Abnormalities in the gene that regulates body weight have a "major" impact on the tendency to obesity, bumping up the body mass index (BMI) score, reveals research in the Journal of Medical Genetics.
But the mutations in the melanocortin 4 receptor (MC4R) gene are rare. Although there are up to 34 mutations, only 2% to 3% of very obese people carry them.
German researchers compared the body weights of 181 relatives from 25 families of extremely obese people carrying the MC4R mutations.
Carriers of the relevant mutations had a significantly higher body mass index (BMI), which is the formula used to calculate appropriate weight for height, than those who did not.
BMI is calculated by dividing weight by height squared and multiplying by 703 (imperial measures) or 10,000 (metric measures). A BMI of 30 indicates clinical obesity.
The impact of the genetic mutations was twice as great for women as for men. In women the mutations accounted for 9.5 kg/ metre2 height in middle aged women and for 4 kg in middle aged men.
While rates of obesity dropped substantially between first and second degree relatives of obese patients without the gene mutations, this was far less obvious among relatives of those who were carriers. This further emphasises the impact of the mutations, say the authors.
"MC4R mutations entail a strong predisposition to obesity," conclude the authors. But they add that the high rate of body fat among the relatives of the carriers suggests that the mutations do not, by themselves, account for obesity.
Other genetic and environmental factors play their part, they say, perhaps illustrated by the fact that thin people can also carry the genetic mutations.
A measure of body weight relative to height. BMI can be used to determine if people are at a healthy weight, overweight, or obese. To figure out BMI, use the following formula:
A body mass index (BMI) of 18.5 up to 25 refers to a healthy weight, a BMI of 25 up to 30 refers to overweight and a BMI of 30 or higher refers to obese.