Contrary to one theory on obesity, people with extra body fat may not have a lower body temperature than thinner folks, a new study finds.
Many factors, from super-sized fast-food portions to increasing time in front of computers, have been blamed for the rising rates of obesity worldwide.
But physiological factors also may make some people more vulnerable to becoming obese than others. One theory is that people with a relatively lower core body temperature might be predisposed to weight gain, while those with a slightly higher core temperature pack on pounds less easily.
The idea stems from the fact that the body has to burn calories in order to rid itself of excess heat and return to a desirable internal temperature. A cooler core temperature would mean less heat to shed and require fewer calories to be burned.
"Temperature could be a marker for the 'slow metabolism' that some people think they have," explained senior researcher Dr. Jack A. Yanovski of the U.S. National Institute of Child Health and Human Development.
Animal research, he told Reuters Health, has suggested this could be the case.
Researchers have found, for example, that genetically altered obese mice display a decreased core temperature -- along with a slower than normal metabolism and bigger appetite.
But, Yanovski said, he and his colleagues found no evidence that core body temperature is related to obesity in people.
For their study, reported in the American Journal of Clinical Nutrition, the researchers compared the average core temperature of a group of obese adults with that of thinner men and women.
In one experiment, 46 obese and 35 normal-weight or overweight adults swallowed a wireless, temperature-sensing capsule that continuously monitored their body temperature over 24 hours.
On average, the study found, there was no difference in the two groups' core temperatures — with both around 36.9 degrees Celsius or 98.4 Fahrenheit.
In a second experiment, the researchers used the capsules to measure core temperature in 19 obese and 11 normal-weight people over 2 days, while the participants kept a record of their daily activities.
Again, the two groups were similar — with no clear differences in body-temperature fluctuations throughout the day.
The study is the largest so far to look at core temperature and obesity in humans, Yanovski said. But it's unlikely to be the final word.
There may be certain people for whom a lower core temperature has some effect on weight, Yanovski noted. He and his colleagues say that studies of people with alterations in genes that regulate core temperature could offer more insight into whether body temperature has a role in obesity risk.
However, Yanovski said, "For most obese individuals, it's not that they're just cooler inside."
Studies into core temperature and obesity have been done with practical goals in mind, according to Yanovski. If lower body temperature were found to play a role in body weight, then measures to raise core temperature a bit might help treat obesity.