Scientists say such a test already works in lab rats, revealing which ones will become obese if given access to the rodent equivalent of limitless hamburgers, potato chips and fried chicken.
Whether such a test will work in people remains to be proven.
However, researchers say they are amazed at how similar the underlying machinery of appetite and weight gain are in rats and people.
"I think something like this could be applied to the human situation," said Dr. Sarah Leibowitz of Rockefeller University in New York. She presented her research Monday at a meeting of the American Association for the Advancement of Science.
Her research involves the link between high-fat food and weight gain.
She and other scientists believe that too much fat in the diet -- probably anything more than 30 percent of the day's intake of calories -- triggers weight gain by prompting the body to store new fat and making it crave still more fat to eat.
While such a system may have helped people survive when food was chronically scarce, it leads to rampant obesity when fat-loaded food and sugary soft drinks are cheap and available everywhere.
"For humans, the overabundance of food is a very recent problem, just in the past few hundred years," said Dr. Joel Elmquist of Beth Israel Deaconess Medical Center in Boston. "The body is designed to store as much energy as possible."
One approach to weight control is helping people know they have a problem before it develops. A test that reveals this, Leibowitz said, "would tell us how much we can splurge. People would like to have an early warning sign."
In rats, this warning sign is the production of triglycerides, fats that circulate in the blood.
Leibowitz raised rats on standard lowfat chow. When they got to be normal-size juveniles, she fed them a single high-fat meal, then measured their triglyceride levels.
Ordinarily, about one-third of run-of-the-mill rats will have a weight problem if given a chance. Leibowitz found that rats whose triglycerides shot up the highest after the high-fat meal were also the most likely to become obese.
The research is part of a larger effort at many labs to sort out the hundreds of genes and chemical signals inside the brain that control appetite and body weight. In particular, Leibowitz is interested in how high-fat food raises triglycerides, which in turn may activate fat-sensitive genes deep within the brain.
The researchers believe that triglycerides, or something that travels with them, turn on genes in the part of the brain called the hypothalamus. These genes promote overeating and fat storage.
The research shows that one high-fat meal is enough to turn on these genes.
The genes work overtime in normal-size rats that are prone to obesity, churning out high levels of peptides after a fatty meal. In particular, the researchers found that animals with high levels of triglycerides are likely to produce appetite-stimulating substances called galanin and orexin.
High triglycerides also interfere with the ability of a hormone called leptin that ordinarily dampens appetite. In females, they also appear to stimulate the production of the sex hormones estrogen and progesterone, which in turn trigger the release of more weight-gain peptides in the brain.