---
title: "A Balanced Diet"
book: "Primary & Middle School Biology"
subject: biology
language: en
chapter: 18
exercises: 10
source: https://one-course.com/books/biology/1/en/chapter/18-a-balanced-diet
---

# Chapter 18 — A Balanced Diet

A builder’s site needs bricks, cement, planks — and fuel for the machines. Your body is a building site that never closes: it grows, repairs itself and runs about all day. Its deliveries arrive three or four times a day, on a plate. What must the deliveries contain? That is the question of diet.

## 18.1 Why the body needs food

**Proposition 18.1 (Food’s two jobs).**

Food does two different jobs:

1. it provides *energy* — the power to move, to keep warm, to keep heart and brain running, even during [sleep](https://one-course.com/books/biology/1/en/chapter/6-healthy-habits#prop-g1-healthy-habits-needs) ;
2. it provides *building material* — what the body uses to grow taller, build [muscle](https://one-course.com/books/biology/1/en/chapter/17-bones-and-muscles#def-g3-bones-and-muscles-muscle) and [bone](https://one-course.com/books/biology/1/en/chapter/17-bones-and-muscles#def-g3-bones-and-muscles-skeleton) , repair scrapes and replace worn parts.

A diet must deliver both, every day.

**Proof.** *Admitted at this level.* ∎

**Example 18.2 (Fuel even at rest).**

Even lying perfectly still you spend [energy](#prop-g3-balanced-diet-jobs): the heart beats, the chest rises and falls, the body stays warm at about $37{}^{\circ}\mathrm{C}$ whatever the weather. That standing charge is why missing meals makes you weak and cold, not just hungry.

## 18.2 The food families

**Definition 18.3 (The food families).**

Foods are sorted into *food families* by what they bring:

- *[fruit](https://one-course.com/books/biology/1/en/chapter/16-flowers-fruits-and-seeds#prop-g3-flowers-fruits-seeds-transform) and vegetables* — vitamins, fibre and water; the freshness family;
- *cereals and starchy foods* (bread, pasta, rice, potatoes) — the steady [energy](#prop-g3-balanced-diet-jobs) family;
- *dairy* ( [milk](https://one-course.com/books/biology/1/en/chapter/8-how-animals-grow-up#def-g2-animals-grow-up-born) , yogurt, cheese) — the bone-building family;
- *meat, [fish](https://one-course.com/books/biology/1/en/chapter/14-sorting-living-things#prop-g3-sorting-living-things-groups) and [eggs](https://one-course.com/books/biology/1/en/chapter/8-how-animals-grow-up#def-g2-animals-grow-up-born)* — the body-building family;
- *fats* (butter, oil) — concentrated [energy](#prop-g3-balanced-diet-jobs) , needed in small amounts;
- *sugary foods* — quick pleasure, needed not at all: the treat family;
- and *water* — the only drink the body requires, all day long.

![The freshness family at the market: the corner of the plate most often forgotten, here in no danger of being missed.](https://one-course.com/images/onecourse/chapters/biology-1/g3-balanced-diet/img-0884083a4b70.jpg)

*The freshness family at the market: the corner of the plate most often forgotten, here in no danger of being missed.*

![A well-balanced plate: half freshness, a quarter steady energy, a quarter builders — and water in the glass.](https://one-course.com/images/onecourse/chapters/biology-1/g3-balanced-diet/fig-8759684930d5.svg)

*A well-balanced plate: half freshness, a quarter steady [energy](#prop-g3-balanced-diet-jobs), a quarter builders — and water in the glass.*

**Example 18.4 (Balancing a lunch).**

Pasta with tomato sauce, a piece of [fish](https://one-course.com/books/biology/1/en/chapter/14-sorting-living-things#prop-g3-sorting-living-things-groups), green beans, a yogurt, an apple, water: every family present, treats absent — a balanced lunch. Chips, a soda and a chocolate bar feed exactly two families (fats and sugar) and skip the ones the building site was waiting for.

**Remark 18.5 (No forbidden foods, only forgotten ones).**

Balance is measured over days, not single bites. A slice of birthday cake breaks no rule — the trouble is never one treat, it is treats *instead of* the families, day after day. The practical question at each meal is not “is this food bad?” but “which family have I forgotten today?”

## 18.3 Matching food to the day

**Proposition 18.6 (Needs change with the day and the person).**

How much [energy](#prop-g3-balanced-diet-jobs) a body needs depends on who it is and what it does: a growing child needs proportionally more building material than a grown-up; a day of sport or winter cold burns more [energy](#prop-g3-balanced-diet-jobs) than an afternoon of reading. Hunger grows with the body’s spending.

**Proof.** *Admitted at this level.* ∎

**Example 18.7 (Why breakfast matters).**

By morning, the body has run all night — heart, breath, warmth — on no deliveries at all. Breakfast restocks it for the morning’s lessons and running about. Skip it, and by mid-morning the site runs short: rumbling stomach, wandering attention, no strength at play.

**Method 18.8 (Checking a day’s balance).**

At the end of the day, run through the list:

1. [fruit](https://one-course.com/books/biology/1/en/chapter/16-flowers-fruits-and-seeds#prop-g3-flowers-fruits-seeds-transform) or vegetables at every meal? (aim: about five helpings in the day)
2. starchy food at each meal for steady [energy](#prop-g3-balanced-diet-jobs) ?
3. builders present — dairy a few times, and meat, [fish](https://one-course.com/books/biology/1/en/chapter/14-sorting-living-things#prop-g3-sorting-living-things-groups) or [eggs](https://one-course.com/books/biology/1/en/chapter/8-how-animals-grow-up#def-g2-animals-grow-up-born) once or twice?
4. water as the day’s drink?
5. treats: a small amount, not a main part?

Whatever family is missing, invite it tomorrow.

## 18.4 Exercises

**Exercise 18.1 ★.**

What are food’s two jobs? Give one [food family](#def-g3-balanced-diet-families) that mainly serves each job.

**Solution of Exercise 18.1.**

Providing [energy](#prop-g3-balanced-diet-jobs), and providing building material. [Energy](#prop-g3-balanced-diet-jobs): cereals and starchy foods (or fats). Building: meat, [fish](https://one-course.com/books/biology/1/en/chapter/14-sorting-living-things#prop-g3-sorting-living-things-groups) and [eggs](https://one-course.com/books/biology/1/en/chapter/8-how-animals-grow-up#def-g2-animals-grow-up-born), or dairy.

**Exercise 18.2 ★.**

Name the [food families](#def-g3-balanced-diet-families), and the one drink the body requires.

**Solution of Exercise 18.2.**

[Fruit](https://one-course.com/books/biology/1/en/chapter/16-flowers-fruits-and-seeds#prop-g3-flowers-fruits-seeds-transform) and vegetables; cereals and starchy foods; dairy; meat, [fish](https://one-course.com/books/biology/1/en/chapter/14-sorting-living-things#prop-g3-sorting-living-things-groups) and [eggs](https://one-course.com/books/biology/1/en/chapter/8-how-animals-grow-up#def-g2-animals-grow-up-born); fats; sugary foods (the treat family). The one required drink is water.

**Exercise 18.3 ★.**

Sort this shopping into families: rice, yogurt, carrots, [eggs](https://one-course.com/books/biology/1/en/chapter/8-how-animals-grow-up#def-g2-animals-grow-up-born), butter, apples, bread.

**Solution of Exercise 18.3.**

Rice, bread: cereals/starchy. Yogurt: dairy. Carrots, apples: [fruit](https://one-course.com/books/biology/1/en/chapter/16-flowers-fruits-and-seeds#prop-g3-flowers-fruits-seeds-transform) and vegetables. [Eggs](https://one-course.com/books/biology/1/en/chapter/8-how-animals-grow-up#def-g2-animals-grow-up-born): meat-fish-eggs. Butter: fats.

**Exercise 18.4 ★.**

Why does the body spend [energy](#prop-g3-balanced-diet-jobs) even during [sleep](https://one-course.com/books/biology/1/en/chapter/6-healthy-habits#prop-g1-healthy-habits-needs)? Name two of the night’s expenses.

**Solution of Exercise 18.4.**

Because the body never stops: the heart beats all night, the chest keeps breathing, and the body stays warm at about $37{}^{\circ}\mathrm{C}$ — all paid for in [energy](#prop-g3-balanced-diet-jobs).

**Exercise 18.5 ★.**

Check the lunch “pasta, [fish](https://one-course.com/books/biology/1/en/chapter/14-sorting-living-things#prop-g3-sorting-living-things-groups), green beans, yogurt, apple, water” against the balanced plate of the figure: which part of the plate is each item?

**Solution of Exercise 18.5.**

Green beans and the apple: the freshness half. Pasta: the starchy quarter. [Fish](https://one-course.com/books/biology/1/en/chapter/14-sorting-living-things#prop-g3-sorting-living-things-groups) and the yogurt: the builders’ quarter. Water: the glass. Every part of the plate is served.

**Exercise 18.6 ★.**

Why is breakfast especially important? What happens mid-morning without it?

**Solution of Exercise 18.6.**

The body has run all night on no deliveries; breakfast restocks it. Without it, by mid-morning [energy](#prop-g3-balanced-diet-jobs) runs short: rumbling stomach, wandering attention, weak play.

**Exercise 18.7 ★.**

Who needs proportionally more building material: a child or a grown-up? Why?

**Solution of Exercise 18.7.**

The child — a child’s body is still building itself taller and bigger, on top of repairing and running, so building material matters proportionally more.

**Exercise 18.8 ★.**

Try it: run [Method 18.8](#met-g3-balanced-diet-check) on your own day, honestly. Which family did you forget — and which appeared more than its share?

**Solution of Exercise 18.8.**

Open answer. The commonly forgotten family is [fruit](https://one-course.com/books/biology/1/en/chapter/16-flowers-fruits-and-seeds#prop-g3-flowers-fruits-seeds-transform) and vegetables; the one over its share, sugary foods.

**Exercise 18.9 ★★.**

“Sweets are forbidden!” Is that what this chapter says? Give the more precise rule of [Remark 18.5](#rem-g3-balanced-diet-balance).

**Solution of Exercise 18.9.**

No food is forbidden: balance is judged over days, not bites. The rule is that treats must stay small and must not take the place of the families — the question is “which family have I forgotten?”, not “which food is bad?”.

**Exercise 18.10 ★★.**

On a ski holiday, everyone is hungrier than at home. Explain with [Proposition 18.6](#prop-g3-balanced-diet-needs) — name the two extra expenses a cold, sporty day adds.

**Solution of Exercise 18.10.**

Sport spends extra [energy](#prop-g3-balanced-diet-jobs) in movement, and cold spends extra [energy](#prop-g3-balanced-diet-jobs) keeping the body warm. Spending more, the body asks for more: hunger grows with the day’s expenses.