Food, drink and exercise: why timing is everything

How to maximise muscle glycogen

For athletes, high-carbohydrate diets optimise muscle-glycogen levels, and high muscle-glycogen concentrations improve endurance-exercise performance. However, achieving these magnificent glycogen levels is not just a matter of eating plenty of carbs but also of timing your carbohydrate intake appropriately. If, for example, you load up on carbohydrate immediately after either endurance or resistance exercise, you will enhance your total daily muscle glycogen re-synthesis much more than you would by consuming the same amount of carbs earlier in the day or postponing your carb consumption until a few hours later.

 Interestingly enough, taking in carbohydrate immediately after exercise also seems to have a pronounced effect on protein metabolism, decreasing the rate of protein degradation in muscles and increasing whole-body protein synthesis. These twin effects are, of course, highly desirable for athletes, whose performances tend to drop off if significant quantities of protein are lost. (Remember that proteins are the building blocks of muscles and that certain proteins can also serve as energy-releasing enzymes within muscle cells.) Why do carbs save – and even boost – the production of proteins?

For years, exercise scientists have assumed simply that ample carbs give muscles enough energy to stimulate protein production. But, in fact, the true explanation is slightly more complex. Proteins are made of subunits called amino acids, and the breakdown of a very important group of amino acids, called the branched-chain amino acids, is regulated by the activity of an enzyme called BCOAD. As you might expect, a diet which is very rich in protein leads to an increase in BCOAD activity in the liver.

By contrast, when athletes step up their carb intakes BCOAD activity drops. Thus, high intakes of carbohydrate seem to spare protein by calming down BCOAD. Training per se also has an impact on BCOAD activity, with chronic training producing a drop in BCOAD activity in the muscles. In general, ‘down-regulation’ of BCOAD is considered a positive adaptation since it means that muscles, which are primarily composed of protein, will be less apt to ‘break down’ and more inclined to build new structures during periods of heavy exertion.

When training increases in volume or intensity, considerations related to total carbohydrate intake, the timing of this intake and the impact of diet and training load on protein metabolism become particularly crucial. This is because increases in training can deplete muscle glycogen stores and throw athletes into a state of ‘negative nitrogen balance’, in which they lose more protein than they are making.

How to maximise muscle glycogen

In order to work out what sort of nutritional strategy might be best for athletes undergoing an increase in total training load, who want to max-out muscle glycogen and stay positive with protein, exercise scientists at McMaster University in Hamilton, Ontario recently studied 10 female athletes over two separate, one-week periods. The choice of female athletes as subjects was particularly appropriate, since many sportswomen have abnormally low protein and total-calorie intakes.

Before the study began, all 10 women had participated regularly in some form of endurance activity, with an average training load of three 45-minute workouts per week. All were menstruating regularly, and they were tested only during the mid-follicular phases of their menstrual cycles (days four to 11). Average VO2max, measured during progressive cycling to fatigue, was 46.3 ml/kg-min. The athletes completed two separate seven-day interventions, one involving carbohydrate supplementation before exercise and the other immediately afterwards.

During both of these trials the athletes’ energy, carbohydrate and protein-intake patterns were exactly the same: approximately 2,160 calories per day, including 1.4g of protein per kilogram of body weight per day, with 58% of energy derived from carbs, 26% from fat and 16% from protein. On days one, three and four of the seven-day trials, the women worked out in the mornings by cycling for one hour at an intensity of 65% of VO2max (about 76% of max heart rate). On day three, the women completed an additional one-hour workout at 65% VO2max in the afternoon, and on day six they completed an additional 90-minute exertion at the same intensity. On the seventh day they didn’t rest but cycled for as long as possible at 75% VO2max. Thus, the volume associated with each week’s training schedule was nearly 150% above the usual level (335 minutes compared with 135).

 In both trials, the women used a liquid supplement called Results (produced by Mead-Johnson Canada Inc), in which 66% of the calories were derived from carbohydrate, 23% from protein and 11% from fat. Athletes on the post-exercise supplementation regime imbibed the drink immediately after their one-hour bike rides, while the pre-exercise athletes consumed the same amount of the product at breakfast, well before their exertions. Remember, total energy, carbohydrate, and protein intakes were identical in the two groups: the only difference was in the timing of the supplementation. And that timing made a significant difference to a whole range of variables, including fat oxidation, carbohydrate concentrations, protein breakdown, exercise capacity and body weight!

As the researchers might have expected, carbohydrate breakdown during exercise tended to be greater when the supplement had been taken beforehand rather than afterwards. Also, not surprisingly, fat oxidation during exercise was greater when supplementation was postponed until afterwards. These findings are simple to explain: when athletes take in extra carbohydrate at breakfast, their liver and muscle stores of glycogen tend to rise, and thus they have more carbohydrate fuel available for prolonged exercise later in the day. When carbohydrate intake is more minimal, fat is forced to supply more of the energy needed for prolonged exertion.

As usual, timing is everything!

The key results, though, related to nitrogen balance, body mass, and performance. When Results was imbibed after exercise, nitrogen balance was positive: this simply means that the athletes were taking in more nitrogen than they were losing – another way of saying that their protein stores were increasing. When the supplement was taken at breakfast, however, nitrogen balance during the heavy training was negative – i.e. protein was being lost. As well as saving protein, drinking Results right after workouts also prevented excessive losses in body weight during the heavy training. When the supplement was taken after training sessions, the athletes lost 1.5lb in weight during the seven-day trial period, compared with 3.1lb when the supplement was taken at breakfast.

Finally, taking the drink after workouts allowed the athletes to exercise an average of 47% longer during the 75% VO2max effort on day seven than they were able to after breakfast supplementation. But why does carb intake right after exercise produce such great results? Well, carbs control our old friend BCOAD, an effect which helps muscles hang on to more of their very desirable protein. Also, post-exercise carbs drive up blood insulin levels, and insulin can have a very powerful effect on muscles.

To determine insulin’s specific effects, scientists at the University of Texas recently infused insulin into the femoral arteries of athletes following heavy-duty resistance exercise involving the legs. Without insulin, the rate of leg-muscle protein degradation was about 15% higher than the rate of protein synthesis following exercise – i.e. the athletes’ leg muscles were losing protein. When insulin was supplied to the leg muscles after exercise, the rate of protein synthesis was not augmented, but the rate of protein degradation dropped by about 30%! This created a situation of positive protein balance, with the athletes immediately building up their muscles in response to their training.

One of the fascinating aspects of this research was that insulin preferentially drove certain amino acids into the leg muscles. For example, insulin increased the muscles’ rate of absorption of an amino acid called alanine threefold after exercise but had little effect on the absorption of other amino acids, such as phenylalanine, leucine, and lysine. This raises the possibility that well-planned ‘cocktails’ of amino acids might have the greatest possible effect on intramuscular protein synthesis.

In this study, protein synthesis was not enhanced, but this may have been due to the fact that no additional amino acids were given to the athletes, either orally or intravenously, and they had to rely on the normal concentrations of amino acids already found in their tissues and blood plasma. The combination of an amino-acid cocktail and high insulin levels might have incredibly positive effects on protein synthesis and recovery from exercise in serious athletes. As demonstrated, when athletes time their carb intake properly they achieve a better nitrogen balance and – as a result of greater glycogen – better performance on the following day (as they did on day seven of the McMaster University study).

A sceptic might ask: why couldn’t breakfast carbs do the same thing? In theory they should be able to, but the ‘catch’ is that muscles are most receptive to taking carbohydrate on board and holding onto it during the two hours immediately after exercise. What are the practical implications of this research for your training? To promote recovery and increase your chances of performing well during subsequent training sessions, take in carbohydrate and protein immediately after your workout ends. Aim for about 2g of carbohydrate per kg of body weight and approximately 40g of protein. If, for example, you weigh in at 65 kilos, you would consume 130 grams (520 calories) of carbs soon after exercise along with your 40g of protein. Note, too, that the new research on nutrient timing means that the composition of any workout consists of more than the exercise itself. It also includes a ‘nutritional warm-up’, consisting of the pre-exercise amino acids you plan to take (30-40g should be about right) and a post-exercise warm-down, when you ingest the right amounts of carbohydrate and protein. These nutritional strategies should be planned just as carefully as your actual physical training.

Owen Anderson

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