Train carb-free to boost your body’s fat-burning ability

Training without carbs will boost your fat metabolism and ultimately enable you to go harder, longer. James Witts reports on how to fast to get fast...

Train carb-free to boost your fat metabolism

One of the most popular nutritional trends of recent times has been zero-calorie drinks. Companies like High5 and Powerade, known for creating potions packed with sugar, now offer carb-free electrolyte versions. So it’s out with the glucose and in with a gulp of performance-boosting minerals like sodium, magnesium and potassium. But why?


“It’s simple – zero-calorie sessions boost your ability to burn fat,” says Torbjørn Sindballe, who finished third at Ironman Hawaii 2007 and then revealed many of his sessions were fuelled on nothing more than water.

On a basic level the physiology is straightforward. As triathletes we’re looking to increase fat metabolism – the amount of energy we derive from fat – because this enables us to burn more fat when carbohydrate stores are limited, which in an event like Ironman is often.

Dr James Morton of Liverpool John Moores University, an expert on the matter, delves a little deeper: “The main adaptation to a zero-calorie or water-only session is enhanced mitochondrial volume in your muscles. That means all the enzymes and sites of aerobic metabolism are up-regulated to a greater extent.”

The phenomenon is known as mitochondrial biogenesis and, as a result of these changes, you become more efficient at using fat for fuel at a given exercise intensity, which means you produce less lactate – and less fatiguing hydrogen ions – so conserving glycogen for the harder parts of the race.

Intensity limitations

The key word is ‘intensity’. How hard you can swim, bike or run is affected by muscle and liver glycogen levels. If they’re low, so too will be your blood glucose levels, which’ll make the session feel harder. That’s why it’s appropriate to pencil in water-only efforts during the off-season when you’re looking to increase aerobic capacity and fat-burning capabilities by training at a moderate intensity.

How many times a week, which discipline and for how long is a matter for each individual. “Many elites will ride for three or four hours, though may top up with a bit of protein,” says Dr Morton. “Then you look at beginners who might go out for less than 45 minutes and they’ll have enough fuel in their bodies without needing to top up.”

Sindballe and coach Tom Bennett offer their ideas on integrating water-only sessions, but key is that this isn’t the sole domain of the elite. “Having worked with many pro triathletes, it’s fair to say they were often already close to optimised levels of fat oxidation,” explains Bennett. “That’s not the same for age-groupers, so these sessions could result in significant performance gains.”

Though Bennett throws in a caveat: “If your day job requires mental effort, being carb-deficient can impact on clarity and you may underperform at work.” Low glycogen levels can also impair immunity, so bear that in mind if you feel a sniffle coming on.

Mouth rinsing

Of course, necessity will have meant that many of you have trained in this state for years, squeezing in a session before breakfast and work. So nothing new with water-only sessions for you… but what about mouth-rinsing carbohydrate?

This is an area of increasing interest for researchers and involves swilling a carb solution around your mouth before spitting it out. Brain imaging has revealed oral sensors tell the brain that carbs are on the way, which stimulates the central nervous system to have the body work harder.

You might ask, if mouth-rinsing is stimulating the same neural pathways as carbohydrate ingestion, why not simply swallow? Well, firstly it’s been shown that rinsing a carb-only solution for 5-10secs improves high-intensity endurance performance lasting 30-70mins.

If you’re running at, say, 85% max, your body can struggle to absorb and assimilate sugar, causing gastro-distress. But not if you spit. It’s also a potential strategy for athletes experiencing stomach discomfort in an Ironman, for instance, and temporarily finding it hard to keep fluid down. It’s a useful tool for weight management too.

That’s not all. Topping up that carb rinse with a shot of caffeine could increase the benefits further. “We’ve just finished a study where we’ve seen exercise capacity increase with mouth rinse plus caffeine,” says Dr Morton. “We had people running 30mins longer!”

If you do struggle with water-only and fasted sessions, you can always consume a no-carb protein-only breakfast such as scrambled eggs, and a small amount of caffeine (about 150mg) as this will reduce the risk of your fasting affecting your exercise intensity.

Fasted sessions can be a vital weapon in your off-season armoury before intensity – and carb intake – rises as race season approaches. But, as Bennett concludes, this should be one strand to your nutritional strategy.

“It shouldn’t be ‘fat versus carbohydrate’ as the body needs both during endurance exercise. Carbohydrate energy rates are capped (independent of athlete weight), so greater fat oxidation helps minimise calorie deficit. But ultimately, when exercise is hard, you need carbohydrates.”

Double training days

Tom Bennett, head coach at T2 Coaching, trains athletes of the calibre of Tamsin Lewis, pictured above. Here’s how they – and you – can implement water-only sessions when training twice a day…

Training low can be applied a number of ways. The least taxing is what we call a riser session and is the first workout of the day, fuelled on water or a caffeinated drink after an overnight fast. If athletes have a moderate carbohydrate meal post-session, glycogen stores are sufficiently replenished for a second workout later in the day.

“An alternative method is to perform a high-intensity session in the afternoon/evening followed by a low-carb meal after the workout, before a low- to moderate-intensity workout prior to breakfast the next day. Athletes typically report that this workout is at a higher rate of perceived exertion [RPE] for a given speed than when carb-fuelled, with many suggesting this protocol feels like the back end of the Ironman marathon. This morning workout is followed by a high-carb meal to fuel the rest of the day.

“Of note is that we don’t use fasted sessions in the pool, preferring to do most of our fasted exercise on the bike because training load is less.”


Images: Delly Carr