Is there an enemy in your coffee cup?

Coffee and road cycling. A relationship so entwined that for many, the thought of riding without a pre-ride injection of caffeine is simply not possible. Sipping an espresso as we watch our fellow riders roll in, pimped, pumped and ready to ride, is as close as many of us will ever get to looking pro, yet new research offers some startling evidence that for some of us, far from putting a little extra va-va-voom in the tank, a quick coffee is going to seriously undermine our performance. 

For several years now, nutritional scientists have known that our genetic profile, or more specifically, the variant of a specific gene, CYP1A2, determines how quickly we metabolise and clear caffeine from our bodies.

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For around 50% of the population – the lucky ‘fast metabolisers’ in the bunch – caffeine is efficiently cleared from the body and a growing body of evidence suggests that caffeine has a beneficial effect on their cardiovascular health. Roughly 40% of us are ‘medium metabolisers’, for whom the effects of caffeine are negligible. The remainder are ‘slow metabolisers’ and for this small, but significant, proportion, caffeine is about as welcome as an asthma convention in the Team Sky hotel. 

Somewhat surprisingly, it is only recently that a comprehensive study set out to examine the impact of caffeine on the athletic performance of the respective fast, medium and slow metabolisers. The research team, led by Dr Ahmed El-Sohemy, a leading expert in nutritional sciences at the University of Toronto, recruited 101 male cyclists, all of whom undertook a genetic test to establish which variant of the CYP1A2 gene they possessed.


They were then required to cycle three 10km rides in laboratory conditions and prior to the turbo sessions were given either a placebo, a low dose of caffeine (two milligrams per kilogram of body weight, roughly equating to one strong cup of coffee) and a higher dose of caffeine (four milligrams per kilogram of body weight). 

Having taken the low and higher dose respectively, the fast metabolisers recorded speeds averaging 5% and 7% faster, compared to their times after taking the placebo – a performance improvement that marries with many other studies that suggest caffeine enhances the athletic performance of the majority. The medium metabolisers recorded little difference, but for the slow metabolisers the results were staggering; having taken the larger dose of caffeine, the average time recorded was 17% slower. That’s a significant figure and one that even surprised the research team. 

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“We had no idea what to expect with the slow metabolizers,” says Dr El-Sohemy. “At one point, we thought slow metabolizers might benefit more because caffeine was considered to be an ergogenic aid and would, therefore, have a greater effect in those who are not clearing it or eliminating it quickly from their system.”

So why did the slow metabolisers experience such a dramatic loss of form? Dr Sohemy offers this possible explanation: “We believe that the decreased blood flow caused by caffeine occurs for a longer period of time in slow metabolizers, so their muscles get fatigued faster on caffeine than when taking nothing (placebo).”

It’s a plausible theory that marries with an extensive study of 4000 adults undertaken by Dr El-Sohemy in 2006 that suggested coffee consumption was more likely to cause heart attacks in slow caffeine metabolisers. Their livers, the researchers concluded, could not efficiently clear the caffeine in their bloodstream, which caused their blood vessels to constrict, increasing the risk of a heart attack.


For some slow metabolisers the impact of caffeine on athletic performance could be substantially greater. Pity the poor soul who took over 24 minutes to complete 10km on the 4mg/kg dose of caffeine. The same athlete, having taken the placebo, recorded a time of under 16 minutes. “Now, there could be a number of reasons for this, including just having an off day,’ offers Dr El-Sohemy. 

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Well, we’ve all experienced a few of those, but how many of us have ever entertained the idea that the quick shot of coffee before we left home or the flat white at the café stop was the cause of our catastrophic drop in form? Perhaps this new research may prompt some of us to question our choice of beverage before and during a ride.

So here’s the bad news: if you suspect you are a slow metaboliser (tell-tale signs are inability to sleep at night if you drink coffee later in the day, jitters if you drink too much), you may have to consider the unthinkable and swop your espresso for a herbal tea. 

The flip-side, of course, as you watch your fellow riders disappear up the road, is that these new findings provide you with (another) excuse to explain your lacklustre performance: “I’m a slow caffeine metaboliser” you can casually explain, as you sit there, enjoying your post-ride skinny latte.


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