Fast food diets, youngsters be warned, can cost you your health in later life.
Nutritionists strongly advise that an appetite for junk food is feeding a "ticking timebomb" of disease and ill health.
They found teenagers of both sexes were among the biggest guzzlers of salt, alcohol and sugar-laden soft drinks. At the same time, they shun fruit, vegetables and oily fish.
According to experts, millions of teenagers are dangerously low in key vitamins and minerals - with girls faring worst.
Carrie Ruxton, an independent nutritionist, and Emma Derbyshire, a nutritionist at Manchester Metropolitan University, analysed the results of 38 studies into diets and their consequences for health.
"While things like heart disease and cancer affect people in their 40s, 50s and 60s, the very early stages happen several decades before," the Daily Mail quoted Dr Ruxton as saying.
"It may be boring thinking about your health when you are 14, 15, or 16, but it is really important to lay down the balanced diet you are going to follow of the rest of your life.'
The study was published in the journal Complete Nutrition.