Type 1 vs Type 2 Diabetes: Insulin, Blood Glucose and What Changes
Type 1 and Type 2 diabetes can produce the same number on a blood-glucose meter, but they do not begin with the same physiological problem. That difference explains why insulin is essential in Type 1 diabetes, why Type 2 diabetes can be treated through several different metabolic pathways, and why the two conditions can deteriorate differently.
The easiest way to understand diabetes is not to memorise two lists. Start with what insulin normally does, then follow what changes.
First, what is insulin actually doing?
After carbohydrate is digested, glucose is absorbed from the gastrointestinal tract into the bloodstream. As blood glucose rises, beta cells in the pancreatic islets increase insulin secretion.
Insulin then travels through the circulation and binds to insulin receptors on target cells. It is often described as a key that “unlocks” the cell, but that is only a teaching shortcut. Insulin does not open a physical door. Receptor binding starts an intracellular signalling pathway.
In skeletal muscle and adipose tissue, this signalling promotes movement of glucose transporter type 4 (GLUT4) to the cell membrane. With more GLUT4 transporters available, glucose uptake into these cells increases. Insulin also promotes glycogen synthesis and reduces hepatic glucose production, helping move circulating glucose back towards its usual range.
In skeletal muscle and adipose tissue, insulin signalling moves GLUT4 transporters to the cell membrane, increasing glucose uptake.
Follow the physiology: glucose rises after a meal → pancreatic beta cells release insulin → insulin binds to receptors → intracellular signalling increases glucose uptake and storage → blood glucose falls towards its usual range.
Between meals, insulin concentrations fall and glucagon has relatively greater influence. The liver can then release stored glucose and produce glucose to help maintain the circulating supply. Glucose homeostasis is therefore a continuously regulated balance involving the pancreas, liver, skeletal muscle, adipose tissue, gut hormones and other metabolic signals.
Now break the system: Type 1 and Type 2 fail in different places
Type 1 diabetes: not enough insulin
Type 1 diabetes is an autoimmune condition in which pancreatic beta cells are destroyed. As functioning beta-cell mass falls, endogenous insulin production becomes severely deficient.
Now go back to the normal pathway. Glucose is still entering the blood, but there is not enough insulin to regulate its use and storage normally. Insulin-responsive tissues cannot respond appropriately, while hepatic glucose production is insufficiently suppressed. Blood glucose therefore rises.
Once the filtered glucose load exceeds the kidney's capacity for reabsorption, glucose remains in the urine and draws water with it. This osmotic diuresis contributes to polyuria, thirst, dehydration and electrolyte loss. Weight loss can occur despite eating because insulin deficiency promotes breakdown of fat and protein rather than normal storage of metabolic fuel.
Why ketones matter in Type 1 diabetes
Insulin normally restrains lipolysis. With marked insulin deficiency, stored triglyceride is broken down and free fatty acids are delivered to the liver, where some are converted into ketone bodies. If ketone production becomes excessive, metabolic acidosis develops.
This is why diabetic ketoacidosis (DKA) is more than “very high sugar”. It is a metabolic emergency involving insulin deficiency, ketonaemia, acidosis, dehydration and electrolyte disturbance. Deep, laboured breathing can occur as respiratory compensation for metabolic acidosis. Nausea, vomiting and abdominal pain may worsen fluid loss.
Importantly, the glucose concentration does not have to be exceptionally high for DKA to occur. Euglycaemic or relatively low-glucose DKA can occur, including in association with sodium-glucose cotransporter-2 (SGLT-2) inhibitors. Suspected DKA therefore requires assessment of the whole metabolic picture rather than glucose alone.
Because the fundamental problem in Type 1 diabetes is severe insulin deficiency, insulin replacement is essential. It is not a treatment that is added only after tablets fail.
Type 2 diabetes: insulin is present, but the response is impaired
Type 2 diabetes usually develops through a combination of insulin resistance and progressive beta-cell dysfunction. Early in the condition, the pancreas may still produce substantial amounts of insulin. The problem is that skeletal muscle, liver and adipose tissue respond less effectively to it.
The beta cells initially compensate by secreting more insulin. For a time this can keep glucose close to the usual range. In susceptible people, however, beta-cell function progressively declines and insulin secretion can no longer compensate adequately for insulin resistance.
The liver is particularly important. Hepatic insulin resistance means glucose production may remain inappropriately active even when circulating glucose is already raised, contributing especially to fasting hyperglycaemia.
Type 2 diabetes is influenced by multiple interacting factors, including genetics, age, ethnicity, body composition, previous gestational diabetes, physical activity, medicines and wider social determinants of health. Overweight and obesity can increase risk, but neither is required for Type 2 diabetes and reducing the condition to personal blame is scientifically inaccurate.
DKA and HHS: understand the difference rather than memorising the names
In profound insulin deficiency, uncontrolled lipolysis and ketone production make DKA a particular concern. In Type 2 diabetes, enough insulin activity is often present to suppress major ketogenesis, but not enough to control blood glucose adequately.
Severe, sustained hyperglycaemia can then drive extensive osmotic diuresis. The resulting water loss, hyperglycaemia and rising serum osmolality can produce hyperosmolar hyperglycaemic state (HHS). HHS generally develops more gradually than DKA and is characterised by severe dehydration and hyperosmolality, often with marked hyperglycaemia and neurological change.
Do not turn this into a rigid rule. People with Type 2 diabetes can develop DKA, and mixed DKA/HHS presentations occur. The diabetes label does not diagnose the emergency.
Think clinically: vomiting, abdominal pain, raised ketones and deep breathing should make you think about DKA. Profound dehydration, very high glucose and altered cognition developing over days should make you think about HHS. Both require urgent assessment and treatment according to local and specialist guidance.
What do the diabetes tests actually tell you?
A capillary blood-glucose measurement is a snapshot. It tells you the glucose concentration at that moment and can change quickly with food, insulin, physical activity, infection, physiological stress, steroids and other medicines.
HbA1c answers a different question. Glycated haemoglobin reflects glycaemic exposure over the preceding weeks, with greater influence from more recent glucose levels. It is useful for diagnosis and longer-term monitoring in appropriate circumstances, but it is not a real-time measure and should not delay assessment when acute Type 1 diabetes, DKA or another hyperglycaemic emergency is suspected.
HbA1c also needs context because conditions that alter red-cell lifespan or haemoglobin can affect its interpretation.
If the diabetes type is uncertain, specialist assessment may include pancreatic autoantibodies and C-peptide. Autoantibodies can support a diagnosis of autoimmune diabetes. C-peptide is produced when endogenous proinsulin is cleaved into insulin and C-peptide, so it can provide information about the person's own insulin secretion. Injected insulin does not contain C-peptide.
Age and body size are clues, not diagnostic rules. Adults can develop Type 1 diabetes, and Type 2 diabetes can occur in younger people and in people without obesity.
Why treatment is different
In Type 1 diabetes, treatment has to replace the missing insulin. Regimens vary, but insulin must cover background metabolic requirements as well as rises associated with meals. Management also involves glucose monitoring, carbohydrate estimation where appropriate, physical activity, illness management, ketone testing and prevention and treatment of hypoglycaemia.
Type 2 diabetes has more possible treatment targets because several metabolic abnormalities contribute. Management can include structured education, nutritional and physical-activity support and glucose-lowering medicines. Current NICE guidance individualises treatment according to factors including cardiovascular status, renal status, obesity, frailty, early-onset Type 2 diabetes, treatment goals and medicine safety.
This is an important update for students: Type 2 treatment should no longer be remembered as a simple fixed ladder of “metformin, then another tablet, then insulin”. NICE updated NG28 in February 2026. Modified-release metformin and SGLT-2 inhibitors now have prominent first-line roles for many adults, with treatment choices shaped by comorbidities and individual clinical characteristics. GLP-1 receptor agonists, tirzepatide, DPP-4 inhibitors, sulfonylureas, pioglitazone and insulin-based treatment may be appropriate in selected circumstances.
Insulin can also be required in Type 2 diabetes when endogenous secretion becomes insufficient or temporarily during significant illness or metabolic decompensation. Needing insulin does not turn Type 2 diabetes into Type 1 diabetes.
Put it together: the same glucose result can mean something different
Imagine a 19-year-old with several weeks of thirst, polyuria and weight loss who now has vomiting, abdominal pain and deep breathing. Their capillary glucose is 18 mmol/L.
Now imagine a 78-year-old with known Type 2 diabetes who has a chest infection, poor oral intake and increasing confusion over several days. Their glucose is also markedly raised.
The glucose meter confirms hyperglycaemia in both people, but it does not tell you the diagnosis.
In the first patient, the history should make severe insulin deficiency and DKA an urgent concern. Assessment would typically include blood ketones, venous blood gas measurements including pH and bicarbonate, electrolytes, renal function, fluid status and investigation of the precipitating cause.
In the second patient, HHS may be a major concern because of the combination of illness, poor intake, severe dehydration and neurological change. Osmolality, glucose, electrolytes, renal function, fluid status and the underlying infection become central to assessment. DKA or a mixed presentation must still be considered.
The point is not to diagnose from the vignette. The point is to stop treating glucose as an isolated number.
What changes for you on placement?
When caring for somebody with diabetes, first establish how they normally manage it. They may use insulin pens, a pump or a hybrid closed-loop system, and may monitor glucose using finger-prick testing, continuous glucose monitoring or both. Where clinically appropriate and safe, people who normally self-manage should be involved in decisions about their diabetes care.
Meal timing matters. Rapid-acting insulin followed by delayed food, vomiting, interruption of enteral feeding or an unexpected procedure can create immediate hypoglycaemia risk. Do not independently omit, delay or alter prescribed insulin. If intake or treatment timing changes, escalate promptly and follow the prescribed plan and local policy.
Acute illness can raise glucose through counter-regulatory hormones such as cortisol and catecholamines, even when food intake falls. Glucocorticoids can also cause substantial hyperglycaemia. A changing glucose pattern may therefore be telling you something about the wider clinical picture.
Look beyond the number. Consider hydration, consciousness, respiratory pattern, nausea, vomiting, abdominal pain, urine output and ketones when indicated. Renal function matters because it can alter medicine safety and insulin requirements.
Hypoglycaemia also needs more than a quick correction. Sweating, tremor, palpitations, hunger, confusion, behavioural change, drowsiness, seizure or loss of consciousness may occur, although warning symptoms vary. Treatment depends on consciousness, swallowing safety and local protocol. Once the immediate episode is treated, ask why it happened. Recurrent or unexplained hypoglycaemia needs escalation and review of the person's regimen and clinical condition.
Remember the physiology: Type 1 diabetes is primarily severe insulin deficiency caused by autoimmune beta-cell destruction. Type 2 diabetes usually combines insulin resistance with progressive beta-cell dysfunction. In either condition, a glucose result only makes sense when you interpret it alongside the person in front of you.
Escalate reduced consciousness, severe or recurrent hypoglycaemia, vomiting with hyperglycaemia, raised ketones, deep or laboured breathing, significant dehydration, neurological change or any concern for DKA or HHS.
References
National Institute for Health and Care Excellence (NICE) (2015, updated February 2026). Type 2 diabetes in adults: management (NG28). Available at: NICE NG28.
National Institute for Health and Care Excellence (NICE). Type 1 diabetes in adults: diagnosis and management (NG17). Available at: NICE NG17.
Joint British Diabetes Societies for Inpatient Care (2023). The management of diabetic ketoacidosis in adults. Available via the Association of British Clinical Diabetologists.
Joint British Diabetes Societies for Inpatient Care. The management of the hyperosmolar hyperglycaemic state (HHS) in adults with diabetes. Available via the Association of British Clinical Diabetologists.
Joint British Diabetes Societies for Inpatient Care (2023). The hospital management of hypoglycaemia in adults with diabetes mellitus. Available via the Association of British Clinical Diabetologists.
Nursing and Midwifery Council (NMC) (2018, updated 2025). The Code: Professional standards of practice and behaviour for nurses, midwives and nursing associates. Available at: NMC Code.
This article is for education and placement preparation. Follow the diabetes, medicines and escalation procedures used in your placement area and work within your level of competence.