Summer conception linked to babies that are fat for life

WINTER sex is more likely to produce naturally skinny individuals who are unlikely to put on excess weight, a study suggests.

On the other hand, babies conceived in the heat of summer may face a lifelong struggle to stay slim.

Scientists have linked cold-weather conception to larger amounts of 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 a higher than average amount of the 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 calories, generating heat, and reduces the risk of being overweight or developing metabolic disorders.

Swiss scientists examined CT 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 at a cool 46F (8C) 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 more brown fat.

Prof 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 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.