Warmer Houses May Increase Obesity and Diabetes

In recent decades, residents of affluent Western countries have increasingly been turning up room temperatures in winter -- a practice that may be contributing to the obesity epidemic by turning down the body's own thermostat, researchers suggested.  

Fiona Johnson, MRCPsych, of University College London wrote, "Since the 1960s, a cultural shift in norms of thermal comfort and expectations of 'thermal monotony' has been driven by the widespread uptake of central heating and air conditioning. This reduced exposure to indoor cold may have minimized the need for higher energy expenditure for the body to stay warm." 

They noted that in the U.K., average living room temperatures rose from 18.3°C (64.94°F) in 1978 to 19.1°C (66.38°F) in 2008 -- and bedroom temperatures increased from 15.2°C (59.36°F) to 18.5°C (65.3°F). And while living room temperatures in American households remained stable, at 21.3°C (70.34°F) between 1987 and 2005, the average bedroom temperature increased from 19.3°C (66.74°F) to 20.2°C (68.36°F).  

Johnson and colleagues noted that humans respond to cold through vasoconstriction and lowering of skin temperatures, by skeletal muscle fiber contraction leading to shivering, and through adaptive metabolic thermogenic responses generating heat in tissues. The low end of the thermoneutral zone of temperature for humans, when metabolic rate is lowest, is about 25° to 27°C (77° to 80.6°F). 

In background material provided by Johnson and colleagues, they noted that studies have shown that energy expenditure is inversely associated with the ambient thermal environment. Recent research has also helped to explain how thermogenesis influences energy balance and weight maintenance. 

One area of particular interest has been the role of brown fat, previously thought to have little importance in adult thermogenesis. "Unlike white adipose tissue, which acts predominantly as an energy storage depot, [brown adipose tissue] is a thermogenic organ, dissipating energy in the form of heat," explained Johnson and co-authors. 

When brown adipose tissue is activated, it can be detected using positron emission tomography, and studies have shown that activation -- and the resulting energy expenditure -- is much more likely to occur in cold environments. For instance, Johnson and colleagues noted that in one experiment, decreasing the ambient temperature from 22°C (71.6°F) to 16°C (60.8°F) led to the detection of brown adipose tissue in almost all healthy volunteers. 

The model also suggested that under conditions of full activation of the brown adipose tissue there would be an increased energy expenditure leading to a 4 kg (8.82 lb) weight loss over a year, they explained.

Food consumption also can be influenced by thermal conditions. In both humans and animals, appetite decreases under conditions of warmth, but in humans food intake is much more influenced by the social environment and the palatability of food. Palatability also has been associated with changes in food intake in animals.

Unlike mice fed a typical low-fat laboratory diet, adjusting their intake to the ambient temperature, mice fed high-fat diets do not decrease their food intake at higher temperatures and gain weight.  

"These human and animal experimental models suggest that while intake is somewhat suppressed at higher temperatures, this is unlikely to fully compensate for the reduced energy expenditure of a warm environment particularly where highly palatable foods are available," Johnson and colleagues observed. 

This research argues in favor of a causal link between increased time spent in thermal comfort and weight gain in the population, they said. "Establishing the significance and magnitude of the effects of both short-term and long-term thermal exposures on body weight could lead to the development of novel therapies to address obesity on an individual and a population level," they concluded.

• Explain that some researchers believe that an increase in the average indoor temperatures during winter may be contributing to the obesity epidemic by turning down the body's own thermostat that not only keeps people warm -- but also helps burn fat.
  

Obesity Rev 2011; Johnson F, et al "Could increased time spent in a thermal comfort zone contribute to population increases in obesity?"