Fat men's sperm "could make their kids overweight"
Sins of the fathers really can be passed on to their children as a result of a man's weight affecting the genes in his sperm, research suggests.
The biblical link was discovered when scientists examined the sperm of lean and obese men, as well as those undergoing weight-loss surgery.
A host of "epigenetic" alterations - heritable chemical changes that can switch genes on or off - were found that differed between the three groups.
Their exact effects are unknown, but in many cases they impact on regions of DNA linked to the control of appetite. Scientists believe this could be one explanation for why children of obese fathers are also vulnerable to being overweight.
In the case of six men who were studied before, during, and a year after gastric by-pass surgery to help them lose weight, an average 5,000 changes affecting sperm cell DNA were seen.
Surgery-induced weight loss led to a "dramatic remodelling" of epigenetic influences in sperm, again focused on areas involved in appetite.
Lead scientist Dr Roman Barres, from the University of Copenhagen in Denmark, said: "Our research could lead to changing behaviour, particularly pre-conception behaviour of the father.
"It's common knowledge that when a woman is pregnant she should take care of herself - not drink alcohol, stay away from pollutants, etc - but if the implication of our study holds true, then recommendations should be directed towards men, too."
Epigenetics is increasingly becoming recognised as a mechanism that allows the health effects of environment or lifestyle to be passed on to future generations.
The new research, published in the journal Cell Metabolism, was inspired by a 2005 study showing that food availability in a small Swedish village hit by famine correlated with the risk of grandchildren developing heart disease or diabetes.
The stress of not having enough to eat was thought to have an effect on the activity of health-related genes that could be inherited.
Animal studies have confirmed that the influence of nutrition or exposure to chemicals really can be passed from one generation to the next.
In previous eras when food was often scarce, inheriting genetic factors associated with a big appetite and putting on weight may have aided survival, the scientists believe.
"It's only recently that obesity is not an advantage," said Dr Barres. "Only decades ago, the ability to store energy was an advantage to resist infections and famines."
Epigenetics works in different ways, either by altering the protein that wraps DNA, adding or removing chemical "tags" that change the structure of DNA, or via genetic molecules called small RNAs.
The alterations seen in Dr Barres' study only involved small RNAs and the addition of methyl group "tags". Methylation occurred in genes associated with appetite and brain development. What effect the variations in small RNAs might have is unknown.
The team is now working with a fertility clinic to study epigenetic differences in discarded embryos produced from the sperm of men with varying body weight.
Fertility expert Professor Allan Pacey, from the University of Sheffield, said: "This is an interesting study which provides further evidence to support the theory that some characteristics can be passed by sperm from a father to his children, without altering the basic structure of the genetic code.
"Whilst the study examines a relatively small number of individuals, the fact that such significant differences can be found in the epigenetic markers of lean and obese men is intriguing and, in my opinion, worthy of more detailed investigation.
"In addition, the fact that changes can be seen in men before and after significant weight loss also adds some validity to the findings."