WASHINGTON (Reuters) – Can't lose weight on a low-fat diet? Maybe you need to cut carbs instead, and a new genetic test may point the way, maker Interleukin Genetics Inc reported last week.

The small study of about 140 overweight or obese women showed that those on diets "appropriate" for their genetic makeup lost more weight than those on less appropriate diets, researchers told an American Heart Association meeting.

"The potential of using genetic information to achieve this magnitude of weight loss without pharmaceutical intervention would be important in helping to solve the pervasive problem of excessive weight in our society," Christopher Gardner at Stanford University in California, who worked on the study, said in a statement.

Massachusetts-based Interleukin's $149 test looks for mutations in three genes, known as FABP2, PPARG and ADRB2.

The company says 39 percent of white Americans have the low-fat genotype, 45 percent have the type that responds best to a diet low in processed carbohydrates and an unlucky 16 percent have gene mutations that mean they have to watch both fat and processed carbohydrates.

The researchers randomly assigned around 140 women to one of four diets – the low-carb Atkins diet, the ultra low-fat Ornish diet, the very low-fat LEARN diet or the more balanced Zone diet.

Interleukin went back and tested about 100 of the women for their DNA by using a cheek swab and then looked to see if the women on the "right" diets lost more weight.

Over a year, people on diets appropriate to their genetic makeup, as determined by the test, lost 5.3 percent of body weight. People on mismatched diets lost 2.3 percent, the Stanford researchers told the meeting.

Cholesterol levels improved in line with weight loss, they said.

The company said the test looks for genes that affect metabolism.

"One of the gene variations affects absorption of fats from the intestine," Ken Kornman, chief scientific officer at Interleukin, said in a telephone interview.

He said people with that particular mutation absorb more fat from their food and thus should avoid fat if they want to lose weight.

Another of the variations affects insulin response – the body's production of insulin to metabolise sugar, he said. Simple carbohydrates such as sugar and processed flour stimulate people with that particular gene type to store more of the energy as fat.