Nutrition and digestion · Contrast
When diet goes wrong
Three different things can be wrong with a diet, and they are not three degrees of the same thing. Two of them can happen at once, in the same person, on a full plate.
Start here
A patient is eating 13 000 kJ a day and is severely malnourished.
Not going short of food. Not skipping meals. Taking in more energy than an adult needs, every day, and a doctor has diagnosed malnutrition. The diagnosis is correct and the intake figure is correct.
How can both be true at once?
Malnutrition means the balance is wrong, not that the quantity is small. Seven nutrients have to arrive; energy is only what three of them carry. A diet can be generous in kilojoules and missing iron, or vitamin D, or fibre entirely — and the body fails at the missing thing regardless of how much of everything else showed up.
Three separate failures
Not one problem with a dial on it.
Imbalance 1 · Long-term energy surplus
What is wrong Energy taken in is greater than energy transferred, day after day, for months or years. What the body does The surplus is stored as lipid in adipose tissue. Storage is a normal, healthy mechanism; the problem is only the size of the store when it keeps growing. What follows Obesity raises the risk of type 2 diabetes, high blood pressure, heart disease and joint damage. Risk raised, not certainty — these are probabilities, not verdicts. What changes it Any lasting change to the balance between intake and activity. Slow, because the store took a long time to build.
Imbalance 2 · Long-term energy shortfall
What is wrong Energy taken in is less than energy transferred, for weeks or months. Causes range from food being unavailable to illness that prevents eating or absorbing. What the body does Lipid stores are broken down first. When those are gone it breaks down its own protein — muscle, including heart muscle. What follows Growth stops, the immune system weakens, and body temperature is hard to maintain. In children the effect on growth may not be recoverable. What changes it Refeeding, done carefully and medically — restarting food too fast after prolonged starvation is itself dangerous.
Imbalance 3 · A specific nutrient missing
What is wrong One nutrient is absent or too low. Energy intake can be high, normal or low — it is a separate question. What the body does Whatever that nutrient was needed for stops working properly. Nothing else can substitute: no amount of carbohydrate replaces 14 mg of iron. What follows A named deficiency disease. No vitamin C, scurvy. No vitamin D, rickets. No iron, anaemia. Each has its own signs. What changes it Supplying that one nutrient. Often a fast and complete recovery, which is what makes deficiency diseases so striking to treat.
Each of the three has a different cause, a different mechanism inside the body, and a different remedy. Compare them line by line.
Key fact
Too much energy, too little energy and a missing nutrient are three different imbalances. A deficiency can occur at any level of energy intake, because a specific nutrient cannot be substituted by more of the others.
Your turn · five clinics
Which imbalance is this?
0 of 5 diagnosed
Each description gives you a diet and a set of signs. Say which of the three imbalances fits. Two of the five have more than one answer, and being unwilling to pick two is itself a mistake.
Clinic 1
An adult reports tiredness, breathlessness on stairs and pale skin. Blood tests show low haemoglobin. Their diet is bread, pasta, cheese and fruit, with no meat, fish or leafy vegetables. They are not losing mass.
Intake about 10 000 kJ / day — close to requirement
Tick every imbalance that applies
One imbalance applies here
Iron-deficiency anaemia. A nutrient missing, and nothing else.
Energy intake is fine, mass is stable, and the signs all point at haemoglobin. Iron is needed in milligrams and this diet supplies almost none. Adding more of the same food would change nothing.
Clinic 2
A 14-year-old has stopped growing in height over eighteen months, feels cold constantly, and bruises easily. Their family has been unable to afford regular meals.
Intake about 4500 kJ / day — far below requirement
Tick every imbalance that applies
Two imbalances apply here
Energy shortfall, and almost certainly deficiencies alongside it.
Both apply, and this is the pair most students refuse to tick together. Too little food means too little of everything in it — so a prolonged shortfall drags specific deficiencies along behind it. The growth arrest is the shortfall; the bruising suggests vitamin C or K as well.
Clinic 3
An adult has gained mass steadily over ten years. Blood glucose is raised and a doctor has diagnosed type 2 diabetes. They eat large portions, mostly processed food, and have a desk job and a car commute.
Intake about 14 000 kJ / day — well above requirement
Tick every imbalance that applies
One imbalance applies here
Long-term energy surplus.
Ten years of intake above requirement, stored as lipid, and one of the risks that follow has now appeared. Note what this case is not: it is not a moral failure, and the fix is a change in the long-term balance, not a punishment.
Clinic 4
A toddler in a smoky northern city in 1900 has soft, bowed leg bones and a delayed walk. They are fed bread, potatoes, tea and a little meat. They are not thin.
Intake roughly at requirement
Tick every imbalance that applies
One imbalance applies here
Rickets — vitamin D missing.
Enough food, enough energy, no vitamin D. It comes from oily fish or from sunlight on skin, and a smoke-darkened street supplies neither. Calcium cannot be absorbed properly without it, so the bones stay soft. This is the classic case of a deficiency in a fed child.
Clinic 5
An adult recovering from major surgery cannot absorb nutrients properly from their shortened intestine. They eat willingly and in normal quantities but are losing mass and have low levels of several vitamins.
Intake about 9500 kJ / day — at requirement
Tick every imbalance that applies
Two imbalances apply here
Both — and the cause is not the diet at all.
The plate is fine. What has failed is absorption, so what reaches the blood is a shortfall even though what reaches the mouth is not. This is the case that shows why ‘diet’ and ‘nutrition’ are not the same word: the next three lessons are about the part of the system that has broken here.
Now every diagnosis is showing — how many of your five did you have?
Nobody marks this but you. Clinics 2 and 5 each have two answers, and those are the two worth reading again.
Think again
“Malnourished means not having enough to eat.”
The word means badly nourished, not under-nourished, and the difference is not nit-picking — it changes what you look for and what you do. Rickets was widespread in industrial British cities among children who were fed enough, because vitamin D needs either oily fish or sunlight and a smoke-filled street supplies neither. Iron deficiency is the most common nutritional disorder in the world and it is not concentrated in places where food is scarce. If you diagnose every dietary problem as a shortage of food, you will treat these by handing over more of the same food, and nothing will improve, because the missing nutrient is still missing.
“Deficiency diseases are all in the past.”
Scurvy still appears in hospitals. So does rickets, so does severe iron deficiency, and vitamin B12 deficiency is common enough that it is a routine blood test. The reason is not that food is unavailable; it is that a nutrient needed in milligrams can be absent from a diet that looks entirely ordinary, and the early signs — tiredness, aching, slow healing, low mood — are the least specific symptoms in medicine. A deficiency is easy to miss precisely because it does not announce itself as a food problem.
“You can tell what someone eats by looking at them.”
You cannot, and this is the most important sentence in the lesson. Body mass is affected by genetics, by illness, by medication, by how much someone can move, by sleep, by stress and by whether they can afford or reach particular foods — as well as by what they eat. Two people on the same diet can differ a lot, which is exactly what the hook in the energy lesson showed. The three imbalances in this lesson are described by measurements and by clinical signs, not by appearance, and a biologist reasons from the measurements.
Mastery ladder
Not started yet.
Rungs 3 and 4 you mark yourself.
Rung 1 · Name the imbalance
A patient eats 12 000 kJ a day, is gaining mass slowly, and has been diagnosed with scurvy. Which imbalance or imbalances apply?
Rung 2 · The one that catches people
Two children are the same age and height. Child A eats 6000 kJ a day; child B eats 11 000 kJ. Both have been diagnosed with iron-deficiency anaemia. What does this tell you?
Rung 3 · Explain why more food does not fix it
A charity responds to a region with widespread anaemia by shipping large quantities of white rice. Explain why this will reduce hunger but not reduce the anaemia, and what would need to be added.
Rung 4 · Take it somewhere new
The UK adds calcium, iron and B vitamins to white flour by law. Using the three imbalances, explain what problem this policy targets, why it works at the flour rather than at the person, and one limitation you would expect it to have.
Key note
An imbalanced diet can mean a long-term energy surplus, a long-term energy shortfall, or a specific nutrient missing. Obesity raises the risk of type 2 diabetes, heart disease and joint damage. Prolonged energy shortfall breaks down first fat and then muscle, and stops growth. Deficiency diseases — scurvy, rickets, anaemia — are caused by one missing nutrient and can occur at any level of energy intake.
Going further
In 1747 the naval surgeon James Lind ran what is often called the first controlled clinical trial, on twelve scurvy patients divided into six pairs, each pair given a different treatment: cider, sulfuric acid, vinegar, seawater, a paste of herbs, or two oranges and a lemon. The citrus pair recovered. The trial was tiny, it was not blinded, and Lind himself did not fully accept the result — he thought the fruit worked by helping digestion and later recommended a boiled-down concentrate that destroyed the vitamin C entirely. The Navy took another forty years to adopt lemon juice, and it saved more sailors than any battle of the period. Good evidence and correct interpretation are different achievements, and the gap between them is measured in lives.
Before this lesson
Connects to
At GCSE this becomes
- Risk factors for non-communicable disease, and the evidence linking diet to cardiovascular disease and type 2 diabetes.
Where to next
Ask Mr Badmus AI
Want to work through why a deficiency can happen on a full plate?
If any of this is about you or someone you know, talk to someone you trust — a parent or carer, a teacher, your school nurse, a pharmacist or your GP. Out of school hours: Childline — 0800 1111, free and confidential.
This lesson describes conditions clinically, as biology. It is not advice about anybody's own body or eating, and it sets no targets. If any of it worries you about yourself or someone you know, speak to a trusted adult, your school nurse or a doctor — that is the right next step, not a change to what you eat.
Lesson content © MrBadmusAI.