Tuesday May 26 2015
Could noise pollution lead to stress and overeating?
"Living near a main road causes people to gain weight with the risk of obesity," is the slightly dubious claim in The Daily Telegraph. While a Swedish study did find an association between noise pollution and obesity, cause and effect has not been proved.
The study involved more than 5,000 adults. It looked at the traffic noise exposure where participants lived and whether they were obese according to measurements such as their body mass index (BMI) or waist circumference. The researchers also looked at exposure to road, rail and aircraft noise.
Researchers found people with more exposure to traffic noise from any of the sources had greater waist circumferences. The more sources of traffic noise a person was exposed to, the more likely they were to be obese around the waist. However, there was no link between traffic noise exposure and being obese based on BMI measurement.
Because this study measured noise exposure and obesity at around the same time, it's not possible to say whether noise could contribute towards causing obesity. While the researchers did try to take into account factors (confounders) such as people's lifestyles and socioeconomic status, these factors could still be influencing the results.
The link between noise exposure and health outcomes is likely to continue to be studied, but for now a healthy diet and being physically active are the best ways to maintain a healthy weight.
Where did the story come from?
The study was carried out by researchers from the Karolinska Institute in Sweden and other research centres in Sweden and Norway.
It was funded by the Swedish Research Council for Health, Working Life and Welfare, the Swedish Heart and Lung Foundation, Stockholm County Council, the Swedish Research Council, the Swedish Diabetes Association, Novo Nordisk Scandinavia and GlaxoSmithKline.
The study was published in the peer-reviewed journal Occupational and Environmental Medicine on an open-access basis, so it is free to read online or download as a PDF.
The Daily Telegraph, along with the Daily Mirror and the Daily Express, overstates what can be concluded based on the findings of this study. For example, the first sentence in the Telegraph's story states that traffic noise "causes people to gain weight".
We cannot say for certain whether this is the case, or whether people were already obese before they were exposed to road noise. We also can't say that moving to less urban environments would help people lose weight, as the paper suggests.
It also says at one point that: "Living under a flight path doubled the rate of obesity."
To its credit, however, a balanced comment from an expert was included at the end of the article noting that: "It's definitely too soon to be able to blame your increasing waistline on traffic noise!".
Other UK newspapers, such as The Guardian and The Independent, were more reserved, explaining that a causal relationship has not been proven.
What kind of research was this?
This cross-sectional study looked at whether exposure to traffic noise was linked to obesity. Some studies have suggested that this is the case. The suggestion is that this may relate to noise exposure increasing stress hormones such as cortisol, or disrupting sleep.
Other studies have also suggested that traffic noise may be linked to cardiovascular disease, and a link with obesity might be one way this might occur.
But the evidence so far is limited, and studies have not looked at whether the different types of traffic noise (road, rail or aeroplane) show differing associations with obesity.
What did the research involve?
The researchers studied 5,075 adults in suburban and semi-rural areas of Stockholm County. They assessed the participants' exposure to noise from road traffic, railways and aircraft at their homes, and took various measurements of the participants' fatness, such as their weight and waist circumference. They then analysed whether there was a relationship between these factors.
The participants were taking part in the Stockholm Diabetes Prevention Program, which looked at risk factors for type 2 diabetes. About half were selected to participate because of a family history of type 2 diabetes, but none had the condition at the start of the study.
The assessments for the current study took place when participants were followed up between 2002 and 2006, when they were aged between 43 and 66 years. Participants filled out questionnaires on their lifestyles and health, and had a medical examination by trained nurses.
The researchers obtained information on where the participants lived since 1991 from various national sources. They combined this information with maps of road traffic noise exposure from the local regions to assess exposure, and also calculated exposure to railway noise and aircraft noise based on distance from rail lines or Stockholm's Arlanda airport flight paths. The average exposure between 1997 and 2002 for each participant was estimated, taking into account if they moved house.
The researchers analysed whether there were links between the different forms of traffic noise (road, rail or aeroplane) and measures such as BMI, waist circumference and waist to hip ratio. Individuals were considered to have "central obesity" if they had a:
- waist circumference of 88cm or above for women and 102cm or above for men
- waist to hip ratio of 0.85 or above for women and 0.90 or above for men
In their analyses, the researchers took into account confounders such as the participants':
- physical activity
- dietary habits
- self-reported noise sensitivity
- self-reported annoyance with road traffic noise
- road traffic air pollution
- socioeconomic status (based on household income)
What were the basic results?
The researchers found that:
- 62% of participants were exposed to road traffic noise of 45 decibels (dB) or higher – 45dB is just a bit louder than a bird call
- 22% of participants were exposed to airplane traffic noise of 45dB or higher
- 5% of participants were exposed to rail traffic noise of 45dB or higher
- 30% of participants were classified as having no exposure to traffic noise of 45dB or higher
Fewer people were obese based on BMI measurement (19% of men and 17% of women) than based on waist circumference (23% of men and 36% of women) or waist to hip ratio (63% of men and 50% of women).
All forms of traffic noise were linked to waist circumference – every 5dB increase in exposure was associated with a:
- 0.21cm increase in waist circumference for road traffic noise
- 0.46cm increase in waist circumference for rail traffic noise
- 0.99cm increase in waist circumference for aircraft traffic noise
Road and aircraft traffic noise were linked to waist to hip ratio, but rail traffic noise was not. None of the traffic noise sources was linked with BMI.
The odds of having central obesity based on waist circumference and waist to hip ratio was significantly higher in those exposed to any traffic noise source of 45dB or higher, with the odds increasing with the more sources of traffic noise participants were exposed to.
For example, exposure to all three traffic noise sources was associated with almost twice the odds of central obesity based on waist circumference (odds ratio [OR] 1.95, 95% confidence interval [CI] 1.24 to 3.05).
Obesity based on BMI measurement was not significantly associated with any traffic noise source of 45dB or higher.
How did the researchers interpret the results?
The researchers concluded that their results "suggest that traffic noise exposure can increase the risk of central obesity" and that, "combined exposure to different sources of traffic noise may convey a particularly high risk".
This cross-sectional study found a link between traffic noise exposure from cars, railways or aircraft and obesity around the waist (central obesity – having a bigger belly), but not obesity defined by a high BMI (30 or over).
The main limitation of this research is that, as it is cross-sectional, it cannot determine whether the exposure to the traffic noise came before the central obesity. Therefore, we cannot say the traffic noise definitely causes the obesity.
Factors other than traffic noise (confounders) may be contributing to the link seen. The researchers did try to take a number of these factors into account, but their impact may not be removed completely.
For example, where a person lives is likely to be strongly linked to their socioeconomic status, and this is in turn likely to be linked to a range of lifestyle behaviours. Likewise, areas with high levels of noise pollution tend to be located in the poorer parts of towns and cities, and poverty is known to be associated with a higher risk of obesity. Disentangling these factors to identify the exact impact of each is very difficult.
The estimation of traffic noise exposure was based on the person's place of residence, but did not take into account whether they had noise-reducing measures such as double or triple glazing. It also did not assess noise exposure from other sources – for example, at work.
One way that results were expressed (odds ratios) can make it sound like the differences are greater than they are when you look at the groups. Adjusting for other factors does help to remove their effects, but can contribute to this. "Twice the odds" of being obese might not actually translate to twice as many people being obese when you look at the actual numbers.
So, while 33% of women who were exposed to less than 45dB of road traffic noise had central obesity based on their waist circumference, 36% of those experiencing 45-55dB fell into this category and 39% of those experiencing more than 55dB. These are increases, but are not as drastic as the "doubling" figure might suggest.
While the study suggests a link likely to warrant further investigation, we cannot as yet say for certain that noise pollution causes obesity.
You can take other steps to reduce your waist circumference if it is moving into the danger zone (94cm or more for men, 80cm or more for women). The NHS Choices Weight loss plan uses a combination of healthy diet choices and exercise to get your belly back to a healthier size.