower releasing carbohydrates.
For glycogen replenishment after long training sessions (over 2 hour’s duration), follow protocol above, but after 120 minutes, switch the emphasis mainly to low GI carbohydrates.

References