Spanish Study Links Air Pollution to Body Fat and Muscle Loss

Most people assume that body fat and muscle are shaped by everyday habits. Eat sensibly, keep moving, and the figures from a body scan should generally follow. That is the view held by many people and clinicians.

But researchers who followed older adults in five Spanish cities identified an additional factor that had not previously been assessed: air pollution.

An existing Spanish trial

The results were produced by a group led by Ariadna Curto, an environmental epidemiologist at the Health Research Institute of the Balearic Islands (IdISBa). Rather than launching a new study, Curto and her team used an existing dataset.

They added air-pollution information to PREDIMED-Plus, a Spanish trial initially designed to examine whether a Mediterranean diet and exercise programme could help slow cardiovascular disease in older adults.

The study involved 1,454 participants aged 55 to 75 who were overweight and had metabolic syndrome, which combines high blood sugar, high blood pressure and unhealthy cholesterol levels.

Five cities under watch

Participants were based in five Spanish cities: Pamplona, León, Palma, Reus and Barcelona. Of these, Barcelona had the most polluted air.

The team calculated pollution exposure at every participant’s home address using three widespread urban pollutants: PM2.5 fine particles, nitrogen dioxide and black carbon. Diesel engines are responsible for most black carbon emissions.

The researchers also took an approach often absent from pollution studies. Rather than relying on weighing scales, they invited participants for DXA scans, the imaging technique also used to assess bone density.

Scans were carried out at the start of the study, after one year and after three years. They distinguish fat from lean tissue and indicate where these tissues are located in the body.

More fat, less muscle

At the one-year and three-year assessments, more polluted air was associated with measurable physical differences.

Those exposed to higher pollution levels had a greater proportion of total body fat and less lean mass: the bone, organ and muscle tissue that helps maintain strength as people age.

Previous research had suggested a similar pattern. One study of almost 1,700 women connected PM2.5 and nitrogen dioxide with increasing fat mass and declining lean mass over time.

The Spanish findings bring that evidence to older adults who are already overweight and living with metabolic syndrome.

Soot from busy streets

Black carbon was the pollutant with the clearest association. Over three years, people living in areas with higher black carbon levels lost about 1 kilogram of lean tissue, despite having no differences in diet or exercise.

“Living in zones with higher levels was associated with a loss of nearly one kilogram of lean mass over three years,” said Curto.

PM2.5 and nitrogen dioxide showed associations in the same direction, although they were less pronounced. This order fits with existing scientific suspicions about black carbon.

These particles are extremely small and coated in residue from diesel combustion. They can also enter the bloodstream, and animal studies have linked them with inflammation extending far beyond the lungs.

Belly fat under 65

The paper’s most original result concerns what the researchers described as a first. Earlier studies had not examined whether outdoor pollution affects visceral fat, the deep abdominal fat surrounding the liver and intestines.

In contrast with fat directly beneath the skin, visceral fat releases inflammatory signals and repeatedly features in research into the risks of heart disease and diabetes.

“The effect was only evident in participants under 65, which suggests pollutants could favor greater visceral fat accumulation in younger adults, whose adipose tissue still has capacity to grow,” said Dora Romaguera, the study’s senior author.

Fat storage may be more sensitive to pollution while this system is still expanding actively. At later ages, the deep abdominal fat compartment may already have become established.

What scans cannot show

DXA scans reveal the changes that occurred, but not the reason behind them.

Fine particles and traffic-related soot are sufficiently small to reach the bloodstream. Animal research has associated them with inflammation, disturbed blood-sugar control and altered fat storage.

The available data can point towards that sequence in these participants, but cannot demonstrate that it was responsible.

The authors emphasise this limitation. They do not claim that pollution caused the changes in body composition. However, the two changed together over several years, across age, sex, diet, exercise and income, and the association remained after those factors were taken into account.

A specific population

Every one of the 1,454 participants had metabolic syndrome and excess weight, so the findings may not apply in the same way to healthier adults.

Nor was the analysis originally designed specifically to investigate air pollution; the researchers added it to data gathered for a separate cardiovascular trial.

Before this work, nobody had measured changes in deep abdominal fat among older adults living ordinary lives in several cities.

Over three years, the Spanish team documented those changes in real people, recording them in kilograms and percentages.

A separate study involving almost four million US veterans had already associated PM2.5 with weight gain. This research adds information about where that weight is stored and the muscle loss that accompanies it.

Where this could lead

For doctors, the findings introduce another consideration when patients with metabolic syndrome follow dietary and exercise guidance but continue to lose muscle.

For urban planners, they expand the health implications of pollution beyond asthma and lung disease.

The pattern among people under 65 also presents researchers with a more focused question: why polluted air seems to alter some bodies while others, often a decade older, appear to be less affected.