Carbohydrates-forget the rest

The role of carbohydrates in sports performance might be one of the most thoroughly researched topics in the field of sports nutrition, but that doesn’t stop it constantly throwing up new surprises! Read any biochemistry textbook on carbohydrates nutrition and you will find no mention of variation in carbohydrates metabolism between different groups of people. But now new research indicates that both gender and age can affect the way our bodies utilise this vital fuel.

And just in case you have any lingering doubts about the crucial contribution of carbohydrates to optimum performance, scientists have also been busy investigating the link between low carbohydrates intakes and exercise-induced free radical damage, leading to impaired muscle function.

According to evolutionary theory, one of the reasons the average female carries more fat than the average male is because of her role in child rearing. More fat stores and a more efficient fat metabolism add up to an enhanced ability to survive a period of famine – crucial for the survival of any infant, born or unborn. This difference in fat metabolism is thought to underlie the observation that females are able to oxidise proportionately more fat and less carbohydrate during long periods of endurance exercise, when normal fuel reserves run low, and also why women perform proportionately better at ultra-distance events than their male counterparts.

New research on gender differences

Until recently, these gender differences in carbohydrates metabolism have been thought to be minimal. But new research published at the end of last year seems to throw this assumption into doubt(1). In this study, 14 healthy but untrained volunteers were split into two equal groups of men and women. Each group completed two exercise trials in which they pedalled on a stationary bike for 90 minutes at 60% of VO2max.

In the first exercise trial, both groups were given a sweetened placebo drink to consume during the session. In the second, carried out a week later, they were given an 8% carbohydrates drink, supplying carbohydrates at a rate of 1 gram per kg of body weight per hour. This drink contained radio-labelled glucose which, when metabolised for energy, breaks down to form radio-labelled carbon dioxide and water, both of which can be distinguished from ordinary carbon dioxide and water (the breakdown products from fat metabolism and any stored carbohydrate). The more carbohydrates used from the drink to supply energy, the higher the ratio of labelled expired carbon dioxide and water to the unlabelled variety.

During the placebo drink trial, fat oxidation (‘burning’ to produce energy) was higher in females than in males when measured at 30 minutes of exercise. But, when averaged out over the final 60 minutes of exercise, the relative contributions of fat, total carbohydrates and protein to energy were similar for both groups.

However, clear differences emerged during the carbohydrates drink trial. At 75 and 90 minutes, both the ratio of labelled-to-unlabelled carbon dioxide and the proportion of energy derived from the carbohydrates relative to lean body mass were higher in the women than the men. Moreover, when averaged over the final 60 minutes of exercise, the contribution of ingested carbohydrates to the total energy used tended to be higher in the female group – 14.3% compared with 11.2% for the males.

This finding is rather surprising because it is counter-intuitive; in other words, one might expect that women, being more efficient at burning fat than men, might derive less energy from ingested carbohydrates during exercise.

Women burn more carbohydrates during endurance exercise than men

Nevertheless, the researchers concluded that: ‘compared to males, females may oxidise a greater relative proportion of ingested carbohydrates during endurance exercise which, in turn, may spare more endogenous fuel [ie fat]. Based on these observations, ingested carbohydrates may be a particularly beneficial source of fuel during endurance exercise for females’.

This study was small and there was no suggestion that the two groups were matched for aerobic fitness/training levels (remember that high aerobic fitness levels and training volumes increase the efficiency of fat metabolism). This means that further studies are required before firm conclusions can be drawn. However, the notion that carbohydrates replenishment for female endurance athletes may be less important than for men because of their inherent advantage with fat metabolism is certainly going to need revising!

In a related study, researchers set out to see what effect age might have on carbohydrates usage during exercise(2) This time, 12 boys aged just under 10 on average were compared with 10 adult men (average age 22.1 years). As in the previous study, both groups completed two exercise trials on a stationary bike, consuming a placebo drink with the first and a radio-labelled carbohydrate drink with the second. However, this time the trials lasted only 60 minutes and were performed at 70% VO2max, while the carb drink was of 6% concentration, given at the rate of 24ml per kg of body weight over the hour (just over a litre for a 50kg subject).

In both exercise trials, the researchers measured the rate of ingested radio-labelled carbohydrates utilisation over the final 30 minutes and compared it with that of other fuels (primarily fats and stored carbohydrates).

In both trials, total fat oxidation was higher and the total ingested carbohydrates oxidation lower in the boys than in the men. But in the carbohydrates drink trial, the rate of carbohydrate oxidation was increased and made a relatively greater contribution to total energy in the boys – 21.8% compared with 14.6% for men.

These results suggest that, although stored carbohydrates utilisation during exercise is lower, the relative oxidation of ingested carbohydrates is considerably higher in boys than in men. The researchers concluded that the greater reliance on ingested carbohydrates in boys may be an important mechanism in preserving stored fuels and may also be related to pubertal status.

To put it another way, there may be biochemical/physiological mechanisms operating in children that are designed to conserve stored glycogen and body fat. If you consider these results in relation to those of the male-female study, it begins to look like a carbohydrate-rich diet may be more important for young female athletes than has previously been realised.

Exercise-induced oxidative stress and the role of dietary antioxidants have been covered in depth in a recent issue (PP 199, July 2004) and we’ve also examined the role of high-carbohydrates diets in reducing post-exercise immune suppression (PP194, March 2004). Now some researchers are wondering whether the two issues may be linked.

Free radical damage and carbs

High-carbohydrates diets are associated with reduced secretion of the immune-suppressing stress hormones cortisol and the catecholamines, and it is known that the latter can undergo a biochemical transformation in the body known as ‘auto-oxidation’, forming highly reactive oxygen species (ROS), more commonly known as ‘free radicals’.

The obvious question, therefore, is whether the ingestion of carbohydrates during intense exercise can diminish the production of ROS, thereby reducing oxidative stress.

In a bid to answer this question, researchers at the University of Montana studied 16 experienced marathon runners, who ran on treadmills for three hours at approximately 70% VO2max on two separate occasions under the following conditions(3):

  • with a carbohydrates beverage taken throughout the run;
  • with an identical-tasting placebo beverage containing no carbohydrate.

Blood samples were taken before and after training and analysed for isoprostanes and lipid hydroperoxides (both markers of free radical damage within the body), levels of the stress hormone cortisol and the so-called ‘ferric reducing ability of plasma’ (FRAP), which is basically a measure of the body’s ability to neutralise free radicals.

As expected, the pattern of change in cortisol levels was significantly different between trial conditions, with higher post-exercise levels recorded after the placebo trial. The researchers then went on to examine the markers of free radical damage and demand on the antioxidant defence systems of the body.

Although these markers were increased after both exercise trials, there was no significant difference between trial conditions. In other words, the excess stress hormone secreted in the placebo condition did not lead to a significant increase in oxidative stress.

We cannot conclude from this that stress hormones do not aggravate oxidative stress. The effect may be small, for example, and submerged in the overall increase in oxidative stress induced by the exercise alone. Also, these results were obtained at a training intensity of 70% VO2max and it is not possible to extrapolate these results to other intensities. It may be a cliché, but more research will be needed before we can draw definite conclusions!

Central nervous fatigue and carbs

Every athlete knows that ingesting carbohydrates during prolonged exercise can improve endurance, while an insufficiency of carbs reduces glucose availability to the muscles which, in turn, leads to hypoglycaemia and fatigue.

Fatigue, normally defined as a loss of force-generating capacity, may set in for a variety of reasons, but in long bouts of endurance exercise it is generally believed to occur principally as a result of reduced availability of muscular adenosine triphosphate (ATP), the high-energy molecule that fuels muscle contraction and is generated by the oxidation of glucose. However, some exercise physiologists have questioned whether this is the whole story, arguing that the central nervous system (CNS) may also play a role in fatigue.

The CNS is responsible for sending the electrical signals required to ‘fire’ muscle fibres, thereby releasing the stored energy of ATP to produce muscular contraction. However, the CNS itself also requires carbohydrates, in the form of glucose, to function, and the key question is whether the reduced levels of blood glucose typically present after long bouts of exercise can impair the efficiency of the CNS, thereby reducing the firing ability of the muscles, regardless of ATP levels.

To resolve this question, a study was recently carried out to examine the degree of CNS activation before and after three-hour cycling sessions performed with and without supplemental carbohydrate(4).

Eight endurance-trained male cyclists were randomised to one of two groups, one given a carbohydrate beverage to take throughout the bike ride and the other a no-carb placebo.

Before the trial, all the cyclists completed a two-minute sustained maximal knee extension session during which voluntary force production and central nervous activation ratios were assessed by means of a technique known as ‘twitch interpolation’, which measures the efficiency of the CNS in sending electrical impulses to the muscle fibres.

Blood glucose concentrations were monitored in both groups. In the placebo trial, these fell from 4.5mM (moles) per litre before the ride to around 3.0mM per litre afterwards. By contrast, blood glucose concentrations were maintained in the carbohydrates trial.

After the ride, both groups were reassessed for knee extension force and CNS activation. Before the ride, the average force during sustained maximal voluntary muscle contraction was 248 newtons (N). This force fell to an average of 222N in the carbohydrate group and 197N in the placebo trial group.

However, this result could not simply be attributed to reduced muscle stores of glycogen (and therefore reduced ATP availability) because in the placebo group the lowered force production was accompanied by a significantly reduced level of CNS activation, which remained stable in the carbohydrates group.

The researchers concluded that exerciseinduced hypoglycaemia can reduce CNS activation during sustained muscle contractions, but that this effect can be mitigated by ingestion of a suitable carbohydrate drink.

This latest research continues to emphasise the absolutely pivotal role of carbohydrates nutrition in sports performance. Forget fancy supplements: the most useful performanceenhancing change any athlete can make to his or her dietary regime is to ensure a plentiful carbohydrates intake, before, during and after exercise!

This may be particularly critical for young and female athletes, because it appears that their bodies may be ‘preferentially programmed’ to conserve stored body fat and carbohydrates by comparison with other groups.

The work on CNS activation also has implications for power and strength athletes, who have traditionally been less assiduous in maintaining optimum carbohydrates intakes.

The fact that reduced blood glucose appears to reduce CNS activation, thereby reducing the peak power of sustained muscle contractions, means that these athletes, too, neglect carbohydrates nutrition at their peril!

Andrew Hamilton


  1. Int J Sport Nutr Exerc Metab, 13(4): 407-21, 2003
  2. J Appl Physiol, 94(1): 278-84, 2003
  3. Free Radic Res, 37(8): 835-40, 2003
  4. Med Sci Sports Exerc, 35(4): 589- 94, 2003



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