When should insulin therapy be initiated in type 1 and type 2 diabetes, and what are the recommended initial dosing regimens?

Insulin Therapy in Diabetes Management

Type 1 Diabetes: Immediate Insulin Initiation Required

Patients with type 1 diabetes require insulin therapy immediately at diagnosis to sustain life, with multiple daily injections (MDI) as the standard initial regimen. 1, 2

Initial Dosing Regimen for Type 1 Diabetes

• Start with a total daily dose of 0.2-0.4 units/kg body weight 1
• Divide this dose: one-third to one-half as basal insulin (long-acting), with the remainder as rapid-acting insulin split between meals 1, 3
• Administer rapid-acting insulin analogs (lispro, aspart, or glulisine) 0-15 minutes before each meal 2, 4
• Use long-acting basal insulin (glargine, detemir, or degludec) once or twice daily 1, 2

Example calculation: For a 70 kg patient, start with 14-28 units total daily dose. If using 20 units total: give 7-10 units as basal insulin once daily, and 3-4 units before each of three meals as rapid-acting insulin 1.

Preferred Insulin Types for Type 1 Diabetes

• Basal insulin analogues (glargine, detemir, degludec) are strongly preferred over NPH due to reduced hypoglycemia risk, especially nocturnal episodes, and lower intraindividual variability 4
• Rapid-acting analogues (aspart, lispro, glulisine) are preferred over regular human insulin for better postprandial control and reduced delayed hypoglycemia 2, 4

---

Type 2 Diabetes: Stepwise Approach to Insulin Initiation

For type 2 diabetes, insulin should be initiated when oral agents fail to achieve glycemic targets after 3 months, or immediately at diagnosis if presenting with severe hyperglycemia. 1

When to Start Insulin in Type 2 Diabetes

Immediate insulin initiation is required when: 1

• Blood glucose ≥300-350 mg/dL (16.7-19.4 mmol/L) AND/OR
• HbA1c ≥10-12% with symptoms (polyuria, polydipsia, weight loss, ketosis)
• In these cases, start basal insulin PLUS mealtime insulin (basal-bolus regimen)

Consider insulin when: 1, 5

• HbA1c ≥9% despite lifestyle modification and metformin
• HbA1c remains above target after 3 months of optimal oral combination therapy

Short-term intensive insulin therapy (2 weeks to 3 months) should be implemented in newly diagnosed patients with HbA1c >9.0% or fasting glucose ≥11.1 mmol/L with symptomatic hyperglycemia 1

Initial Insulin Regimen for Type 2 Diabetes

Start with basal insulin as the simplest and most effective initial approach: 1, 6

• Begin at 10 units once daily OR 0.1-0.2 units/kg body weight, depending on degree of hyperglycemia 1, 3
• Continue metformin (proven to reduce mortality and cardiovascular events) 1, 6
• May continue one additional oral agent 1
• Inject at any time of day (flexibility advantage in adults) 3

Titration strategy: 1

• Adjust dose every 3-4 days based on fasting blood glucose
• Equip patients with self-titration algorithms based on self-monitoring

Advancing Beyond Basal Insulin

If basal insulin achieves acceptable fasting glucose but HbA1c remains above target, add coverage for postprandial excursions: 1

Option 1: Add GLP-1 receptor agonist (preferred for cardiovascular benefits and weight loss) 1, 5

Option 2: Add mealtime rapid-acting insulin 1

• Start with one injection before the largest meal
• Progress to 1-3 injections before meals as needed
• Alternative method: Split total daily insulin 50% basal/50% bolus (divide bolus evenly between three meals) 1

Option 3: Switch to premixed insulin 1

• Administer 2-3 times daily
• Less flexible but simpler for some patients

---

Critical Dosing Considerations and Pitfalls

Pediatric Patients (Type 1 or Type 2)

• When switching from another insulin to degludec, start at 80% of previous long-acting insulin dose to minimize hypoglycemia risk 3
• Must inject at the same time every day (unlike adults who have flexibility) 3
• For doses <5 units daily, use U-100 vial formulation 3

Common Pitfalls to Avoid

The most critical error is delaying insulin intensification when targets are not met 5. Reassess every 3 months and adjust promptly 1, 5.

Never mix or dilute insulin degludec with other insulins or solutions 3

Do not transfer insulin from pens to syringes - this causes dosing errors 3

Do not administer long-acting insulin intramuscularly - severe hypoglycemia may result from altered absorption 2

Avoid injecting into lipohypertrophy sites - rotate injection sites properly to prevent this complication which distorts absorption 2

Dose Adjustments Required For

• Changes in physical activity, meal patterns, or timing 3
• Renal or hepatic dysfunction 3
• Acute illness 3
• When oral agents are discontinued (risk of rebound hyperglycemia if stopped abruptly) 2

Medication Continuation During Insulin Therapy

Continue metformin when adding insulin - it reduces insulin dose requirements, weight gain, and hypoglycemia compared to insulin alone 1, 2, 6

Discontinue sulfonylureas, DPP-4 inhibitors, and GLP-1 agonists when advancing to complex insulin regimens (beyond basal insulin alone) 1

May continue SGLT2 inhibitors or pioglitazone to reduce total insulin dose, but monitor for ketoacidosis risk with SGLT2 inhibitors and heart failure/fracture risk with pioglitazone 1

Monitoring Requirements

Titrate insulin doses based on self-monitoring of blood glucose: 1

• Use fasting glucose to adjust basal insulin
• Use both fasting and postprandial glucose to adjust mealtime insulin

Check HbA1c every 3 months until target achieved, then every 6 months 1

Monitor vitamin B12 levels periodically with long-term metformin use, especially if anemia or peripheral neuropathy develops 5