Students are shunning toast and cereal for breakfast in favour of fried chicken, and chips, pies and fizzy drinks - a habit nutritional experts say is as dangerous as smoking or alcoholism.

Despite banning junk food from tuckshops, schools appear powerless to stop students buying breakfast on their way to school from dairies, takeaway bars and fast-food outlets like McDonalds.

And it seems few parents - especially busy working parents - have any idea what their children are eating.

A Herald on Sunday investigation found takeaway breakfasts washed down with two-litre bottles of soft drink were the norm for students at six Auckland secondary schools.

De La Salle College students eat about 6kg of fried chicken drumsticks every morning, Rangitoto College pupils prefer energy drinks and McDonald's, and Avondale College students buy 1600 pies and 1200 cans of soft drink from one dairy in the area every week.

The Herald on Sunday also found students from Auckland Grammar and Epsom Girls Grammar tucking into McDonald's, pastries and 1.5 litre fizzy drinks at 8am.

Jim Mann, professor of human nutrition and medicine at Otago University, called the trend an "epidemic".

He said if the Ministry of Health's plans to educate the public about healthy eating did not work, it should press for legislation to ban junk food advertising aimed at children, he said.

A new University of Auckland study showed that on average, students who ate takeaway breakfasts were at least 2kg heavier than schoolmates who ate breakfast at home.

Those who drank at least one soft drink a day were also 2kg heavier than their healthier peers.

Nutrition expert Sarah Ley said the fat-laden breakfasts meant students risked diabetes, heart disease and strokes.

A chicken and chips breakfast would provide double the calories and perhaps three times the fat of a typical breakfast, said dietitian Victoria Nealie.

"Obviously, they are also missing out in terms of sufficient fibre ... the vitamins and minerals and calcium and things like that."

But students interviewed by the Herald on Sunday said they did not like cereal and had no time for breakfast at home.

"It tastes good, we don't think about our health, we just go for taste," said 16 year-old St Peter's College student Orlando Brown-Esera. He and his friends ate at the Newmarket McDonald's every morning. Duty manager Shuyi Zeng said up to 80 children - some of primary school age - bought breakfast there every day.