“I only ate 27 grams of fat today.”

“Liar!”

Pretty soon that’s the conversation you’ll be able to have with your nutritionist, trainer, diet counselor, or doctor, say scientists who’ve developed a simple five-minute urine test that accurately reveals how much fat, sugar, fiber, and protein a person has consumed, the journal Lancet Diabetes and Endocrinology reports.

“This test could be the first independent indicator of the quality of a person’s diet, what they’re really eating,” said senior study author Gary Frost, of Imperial College London, citing evidence that shows that, when left to their own devices—say, logging their daily meals or keeping a mental list—about 60% of people misreport what they’ve eaten to some degree.

This is especially true for the overweight and obese, who could particularly benefit from keeping track of their daily macros to ensure healthy weight loss.

In Frost’s experiment, 19 subjects strictly followed four different diets, ranging from very healthy to very unhealthy, for three days while holed up in a London study facility to avoid cheating. Urine samples were collected in the morning, afternoon, and evening, and then analyzed for “metabolites,” compounds created when the body breaks down.

It’s hoped that the test—which needs some further refining but is expected to be available to the public in about two years—will help dieters, people with health problems that could be helped with diet (like heart disease), and, basically, anyone interested in maintaining a healthy lifestyle.