Scientists say they have discovered a gene linked to diabetes and obesity, offering hope for a diagnostic test.
Children as young as five with the gene variant were obese
Faulty versions of the gene ENPP1 disrupt the way the body stores energy and handles sugar by blocking the hormone insulin.
Children with faulty versions were obese at as young as five years old.
The French and UK team told Nature Genetics that spotting the problem early and intervening could save lives.
Experts have already predicted that the UK is facing an obesity and diabetes time bomb, with rates increasing.
The number of people in the UK with diabetes is predicted to reach three million by 2010.
Diabetes and obesity increase the likelihood of potentially fatal diseases such as heart attacks.
While inactivity and poor diets are much to blame for obesity and the metabolic problems that can lead to full-blown diabetes, the authors of the latest study say some people are genetically prone as well.
In these individuals - up to 20% of Caucasians and 50% of black communities - leading a healthier lifestyle from the outset is imperative if they are to avoid problems in later life, say Dr Philippe Froguel and his team.
The researchers from Imperial College London and the Institut Pasteur in Lille looked at French families with a strong history of diabetes and obesity and compared them with families that did not.
When they compared the genes of 1,225 children who were grossly obese or overweight at ages five and 11 with 1,205 normal weight children they found an obvious pattern - many of the obese children possessed culprit versions of ENPP1.
When they looked at the adults in the families, they found a similar link between the ENPP1 variants and obesity, as well as between the gene variants and early warning signs of diabetes.
ENPP1 was also linked to full-blown type 2 diabetes in the adults.
Storing up trouble
Dr Froguel said: "What is extremely worrying is the children who had these mutations developed obesity at an early age - age five to six - and diabetes occurred in middle age."
He said that although the discovery would not lead to a 'magic pill' for curing obesity and type 2 diabetes, it could help in identifying groups and individuals at increased risk.
"If we can identify those at risk at an earlier age, it may be possible to take preventative measures earlier on, and reduce the burden of ill health caused by obesity in later life."
He said that it was technically possible to screen people for the ENPP1 variants. However, he did not think it was the most appropriate route to tackle the problem.
Instead, he said it would be better to introduce public health measures such as encouraging food and drinks manufacturers to lower the sugar and fat contents in food and encouraging families to do more exercise.
The researchers did not assess how much the genes were to blame for weight gain and metabolic problems as opposed to poor diet or lack of exercise.
However, Dr Froguel said those with a family history of diabetes and obesity should be particularly vigilant with regards to lifestyle.
Dr Angela Wilson of Diabetes UK said: "This research is an important step in helping us unravel the genetics of Type 2 diabetes. We will be following its progress with interest."