Does central heating make us fat?

“If you want to lose weight you should open a window or turn down the heating,” according to The Daily Telegraph. The newspaper says that central heating and double-glazing are helping drive up obesity rates.

The story is based on a scientific article that suggests that indoor temperatures have increased, reducing the calories we use to stay warm and leading us to store the excess energy as body fat. The article says that the “thermal comfort zone” provided by modern living may therefore be a cause of the Western world’s increasing rates of obesity.

While the research paper proposes an interesting and plausible scientific theory, it is not conclusive and does not prove that simply turning down your thermostat will make you thinner. There are many different reasons for obesity, including genetics, poor diet, lack of exercise and general lifestyle. As the authors note, there is a lack of direct evidence to support this theory, which needs to be evaluated by future research.

Where did the story come from?

The study was carried out by researchers from University College London and the University of Cambridge. It was funded by a number of bodies, including the UCL Crucible Centre. The study was published in the peer-reviewed medical journal, Obesity Reviews.

The Daily Telegraph and the Daily Mail have reported the study accurately, but they both imply that the evidence for rising temperatures causing obesity is conclusive, which it is not.

What kind of research was this?

This was a narrative review discussing the evidence of a possible link between rises in indoor temperatures and increases in obesity rates. A narrative review discusses and summarises selected literature to give a general overview on a particular topic or theory. It differs from a systematic review, which follows a strict protocol to identify and appraise relevant studies. Narrative reviews are more suited to topics of a discursive nature rather than, for example, quantifying the effects of treatment.

A sample of studies have been included in this review to provide evidence supporting the authors’ discussion, but the methods by which these studies were identified and selected for inclusion are not given.

The authors point out that obesity is a growing global health problem. While diet and more sedentary lifestyles are undoubtedly the major causes, other factors may also contribute. The authors present a hypothesis that exposure to seasonal cold weather increases the body’s capacity for thermogenesis, the process of burning energy gained from food to produce heat and regulate the body’s temperature.

The authors say that access to central heating, cheap fuel prices and spending an increased amount of time indoors all place modern humans under less “mild thermal stress”, which may therefore lead us to burn less food energy to stay warm and store this excess energy as additional body fat.

What did the research involve?

The authors reference 72 articles in their narrative review, which provides broad overviews on trends in indoor winter temperatures, human responses to cold, human energy expenditure in response to mild cold and the role of brown adipose tissue in creating heat. Brown adipose tissue is one of two types of fat tissue in the human body, and is believed to play a role in generating body heat.

The narrative does not report on how the authors searched for relevant studies or how they decided which studies were relevant to include, so it is uncertain if there are other studies in this field that contradict their hypothesis. The researchers also provide detail on only seven of the included studies, all of which are non-randomised, experimental studies examining energy expenditure at varying ambient temperatures. These studies had very small sample sizes, ranging from eight participants in the smallest group to 20 in the largest.

What were the basic results?

The narrative is detailed, and the following is an overview of the main discussion points:

Trends in indoor winter temperatures
The authors cite evidence to show:

a trend in affluent populations to heat their homes to temperatures closer to the lower limit of the “human thermoneutral zone” (TNZ). The TNZ is defined as the range of temperatures at which the metabolic rate (and therefore energy expenditure) is minimal (25C-27C for a naked adult human).
that widespread uptake in central heating and air conditioning have led to expectations of “thermal monotony” and rises in living-room temperatures have been accompanied by rises in bedroom and hallway temperatures, which were previously maintained at cooler temperatures
that workplace temperatures are also thought to be increasing
that reduced seasonal cold exposure is exacerbated by reductions in walking and cycling in favour of temperature-controlled cars

Human responses to cold

Humans exposed to cold maintain body temperature and conserve heat through different modes of thermogenesis (heat production).
There is “indirect evidence” to show that thermogenesis plays a significant role in energy balance. A small number of studies have indicated that effects of mild cold can increase human energy expenditure, with one study suggesting that the energy expenditure of being exposed to mild cold for 10% of the time could be equivalent to an 8kg difference in body weight over 10 years.
Studies indicate that this reduced exposure to seasonal cold may minimise the need for thermogenesis, thereby reducing energy expenditure.

Role of brown adipose tissue (BAT)

The main role of BAT (also called brown fat) is to generate body heat, usually in newborns who cannot shiver to keep warm, although the authors cite studies to show it also has a role in adult heat production. They say that studies show that BAT is activated in conditions of mild cold and that it increases energy expenditure. They propose that increased time spent in “thermal comfort” can lead to a loss of BAT and discuss a link between reduced exposure to cold temperatures and reduced BAT activity.

The authors also point out that the reduced energy expenditure needed in higher temperatures should be counterbalanced by a reduced food intake, although for adults in Western countries the constant availability of highly palatable food may override this adjustment.

How did the researchers interpret the results?

The researchers conclude that increased time spent in conditions of “thermal comfort” leads to a reduced need for thermogenesis, a process that expends energy gained from food through various biological pathways. Therefore, this loss of energy expenditure could be one cause of increased obesity.

Conclusion

The theory that the trend for higher temperatures at home and work is leading to reduced energy expenditure and increased obesity rates is an interesting one, but as the authors of this review point out, further study is required to prove whether or not this is actually the case. In particular, there is a need for studies which specifically look at the individual’s exposure to cold and how it impacts upon their energy expenditure and body weight.

Also, as the authors note, there are significant gaps in understanding the biological pathways which might be involved in how time spent in warmer temperatures may reduce energy expenditure. Crucially, there have been no direct studies of the effects of varying temperature on energy balance or long-term body weight in humans. As the authors say, designing such a study would present considerable challenges.

Regarding this particular review, although various studies have been included to provide evidence supporting the authors’ discussion, the methods by which these studies were identified and selected for inclusion are not given. It may be the case that other studies not included may contradict their hypothesis.

Overall, this review is not conclusive and does not prove that simply opening a window or turning down the thermostat will make people thinner. An alternative explanation could be that people put on winter weight because they stay indoors to keep warm and therefore do less exercise.

There are many different reasons for obesity in any individual, including genetics, poor diet, lack of exercise and general lifestyle. While a consistently ambient temperature may or may not affect weight gain, a balanced diet and regular exercise still remain the best ways to maintain a healthy weight.