Winter sex produces skinnier babies who never have to worry about their weight, new research finds
Winter sex is more likely to produce naturally skinny individuals who never have to worry about their weight, a study suggests.
On the other hand, babies conceived in the heat of summer may face a life-long struggle to stay slim.
Scientists have linked cold weather conception to larger amounts of calorie-burning brown fat in both mice and humans.
Only small amounts of brown fat are found in human adults, mainly under the tongue, around the collar bone and along the spine.
But people who have more than their fair share of the precious tissue can count themselves lucky.
Unlike white fat, which contributes to heart disease, cancer and diabetes, brown fat is widely seen as beneficial. It burns up calories, generating heat, and reduces the risk of being overweight or developing metabolic disorders.
Swiss scientists examined CT (computed tomography) scans of 8,400 adults, comparing those conceived at colder times of year and born between July and November and others born between January and June.
They found that the first group had significantly more active brown fat than the second.
Tests on mice encouraged to mate in hot or cool conditions confirmed the finding and showed that the temperature difference only affected sexually active males.
Offspring of male mice kept in a cool 8C environment for several days prior to mating had larger amounts of brown fat. They were also better protected against weight gain and metabolic disorders when fed a high fat diet.
Previous studies have suggested that people living in colder regions of the world tend to have higher levels of brown fat.
Professor Christian Wolfrum, from ETH Zurich University, said: "Until now, the assumption was that this had something to do with the temperatures people experienced during their lifetime. But our observations suggest that temperatures prior to conception might also affect later levels of brown fat."
More work is needed before advice based on the research can be offered to couples, say the scientists.
The study, reported in the journal Nature Medicine, showed that temperature affected chemical changes to sperm DNA that could be inherited, a process known as "epigenetic programming".
Prof Wolfrum added: "We need to study the correlation in people more closely. But it is likely that the exposure to cold needs to persist over a longer period for it to have an effect on epigenetic programming.
"Taking a plunge in cold water or spending a short time lying on a block of ice probably won't be enough."
He pointed out that average indoor temperatures in the US had increased in recent decades. Studies had also shown that the temperature people experienced at home was linked to how much they weighed.
"Our work highlights a possible mechanism for this," said the professor.
His team is now planning a new study comparing the epigenetic programming of human sperm in summer and winter.