Can overweight men pass obesity risk to their children?

"Are you fat because of your dad?" is the Mail Online's bold question to its readers, explaining that "Men's weight directly affects genes in sperm linked to appetite and brain development".

This was based on a new study that found a man's weight influences the genes in his sperm.

This small study showed that DNA in the sperm of obese men differed from that of healthy weight men. The DNA itself wasn't changed, but modifications affecting how it's used by the body were.

These differences raised the possibility that children of overweight men could inherit genetic traits that make them more likely to be overweight themselves.

The researchers then looked at the sperm in six men before and after weight loss surgery, and found that the genes in their sperm changed considerably as they rapidly lost weight. This suggests that losing weight may be able to reverse the genetic changes associated with being overweight.

The results of this study are interesting, but should be treated with caution. The study only compared 13 healthy weight men with 10 overweight or obese men in the first part of the study, and just six men having weight loss surgery. These are very small numbers.

Similarly, we don't yet know whether the genetic changes identified in the overweight men will make their children more or less likely to be obese themselves, as this was not investigated in the study.

If the results of this research are confirmed in larger groups, the findings could lead to men, like women, being advised to eat healthily when trying to start a family.

Where did the story come from?

The study was carried out by researchers from Universities in Copenhagen and Sweden, and was funded by Novo Nordisk Foundation – Endocrinology Research.

The study was published in the peer-reviewed science journal Cell Metabolism, and is free to read online (PDF, 2.67Mb).

Generally, the media reported the study accurately, but most didn't emphasise the significant limitations, particularly the small number of men involved.

Some headlines also implied that the study had found children of overweight men are more likely to be overweight as a result of inherited genetic changes, but this study didn't look at whether these changes do actually affect the chances of a child becoming obese. More research is needed to explore this.

The Mail Online also reported that the genetic changes may explain "why autism is more common in those whose fathers are very fat", but that wasn't investigated directly in this study. Autism was briefly discussed in the scientific paper, but only in reference to other research.

This study didn't add anything new to any existing research on a potential link between obesity and autism.

What kind of research was this?

This was a small human study looking at whether the genetics of men's sperm differed, depending on their weight.

This study was exploratory and used just a small group of men. This is useful to investigate a new theory or generate ideas, but cannot provide reliable proof. Much larger studies are needed to confirm or refute the initial results.

What did the research involve?

Researchers compared inheritable genes in the sperm of 23 white men aged 20 to 40 – 13 who were a healthy weight (with a BMI of 20-25) and 10 who were overweight or obese (with a BMI of more than 29.7). Single sperm samples were taken from all 23 men.

Separately, three sperm samples were collected from six men undergoing weight loss surgery (average BMI 42.6). They had samples taken a week before the operation, one week after, and a final sample a year after the operation.

The study looked at differences in three aspects to DNA known to alter how the DNA is used by cells (gene expression):

• the way DNA is folded and packaged in a cell
• small pieces of genetic material called small non-coding RNA (sncRNA)
• chemical groups fused to the DNA – called DNA methylation

The main analysis was split in two. The first part looked at genetic changes between obese and healthy weight men, while the second looked at changes before and after weight loss surgery in a different group of men.

What were the basic results?

The main results showed differences in sncRNA regulation and DNA methylation in the sperm of obese and healthy weight men, but no significant differences in the way DNA was folded and packaged.

The differences related to genes were thought to be involved in brain function.

For men having weight loss surgery, the results showed a significant number of changes in the sperm DNA. For example, a week after surgery there were about 1,500 changes in DNA methylation, which rose to almost 4,000 after a year. The most extensive changes occurred at genetic locations implicated in appetite control.

How did the researchers interpret the results?

The researchers said their data provide evidence that the genetic signature of sperm can quickly change as a result of environmental factors such as weight loss, and offers insight into how obesity may be passed on to the next generation.

Conclusion

This small study showed there were differences in sperm DNA in obese men compared with men of a healthy weight – and some of the differences related to brain function.

Interestingly, rapid weight loss also led to changes in a specific type of DNA change – called methylation – in a sample of six men before and after weight loss surgery. The genes affected appeared to be related to appetite control.

These findings strongly suggest a man’s weight leads to changes in his sperm DNA. The implication is that these might be passed on to his children, increasing their chance of being obese themselves.  

We know that children of obese parents are more likely to be obese, but the extent to which genetic and lifestyle factors influence this isn't clear. Despite some media headlines, this study doesn't prove that obesity risk is inherited, as this wasn't investigated by the researchers. However, it does give researchers a better idea of some of the specific DNA changes to investigate in the future.

The Mail Online reported that the changes in sperm DNA and their potential effect on brain development may explain "why autism is more common in those whose fathers are very fat", but this wasn't investigated directly in this study. Autism was only mentioned briefly as a discussion point, in reference to other research.

Overall, the results of this research are fascinating, but we need to be cautious. It's not possible to say whether findings in less than 30 people affect all men. Studying larger groups of men will indicate if these results are typical.