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鈥檛 distinguish between loss of fat, muscle and water. Smartwatches and gym equipment can also only infer whether you are in a 鈥渇at-burning zone鈥� 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.

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.

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.聽聽

To overcome these problems, 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 鈥� some of whom are shareholders or on the board of Alivion, a spin-off of ETH Zurich that makes Nutrion 鈥� 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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鈥淲e were able to resolve even the finest changes in breath as someone started to burn fat,鈥� says G眉ntner, who is an advisor to Alivion. 鈥淎fter 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.鈥�

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 鈥� high-fat, very low-carbohydrate diets that trigger ketone production 鈥� are used to treat some forms of drug-resistant epilepsy, particularly in children. But monitoring ketosis usually involves repeated finger-prick blood tests.聽An ongoing trial is evaluating Nutrion as an alternative.

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鈥淚t 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,鈥� says at Elevate Performance Testing in Wymondham, UK, which provides personalised testing to athletes.

The effects of Nutrion are also being investigated alongside , GLP-1 weight-loss therapy and ketogenic diets during cancer treatment, says G眉ntner. 鈥淭here are many situations where monitoring ketone production is important, and it would be useful to have this information immediately available.鈥�

Dugdale-Duwell 鈥� who has that measures metabolism via exhaled carbon dioxide 鈥� 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. 鈥淚 wouldn鈥檛 be surprised to see the device used from a marginal-gains perspective,鈥� he says.

A brain structure that seems to influence food consumption could one day be targeted to enhance interventions for weight loss or gain.

Studies have shown that activating neurons in this structure, called the bed nucleus of the stria terminalis (BNST) 鈥� located around the centre of the brain and about in people 鈥� . But it was unknown whether taste influences its activity.

To learn more about its function, at Columbia University in New York and his colleagues first imaged the brains of mice while they drank water flavoured with one of the five basic tastes 鈥� sweet, bitter, sour, salty and umami. The researchers previously linked in a region called the amygdala, and have now pinpointed neurons there that only became activated in response to sweet water.

These neurons then activated others in the BNST, 鈥渆xtended amygdala鈥�. This is the first evidence that this structure receives taste signals, says at the University of Arizona, who wasn鈥檛 involved in the study.

The researchers then wanted to understand whether these activated BNST neurons influence dietary consumption, so they genetically engineered the cells so that they didn鈥檛 activate when mice tasted sweet water. Over 10 minutes, these mice drank substantially less than normal ones, suggesting that activation of BNST neurons enhances consumption of sweet tastes.

But the researchers also found that this artificial activation prompted mice to consume more water of any taste, including whether it was unflavoured, salty or bitter, despite them usually avoiding the latter taste.

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In further experiments, the team found that substantially more BNST neurons were activated by sweet and salty signals in hungry or salt-depleted mice, respectively, compared with those that were fed until they were full or had normal salt levels. This suggests that the BNST integrates signals for hunger and nutrient depletion, in addition to taste, to determine food intake, says Cai.

The findings are highly relevant to people, as our BNST is very similar to that of mice, says Cai. They suggest that developing drugs that activate BNST neurons could potentially help to encourage eating in people with a severe loss of appetite, such as those undergoing cancer treatment, he says.

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However, more than a dozen brain pathways have been linked to food intake, says Cai, and some of these may compensate for any prolonged drug-related changes in BNST activity, so targeting multiple feeding circuits simultaneously will probably be needed, he says.

The study could also help us achieve better results with weight-loss treatments, such as the GLP-1 drug semaglutide. These , so a better understanding of how it alters food consumption could help us gain a clearer picture of how such drugs work and how to make them more effective in people , says at the Max Planck Florida Institute for Neuroscience.

It seems you can lose twice as much weight if you eat a diet based around minimally processed, homemade food, compared with ultra-processed meals and snacks.

Food is generally considered to be ultra-processed if it includes ingredients that are , such as high-fructose corn syrup, or additives that make the product more palatable or appealing, such as flavourings and thickeners.

Many studies have linked eating ultra-processed food to adverse health outcomes, such as and , but these studies have been observational. Ultra-processed food also tends to be high in sugar, salt or fat 鈥� like cookies and microwave meals 鈥� sparking a debate over whether it is simply the ingredients that make ultra-processed food unhealthy or if there is something intrinsically detrimental about the processing itself.

To better understand this in the context of weight loss, at University College London and his colleagues have carried out a trial in which 55 people who were overweight or had obesity were randomised to eat a diet of either ultra-processed or minimally processed foods.

鈥淥bviously everyone imagines pizza, chips and that kind of stuff when they think of ultra-processed foods,鈥� says Dicken. However, the researchers made sure that both diets aligned with the , which encourages a healthy, balanced diet that includes at least five portions of fruit and vegetables a day and several sources of protein, such as beans, fish, eggs and meat. The two diets were also matched so they contained roughly the same levels of fat, sugar, salt and carbohydrate.

Food was delivered to the participants, making it the first trial to compare such diets in real-world conditions, rather than in a hospital or lab. With the ultra-processed group, this involved things like breakfast cereals, protein bars, chicken sandwiches and ready-meal lasagnes, but versions that were low in fat and salt. 鈥淭he kind of foods that if you go into a supermarket, they鈥檙e slapped with nutritional health claims,鈥� says Dicken.

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The minimally processed diet featured homemade foods like overnight oats, chicken salad, bread made from scratch and spaghetti bolognese. Both groups were given enough food for about 4000 calories a day and were told to eat as much as they wanted. The researchers set it up so that half of the participants were on one diet for eight weeks, half on the other, and then they switched after a four-week break.

The participants were told the study was investigating the health effects of balanced meals made in different ways, rather than looking for weight loss specifically, but both diets still led to people shedding pounds: the minimally processed food diet resulted in a 2 per cent average reduction in weight, and the ultra-processed diet led to a 1 per cent reduction.

鈥淲e saw more weight loss on the minimally processed diet, and it鈥檚 not just that, we also saw greater fat loss and also a greater reduction in craving,鈥� says Dicken.

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The researchers also looked at other measures of health and found that the minimally processed diet reduced the amount of fat in their bodies and its levels in blood. Perhaps surprisingly, the ultra-processed diet resulted in lower levels of low-density lipoprotein, or 鈥渂ad鈥� cholesterol.

However, at Wageningen University in the Netherlands says the ultra-processed diet was more calorie-dense than the minimally processed one, which could have driven the difference in weight loss. 鈥淭he basic question remains unanswered as to what type of processing or ingredient is driving the observed effects,鈥� he says.

Forde adds that it isn鈥檛 surprising the participants lost weight considering they were overweight or had obesity to start with and then went onto a healthy diet. This may mean the weight-loss results won鈥檛 apply to the wider population.