Nutrition and digestion · Classify
A balanced diet
Seven things your diet has to contain. You need about 300 g of one of them a day and 0.000001 g of another. Both are essential. What does “balanced” actually mean?
Start here
Two plates. One of them will kill you in about four months.
Plate A is white rice, chicken, oil, water and salt — every day, nothing else. It has carbohydrate, protein, lipid, minerals and water in roughly the right amounts. Plate B is the same food plus one orange. The person eating Plate A dies. The person eating Plate B is fine.
One orange. What is it doing?
The orange supplies about 50 milligrams of vitamin C — one twenty-thousandth of the mass of the meal. It carries no useful energy at all. Without it, the protein holding your body together cannot be made properly, and you bleed to death from the inside. Balance is not about amounts being similar.
At the bench · build a day
Set the amounts yourself, then see the real ones
0 of 7 set
For each of the seven, choose how much a 13-year-old needs in one day. Commit to all seven before you open the answers.
Carbohydrate
bread, rice, pasta, potato
You had itActually hundreds of gramsabout 300 g
The bulk of the plate, and the body’s first choice of fuel. Almost every cell can respire glucose, and your brain can use very little else.
Lipid (fat and oil)
oils, butter, nuts, oily fish
You had itActually tens of gramsabout 70 g
Tens of grams, not hundreds — but not zero. It carries more than twice the energy per gram of carbohydrate, builds every cell membrane, and is the only way vitamins A, D, E and K can be absorbed.
Protein
meat, fish, eggs, beans, lentils
You had itActually tens of gramsabout 45 g
Tens of grams. Protein is the only nutrient that supplies the nitrogen needed to build and repair tissue, so a growing 13-year-old needs a steady supply rather than a large one.
Vitamins
fruit, vegetables, dairy, liver
You had itActually milligrams or lessabout 0.2 g in total
A fifth of a gram covers all thirteen of them together. They release no energy at all — they let the reactions that do release energy actually run.
Minerals
dairy, red meat, leafy greens, salt
You had itActually a few gramsabout 5 g in total
A few grams, mostly calcium, and then iron, iodine and the rest in milligrams. Iron is 14 mg a day; miss it and your blood cannot carry oxygen properly.
Dietary fibre
wholegrains, skins, pulses, vegetables
You had itActually tens of gramsabout 25 g
Tens of grams of something you never absorb. It is not digested and not respired — it gives the gut muscles something to push against, which is what keeps everything moving.
Water
drinks, and most of your food
You had itActually hundreds of gramsabout 2000 g
Two kilograms — by far the largest requirement on the list, and the one you die from fastest without. Every reaction in you happens in solution.
Your day, scored
You put all seven in the same band — which is exactly the wrong idea this lesson exists to kill. Read the amounts again: 2000 g of water and 0.2 g of vitamins are both correct, and they are ten thousand times apart.
Notice what the spread looks like: two nutrients in hundreds of grams, three in tens, one in single grams and one in milligrams. Balanced describes seven separate targets, not seven similar piles.
The usual misses are lipid and fibre. Lipid feels like it should be near zero and is not; fibre feels like a trace and is 25 g — more than half a day’s protein, of something you never absorb at all.
The seven
What each one is for, and what goes wrong without it.
- CarbohydrateBroken down to glucose and respired for energy. The store the body reaches for first.Tiredness, and the body starts breaking down its own fat and then its own protein for fuel.
- LipidEnergy store, insulation, and the raw material of every cell membrane. Carries vitamins A, D, E and K.Those four vitamins cannot be absorbed. Skin and nerves are affected before anything else shows.
- ProteinBroken into amino acids and rebuilt into muscle, enzymes, antibodies and haemoglobin. Growth and repair.Growth stops, wounds heal slowly, and fluid collects in the tissues. Kwashiorkor in severe cases.
- VitaminsSmall molecules that make specific reactions possible. Thirteen of them, each with its own job.A specific deficiency disease per vitamin — scurvy without C, rickets without D, beriberi without B1.
- MineralsElements built into structures and carriers: calcium into bone, iron into haemoglobin, iodine into thyroid hormone.Anaemia without iron, weak bones without calcium, goitre without iodine.
- Dietary fibreNot digested and not absorbed. Adds bulk so gut muscles have something to grip and push.Constipation, and a raised long-term risk of bowel disease.
- WaterThe solvent every reaction in the body happens in, and the transport medium for blood, urine and digestion.Concentration and temperature control fail within hours. Death in days — faster than for any other nutrient.
Key fact
Balanced means each nutrient in the amount that nutrient needs — not the same amount of each. Two of the seven are measured in hundreds of grams. One is measured in milligrams. None of them is optional.
Think again
“A balanced diet means equal amounts of each food group.”
Look at the two ends of the table you have just filled in. A 13-year-old needs roughly 300 g of carbohydrate and about 0.0000015 g of vitamin B12 in a day — a difference of two hundred million times. Eating them in equal amounts would mean 300 g of vitamin B12, which would be a year’s supply for about half a million people and would poison you long before you finished the plate. The word balanced describes a set of seven different targets being met at once, like seven dials each reading correctly. It has never meant seven equal piles.
“Vitamins give you energy.”
They do not, and this is worth being exact about. Only three of the seven nutrients release energy when they are respired: carbohydrate, lipid and protein. A vitamin tablet has an energy content of zero — you could eat the whole bottle and be no better fuelled than before. What vitamins do is let the reactions that release energy actually happen: vitamin B1 is part of the machinery that gets glucose into respiration, so someone short of it is exhausted despite eating plenty. Feeling tired because you lack a vitamin is not the same as feeling tired because you lack fuel, and the fix is different.
“Fat is bad for you, so a healthy diet has none in it.”
Lipids are a required nutrient, not a permitted vice. Every cell membrane in you — and there are about thirty trillion of them — is built from lipid, and vitamins A, D, E and K only dissolve in fat, so a diet with no lipid in it cannot deliver them however much you eat. What is true is that lipid carries more than twice the energy per gram of carbohydrate, so it is easy to take in far more energy than you need without noticing. That is a statement about quantity, not about the nutrient being harmful.
Mastery ladder
Not started yet.
Rungs 3 and 4 you mark yourself.
Rung 1 · Which three
Which nutrients release energy when the body respires them?
Rung 2 · The one that catches people
A student is tired all the time. They eat plenty of bread, pasta and rice but almost no meat, fish or leafy vegetables. A doctor finds their blood cannot carry oxygen properly. What is missing?
Rung 3 · Explain the two ends
A 13-year-old needs about 300 g of carbohydrate and about 14 mg of iron each day — twenty thousand times less iron. Explain why both are described as essential, and why the amounts are so different.
Rung 4 · Take it somewhere new
A sports drink advertises itself as ‘packed with energy-giving vitamins’. The bottle lists sugars 32 g and eight vitamins. Say what is wrong with the claim, what is actually supplying the energy, and why the vitamins might still be worth having in it.
Key note
A healthy diet contains carbohydrate, lipid, protein, vitamins, minerals, dietary fibre and water. Carbohydrate, lipid and protein are the only three that release energy; protein also builds and repairs. The other four are needed in far smaller amounts and are needed just as absolutely — fibre and water are not even absorbed as fuel, and you cannot live without either.
Going further
In the 1880s the Japanese Navy was losing sailors to beriberi — weakness, swelling, heart failure — while the Army was not. Takaki Kanehiro noticed the two forces ate differently and ran what amounted to a fleet-sized experiment: one ship on the usual polished white rice, another with barley and vegetables added. The barley ship came home almost untouched. He had no idea why, because vitamin B1 would not be identified for another forty years, and he was ridiculed for suggesting food could be the cause of a disease everyone assumed was an infection. The interesting part is that he was right about the fix while being wrong about the mechanism, which happens in science far more often than the tidy version admits.
Before this lesson
- Nothing — this is where the unit starts.
Connects to
- Food tests
- When diet goes wrong
- Energy stores
Where the chemical store in food is developed as energy rather than as nutrition.
At GCSE this becomes
- Nutrient requirements, malnutrition, and the biochemistry of carbohydrates, lipids and proteins as polymers.
Where to next
- Next: Food tests
- Previous: Biomechanics: forces in the body
Movement: skeleton and muscles
Ask Mr Badmus AI
Still not sure why a milligram can matter as much as a gram?
Figures are typical daily requirements for a 13-year-old and are for learning the principle, not for planning anyone's meals. Individual needs differ; dietary advice comes from a doctor or a registered dietitian.
Lesson content © MrBadmusAI.