Sports Nutrition, Sports Supplements

There’s no doubt that ensuring optimum fluid and carbohydrate replenishment is vital as a sports supplement for maximising sport performance. But, as Richard Godfrey explains, while this strategy is fine for competition, some scientists are wondering whether the routine use of carbohydrate/fluid replacement drink during training could actually hinder the process of training adaptation rather than enhance it

During the last 20 years an increasing body of research literature has suggested that appropriate fluid intake is imperative to ensure good performance (1,2). Research has also demonstrated that a 2% weight loss attributable to dehydration is the first time an individual will feel thirsty, but that this level of dehydration can be associated with a 20-30% drop in performance (2).

Given this evidence, the practice of using carbohydrate-electrolyte (sports) drinks to enhance the rate of fluid uptake by the tissues has become widely accepted – a practice for which there’s also good evidence (3). Indeed, current guidelines suggest that fluid composition should be around 2-8% carbohydrate and 10-60mM (0.58-3.48g per litre) of salt (3).

Recently however, Tim Noakes has questioned the extent to which such guidelines for fluid intake are efficacious or even necessary (4). He disputes, for example, the ‘fact’ that high levels of fluid intake are necessary to prevent heat stroke in athletes, describing this as a ‘foundational myth’ (see this issue’s What The Papers Say). That is, it has been stated as so by well-known, well-respected scientists and so has been automatically accepted without challenge and without good evidence to support it. Noakes further suggests that research findings which contradict the accepted wisdom do not receive as much exposure by the devotees of the ‘foundational myth’ so further adding to the perpetuation of the ‘myth’.

Can sports drinks be disadvantageous?

There remains some disagreement about how beneficial sports drinks are, but the question posed here is whether there are any circumstances where ingesting sports drinks could actually be disadvantageous. In most situations, where maximising performance is the aim, following the normal guidelines will reduce any chance of underperformance. However, in extremely hot conditions, simply drinking sports drinks ad lib could, in the medium to long term, actually cause problems.

I recall an incident with a table tennis player in just such circumstances. Over a number of days of competition in a non-air-conditioned hall in Malaysia this player was consuming around 12-15 litres of sports drink per day! At 80g per litre that means consuming an extra 1,200g of carbohydrate per day and the athlete was concerned because he appeared to be putting on weight. Clearly, in this situation, occasionally the player should have been drinking plain water and limited the amount of sports drink taken.

During training, an inadequate water intake can limit human growth hormone (hGH) secretion (5). However, ingesting only water can also have its drawbacks. In extreme events, such as the 52-mile Comrades Marathon in South Africa, there have been reports of hyponatremia (an excessive dilution of blood sodium, which can be fatal) (4,6,7). In a sporting context it should be pointed out that this is extreme and as such is very rare. The point here is that both extremes – only ever drinking copious amounts of water alone or copious amounts of sports drink – are unlikely to be the best thing to do, even though sports drink manufacturers would prefer the latter!

Sports drinks and growth hormone suppression

It’s possible to speculate that the ingestion of sports drinks may lead to the suppression of growth hormone response associated with exercise (a bad thing) because we know that elevated levels of blood sugar can inhibit the secretion of hGH (see box opposite).

The exact benefit or purpose of exercise-induced growth hormone response (EIGR) has yet to be determined. However, there are a small number of adults who have growth hormone deficiency (GHD) and can therefore provide us with an insight into some of the likely benefits of a normal hGH profile.

Those with GHD have central obesity (ie excess fat deposition in the abdomen), increased blood fat levels and a restricted exercise capacity. Accordingly, it is logical to assume that a normal growth hormone profile is associated with, and may even directly contribute to, improvements in exercise capacity and adaptation.

From research evidence, we know that hGH does promote the use of fat as an energy source (8). In addition, hGH has been demonstrated to enhance protein/muscle synthesis (9,10,11). This is partly accomplished by hGH causing release of insulin-like growth factor-1 (IGF-1) from the liver (12) and from inhibition of myostatin, the protein that normally acts as a ‘brake’ to the production of more muscle than is necessary (13).

At the 1998 World Swimming Championships in Perth, Australia, a female Chinese swimmer was sent home for allegedly trying to smuggle a flask containing numerous vials of rhGH into the country. Wild speculation followed in the Australian press and much of the discussion centred around the perceived fat reducing/muscle building properties of growth hormone. However, given that it takes weeks to see any benefits of this type affecting performance, the puzzle remains as to why, within days of competition, the Chinese would want to use rhGH. In fact, a more likely explanation is that rhGH could be taken on one day and affect the way in which the muscle uses energy the next.

So how does all of this relate to sports drink intake? Well, drinking sports drinks will elevate blood glucose, and this in turn tends to halt the secretion of hGH. So perhaps, if hGH is integral to maximising adaptation to training, taking sports drinks routinely will not result in the best adaptation.

A sustainable rise in hGH secretion is seen with exercise of more than 10 minutes’ duration above an intensity associated with lactate threshold (14) (see figure 1). So, perhaps before, during and for 90 minutes after exercise at or above lactate threshold, where the objective is to optimise training-induced adaptation, only water plus electrolytes should be consumed in training.

However, in other training conditions, and especially before, during and after competition, where the objective is to maximise performance, carbohydrate-electrolyte (sports) drinks should be used. Clearly, extremes are less desirable – eg always drinking only water or only sports drinks – so the aim of training should be considered when deciding which fluid to ingest. Consequently, the suggestion is that ‘periodisation’ of fluid intake be considered.

Periodisation

Traditionally, periodisation refers to the cycling of training intensity and duration in blocks known as microcycles (up to 14 days), mesocycles (2 weeks – 6 months) and macrocycles (6 months – 4 years). These were developed in recognition of the training principles of specificity and reversibility. In other words, ‘use it or lose it’.

The problem is that it is not possible to train every physiological system maximally all of the time, but if a stimulus is not applied at least every two weeks then detraining can begin. So the best coaches draw up long-term plans which are of increasing detail and complexity in the short term and increasing flexibility and less detail as we move from medium to long term.

If certain foods or drinks affect the rate of adaptation then a form of periodisation could, and perhaps should, be implemented. Generally, this would be best applied with reference to the aims and objectives of individual training sessions. Currently, this is an untested speculation and only a randomised, controlled study will provide sufficient evidence that this suggestion results in an improvement in adaptation and longer-term performance.

However, adding weight to the supposition that carbohydrate should be periodised, Professor Bente Pedersen gave a lecture at the recent American College of Sports Medicine (ACSM) conference held in Denver, Colorado in June this year. Her lecture was entitled ‘Signalling muscle to adapt – training low and competing high’. Her talk referred to mounting evidence that partially depleted muscle carbohydrate stores ensure an environment in which muscle adapts to the training stimulus more powerfully (15, 16).

This in contrast to much of the previous work in which training with partially depleted muscle carbohydrate stores has been demonstrated to be sub-optimal (17) (see figure 2) and linked with the risk of unexplained underperformance syndrome (UPS)(18). Hence there may be a case to be made for the use of training when muscle carbohydrate stores are low, but because of the known risks of UPS, such training should be used with great caution. Indeed, Pedersen quite rightly urges caution as it appears that getting the balance right has yet to happen by design so the risks are high.

More than 1,000 genes are activated by exercise and many of these regulate adaptation to training. Such diverse areas as response to stress, aerobic metabolism, anaerobic metabolism and strength are affected. Examples include genes activated in exercised muscle that have low glycogen stores such as PGC-1 (a gene that is upregulated in human muscle during recovery from exercise training) and PDK4 (a key regulator of fat oxidation in human skeletal muscle).

In addition, it is known that acute exercise increases mRNA and protein synthesis, and Professor Pedersen presented data to demonstrate that this is enhanced further in muscle that is low in glycogen. A training study was presented in which one leg was exercised hard, using single-leg cycling to partially carbohydrate-deplete the muscles of that leg. Twice daily training was then used on both legs and then a test administered to examine the time to exhaustion. The leg trained while having low glycogen stores took longer to become exhausted. Muscle biopsies subsequently revealed a greater increase in some oxidative genes for mitochondrial enzymes in the low-glycogen leg in comparison with the high-glycogen leg.

In addition to increased gene activation, the release of certain cytokines has been shown to be further enhanced in exercise in association with low muscle glycogen. Cytokines are factors associated with the immune system and are increasingly being demonstrated to have signalling roles – ie their release can inhibit or activate metabolic pathways in cells or tissues in various parts of the body. One such cytokine is IL-6; studies in which IL-6 is infused during exercise have demonstrated an increase in the use of fat in muscle. Muscle with sufficient carbohydrate will not release as much IL-6 hence demonstrating that a more pronounced adaptation is seen with low muscle glycogen stores.

Practical advice

The primary purpose of this article is to provoke thought on the issue, make athletes and coaches aware of such possibilities and alert them to this issue so they can be looking for it in the literature. Clearly there is a lot more research required and it is likely that applied scientists rather than pure scientists will find the answer. I believe it is well worth reiterating the advice above that this should not be experimented with until more is known. Athletes are only human and, as long as there is a risk, underperformance is all too likely.

The occasional use of water containing only electrolytes before, during and after training sessions may be worth experimenting with, but again let me caution coaches out there ‘less is more’. My recommendation for the time being would be to attempt this no more than once per month with exercise at an intensity associated with lactate threshold and above. Athletes and coaches may also want to experiment with the practice of twice daily training every second day as another means of enhancing growth-hormone mediated adaptation.

Dr Richard Godfrey is a senior research lecturer at Brunel University and has previously spent 12 years working as a chief physiologist for the British Olympic Association

References
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