English
中文Red Bull is a carbonated, non-alcoholic drink, sold in aluminium cans, with a highly visible marketing strategy. This emphasises one desirable function, “energy”, and one ingredient, taurine. In fact, the energy in its true sense comes from the oxidation of glucose, present in the drink both in its pure form and as its digestive precursor sucrose. “Energy” is also, more loosely, an interpretation of brain arousal or alertness, and this effect can be ascribed to caffeine.
The evidence for the physiological effects of Red Bull can be sought in two categories: (1) studies and nutritional consensus on the effects of each ingredient, leading to prediction of the effects of the commercial drink, and (2) studies of the effects of Red Bull, the complete product, itself. The evidence from strategy (2) is weaker.
Caffeine, present in Red Bull in the concentration of relatively weak coffee (320 mg/L), is a naturally occurring xanthine which is an inhibitor of phosphodiesterase. This property gives it a number of physiological effects: diuresis, smooth muscle relaxation in the bronchi and increased contractility of heart muscle. Caffeine has endocrinological as well as direct effects, releasing catecholamines, and these may well be mediators of its well-known brain effects of alertness and euphoria. Unlike taurine (Kang et al, 2002), caffeine readily diffuses across the blood brain barrier. See Leonard et al, 1987, for a comprehensive review of the effects of caffeine.
Taurine is, after the carbohydrates, the compound with the highest concentration in Red Bull (4 g/L). This is a sulphur amino-acid that is found in high concentration in tissues rich in mitochondria, with a high dependence on oxidative metabolism: brain and skeletal muscle. It is believed to provide protection against oxidative (free radical) damage in these tissues (Dawson et al, 2002). It is also claimed that taurine increases contractility by enhancing Ca2+ accumulation and release from sarcoplasmic reticulum ( Bakker and Berg, 2002).
However, this is not evidence that ingestion of the compound will have any immediate effect either on muscle contractility or central nervous system function. It seems unlikely that an increase in plasma level of taurine following a bolus by gastrointestinal absorption would raise the intracellular concentration of taurine in the muscle. Similar considerations apply to the brain. Woojae (2003), in her critical review of the claims of Red Bull to enhance performance in any way other than through sugar and caffeine, points out that the brain already contains large amounts of taurine. Furthermore, except in conditions of inflammation – when the neuroprotective effect of taurine is needed (Kang et al, 2002) – the blood brain barrier blocks the ingress of this amino-acid.
Why then, is taurine so emphasised in the marketing of Red Bull? The name is derived from Taurus, the bull (Latin), one of the signs of the Zodiac, a potent symbol of strength. The logo on the can depicts two bulls locking horns. This symbol has effects on the higher functions of the human brain, the seat of the mind, which are not physiological in the narrow sense, but which can nevertheless be mediated through the nervous system to enhance effort and performance.
Apart from caffeine and taurine, the Red Bull ingredient for which performance claims are made is d-glucuronolactone. This is present in Red Bull in quite high concentration, 2.4 g/L. This compound, naturally found in mammalian tissue, is an intermediary metabolite in many pathways. One of these is the glucuronic acid pathway, so it is has an important role in the formation and repair of connective tissue. In animals other than man and the guinea pig it is a direct precursor of l-ascorbic acid, or Vitamin C. However, these facts provide no direct evidence at all that there is any health – or indeed any physiological – effect of ingestion of the compound. The heyday of research into the role of glucuronolactone in human metabolism was in the 1940s and 1950s, with no conclusive results. A PubMed search provided no recent citations. As far as any benefit of its addition to Red Bull is concerned, there is no evidence. Therefore, from the “precautionary principle”, i.e. the a priori assumption of possible harm, there must be some concern.
Having considered the three supposedly active ingredients of Red Bull separately, what of physiological studies of Red Bull itself? Greiss et al (1994) and Baum and Weiss (2001) conducted studies to compare the effect of the commercial product to that of preparations missing one or more of the three supposedly active ingredients. Although positive effects of the full mixture on catecholamine production (Greiss et al) and cardiac filling/stroke volume (Baum and Weiss) were shown, Woojae (2003) has criticised the study of Baum and Weiss. Two of her criticisms may be true but still lack force: that the sample size was small, and that the drinks with altered composition were prepared and supplied by the Red Bull Company, rather than in the experimenters’ laboratory. Sample sizes are always small in physiological experiments and the main risk of this is in generating a Type 2 statistical error, i.e. failing to show a real effect that is truly present. A statistically significant result has the same probabilistic force whatever the sample size. A far more devastating criticism was that in one experiment the mean cardiac filling was higher in the placebo-taking group than in the pure caffeine group, showing that the variability in experimental conditions for runs of the experiment was not acceptable. The Greiss et al experiments seem to have been rigorous, and produced statistically significant results, but the actual effect size was extremely small.
Other studies on Red Bull, such as those of Ashford et al (2001) and Warburton et al (2001), compared performance effects of Red Bull with carbonated water, or a similar looking and tasting placebo drink, respectively. These studies were therefore unable to differentiate the effects of the specific ingredients, although Warburton et al could at least control for the effects of a carbohydrate load. Seidl et al (2001) studied event-related waveforms and other neurophysiological and psychological outcomes, showing a significantly beneficial effect from Red Bull in a double-blind crossover trial. But again they had no way to separate the effects of caffeine from the other ingredients, or to show that the combination was superior to caffeine alone.
Conclusion:
Despite the emphasis on taurine as the active ingredient, and the claims that caffeine, taurine and glucuronolactone combine in their effect, there is insufficient evidence to reject the sceptical view that the performance enhancing effects of Red Bull are mediated only by caffeine, psychology and a carbohydrate load.
