Diabetes is a condition in which the amount of glucose in the blood stays too high, because the body either cannot make insulin, cannot make enough of it, or cannot use what it makes.
Insulin is a hormone produced by the pancreas. Carbohydrate in food is broken down into glucose, which the blood carries to every cell in the body as fuel, and insulin is what allows the cells to take it in. Without working insulin the glucose stays in the bloodstream, where over time it damages blood vessels and nerves — and in the short term, with no usable fuel reaching the cells, the body starts breaking down fat and muscle instead.
That mechanism is shared by every type of diabetes. Almost everything else about them differs, and the differences matter a good deal more than the shared name suggests.
The symptoms, and which are urgent
This comes first because it is the part that occasionally matters within hours. The classic four are taught by Diabetes UK as the 4 Ts:
- Toilet — passing urine far more often than usual; bedwetting in a child who was previously dry; heavier nappies in a baby.
- Thirsty — a thirst that does not go away, however much is drunk.
- Tired — exhausted beyond anything the circumstances explain.
- Thinner — losing weight, or simply looking thinner, without trying.
Alongside those: blurred vision, cuts and grazes slow to heal, recurrent thrush or genital itching, and frequent infections.
The timescale separates the two main types. Type 1 symptoms arrive over days or a few weeks and worsen quickly. Type 2 develops over years, often with nothing noticeable at all, which is why a great many people are diagnosed on a routine blood test or because an optician noticed changes at the back of the eye.
Where the 4 Ts appear, particularly in a child or young person, that warrants being seen the same day rather than waiting. They are easily mistaken for a virus, and the reason for the urgency is diabetic ketoacidosis: without insulin the body burns fat for fuel and produces acidic ketones, which is life-threatening. Close to a quarter of children and young people diagnosed with type 1 diabetes in England and Wales are identified only once it has already developed.
Rapid or deep breathing, breath smelling of pear drops or nail varnish, vomiting, abdominal pain, drowsiness or confusion in someone with these symptoms is a medical emergency. That means 999.
Type 1
Type 1 diabetes is an autoimmune condition. The immune system destroys the cells in the pancreas that make insulin, and once they are gone the body makes none at all. Insulin has to be replaced, by injection or pump, from diagnosis and for life.
It accounts for roughly eight in every hundred cases. It is not caused by diet, by weight, by eating sugar, or by anything a person or their parents did — and the belief that it is causes a great deal of unnecessary guilt. Why the immune system does this is not understood: a genetic susceptibility combined with some environmental trigger is the leading account, and the trigger has never been identified.
Nor is it a childhood condition, whatever the old name suggested. A large proportion of people with type 1 diabetes are diagnosed as adults, at any age.
Type 2
Type 2 diabetes is a different process. The cells become resistant to insulin, so more is needed to do the same job; the pancreas compensates by producing more; and over years it cannot keep up. Both halves — the resistance and the failing production — are part of it, which is why type 2 tends to progress and why its treatment often needs changing over time.
It accounts for around nine in ten cases, and is strongly associated with weight, inactivity, age, family history and ethnic background. Two things are worth saying straight away to head off the usual misunderstanding: it is not caused by eating sugar as such, and plenty of people who develop it are not overweight.
Unlike type 1, type 2 can sometimes go into remission — not cure, not in everyone, but a real phenomenon with real evidence behind it.
Why the old names were dropped
Type 1 used to be called juvenile-onset diabetes and type 2 adult-onset. Both were abandoned because both became wrong. Type 1 is frequently diagnosed in adults, and type 2 is now diagnosed in children and young people, which it essentially never was when the names were coined.
The same goes for "insulin-dependent" and "non-insulin-dependent": a great many people with type 2 diabetes end up using insulin, and that does not make it type 1.
The other types
Together a small minority of cases, but each one changes what the right treatment is.
- Gestational diabetes develops during pregnancy, usually in the second or third trimester, and generally resolves after the birth. It needs managing while it lasts, and it leaves a substantially raised risk of type 2 diabetes later.
- Monogenic diabetes, including MODY, is caused by a single inherited gene fault. It is frequently misdiagnosed as type 1 or type 2, which matters because some forms respond to tablets rather than insulin and some need no treatment at all. A strong family history across generations, diagnosis under about twenty-five, and a presentation that does not quite fit either main type are what prompt testing for it.
- Diabetes secondary to something else — disease of or surgery on the pancreas, cystic fibrosis, or medication, most commonly steroids.
How it is diagnosed
By blood test, normally repeated to confirm unless the symptoms are clear-cut. The usual measure is HbA1c, which reflects average glucose over the preceding two to three months rather than at a single moment.
In the UK, HbA1c is reported in mmol/mol. American sources and older British writing use a percentage, which causes needless confusion when figures are compared.
- 48 mmol/mol (6.5%) or above — diabetes.
- 42 to 47 mmol/mol (6.0 to 6.4%) — the high-risk range.
That middle band goes by several names which all mean the same thing: prediabetes, non-diabetic hyperglycaemia, impaired glucose regulation. It is not a mild form of diabetes. It is a warning, it does not inevitably progress, and it is the point at which doing something works best.
HbA1c is unreliable in some circumstances — in pregnancy, in some anaemias and haemoglobin disorders, in advanced kidney disease, and in children — so other tests are used where that applies.
The rest of this section
Diabetes of either main type is a long-term condition needing long-term care, and what has changed most in recent decades is how much of its damage is now avoidable. That is not a matter of willpower; it is better treatment, better monitoring, and care organised around a set of annual checks that are worth knowing about so they can be asked for.
- Risk factors for diabetes — why type 1 cannot be prevented, what in type 2 can and cannot be changed, and the UK thresholds that decide who should be tested, which are lower than the familiar ones.
- Treating diabetes — insulin in type 1, the type 2 sequence from metformin onwards, gestational diabetes, and the treatments that were withdrawn or never existed.
- Food, activity and diabetes — the delayed hypoglycaemia that follows exercise, how much activity is recommended, and why there is no such thing as a diabetic diet.
- Living with diabetes — glucose sensors and closed loop systems, hypoglycaemia and ketoacidosis, the annual checks, remission, and the DVLA rules.
- Diabetes and mental health — diabetes distress and burnout, depression and anxiety, fear of hypoglycaemia, and the psychological support that care is supposed to include.