Andrew Hamilton looks at optimum fueling strategies for a half marathon, and explains why one size doesn’t fit all MORE
Practical advice on carbohydrates, hydration and antioxidants
How 10 years of sports nutritional research can be utilised in your training programme
In the first decade of the century, there’s been an explosion in the sheer volume of sports nutrition research. But what are the key findings that athletes should seriously consider putting into practice? Andrew Hamilton investigates…
If the 1980s and 1990s were the decades when we came to fully appreciate the importance of carbohydrate for performance, the noughties was the decade when we realised that not all carbohydrate drinks are created equal.
Unlike other fuels such as fat and protein, carbohydrate can be broken down very rapidly without oxygen to provide large amounts of extra ATP (the universal energy currency in the body) via a process known as glycolysis during intense (anaerobic) training. Unless you’re training at a very low intensity, this additional energy route provided by carbohydrate is absolutely vital for maximal performance.
The typical muscle concentration of glycogen in sedentary individuals is around 100-120mmols per kg of ‘wet weight’ muscle mass, which equates to around 300-400g in total. Endurance training such as long-distance running or cycling coupled with a high-carbohydrate diet can raise muscle glycogen concentrations by an additional 50-60%. This in turn can extend the duration of exercise before fatigue sets in by up to 20%(1); studies have shown that the onset of ‘fatigue’ coincides closely with the depletion of glycogen in exercising muscles(2,3).
However, valuable as your muscle glycogen stores are, and even though some extra carbohydrate (in the form of circulating blood glucose) can be made available to working muscles courtesy of glycogen stored in your liver, the level is often insufficient to supply the energy needed during longer events lasting 90 minutes or more.
Consuming carbohydrate drinks during exercise can help offset the effects of glycogen depletion by providing working muscles with another source of carbohydrate. During the late 1980s and 1990s, a number of key studies showed that:
- carbohydrate taken during exercise can be oxidised at a rate of roughly 1g per minute(4-6), supplying approximately 250kcals per hour
- this carbohydrate can be supplied and absorbed well by drinking 600-1200mls of a solution of 4-8% (40-80g per litre of water) carbohydrate solution per hour(7-10)
- ingested carbohydrate becomes the predominant source of carbohydrate energy late in a bout of prolonged exercise(10), and can delay the onset of fatigue during prolonged cycling and running as well as improving power output(11,12).
The rub, however, is that at the concentrations and volumes used in those studies, traditional (glucose/glucose polymer) carbohydrate drinks only supply around 60g per hour (around 250kcals per hour), which provides no more than a modest replenishment of energy compared to that being expended during training or competition (elite athletes can burn well over 1000kcals per hour). Higher concentrations or volumes than this are not recommended because of gastric distress and the fact that the extra carbohydrate ingested is simply not absorbed or utilised. However, the good news is that recent research has clearly demonstrated that by tweaking the type of carbohydrate in a drink, it’s possible to increase the rate of carbohydrate replenishment.
In 2003, researchers from the University of Birmingham in the UK began looking more closely at this issue in a study of eight cyclists pedalling at 63% of VO2max for two hours(13). In the study, the cyclists performed four exercise trials in random order while drinking a radio-labelled solution supplying one of the following:
- 1.2g/min of glucose (medium glucose);
- 1.8g/min of glucose (high glucose);
- 1.2g of glucose + 0.6g of fructose per minute (glucose/fructose blend);
- water (control).
The key finding from the study was that the maximum rate of glucose absorption into the body was around 1.2g per minute because feeding more glucose produced no more glucose oxidation. However, giving extra fructose did increase overall carbohydrate oxidation rates, which indicates that fructose in the glucose/fructose drink was absorbed into the bloodstream from the intestine via a different mechanism.
This study and others have shown that glucose/fructose mixtures do result in higher oxidation rates of ingested carbohydrate, especially in the later stages of exercise(14). Moreover, further studies by the Birmingham team showed that ingesting glucose/fructose drinks also provided the following benefits over conventional glucose-only drinks(15):
- more energy produced from consumed carbohydrate (in drink) and less from stored muscle carbohydrate, thereby preserving muscle glycogen stores
- better hydration due to increased amounts of water absorbed from the stomach
- a reduced perception of stomach fullness after consuming the same volumes of drink
- lower perceived rates of exertion in the later stages of exercise.
Even more importantly, the same team has carried out further research, which shows that the benefits outlined above of a glucose/fructose combination drink do translate into better performance(16). In the study on cyclists performing a one-hour time trial, the ingestion of the glucose/fructose drink resulted in an 8% quicker time to completion of the time trial compared with the glucose-only drink, and a 19% improvement compared with the water-placebo drink. Moreover, the glucose/fructose drink helped to spare the cyclists’ reserves of liver and muscle glycogen.
These research findings are very encouraging; higher rates of energy production from ingested carbohydrate and increased water uptake is very desirable for anyone who participates in endurance sports or activities. Even better, it seems that glucose/fructose drinks actually enhance endurance performance in real athletes under real race conditions. The icing on the cake is that these drinks are no more expensive than conventional glucose/glucose polymer drinks, so it seems that the future for glucose/fructose carbohydrate drinks looks bright indeed!
Maintaining hydration in sport is relatively straightforward compared to other aspects of sports nutrition but research conducted during the past decade has changed perceptions about what constitutes best hydration practice. Moreover, the traditional advice to athletes to drink enough to ‘replace fluid lost in sweat’ during endurance events has come under attack from scientists such as the renowned exercise physiologist Professor Tim Noakes. In 2006, in a hard-hitting leading article in the British Journal of Sports Medicine, Noakes claimed that the case against ‘over-drinking’ in athletes was proved 20 years earlier but that official advice since has been influenced by the marketing needs of the sports drink industry rather than the needs of the athlete(17).
For example, Australian researchers measured core temperature in ten participants in the 2004 Ironman Western Australia and then related this to the triathletes’ hydration status after the event(18). The results showed that while fluid losses led to an average fall in body mass of 2.3kg (about 3% of body weight), the athletes’ core body temperature averaged only a modest 1oC above normal resting temperature, while other measures of dehydration, including plasma levels of sodium and urine concentration, stayed within normal ranges. Following their findings (that even quite large fluid losses don’t lead to dehydration or heat illness), the scientists called on the American College of Sports Medicine (ACSM) and other official bodies to revise their current fluid replacement guidelines.
Another study looked into the effects of 5% dehydration on running economy in ten highly trained collegiate distance runners(19). Two 10-minute runs (one at 70% VO2max and one at 85% VO2max) were performed in a fully hydrated state and two in a dehydrated state (a water loss corresponding to 5.5 and 5.7% body mass – ie about 3 litres). The researchers discovered that there were no significant differences in running economy between any of the combinations of hydration states and workloads. Likewise, there were no differences in perceived rates of exertion or in post-exercise lactateconcentration.
Why is this surprising? Well, 5% dehydration is very severe, equating to a water loss of two and a half times that commonly accepted to be the threshold of reduced performance! To achieve 5% dehydration in reasonably temperate conditions, a runner would have to run for long periods beyond the 2% dehydration point that has traditionally been claimed to produce significant performance drop.
Dehydration and motor skills
When it comes to sports where complex motor skills are important, some recent evidence suggests that the 2% dehydration threshold may be relevant. US scientists set out to investigate the effects of three hydration strategies on 15 basketballers (aged 12 to 15 years) who underwent three separate two-hour exercise sessions in hot conditions with different drinking strategies(20).
1. No drinks consumed leading to 2% dehydration;
2. Consumption of a 6% carbohydrate/electrolyte drink to maintain hydration levels (ie 0% dehydration);
3. Consumption of a flavoured water placebo drink to maintain hydration levels, but with no added carbohydrate/electrolyte.
After each exercise session, the subjects performed a sequence of continuous basketball drills designed to simulate a game during which the researchers looked at a number of performance measures. Compared with the flavoured water drinking strategy, 2% dehydration significantly impaired shooting ability whereas consuming the carbohydrate/electrolyte drink improved it. Moreover, the carbohydrate/electrolyte drinking strategy significantly improved total defensive drill times compared with no drinking.
Oxygen is a double-edged sword; its reactivity provides us with the energy for all life processes including muscular contraction. However, this very same reactivity leads to the production of highly damaging free radicals, which are now believed to play a major role in the process of aging and degenerative diseases such as cancer, heart and autoimmune diseases.
Because athletes use greater volumes of oxygen than their sedentary counterparts, it’s long been assumed that they need greater amounts of protective antioxidant nutrients such as vitamins A, C, E and naturally occurring compounds in brightly coloured fruits and vegetables called phytochemicals.
In the late 1990s and early noughties, the practice of taking large doses of antioxidant supplements such as vitamins C and E was commonplace among athletes, but ten years of research has shown that the benefits of such a strategy are far from clear.
While some studies on athletes taking supplements have shown benefits(22-25), a significant number have produced inconclusive results(26-29) and some have even indicated that large single doses of an antioxidant nutrient may increase cellular damage in the body following exercise!
Although it’s true that the balance of evidence for taking antioxidant nutrients such as vitamin C or E is slightly more positive than negative, recent research indicates that the phytochemical antioxidants found in fruits and vegetables have a far greater antioxidant activity in the body than that achieved from vitamin C and E supplements. This has led to a major shift in thinking about how athletes can best protect themselves and also provides the rationale behind the more general ‘5-a-day’ recommendation for healthy eating.
A good example of this research is a study on oxidative stress generated during a 30-minute treadmill run at 80% VO2max, which showed that a mixed fruit and vegetable powdered extract containing only small amounts of vitamins C and E afforded as much protection as the pure vitamins supplemented at four times the amount found in the extract(30). The clear implication was that it was the phytochemical content of the extract that was providing the protection rather than vitamins C and E.
There are also a number of other studies on the use of fruit and vegetable extracts and juices for antioxidant protection during training, which not only provide encouraging evidence but also indicate possible performance benefits too.
- cyclists taking grape, redcurrant and raspberry concentrates, which lowered measures of muscle damage compared to a placebo(31)
- rowers consuming chokeberry juice before strenuous workouts, which again lowered markers of muscle damage and oxidative stress(32)
- resistance trainers who consumed cherry juice before and after eccentric exercise and who suffered less post-exercise muscle soreness and strength losses than those taking placebo(33).
The evidence gathered over the first decade of the 21st century suggests that antioxidant nutrient supplementation (eg vitamins A, C and E and the mineral selenium) shouldn’t be ruled out, but it may be better to use low-dose synergistic combinations of these nutrients rather than large doses of a single nutrient – in a broad-spectrum multi-vitamin/mineral for example. More importantly, any health-boosting strategy should be based on harnessing the antioxidant power of phytochemical-rich fruits, vegetables and other high-antioxidant foods, which you should try to increase significantly in your diet (see box, left).
Andrew Hamilton BSc Hons, MRSC, ACSM is a member of the Royal Society of Chemistry, the American College of Sports Medicine and a consultant to the fitness industry, specialising in sport and performance nutrition
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