Pearson Edexcel International GCSE in Biology · 4BI1

A Balanced Diet

Why you can eat plenty and still be starving — food does several separate jobs, and having enough of one does not cover the others.

Topic 2 · Structures and functions in living organisms — one of 34 lessons in this topic, and one of 68 in Biology.

What this lesson covers in the specification

Incandio is aligned to this specification. It is not published by, endorsed by or affiliated with Pearson, and it reproduces none of Pearson's wording — the statement numbers are given so you can check every lesson against your own copy.

  • 2.24 — The components of a balanced diet and their appropriate proportions
  • 2.25 — Sources and functions of carbohydrate, protein, lipid, vitamins A, C and D, calcium, iron, water and dietary fibre
  • 2.26 — How energy requirements vary with activity level, age and pregnancy

1 · Understand it

No exam language yet. The only question this section answers is: do I actually understand what is happening?

You could eat nothing but sugar, take in more than enough energy every day, and still die of malnutrition. Not from hunger — you would have plenty of fuel. You would die because supplying energy is only one of the jobs food does, and sugar does only that one. This is the idea the whole topic rests on: a diet is not one requirement that you either meet or fail to meet. It is several separate requirements, in different currencies, and having a surplus in one buys you nothing in another.

The three separate jobs food is doing

  1. FUEL. Carbohydrate and lipid are respired to release energy. Run short and you become tired, then the body begins breaking down its own tissue to burn instead.
  2. BUILDING MATERIALS. Protein supplies the amino acids for growth and repair; calcium builds bones and teeth; iron is built into haemoglobin. Run short of these and you cannot grow or mend, however much energy you have.
  3. TINY-QUANTITY ESSENTIALS. Vitamins are needed in milligrams, and the body cannot make them. Vitamin C is needed to make collagen, the fibre that holds tissue together, so without it structures that are constantly being repaired begin to fall apart.
  4. And two components that fit none of those descriptions: water, which is the solvent every reaction in you happens in, and fibre, which is never absorbed at all.

Fibre is worth pausing on, because it is the odd one out and examiners like it for exactly that reason. You cannot digest it — humans have no enzyme for cellulose — so none of it enters your blood and it supplies you with nothing at all chemically. Its value is purely mechanical. The muscles of the gut wall work by gripping and squeezing what is inside them, and they need something with bulk to grip. Fibre provides that bulk, so the contents move steadily instead of stalling. It is the only part of your diet whose job is to be a physical object.

Why two people need completely different amounts

  1. Energy is spent by respiration in every cell, so anything that means more respiring is happening means more energy is needed.
  2. ACTIVITY: a manual worker or an athlete has muscles respiring hard for hours, so their energy requirement is far higher than someone sitting still all day.
  3. AGE: children and teenagers need more energy for each kilogram of body mass than adults, because they are building new tissue as well as running the tissue they have — and building it needs protein for the same reason.
  4. PREGNANCY: there is now a second body to supply. More energy, more protein to build the fetus, more calcium for its skeleton and more iron for its blood.
  5. Later in life the requirement usually falls again, as activity and muscle mass decrease.

Finally, notice that 'balanced' points in both directions. Take in less energy than you use and the body burns its own fat and then its own protein — weight falls and, if protein intake is also low, growth and repair stop. Take in more energy than you use and the surplus is stored as lipid, whatever food it arrived as, which is why excess carbohydrate makes people gain fat. Neither failure is about a single missing item; both are about proportion.

2 · Grade 9 Notes

A different job from the section above. You have already understood it; this is the precise set of things to LEARN — definitions to reproduce word for word, processes in order, equations with units, and the answers that score full marks.

Learn this definition · Balanced diet

A diet containing all the required nutrients — carbohydrate, protein, lipid, vitamins, minerals, water and dietary fibre — in the correct proportions, and supplying an amount of energy that matches the energy the body uses.

The seven components — source, function, and what deficiency does

Carbohydrate
bread, rice, pasta, potatoes. Respired to release energy; the body's main energy source.
Protein
meat, fish, eggs, beans, dairy. Supplies amino acids for growth and repair of tissues; deficiency stunts growth and slows healing.
Lipid (fat and oil)
butter, oils, nuts, oily fish. An energy store, insulation, and part of every cell membrane.
Water
drinks and most foods. The solvent for all reactions in cells, and the transport medium of the blood.
Dietary fibre
wholegrains, vegetables, fruit skins. Not digested or absorbed; provides the bulk the gut muscles grip during peristalsis.
Minerals
needed in small amounts and built into structures — see the mineral and vitamin lists below.

The named vitamins and minerals — learn all five with their deficiency

Vitamin A
liver, fish oils, carrots. Needed to make the pigment that allows vision in dim light; deficiency causes night blindness.
Vitamin C
citrus fruit, blackcurrants, peppers. Needed to make collagen, which holds tissues together; deficiency causes scurvy.
Vitamin D
oily fish, eggs, and made in the skin in sunlight. Needed to absorb calcium; deficiency causes rickets.
Calcium
milk, cheese, green vegetables. Builds bones and teeth and is needed for blood clotting; deficiency weakens bone.
Iron
red meat, liver, dark green vegetables. Built into haemoglobin to carry oxygen; deficiency causes anaemia.

What raises a person's energy requirement

  • More physical activity — muscles respiring harder and for longer
  • Being a child or adolescent — building new tissue as well as maintaining it, so more energy per kilogram of body mass
  • Pregnancy — energy, protein, calcium and iron for a second developing body
  • A larger body mass — more living tissue to keep respiring
  • Cold surroundings — more energy released to maintain body temperature

Too little energy against too much

A. Energy intake below energy use: the body respires its own fat stores and then its own protein. Body mass falls, and if protein intake is also low, growth and repair stop.

B. Energy intake above energy use: the surplus is stored as lipid whatever food it arrived as, so body mass rises. This is why excess carbohydrate, not only excess fat, causes obesity.

Model answer [4 marks]

A teenage girl is diagnosed with anaemia and is often tired. Explain which part of her diet is most likely to be inadequate, and why this makes her tired. [4]

She is most likely short of iron. Iron is needed to make haemoglobin in red blood cells. With less haemoglobin, the blood carries less oxygen to the tissues. Less oxygen means less aerobic respiration, so less energy is released in her cells, which is why she is tired.

Model answer [3 marks]

Explain why a pregnant woman needs more protein, calcium and iron than she did before pregnancy. [3]

Protein supplies the amino acids used to build the new tissues of the developing fetus as well as maintaining her own. Calcium is needed to build the fetus's bones and teeth. Iron is needed to make haemoglobin for the fetus's red blood cells as well as her own increased blood volume.

Mark-losing trap. Fibre is not digested and not absorbed. Never write that it supplies energy — its whole job is to give the gut muscles bulk to grip.

Mark-losing trap. Name the vitamin AND its function. 'Vitamin C stops scurvy' is a deficiency, not a function; the function is making collagen.

Mark-losing trap. Excess carbohydrate is stored as lipid. Weight gain is about surplus energy, not about which food the energy arrived in.

Mark-losing trap. Vitamin D does not build bone. It lets you absorb the calcium that does, which is why rickets looks like a calcium problem.

3 · Prove it — the five questions

The five questions climb Grade 6 → Grade 7 → Grade 8 → Grade 9 → Grade 9 challenge, and are marked inside Incandio on your own device, by rule, with an authored diagnosis of the mistake you actually made. The mark schemes stay in the app so that the practice is worth doing; the questions themselves are here.

  1. Grade 6 · Identify [1 mark] — Which component of a balanced diet is not digested or absorbed by the body?
  2. Grade 7 · Name [1 mark] — Name the mineral that is built into haemoglobin, and whose shortage causes anaemia.
  3. Grade 8 · Explain [4 marks] — A child eats enough food to meet his energy needs entirely from rice and sugar. He is not underweight, but he grows slowly, bruises easily and his wounds heal badly. Select every statement that belongs in a full-mark explanation.
  4. Grade 9 · Calculate [3 marks] — A meal contains 40 g of carbohydrate, 12 g of protein and 9 g of fat. Carbohydrate and protein each release 17 kJ per gram; fat releases 37 kJ per gram. Calculate the percentage of the meal's energy that comes from fat, to one decimal place.
  5. 9+ · Analyse [5 marks] — Two adults of the same age and body mass are compared. One is a builder; the other works at a desk and both eat identical meals, meeting the builder's energy needs exactly. Over a year the desk worker gains a considerable amount of fat, while the builder's mass does not change. Both remain healthy in every other respect. Select every statement that belongs in a full-mark explanation.

The people behind this science

Three ways into the same idea — the one who ran the first fair test of a cure, and the one who proved bad diet was killing people, and the one who ran the experiment on himself for forty years. Inside Incandio each of them answers knowing exactly which lesson you have just finished.

James Lind — the one who ran the first fair test of a cure

Scurvy — the deficiency disease this lesson names under vitamin C — killed more sailors than battle, and every physician had a theory. In 1747, aboard HMS Salisbury, Lind ignored the theories and arranged a comparison: twelve similar patients, one shared diet, six remedies given to matched pairs. Only the pair on oranges and lemons recovered. It is remembered as the first controlled clinical trial, and it proved the cure for a disease whose cause — a missing tiny-quantity essential — his century had no way to imagine.

  • “How did you choose the twelve men, and why did they all share one berth and one diet?”
  • “What happened to the pair on oranges and lemons?”
  • “Sharper acids than lemon juice failed your trial — what did that tell you?”
  • “How could men who ate their fill every day still be starving?”
  • “Why did the Navy take fifty years to act on what you proved?”

Florence Nightingale — the one who proved bad diet was killing people

At Scutari, Nightingale rebuilt the hospital kitchens as deliberately as the wards, and she recorded the outcomes month by month in numbers nobody could argue with. She was making the argument this lesson makes — that a body which is not supplied properly cannot repair itself — at a time when diet was considered a comfort rather than a medical treatment.

  • “How did you convince officials that the kitchens mattered as much as the medicine?”
  • “What changed in the wards once the food changed?”
  • “Why did you insist on counting deaths rather than describing them?”
  • “How can you tell that poor food caused a death rather than simply accompanying it?”

Mahatma Gandhi — the one who ran the experiment on himself for forty years

Gandhi spent forty years altering what he ate and writing down what happened — removing milk, living on fruit and nuts, fasting for long periods — and published the results in careful detail. Almost nobody has kept a longer record of deliberately unbalancing a diet and observing the consequences, which is this lesson's content approached from the inside.

  • “What happened to your body when you removed whole groups of food?”
  • “How could you tell a change was caused by your diet and not by something else?”
  • “Why did you keep such careful written records of what you ate?”
  • “What did long fasting teach you about what the body does for fuel?”

Then defend it

On Incandio a lesson is not finished when the questions come out right. You teach the idea back to Ember, an AI apprentice who asks the awkward question, and then you argue it against James Lind in a structured debate marked against descriptors you can read before you enter. Learn it, teach it, then defend it — all three happen on this page once the app loads.

Carry on through the course