Prime Minister Christopher Luxon's suggestion of a Marmite sandwich and an apple for lunch wouldn't keep a child full or feed their brain sufficiently, according to local health and education experts.

"If you're just doing a Marmite sandwich and an apple, you're probably not meeting the protein requirements [which depend on age and gender]," paediatric dietician Jenny Douglas explained to RNZ.

"Ideally it would be a Marmite and cheese sandwich at least," she said, noting that the high salt content of Marmite, which could shape one's palate, giving a child a taste for salty snacks like pies and chips early on.

She explained that protein-packed foods like meat, fish, chicken, cheese and yoghurt supply the body with iron, which supports brain development, bone growth and energy. They also keep us full for longer.

"When you're low on iron, you feel tired, you feel low in mood, and children often feel quite sleepy, so, if you're not meeting your iron requirements, then concentration will be affected.

"You're not going to be feeling like you can try and learn something new … you might even be sleeping in class, not even thinking about learning, it's more about just surviving the class.

"Everyone gets a little bit hangry, imagine children are just the same, they're going to be irritable, and you're not going to be able to take on new things, behaviour can be affected … a positive experience at school can be affected."

Luxon made the comment during a Newstalk ZB interview on Tuesday after he was quizzed on the numerous problems with the "revamped" school lunches programme.

"If you are unhappy with it, for God's sake, go make a Marmite sandwich and put an apple in a bag just like you and I had," he said.

Talking to reporters later on Tuesday, Luxon acknowledged not everyone can afford to make lunches but said for many a loaf of bread, some jam, and a piece of fruit isn't "rocket science".

(The cheapest loaf of bread at Countdown comes in at $1.19, you can grab a jar of jam for around $4 while an in-season apple will come in at about 90c.)

But Douglas explained, it's not just the tucker tamariki actually eat at lunch (acknowledging "often fruit and vegetables do just travel to and from school"), supplying a nutritious lunchbox also kicked off healthy habits around food.

"Offering vegetables and fruit is important for installing a lifetime of thought around having those things in the future."

She explained morning tea, lunch and afternoon tea, were opportunities to get 5+ a day in front of children and encourage them to try new foods before the end of the day when they were tired and didn't "want to deal with something that's a little bit different in front of them".

"It's an amazing opportunity to offer social eating for kids to eat alongside other children … It's a really good opportunity for exposure for food, and it's a missed opportunity if you're not."

Education consultant Karen Tui Boyes said one sandwich and an apple was simply not enough lunch for a child, especially teenagers.

With 30+ years as a teacher, Boyes said she'd seen how the food a child eats is essential to learning.

"When you're hungry you just can't learn," she said.

"We absolutely know hungry children can't think, they can't reason, they can't get stuff from their short term, let alone their long-term memory as easily because hunger takes over.

"It [lunch] matters because it helps them concentrate. It helps them focus. It helps them regulate their emotions."

She said a Marmite or jam sandwich and an apple was not enough to keep a child fuelled to focus for a full, six-hour school day, often with added extracurricular activities or homework.

Boyes said a kid's lunch is "the base".

"You can't run your car without fuel … and you've got to put the right petrol in, because if you put diesel in your petrol car, it's not gonna work … it's not going to go to the specs it's designed for."

Luxon, 54, was at school during the late 1970s. Douglas explained we've come a long way in what we know about eating well, so "just like you and I had" is not good enough any more.

"There's plenty of research that we know around what a child's diet needs to look like for them to thrive and when we think of growing, we know what is needed to support adequate growth … that research here wasn't really there 40 years ago," she said.

"Also what we're installing is learning about food and knowing how to eat these foods early, rather than waiting until you're 30-years-old, and finding out that you have a long-term health condition and having to change the way you eat after a lifetime of eating in a way that's not sustainable for health.