Staying longer indoors in winter and turning up the heating may be contributing to the rise in obesity in the UK, the US and other developed countries, according to new research led by University College London, which also suggests this growing expectation of “thermal comfort” may be affecting the balance between brown and white fat in the body, thereby reducing its ability to generate heat.

Led by Dr Fiona Johnson, from the department of Epidemiology & Public Health at University College London (UCL), the researchers reviewed the evidence of a possible link between reduced exposure to seasonal cold and rise in obesity rates in the UK and the US.

You can read how they came to their findings in a paper published online in the journal Obesity Reviews on 24 January.

In a press statement, Johnson summarized their view on what they found:

“Increased time spent indoors, widespread access to central heating and air conditioning, and increased expectations of thermal comfort all contribute to restricting the range of temperatures we experience in daily life and reduce the time our bodies spend under mild thermal stress – meaning we’re burning less energy.”

“This could have an impact on energy balance and ultimately have an impact on body weight and obesity,” she added.

Explaining why they did the study, she said there is a greater tendency in research to focus on behavioural aspects like diet and exercise to find drivers of obesity, and less emphasis on looking for possible environmental drivers.

“However, it is possible that other environmental factors, such as winter indoor temperatures, may also have a contributing role,” she added, and said their review shows there is a need to consider new public health strategies with more emphasis on environmental issues for tackling the obesity epidemic.

Johnson and colleagues write in the background information to the study that over the last few decades, indoor temperatures in domestic homes in the UK and the US have been rising steadily, resulting in “greater expectation of thermal comfort”, such that people are spending more time exposed to milder temperatures and less exposed to seasonal cold.

For their review they investigated potential links between this pattern and rising obesity rates, focusing on both the acute and longer term biological effects of spending time in thermal comfort compared to mild cold.

They also explored the idea that exposure to seasonal cold plays an important role in helping to regulate energy balance and body weight in a population.

They found evidence to suggest that reduced exposure to seasonal cold may have a dual effect on the ability to maintain a healthy weight: it may both minimize the body’s need to generate heat and also reduce its capacity to do so.

They refer to experimental studies that show a “graded association between acute mild cold and human energy expenditure over the range of temperatures relevant to indoor heating trends”.

In looking at the biological effects of spending time in thermal comfort as opposed to mild cold, the researchers found studies that suggest more time spent in thermal comfort conditions can result in loss of brown fat and loss of ability to generate body heat.

Unlike white body fat which is principally an energy store, brown adipose tissue or fat plays a key part in helping the body generate its own heat, and its development in the body is thought to be triggered by exposure to cold temperatures.

However, Johnson and colleagues stress there have been no studies looking at the direct link between exposure to cold and obesity in humans, so they recommend that:

“Research in naturalistic and experimental settings is needed to establish effects of changes in thermal exposures on weight, which may raise possibilities for novel public health strategies to address obesity.”

Co-author, Marcella Ucci , of UCL’s Bartlett School of Graduate Studies, said:

“The findings suggest that lower winter temperatures in buildings might contribute to tackling obesity as well as reducing carbon emissions.”

“Could increased time spent in a thermal comfort zone contribute to population increases in obesity?”
F. Johnson, A. Mavroggiani, M. Ucci, A. Vidal-Puig and J. Wardle.
Obesity Reviews, first published online : 24 JAN 2011
DOI: 10.1111/j.1467-789X.2010.00851.x

Additional source: University College London (press release 24 Jan 2011).

Written by: Catharine Paddock, PhD