How much do teenage boys really need to eat?

You can imagine - or recall - the boasting in the school playground.

"How many Weet-Bix can you eat?" one teenage boy asks his mates. "I ate six in one go yesterday."

"I ate eight the other day."

"Well, I ate 11 after rugby training."

"Yeah, right."

Actually, 11 Weet-Bix isn't so crazy for a teenage boy. They total 583 calories, plus some more for milk, and that's only 25 percent of the 2800 calories a 16-year-old teenage boy might need everyday.

Yes, most teenage boys really do need to eat a lot of food - and that amount is significantly more than the energy requirements of a teenage girl.

"The reason that the energy requirements are lower for girls is that they've got smaller bodies essentially. Their weight and height differs from boys, " Professor Carol Wham from the NZ Nutrition Foundation says.

Teenage boys are busy being active with sport and play. They're also growing their skeletons and putting on muscle mass, particularly towards their late teenage years when their height growth starts to slow, according to Rachel Scrivin, a sports dietitian who has three teenage boys aged 15, 16 and 19.

"It's not uncommon for them to come home and have two packets of noodles and a milkshake and still eat dinner," she says.

As a dietitian, she isn't too happy about her kids eating a lot of instant noodles, but it's a snack that is balanced with wholefoods elsewhere.

Calorie needs: Teen boys vs teen girls

While not everyone will identify with the binary of boy or girl, this is the dividing line that scientific research makes, even though ultimately everyone will have unique needs. A GP will help you with those individual needs.

Typically, the daily calorie needs of girls is stable at about 2000 from age 12 to 18. However, for boys their calorie requirements increase from 2200 at age 12 to 2800 by age 16.

But those calories are general. An individual teen boy's calorie needs will be a complex calculation determined by their activity level and if they are in the midst of a growth spurt.

National guidelines put recommended activity at about an hour of moderate activity four days a week (like walking to school) and strenuous activity (as in sweating and elevated heart rate) three times a week for an hour. Those involved in sport will do much more.

"Swimmers do a lot of training," Scrivin says. "So they could be doing another two hours a day."

Scrivin's teens are playing a sport some mornings and most afternoons after school so their daily calorie needs easily hit 3000, she says. One son is trying to build muscle and is aiming for 3200 daily calories, but bulking up is difficult when a teenage boy is still growing.

What macronutrients do they need?

Macronutrients are the fats, proteins and carbohydrates that make up our daily caloric intake and the recommended guidelines are similar across sexes and age groups, Wham says.

"Half of your total energy is coming from carbohydrates, preferably whole grains, and about a maximum of 35 percent from fat and around 15 percent from protein."

Scrivin has found the protein needs of her boys to be much higher than the recommendation. Generally, males need about one gram of protein per kilogram of body weight. Her teenage boys are eating 1.5 to 2 grams per kilo of body weight.

"So, almost double the recommendations," Scrivin says.

What does a teen boy eat in one day?

For breakfast, Scrivin's youngest is at about eight Weet-Bix with a heap of yogurt and some fruit. Toast with an egg or peanut butter is another carb and protein combo.

Lunch could be something like canned tuna or a peanut butter sandwich for a mix of carbs and protein. Scrivin encourages nuts and fruit for a snack at school and a smoothie when they get home in the afternoons (and there's those instant noodles for extra hungry days).

Dinner will likely have a pasta or rice base with veggie-stuffed sauce (think blitzed celery in almost everything) and a protein like chicken or beef mince.

With the cost of living constantly going up, Wham and Scrivin both recommend extending meals like bolognese sauce with legumes.

"It's delicious and you're adding plant-based protein," Wham says.

Bread is surprisingly a major source of protein in the New Zealand diet. Wholegrain is always preferred but "if people can't afford those whole grains then white bread is okay", Wham says.

Government guidelines recommend five servings of vegetables and two servings of fruit every day, two servings of protein (or three servings of plant-based protein if a teen is vegetarian), three servings of milk products and at least seven servings of breads and cereals.

When is a lot of food too much?

"Most boys gain about 20 kilograms in the four years of high school," Scrivin says. "That's absolutely normal."

And, allowances need to be made for genetics. Some families have larger frames than others.

However, ultimately someone who is eating too much at any age will likely put on body fat.

"The energy will go into growing first and then any excess will be stored just like [adults]," Scrivin says.