Fuel cocktail

Carbohydrates and exercise

Carbohydrate is by far the most important fuel for athletic performance because it provides the most energy/unit of time, explains Jeukendrup. ‘Fats can be quantitatively important, especially in endurance events, but most sports performance is related to the athlete’s ability to use carbohydrate.’ Intake guidelines range from 5g of carbohydrate/kg body fat/day to 12g for endurance athletes expending 5000kcal or more a day.

The body breaks down carbohydrates and stores them as glycogen – a polymer of glucose – in the muscles and liver. Liver glycogen is broken down by enzymes to glucose, which circulates in the blood. The glucose fuels the brain and the central nervous system, and some will be taken up by muscle, explains Williams. Glycogen stored in muscle is broken down to glucose, but does not have the necessary enzyme to transport the glucose into the blood so provides energy for muscle contraction.

The availability of muscle glycogen is key to performance. ‘The evidence is sound, at least in the competition setting, that high [glycogen] availability is desirable,’ Burke states. A daily carb intake based on body size and training regime, combined with replenishing stores during exercise, is crucial if an athlete is to continue moderate to high-intensity exercise for more than a few hours.

Getting the timing right is critical, Burke stresses. Eating carbohydrates before exercise tops up blood glucose levels as well as glycogen stores in the muscle and liver, she says. This is especially important if exercising first thing in the morning, or if the event is high-intensity, or will continue beyond 90 minutes. Replacing carbohydrate during prolonged exercise by eating something easily digestible, can benefit sports performance, both through effects on the muscle and by enhancing concentration and mental agility.

Carbohydrate intake after exercise is essential for glycogen stores to recover. ‘Incomplete or slow restoration of muscle glycogen stores between training sessions can lead to a reduced ability to train and a general feeling of fatigue,’ she warns. Williams agrees: ‘Eat carbs before you shower is a good adage. The metabolic machinery is trying to rebuild its carb stores as quickly as possible directly after exercise and for the next four hours, and then at a slower rate for the next 24 hours. Eating carbs then enhances the rate at which glycogen stores are replenished.’

A fuel cocktail

But carbohydrate is not the only fuel the body needs when exercising. A cocktail of fuels goes into muscle contraction, explains Williams, broken down by a system of energy pathways working in harmony.

‘In low to moderate activity, the working muscles use a little blood glucose, and lots of fatty acids from both the blood and from fat droplets stored inside muscles,’ he continues. ‘It is only as exercise intensity increases that muscle glycogen starts to be broken down. Then the body needs fuel at a faster rate, and only carbohydrate can deliver fuel that rapidly.’

Carbohydrates can be accessed more quickly for their energy, but fat provides a higher concentration of energy. Fat is a very efficient fuel for long duration exercise, explains Jeukendrup. ‘It contains more than twice the amount of energy for the same unit of weight than carbohydrate (1g of fat is 9kcal, 1g of carbohydrate is 4kcal). We store several kg of fat, but only have 400–800g of carbohydrate in our bodies.’

However, fat allows athletes to work only up to approximately 65% of their maximal speed. ‘So if you are racing close to 65%, fat can be the most important fuel. However, even long races, such as bike racing or triathlons, are usually won or lost above that intensity and therefore carbohydrate is essential. This becomes obvious in a marathon. Towards the end when the body runs out of carbohydrate, and fat becomes the predominant fuel, the runner often “hits the wall” and cannot maintain the same pace.’

The body’s muscles also get the energy to contract from adenosine triphosphate (ATP), and creatinine phosphate. ATP is a nucleotide found in all cells and contains a large amount of chemical energy in its high-energy phosphate bonds, which is released when it is hydrolysed by certain enzymes. Creatine phosphate provides a rapidly accessible reserve of high-energy phosphates in muscle and brain cells.

ATP is stored in muscles, but is consumed in only a few seconds of high-intensity exercise, explains Burke. In response, the body uses a range of energy pathways to regenerate ATP, utilising fat and carbohydrates. In most exercise, the body uses a combination of anaerobic (without oxygen) and aerobic (requiring oxygen) pathways so ATP replenishment matches ATP demand.

Take, for example, a 100m sprint that may take 10s. Williams explains: ‘Halfway through the race, our studies have shown that 50% of energy production comes from glycogen, 48% from creatine phosphate, and 2% from stored ATP in muscle.’

What’s happening is that the body first uses up the small reservoir of ATP stored in muscle, then it taps into creatine phosphate, also stored in muscles, that can be broken down into ATP. When this store runs out, the body will move on to either aerobic or anaerobic metabolism (glycolysis) to create ATP. Anaerobic glycolysis creates ATP exclusively from carbohydrates, with lactic acid as a by-product, for short, high-intensity bursts of activity lasting no more than a minute or so. After this, lactic acid builds up and causes muscle pain, burning and fatigue.

Protein power

Unlike carbohydrates and fat, protein is not designed as a fuel, explains Williams. It is used to rebuild tissues. ‘All the research shows that we don’t need that much protein. However, just like the Olympians of antiquity, our athletes today persist in thinking that more protein means bigger, stronger muscles. The fact is the body can’t store protein.’

Burke agrees: ‘In the past 20 years, detailed research has enabled scientists to measure protein metabolism during exercise and recovery, and to monitor protein balance in athletes. Athletes undertaking recreational or light training activities will get all the protein they need from a healthy diet.’

But this is a debatable issue. The 2010 International Olympic Committee suggested that athletes engaged in intensive exercise are likely to need more protein than sedentary people, perhaps 1–1.5g/kg of body mass/day. Endurance athletes in heavy training may require extra protein to help recovery after exercise, says Burke. Strength athletes, who are interested in gaining muscle size and function, may require more protein in the early stages of very intensive resistance exercise.

Again Burke stresses that the key issue is when protein is eaten. Early intake after exercise has been shown to be beneficial, she reports. ‘The consumption of quality protein provides a source of amino acids to act as substrate for the building of new proteins, and in the case of the amino acid leucine, to act as a trigger to activate protein synthesis. Recent studies suggest that maximal rates of protein synthesis are achieved during the early recovery phase by the intake of 15–25g of protein. ‘This heightened state of protein metabolism seems to last for up to 24 hours,’ says Burke.

Significantly, this effect is best when combined with carbohydrate. Burke explains: ‘Carbohydrate intake stimulates an increase in the hormone insulin, which, in turn, stimulates the muscle to take up the amino acids. A protein–carbohydrate snack or meal, such as fruit smoothie or cheese sandwich, after a workout makes good sense, not only for muscle repair and adaptation to training, but to provide carbohydrate fuel to restore muscle glycogen levels.’

Other nutritional issues

As well as an eating plan, athletes should have a drinking plan. Fluid replacement is an issue for all sports, asserts Burke, including those such as swimming and water polo. Thirst is not an adequate guide to acute dehydration or sudden changes in fluid need in the short- term, she warns. Low fluid levels can trigger rises in body temperature and heart rate, causing fatigue, reduced mental function and stomach discomfort.

Typically, athletes only replace 30–70% of sweat losses during exercise. However, they are more likely to drink more if beverages are cool (~15°C), flavoured and contain salt, reports Burke. ‘Sports drinks are not gimmicks. They are legitimate products that are well researched and proven to improve fluid intake and performance. A great deal of science has gone into developing the flavour profile of sports drinks so that they encourage fluid intake during exercise. In addition, sports drinks contain carbohydrate at a concentration (4–8%) that allows refuelling to take place during exercise.’

But what about other products marketed for athletes? There are thousands of supplements and sports foods on the market. But do they work?

In most situations, athletes can obtain all their nutritional needs from their diet, Jeukendrup argues, with a
few exceptions. ‘Apart from carbohydrate and protein, very few supplements have been shown to be effective over many years of research. This was also the conclusion of a recent consensus meeting of the Sports Nutrition expert panel of the International Olympic Committee.’

Sports supplements include special drinks and bars, liquid meals and micronutrients. Protein supplements, for example, are very popular, but Burke doesn’t recommend them. ‘Many tend to provide very large amounts of protein and little other nutrients. There is no need for the amount of protein and there is certainly no justification for the extra cost.’

Jeukendrup disagrees. ‘Supplements can often provide a practical way of delivering accurate amounts of certain proteins in combination with carbohydrate, and can be an easy and convenient way for the athlete to ingest some of the key recovery ingredients’, he says

Nutritional ergogenic aids – substances or practices that enhance an individual’s energy use, production or recovery – promise physiological benefit; they include mega doses of vitamins and some minerals, free-form amino acids, ginseng and other herbal compounds, bee pollen, coenzyme Q10, and inosine. ‘In general, these supplements have been poorly tested or have failed to live up to their claims when rigorous testing has been undertaken,’ explains Burke.

Exceptions to this are creatine, caffeine and the buffering agents bicarbonate and β-alanine, each of which may enhance the performance of certain athletes under specific conditions.

But these aids come with a warning: they may contain impurities and contaminants, or sometimes pro-hormones and stimulants that are banned under anti-doping codes.