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.

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.  

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 – 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 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 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 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.

If you thought body fat was just a passive storage depot for calories, think again. Research increasingly suggests that it plays an important role in our overall health, with two studies shedding new light on its complexity.

Fat exists in several forms. For instance, there’s white fat, which stores energy and releases hormones that influence metabolism; brown fat, which generates heat; and beige fat, which sits somewhere in between, switching on heat production under certain conditions. Even within these categories, location matters: fat under the skin is generally less harmful, while fat deep inside the abdomen – known as visceral fat – is strongly linked to inflammation, type 2 diabetes and heart disease.

The latest research adds further flesh to this picture, suggesting that fat, or adipose tissue, actively helps to regulate blood pressure and coordinate immune responses at key locations.

In one of the studies, at Karolinska University Hospital in Stockholm, Sweden, and her colleagues mapped the cellular architecture of visceral fat from multiple locations within the abdomen. They found that epiploic fat, which wraps around the large intestine, is unusually rich in immune cells, as well as specialised fat cells that produce inflammatory proteins associated with immune activation. Further experiments showed that microbial products originating in the gut trigger these fat cells to activate nearby immune cells.

“Our work shows that fat depots appear to be specialised according to their anatomical location, and those that sit right next to the intestine seem particularly adapted for immune interaction,” says Jalkanen.

Although the study involved people with obesity, Jalkanen suspects that epiploic fat serves similar core functions in people of all body weights, since everyone has some fat surrounding their intestine.

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“The intestine is constantly exposed to nutrients, microbial products and substances coming from our environment,” says Jalkanen. “Having fat tissue nearby that can sense, respond to, and help coordinate immune reactions could provide an additional layer of protection.”

In obesity, however, this system may become chronically overactivated. Eating too much, or too much of certain foods, and having particular bacterial compositions within the gut microbiome could potentially drive persistent immune signalling in intestinal fat, contributing to the low-grade inflammation linked to a range of metabolic conditions, such as type 2 diabetes and obesity.

The second study reveals another unexpected role for fat: controlling blood pressure. at The Rockefeller University in New York and her colleagues set out to understand why obesity, characterised by excess white fat, is linked to high blood pressure, while brown and beige fat appear to be protective.

They focused on perivascular adipose tissue, a fatty layer rich in beige fat calls that surrounds blood vessels. In mice genetically engineered to lose their beige fat, blood vessels became stiffer and overreacted to everyday hormonal signals that constrict arteries, leading to elevated blood pressure.

The team traced this effect to an enzyme called QSOX1, released by dysfunctional fat cells. Blocking it prevented blood vessel damage and normalised blood pressure in mice, regardless of their body weight. “What this nicely shows is that the communication between different organ systems is critical to understand complex diseases such as hypertension and blood pressure regulation,” says Koenen.

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“This study reveals an under-appreciated role for brown or beige fat,” says at The Ohio State University in Columbus. While deposits of perivascular adipose tissue are proportionately smaller in people than they are in mice, they are still probably physiologically relevant in us, she says. “[The study] emphasises a need for nuanced understanding of adipose impacts on health, independent of fat mass or body mass index (BMI) overall.”

The findings point to future therapies that focus less on simply reducing fat and more on preserving or restoring its beneficial functions by targeting specific fat depots, modulating immune-fat communication or maintaining healthy beige fat activity. However, any clinical applications would require further research.

Together, the studies highlight fat as an active, functionally diverse tissue involved in multiple aspects of human physiology. “When I started working in this field in the late 1990s, the prevailing view was that fat was just a simple bag of cells that stored excess nutrients,” says , also at The Rockefeller University, who was involved with the second study. “These studies illustrate a growing shift in the field: recognising fat not as a single cell type, but as a complex tissue with many different types of cells with different roles and diverse processes, extending well beyond just nutrient storage and mobilisation.”

They say timing is everything, and treating cancer may be no exception. Researchers have found that simply shifting when people with cancer receive immunotherapy drugs could improve their survival, adding to evidence that our body’s internal clocks influence how well cancer treatments work.

The activity of our cells and tissues works on 24-hour cycles, known as circadian rhythms, which coordinate everything from hormone release to the timing of cell division and repair. These rhythms are often disrupted in cancer cells, which tend to divide continuously, rather than at set times.

This has prompted efforts to reduce the side effects of chemotherapy, which targets rapidly dividing cells, by administering it when healthy tissues are least active. Increasingly, however, researchers are exploring whether the effectiveness of cancer drugs might also be improved by giving them at particular times.

One such group of drugs is immune checkpoint inhibitors, which help immune T-cells recognise and attack tumours more effectively. “T-cells and other immune defenders are naturally more active in the morning; primed to respond,” says at Ysbyty Gwynedd hospital in Bangor, UK, who wasn’t involved in the study. “Administering immune checkpoint inhibitors during this window may amplify anti-tumour effects and enhance efficacy.”

Earlier this year, at Central South University in Changsha, China, and his colleagues reported that giving the checkpoint inhibitor pembrolizumab alongside chemotherapy to people with advanced non-small cell lung cancer (NSCLC) before 11.30am was associated with seen in those who received most of their treatment in the afternoon.

To investigate whether timing treatments around circadian rhythms – known as chronotherapy – might also benefit people with small cell lung cancer, a faster-growing and more aggressive form of the condition, the same team analysed data from 397 people treated with the checkpoint inhibitors atezolizumab or durvalumab alongside chemotherapy between 2019 and 2023.

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“Compared with patients treated later in the day, those treated before 3pm had significantly longer progression‑free survival and overall survival,” says team member , also at Central South University.

After adjusting for multiple confounding factors, earlier administration was associated with a 52 per cent lower risk of cancer progression and a 63 per cent lower risk of death.

Zhang believes this effect probably exists for other tumour types, pointing to hints from studies of and . As to why this dosing regimen has this effect, the NSCLC trial showed that morning administration boosted circulating T-cell numbers and activation, while late-day dosing had the opposite effect. have also shown that tumour-infiltrating T-cells vary in function over 24 hours, and that the circadian clocks of nearby endothelial cells can regulate when immune cells enter tumours.

Although randomised controlled trials with larger sample sizes are needed, this study “further supports the growing number of reports from all over the world describing better results with early time of day of immunotherapy drugs administration,” says at the University of Warwick, UK.

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But could hospitals realistically implement this? Compared with adding treatments, “adjusting the infusion time is a simple clinical decision that adds almost no cost”, says Zhang.

Still, treating everyone early in the day isn’t practical, says , also at the University of Warwick, and individuals’ internal clocks differ. “The difference in biological time between ‘early birds’ and ‘night owls’, for example, can be many hours.”

But biomarkers to accurately determine people’s chronotypes are being developed. Once tested and validated, chronotherapy “could represent a low-cost, resource-efficient innovation with the power to profoundly improve outcomes: a simple shift in timing that unlocks a new dimension of precision medicine,” says Ismail-Sutton, who recently published on this approach.