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.
“Since the 1960s, a cultural shift in norms of thermal comfort and expectations of ‘thermal monotony’ have been driven by the widespread uptake of central heating and air conditioning,” Fiona Johnson, of University College London, and colleagues wrote online in Obesity Reviews.
This reduced exposure to indoor cold may have minimized the need for higher energy expenditure for the body to stay warm, Johnson and co-authors suggested.
They noted that in England, average living room temperatures rose from about 65°F in 1978 to just over 66°F) in 2008 — and bedroom temperatures increased from about 60°F to 65°F.
And while living room temperatures in American households remained pretty stable at around 70°F between 1987 and 2005, the average U.S. bedroom temperature increased from 67°F to more than 68°F.
The significance of this global warming trend is its effect on human metabolism — human metabolism slows as the temperature goes up, so that the metabolic rate is lowest when the temperature hits 77 to 80°F.
When the temperature drops, humans keep warm in a number of ways — vasoconstriction and lowering of skin temperatures, by skeletal muscle fiber contraction leading to shivering, and through adaptive metabolic thermogenic responses generating heat in tissues.
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 72°F to 61°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.
The authors reported no conflicts of interest.