If you are trying to lose weight, knowing what fuel your body is burning is largely guesswork. Bathroom scales may take weeks to register any change, and even then, they can’t distinguish between loss of fat, muscle and water. Smartwatches and gym equipment can also only infer whether you are in a “fat-burning zone” from measures such as heart rate.
But now researchers have developed a handheld breathalyser that detects whether you are burning fat from a single exhalation.
Read moreThe vital, overlooked role of body fat in shaping your health and mind
When carbohydrate supplies run low, the body increasingly relies on fat for energy, converting some of it into molecules called ketones. This process also produces a molecule called acetone, which passes into breath.
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Although breath acetone has long been used as a window into metabolism, measuring it has typically required bulky laboratory equipment. Blood-ketone tests are also widely used, but require repeated finger pricks.
In recent years, several companies have developed breathalyser-style devices that detect acetone, but accurately measuring the tiny amounts in exhaled breath is challenging. This is because humidity, variations in breathing technique and chemically similar compounds, including alcohol, can interfere with the sensors.
To overcome these problems, Andreas Güntner at ETH Zurich in Switzerland and his colleagues have combined an acetone-specific sensor with miniature filters that remove moisture and other interfering compounds. An accompanying smartphone app also guides users on the correct breathing technique to capture acetone at the end of the third breath, when it most closely resembles levels in the bloodstream.
To test the device, which is commercially available under the name Nutrion, the researchers compared its readings with laboratory measurements of breath-acetone and blood-ketone levels in 12 volunteers, who had no known health problems, under a range of dietary and exercise conditions.
They found that the device closely tracked both laboratory breath-acetone measurements and blood-ketone levels.
The researchers – some of whom are shareholders or on the board of Alivion, a spin-off of ETH Zurich that makes Nutrion – also found that it was sensitive enough to detect subtle short-term fluctuations in metabolism, which is important because the body continually switches between fuels during everyday activities.
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“We were able to resolve even the finest changes in breath as someone started to burn fat,” says Güntner, who is an advisor to Alivion. “After more intensive exercise, we saw breath acetone increase, but if someone then drank a sugary drink or ate a carbohydrate-rich meal, we could see it immediately fall again.”
Such information could be useful when dieting or intermittently fasting, when the goal is to extend the periods during which the body burns fat. But it could also have applications beyond weight loss. Ketogenic diets – high-fat, very low-carbohydrate diets that trigger ketone production – are used to treat some forms of drug-resistant epilepsy, particularly in children. But monitoring ketosis usually involves repeated finger-prick blood tests. An ongoing trial is evaluating Nutrion as an alternative.
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“It is really good to hear that they have moved into clinical trials in epilepsy, which would take some logistical strain off home testing for blood-ketone monitoring and potentially make management of epilepsy easier going forward, while still providing objective data,” says Dirk Dugdale-Duwell at Elevate Performance Testing in Wymondham, UK, which provides personalised testing to athletes.
The effects of Nutrion are also being investigated alongside intermittent fasting, GLP-1 weight-loss therapy and ketogenic diets during cancer treatment, says Güntner. “There are many situations where monitoring ketone production is important, and it would be useful to have this information immediately available.”
Dugdale-Duwell – who has investigated the potential of a similar device called Lumen that measures metabolism via exhaled carbon dioxide – adds that athletes might benefit if Nutrion enables them to tailor their carbohydrate intake to their metabolic responses. But it remains unclear whether this would improve performance. “I wouldn’t be surprised to see the device used from a marginal-gains perspective,” he says.
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