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In recent years, ‘zero-carbohydrate’ drinks have been steadily gaining in popularity among endurance sportsmen and women. But as Andrew Hamilton explains, considerable care is needed to avoid the potential pitfalls they can bring


The idea of using zero-carb drinks in training instead of normal carbohydrate drinks comes from a theory known as ‘train low, race high’. This theory says that in the Late Palaeolithic era (when our ancestors roamed the plains as hunter-gatherers i.e. before farming), humans would have been strongly influenced by the need to ensure survival during periods of famine, with certain genes evolving to regulate efficient intake and utilisation of fat stores – so-called thrifty genes. These genes would have enabled our forebears to burn fat for energy more efficiently, and better endure famine conditions.

Today our genetic makeup is virtually identical to that of our ancient forebears, and it’s theorised that we too can capitalise on our thrifty genes to enhance endurance performance. It seems that by training when your muscles have low levels of stored carbohydrate (glycogen) and not consuming carbohydrate during training, you can boost the activity of the thrifty genes.

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Following the theory through to its logical conclusion, by not consuming carbohydrate during your longer training sessions, you should be able to coax the exercising muscles over time to derive a greater percentage of their energy from fat. This in turn means less stored glycogen in your muscle tissue is used for energy, which means that your reserves of glycogen are depleted less rapidly.

If you then replenish your muscle glycogen fully by consuming plenty of carbohydraterich foods prior to race day and combine this with the use of a normal carbohydrate drink during your race, you should (in theory at least) have the best of both worlds: the ability to derive a large proportion of energy from fat with plenty of muscle carbohydrate on tap too – an ideal combination for maintaining energy levels during a very long race!

What’s in a zero carbohydrate drink?

Zero-carbohydrate drinks typically contain no carbohydrate but do contain electrolyte minerals sodium, potassium, magnesium, calcium and chloride to replace those lost in sweat. Although there’s no clear consensus on what causes muscle cramps, most scientists agree that an electrolyte deficiency can increase the risk of cramping. Zero-carbohydrate drinks may also contain other added ingredients that are claimed to enhance fat burning.

While there’s some supporting evidence for a few of these so-called fat-burning nutrients (e.g. caffeine, green tea), many others lack any solid scientific evidence, and are added more for marketing purposes than anything else.

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Because zero-carbohydrate drinks tend to contain very little other than electrolyte minerals and water, many manufacturers supply them as tablets that are simply added to water, although they can be purchased ready-mixed as well.

Theory and reality

That’s the theory but what about the reality? Well, numerous human and animal studies have shown that curtailing carbohydrate intake before and during exercise does indeed enhance the rate of fat oxidation and the proportion of energy that is derived from fat (thus sparing muscle glycogen reserves).

The problem is that these fat burning adaptations don’t seem to translate into improved performance. For example, a British study examined the effects of restricting carbohydrate drink use during distance running. Three groups of recreationally active men were split into three groups:

  • Group 1 trained twice a day, two days a week, and consumed a 6.4% glucose carbohydrate beverage immediately before every second training session and at regular intervals throughout exercise.
  • Group 2 trained twice a day, two days a week, but consumed no carbohydrate beverage throughout training
  • Group 3 meanwhile trained once a day, four days a week and consumed no carbohydrate beverage throughout training (control group).

The results showed that those in group 2 (who trained in a low-glycogen state during their second run and didn’t consume carbohydrate drinks) had significantly higher post-training levels of key enzymes in aerobic metabolism, indicating that the zero-carbohydrate drink approach to training had induced a greater level of aerobic adaptation – i.e. activated the thrifty genes!

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However, when the researchers looked at actual performance, such as improvements in aerobic capacity and distance covered in the Yo-Yo Intermittent Recovery Test, there were no significant differences between the groups. The researchers concluded that although certain biochemical changes had taken place in the muscles as a result of zero-carbohydrate drink consumption, this did not result in improved running performance.

Other scientists have also questioned the use of carbohydrate restriction during training. For example, in a detailed review of the literature, Professor Louise Burke, a world-leading authority on sports drink use during exercise, looked at all the evidence from previous studies on low glycogen training and/or withholding carbohydrate intake during training sessions.

She concluded: “Despite increasing the muscle adaptive response and reducing the reliance on carbohydrate utilisation during exercise, there is no clear evidence that these strategies enhance exercise performance.”

Zero Carb drawbacks

Although the evidence for zero-carb drink use during training is not exactly overwhelmingly positive, you might be tempted to ‘give it a go anyway’ on the basis that while it might not help much, it can’t possibly do any harm. Unfortunately, this is not necessarily the case because the hazards of low-carbohydrate training are well documented:

  • Increased secretion of stress hormones leading to lowered immunity and increased risk of throat infections.
  • Reduced length of training sessions due to fatigue induced by low glycogen (this hinders your preparation for a long-distance event). n Increased risk of burnout and overtraining.
  • Reduced hydration in hot weather training (some carbohydrate in a drink can help transport fluid from the gut into the body).
  • Increased muscle tissue damage and breakdown, leading to potential losses in muscle mass.
  • Possible strength losses in sports where simultaneous strength and endurance training is required.

Crucially not consuming carbohydrate drinks during longer training sessions can significantly blunt your training performance. If all you’re trying to achieve is to burn more fat, that’s maybe not a problem. But if your goal is for example, to improve your ability to sustain a fast pace, reduced carbohydrate intake during training could be counterproductive. And for very high-intensity efforts such as interval training, any shortfall in muscle glycogen will impair your ability to sustain your training effort.

Remember too that even when you consume carbohydrate drinks, merely performing longer duration training sessions will still teach your muscles how to get dramatically better at burning fat, and this is what will really help your performance for that race or important event!

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