Insulin Therapy in Type 1 and Type 2 Diabetes
Type 1 Diabetes: Insulin is Mandatory and Must Be Physiologic
All patients with type 1 diabetes require insulin from diagnosis, delivered as multiple daily injections (MDI) or continuous subcutaneous insulin infusion (CSII) to provide both basal and prandial coverage. 1, 2
Initiation Strategy for Type 1 Diabetes
- Start with MDI regimen: 3-4 injections daily consisting of basal insulin (intermediate or long-acting) plus rapid-acting insulin analogs before each meal 2, 3
- Rapid-acting insulin analogs (lispro, aspart, glulisine) are preferred over regular human insulin because they reduce hypoglycemia risk while achieving equivalent HbA1c control 2, 4
- Long-acting basal analogs (glargine, degludec, detemir) are preferred over NPH due to markedly lower nocturnal hypoglycemia rates 3, 4
- Alternative: Twice- or thrice-daily premixed insulin (70/30 NPH/regular or analog premixes) may be used, though this requires fixed meal schedules 2, 3
Essential Patient Education for Type 1 Diabetes
- Teach carbohydrate counting and matching prandial insulin dose to carbohydrate intake, premeal glucose, and anticipated activity 2
- Provide comprehensive instruction on hypoglycemia recognition, prevention, and treatment including use of glucagon 1
- Multi-component self-management education by a certified diabetes care and education specialist (CDCES) covering insulin technique, sick-day management, and DKA prevention is mandatory 1
Technology Integration
- Automated insulin delivery (AID) systems are recommended for many patients with type 1 diabetes as they increase time-in-range while reducing hypoglycemia 1
- Continuous glucose monitoring (CGM) is the preferred method of glucose monitoring for all individuals with type 1 diabetes 1
- Sensor-augmented pump therapy with threshold suspend features reduces nocturnal hypoglycemia without increasing HbA1c 2
Type 2 Diabetes: Insulin When Non-Insulin Therapies Fail
In type 2 diabetes, initiate basal insulin when HbA1c ≥10% or random glucose ≥300 mg/dL regardless of symptoms, or when HbA1c remains ≥7.5% despite optimal oral agents after 3-6 months. 5, 6, 3
When to Start Insulin in Type 2 Diabetes
- Immediate insulin therapy required: HbA1c ≥10% (≥86 mmol/mol) OR random glucose ≥300 mg/dL, irrespective of symptoms 5, 6
- Marked hyperglycemia with catabolic features (polyuria, polydipsia, weight loss, ketonuria) mandates insulin even at lower HbA1c levels 6, 3
- Consider insulin when HbA1c ≥7.5% despite optimal use of metformin, GLP-1 RA, and/or SGLT2 inhibitors 6, 3
Basal Insulin Initiation Protocol
Start with 10 units once daily (or 0.1-0.2 units/kg/day) of long-acting basal insulin at bedtime or any consistent time, while continuing metformin. 5, 6, 7
Specific Dosing by HbA1c:
Titration Algorithm:
- Increase dose by 10-15% (approximately 2-4 units) once or twice weekly until fasting plasma glucose reaches 80-130 mg/dL 5, 6, 7
- Provide patients with a self-titration algorithm at initiation—this significantly improves glycemic control 6
- Titration should be guided by self-monitored fasting blood glucose values 3, 7
Basal Insulin Selection
- Long-acting basal analogs (glargine U100/U300, degludec U100/U200, detemir) are preferred over NPH because they markedly lower nocturnal and overall hypoglycemia risk 6, 3
- Glargine U300 and degludec provide even less hypoglycemia than glargine U100 or detemir 6
- NPH insulin remains acceptable when cost is prohibitive, though its pharmacodynamic profile is suboptimal 6
Concurrent Medication Management
- Continue metformin when adding basal insulin—it reduces weight gain, insulin dose requirements, and hypoglycemia 3, 8
- Discontinue sulfonylureas once insulin starts to prevent additive hypoglycemia risk 6
- Maintain SGLT2 inhibitors in patients with heart failure or CKD for cardiovascular/renal protection 6
- Continue GLP-1 receptor agonists for cardiovascular benefit or weight management when adding basal insulin 6
- DPP-4 inhibitors may be co-prescribed with basal insulin without increased hypoglycemia 6
Intensification Beyond Basal Insulin (Type 2 Diabetes)
Recognizing Over-Basalization
If basal insulin dose exceeds 0.5 units/kg/day (or if bedtime-morning glucose differential is ≥50 mg/dL) without meaningful fasting glucose improvement, do NOT continue escalating basal insulin—this is over-basalization. 9, 6
Intensification Options When HbA1c Remains Above Target:
Add prandial insulin before the largest meal: start with 4 units, 0.1 units/kg, or 10% of basal dose. 10, 9, 5, 6
Alternative strategies (choose based on patient preference and clinical context):
- Add or switch to a GLP-1 receptor agonist if not already used—this approach reduces weight gain and hypoglycemia compared to insulin alone 10
- Fixed-ratio combination products (basal insulin + GLP-1 RA) such as glargine/lixisenatide or degludec/liraglutide reduce weight gain and hypoglycemia versus insulin alone 10, 6
- Switch to twice-daily premixed insulin (70/30 NPH/regular or analog premixes) at 6-10 units twice daily before breakfast and dinner 10, 6
Progression to Basal-Bolus Regimen:
- If single prandial injection fails to achieve target, progress to 2 or more prandial injections (basal-bolus regimen) 10
- Prefer rapid-acting insulin analogs (lispro, aspart, glulisine) over regular human insulin for basal-bolus therapy to achieve more physiologic profiles and lower hypoglycemia risk 9, 6
- With significant prandial insulin additions (especially at dinner), consider decreasing basal insulin dose 9
Special Insulin Formulations
Concentrated Insulins
- U-500 regular insulin is indicated for patients requiring >200 units/day—it has delayed onset and longer duration than U-100, with both prandial and basal properties 10, 9
- U-300 glargine and U-200 degludec allow higher basal insulin doses per volume with longer duration of action 10, 9
- U-200 lispro reduces injection volume for patients requiring large prandial doses, potentially improving adherence 10
- Concentrated insulins are available only in prefilled pens (except U-500 regular) to minimize dosing errors 10, 9
Inhaled Insulin
- Inhaled insulin is available for prandial use with limited dosing range 10
- Contraindicated in chronic lung disease (asthma, COPD) and not recommended for smokers or recent ex-smokers 10
- Requires spirometry before and after initiation to identify potential lung disease 10
Critical Safety Considerations
Hypoglycemia Management
- Oral glucose (tablets or fruit juice) followed by protein/carbohydrate snack for conscious patients with glucose <70 mg/dL 1
- Glucagon (intranasal, prefilled liquid, or reconstituted) for patients unable or unwilling to ingest carbohydrates orally 1
Common Pitfalls to Avoid
- Do not abruptly discontinue oral medications when starting insulin—risk of rebound hyperglycemia 3
- Avoid intramuscular injections, especially with long-acting insulins—severe hypoglycemia may result 3
- Do not inject into lipohypertrophic areas—this distorts insulin absorption; correct site rotation prevents lipohypertrophy 3
- Do not frame insulin as "punishment" or "failure"—explain that type 2 diabetes is progressive and insulin is a natural next step 6
- Avoid overbasalization—escalating basal insulin beyond 0.5-1.0 units/kg/day without fasting glucose improvement indicates need for prandial insulin or regimen change 9, 6, 7