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Could the routine use of sports drinks hinder the process of training adaptation?
Everybody knows that achieving maximum sports performance requires optimum carbohydrate and fluid intakes, which is why sports drinks have become so popular. But could the routine use of such drinks actually hinder the process of training adaptation? Richard Godfrey investigates.There’s general agreement among sports scientists that consuming sports drinks containing carbohydrate and electrolytes can enhance fluid uptake and boost athletic performance (1). This is not surprising as studies have shown that as little as a 2% loss of body mass caused by fluid loss can cause a performance drop of 20-30% (2). However, some researchers have recently begun to question the almost universally accepted notion that high levels of fluid intake in the form of sports drinks are always necessary, for example to prevent heat stroke (3). And now some scientists have begun to ask whether there are circumstances in which consuming sports drinks could actually prove counterproductive.
I remember the case of a table tennis player competing in a very hot arena in Malaysia who was consuming between 12 and 15 litres per day of carbohydrate/electrolyte sports drinks. While he remained very well hydrated, he was unwittingly consuming an extra 1.2kgs of carbohydrate per day, containing nearly 5000kcals. No wonder he was concerned about his weight gain and drinking pure water would have been a better option! However, the possible drawbacks of routine sports drink use extend beyond weight gain.
Sports drinks and growth hormone release
Vigorous exercise stimulates an ‘exercise induced growth hormone response’ (EGIR), leading to increased levels of human growth hormone (hGH) circulating in the blood. Although EGIR is poorly understood, scientists believe that increased levels of circulating hGH lead to a number of benefits for the exercising body. These include an enhanced ability to use fat as a fuel (4) for exercise and an increased protein utilisation and muscle synthesis (5-7). Doctors have also long known that those with certain medical conditions involving an hGH deficiency tend to suffer from abdominal obesity and a reduced exercise capacity.
In short, exercise induced hGH appears to improve both exercise capacity and the subsequent adaptation to exercise. So what is the link with sports drink intake? Well, we know that an elevated level of blood glucose tends to halt the secretion of hGH into the bloodstream. And since all carbohydrate drinks produce a rise in blood glucose, the logical conclusion is that consuming sports drinks during and after training might reduce the EGIR. And if the EGIR is blunted, the question is whether the routine use of sports drinks could reduce adaptation to exercise?
We know that a significant hGH release occurs as a result of 10 minutes or more of exercise at or above lactate threshold (8). So, could training adaptation be optimised by consuming just water and electrolytes, and not carbohydrate during and for 90 minutes after exercise? On the other hand, we need to make a clear distinction between competition and training; training is about maximising adaptation, whereas competition is about maximising performance on the day. In the case of competition, supplying ample carbohydrate is clearly desirable. Therefore perhaps we should be considering the ‘periodisation’ of our fluid intake – ie matching what we drink to the objectives that we’re trying to achieve.
Practicalities of fluid periodisation
So far, the concept of fluid periodisation (using carbohydrate drinks during competition and refraining from routine use during training) remains largely theoretical and only properly controlled scientific studies will reveal is this type of strategy could result in an improvement in long-term performance via enhanced adaptation. However, some scientists are adding weight to this theory; in a recent lecture to the American College of Sports Medicine, Professor Pedersen referred to the growing body of evidence suggesting that muscles demonstrate improved adaptation to training stimuli when trained with partially depleted glycogen reserves (9,10).
The reasons for this are unclear; exercise is known to activate over 1000 genes, many of which are involved in processes linked to training adaptation. We also know that exercise stimulates genes to produce more muscle protein. A recent study showed that in single leg cycling to initially deplete glycogen in one leg only, subsequent endurance training on both legs resulted in higher endurance in the low glycogen leg compared to the high glycogen leg – possibly because of a greater concentration of energy producing enzymes in the low glycogen leg9.
However, there are also potentially serious drawbacks with this approach. A number of studies have clearly demonstrated that training habitually when muscle glycogen stores are partially depleted increases the risk of ‘unexplained underperformance syndrome, characterised by fatigue, lethargy, lowered immunity and poor performance. Because of this risk, even the advocates of fluid periodisation are urging extreme caution until we know more.
What does all this mean for coaches and athletes? Well, the goal of this article is not to recommend a new way of using sports drinks; it is however designed to alert athletes and coaches to the possibilities of fluid periodisation, so that they might keep an eye out in the scientific literature. There’s a lot more science needed and in the interim it’s worth reiterating that because of the risk of unexplained underperformance syndrome, coaches should not conduct long-term experiments with their athletes until more is known about the subject.
However, it could be worth experimenting with the occasional use of pure water, or water containing only electrolytes before, during and after training in those sessions where the training intensity is at or above lactate threshold. But this shouldn’t be attempted more than once per month and any effects monitored closely.
1 Optimizing Sport Performance.10: 139-176. Gatorade Sports Science Institute, 2001
2 Med Sci Sports Exerc 1989; 21: 532
3 PNAS 2005;102(51): 18550-18555
4 Biochemistry for the Medical Sciences. John Wiley and Sons Ltd, 1983
5 Am J Physiol 1991; 260: E499-504
6 Diabetes 1992; 41(4):424-429
7 metabolism 1993; 42(9): 1223-1227
8 J Clin Endocrinol & Metab 1992; 75:157-162
9 Biochem Biophys Res Commun 2006; 342(3): 949-55
10 J Appl Physiol 2005; 99(6): 2075-9
Original article by Dr Richard Godfrey
Summary by Andrew Hamilton BSc Hons MRSC ACSM