What are normal insulin levels in a healthy individual and how are they managed in patients with diabetes?

Understanding Insulin Levels

Insulin is a hormone produced by pancreatic beta cells that regulates glucose metabolism by stimulating peripheral glucose uptake and inhibiting hepatic glucose production, with normal fasting levels in healthy individuals typically ranging from approximately 5-7 microU/mL, though these values increase significantly with obesity and insulin resistance. 1, 2

Normal Insulin Levels in Healthy Individuals

Baseline Fasting Values

• Healthy adults with normal glucose tolerance and body weight (BMI <25, fasting glucose <6.1 mmol/L) demonstrate fasting insulin levels of 5.73 microU/mL (SD = 3.99) in men and 7.05 microU/mL (SD = 4.67) in women. 2
• These baseline values are positively correlated with body mass index, increasing by approximately 50% when BMI reaches 25-30, and doubling when BMI exceeds 30. 2

Postprandial Response

• In healthy individuals, insulin levels increase approximately five-fold at 2 hours post-glucose load during oral glucose tolerance testing. 2
• The physiologic insulin response includes both basal (background) insulin secretion throughout the day and bolus (mealtime) insulin secretion in response to food intake. 3

Pathologic Elevations

• Persons with glucose intolerance and newly diagnosed type 2 diabetes demonstrate 2-3 fold increases in fasting insulin levels compared to healthy individuals, depending on BMI and gender. 2
• In individuals with type 2 diabetes and BMI >30,2-hour post-OGTT insulin levels can reach 152 microU/mL (SD = 90) in women and 112 microU/mL (SD = 83.4) in men, indicating severe insulin resistance. 2

Clinical Significance of Insulin Measurement

Early Detection of Metabolic Dysfunction

• Fasting insulinemia serves as a useful indicator of peripheral insulin receptor resistance and may be valuable for early diagnosis of impaired glucose tolerance and type 2 diabetes. 2
• Elevated insulin levels (hyperinsulinemia) often precede the development of overt diabetes and are associated with metabolic syndrome, including dyslipidemia, hypertension, and atherosclerosis. 2

Monitoring Treatment Efficacy

• Insulin levels should be monitored during the course of both non-pharmacological and pharmacological treatment of type 2 diabetes. 2
• Normalization or reduction of hyperinsulinemia represents an important treatment goal in obesity and early phases of type 2 diabetes. 2
• Insulinemia serves as a criterion for selecting appropriate oral antidiabetic medications. 2

Insulin Management in Diabetes

Type 1 Diabetes

• Patients with type 1 diabetes require exogenous insulin as primary treatment, typically initiated with multiple daily injections combining rapid-acting insulin 0-15 minutes before meals with one or more daily injections of intermediate or long-acting insulin. 4
• The regimen mimics normal physiologic insulin levels through basal insulin (providing background coverage) plus bolus insulin at mealtimes. 3
• Target HbA1c for all children with type 1 diabetes is recommended to be <7.5% (<58 mmol/mol). 4

Type 2 Diabetes Insulin Initiation

• Insulin therapy is indicated when HbA1c is ≥7.5% (≥58 mmol/mol) despite optimal oral agents, and is essential when HbA1c ≥10% (≥86 mmol/mol). 4
• The preferred initiation method involves adding long-acting basal insulin or once-daily premixed insulin, alone or combined with GLP-1 receptor agonists or oral antidiabetic drugs. 4
• If targets are not met, rapid-acting or short-acting bolus insulin can be added at mealtimes to control postprandial glucose elevations. 4

Hospitalized Patients

• Insulin therapy should be initiated for persistent hyperglycemia starting at a threshold ≥180 mg/dL (10.0 mmol/L), with a target glucose range of 140-180 mg/dL (7.8-10.0 mmol/L) for the majority of critically ill and non-critically ill patients. 3
• More stringent goals of 110-140 mg/dL may be appropriate for selected patients (such as post-cardiac surgery) if achievable without significant hypoglycemia. 3
• A basal-bolus insulin regimen is strongly preferred over sliding-scale insulin alone, which is strongly discouraged due to poor glycemic control. 3, 5, 4

Pharmacokinetics of Insulin

Rapid-Acting Insulin (e.g., Lispro)

• Rapid-acting insulin analogs like lispro are absorbed more quickly than regular human insulin, with peak serum levels occurring 30-90 minutes after subcutaneous dosing compared to 50-120 minutes for regular insulin. 1
• The half-life of lispro after subcutaneous administration is shorter than regular human insulin (1 hour versus 1.5 hours). 1
• One unit of insulin lispro has the same glucose-lowering effect as one unit of regular human insulin on a molar basis. 1

Factors Affecting Absorption

• The rate of insulin absorption is affected by injection site (abdominal absorption is fastest), exercise (increases absorption rate, especially from leg injections), and other variables. 3, 1
• Insulin is absorbed and peaks faster during exercise, particularly when injected into the leg. 3

Common Pitfalls in Insulin Management

Monitoring and Adherence

• Adults prescribed specific insulin regimens average using only 77% of prescribed doses, and even with good adherence, HbA1c levels often remain above recommended targets, suggesting inadequate dosing or regimen design. 6
• Blood glucose monitoring is integral to effective insulin therapy and should not be omitted—fasting plasma glucose should guide basal insulin titration, while both fasting and postprandial values should guide mealtime insulin. 4

Hypoglycemia Risk

• The hypoglycemia alert value in hospitalized patients is defined as blood glucose ≤70 mg/dL (3.9 mmol/L), with clinically significant hypoglycemia at <54 mg/dL (3.0 mmol/L). 3
• Bedtime glucose testing is especially important because nocturnal hypoglycemia symptoms may go unnoticed, potentially causing severe hypoglycemia. 3
• If blood glucose drops below 100 mg/dL (5.6 mmol/L) at bedtime, patients should consume a small snack to prevent nocturnal hypoglycemia. 3

Weight Management

• Tight glycemic control with insulin can cause weight gain and obesity, which increases blood pressure, LDL cholesterol, and triglycerides while decreasing HDL cholesterol. 3
• Weight gain leads to insulin resistance, making glycemic control more difficult—men should maintain waist size ≤40 inches (102 cm) and women ≤35 inches (88.9 cm). 3
• Metformin combined with insulin is associated with decreased weight gain, lower insulin dose requirements, and less hypoglycemia compared to insulin alone. 4