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Fasting Insulin Test: The Often-Overlooked Sign of Insulin Resistance

Reviewed by Jeremie Walker, MD, MBA · December 27, 2024

If you’ve been to the doctor in the past and labs were ordered, chances are your blood glucose (sugar) level was measured. But when was the last time you checked your insulin levels? After all, insulin is the primary hormone involved in regulating glucose in the body.

Many patients live with insulin resistance for years before developing type 2 diabetes. This means that your body's ability to handle sugar might be impaired long before any noticeable symptoms appear.

But don’t worry. There is a way to catch metabolic disorders such as insulin resistance early. It might be as simple as getting a blood test to evaluate your fasting insulin levels. The fasting insulin test is an effective tool to see how well your body metabolizes glucose before its levels start to rise. 

  • A fasting insulin test can detect higher insulin production even when your glucose levels are still normal. It is a powerful tool for the early detection of impaired glucose regulation.
  • Elevated insulin levels could be a sign of disorders like type 2 diabetes, pancreatic tumors, or Cushing’s syndrome. 
  • You can improve your insulin sensitivity by changing your daily habits: eating a healthier diet, losing weight, exercising, and having a healthy sleep routine. 

Insulin is a hormone produced by the beta cells of the pancreas. It plays a crucial role in human energy metabolism: it drives glucose into cells, thereby reducing its concentration in the blood. 

When you eat, carbohydrates are broken down into glucose and absorbed through the intestines. The presence of glucose in the bloodstream signals the pancreas to release insulin.

Why? An energy-rich molecule like glucose isn’t of much use circulating the body without entering the cells. Besides, it might also damage the blood vessels.1

The main role of insulin is to reduce blood sugar concentration by moving glucose into cells where it can be used and stored. This process is especially significant in muscle cells, liver cells, and fat cells.2 

Insulin Resistance: Where Things Go Wrong

Consuming an excess of calories over an extended period can cause cells to become resistant to the physiologic effect of insulin, preventing them from taking up incoming glucose. This metabolic condition is known as insulin resistance.2

In response, the pancreas increases insulin production as a compensatory mechanism to regulate glucose levels. However, this leads to an excessively high concentration of insulin in the blood.2

Increased insulin production might maintain glucose levels within a normal range for a while. However, over time, the pancreas may become exhausted and produce less insulin, or none at all. This insufficient insulin production, which fails to regulate glucose levels, is a key factor in the development of type 2 diabetes.2

While type 2 diabetes is the most obvious consequence of insulin resistance, this metabolic disorder is linked to a range of other conditions, with ongoing research uncovering more. These conditions include polycystic ovary syndrome (PCOS), obesity, heart disease, and possibly, neurodegeneration.3

Metabolic Health Testing: Standard Diagnostic Options

Modern diagnostics relies on measuring the levels of glucose and insulin in your blood. Your physician might order standard tests like fasting glucose and insulin, glucose levels in response to an oral glucose tolerance test (OGTT), and hemoglobin A1c (HbA1c).1 

Some of these tests, like fasting plasma glucose and insulin, show a snapshot of your current insulin and glucose concentrations. Others, like OGTT and HbA1c, show how your glucose is regulated over time or after meals. 

Fasting plasma glucose and insulin levels in your plasma are measured after 8 to 12 hours of not consuming food. OGTT tracks how your glucose and insulin levels change within 2 hours in response to drinking a mixture of water and 75 grams of sugar.4 

The HbA1c test measures the amount of glucose attached to hemoglobin, a protein in red blood cells that reacts with glucose in the bloodstream. This test provides an overview of average blood glucose levels over the past three months, reflecting the typical lifespan of red blood cells (the actual lifespan for many people may be different).1

Many physicians nowadays rely predominantly on fasting glucose and HbA1c to diagnose insulin resistance and type 2 diabetes. While these markers might provide important information about your metabolic health, OGTT is more reliable but also more difficult to administer.1 Somehow, the fasting insulin test is vastly underused in modern practice. 

The Importance of the Fasting Insulin Test

Insulin levels are typically out of whack long before any abnormalities in blood sugar levels can be detected. Therefore, a blood sugar test might come back completely normal even if you are struggling with insulin resistance or incipient type 2 diabetes.5

A simple test to assess glucose regulation is the fasting insulin test. Normally, your insulin levels should be quite low after fasting for 8 to 12 hours. If your fasting insulin is high, that might be an alarming sign.6

For example, someone might have normal HbA1c and fasting plasma glucose levels, but their fasting insulin concentration may be increased. Using the OGTT test, physicians can then investigate whether the patient is producing extra insulin to manage glucose concentration. Sometimes, the immediate release of insulin following glucose intake can be three or four times higher than normal!5 

Despite having real glucose dysregulation and high insulin levels, this person would not receive an accurate diagnosis based on just HbA1c and fasting plasma glucose measurements. Misdiagnosing insulin resistance at an early stage robs you of the opportunity to improve your metabolic health through lifestyle interventions while there is still time. 

What to Expect: Healthy and Unhealthy Insulin Ranges

Using commercial assays, the normal fasting insulin range is 5-15 µU/mL. More sensitive assays show a normal fasting insulin level to be under 12 μU/mL.7 It’s worth noting that the normal range of fasting insulin varies somewhat between labs. 

However, most healthy individuals would find it to be much lower than the upper limit. In fact, it might be below the lower limit (5µU/mL). Obese people have higher levels of insulin, while those with severe insulin resistance have very high circulating insulin levels (above 15 µU/mL).7

Women with PCOS whose BMI is within the normal range have slightly elevated insulin levels. Higher insulin levels are found in obese women with PCOS.7

Furthermore, the normal ranges of insulin during OGTT are the following:8

  • 30 minutes after glucose intake: 30-230 mlU/L
  • 1 hour after glucose intake: 18-276 mlU/L
  • 2 hours after glucose intake: 16-166 mlU/L
  • 3 or more hours after glucose intake: less than 25 mlU/L

Fasting Insulin Test Reveals Important Metabolic Health Findings

There are several reasons for your insulin production to be dysregulated. Fasting insulin test results are often interpreted in combination with your symptoms, medical history, and other test results like fasting glucose.

Here is what could be causing your fasting insulin to be lower than normal:

  • Type 1 diabetes. The autoimmune disease in which the pancreas cells that normally produce insulin are destroyed is the most common cause of low insulin production. At the same time, your glucose levels will be up.9
  • Pancreatic disease. Inflammation of the pancreas, also known as pancreatitis, might reduce its ability to pancreas insulin.10 If your entire pancreas or a part of it is removed, your insulin levels will also drop.11
  • Insufficiency of the pituitary gland. The pituitary gland dictates the activity of other endocrine glands in your body. Although this condition is very rare, people whose pituitary gland fails to make enough of its hormones typically have lower insulin levels.12

Your insulin levels might be high due to some of the following reasons:

  • Insulin resistance. We already explained how the pancreas starts to produce extra insulin when cells become “resistant” to its effects. 
  • Obesity. High insulin levels increase weight gain, while obesity impairs the way cells respond to insulin.13
  • Type 2 Diabetes. Type 2 diabetes is the most common cause of high insulin before the pancreas becomes damaged and fails to produce enough of this hormone.14
  • Tumors of the pancreas. Insulinomas are benign pancreatic tumors that produce too much insulin.15 In disorders like pancreatic cell hyperplasia, where pancreatic cells appear normal but grow in number, the insulin level is also increased.16
  • Cushing’s syndrome. This rare disease characterized by excess cortisol production causes cells to respond differently to insulin, leading to an increase in insulin production.17
  • Acromegaly. A rare disease marked by high levels of growth hormone and a substance (insulin-like growth factor 1 [IGF-1]) that promotes the production of insulin.18
  • Certain drugs. Taking medications such as corticosteroids, sulfonylureas (used for treating type 2 diabetes), and oral contraceptives might increase your insulin levels.19–21 
  • Pregnancy. Insulin levels normally increase in pregnancy.22 In some cases, pregnant women develop a pregnancy-specific type of diabetes called gestational diabetes. 

I Have a High Fasting Insulin, Now What? 

You might be able to improve your metabolic health by changing your daily habits. It is always best to reach out to your trusted healthcare provider to come up with a plan, which may sometimes involve the use of medication. 

  • Eat a healthy and nutritious diet. Try to get enough carbohydrates from vegetables, fruits, whole grains, and legumes, instead of processed foods rich in sugar, added fats, and sodium.23
  • Lose weight. Losing even a small amount of your body weight can improve your insulin sensitivity and reduce your risk of diabetes.24
  • Move your body. Regular physical exercise is a good way to improve glucose metabolism.25
  • Get enough sleep. Healthy sleep helps lower your insulin levels.26
  • Supplements. Some supplements may help to decrease your insulin levels: chromium, cinnamon, green tea, magnesium, and alpha lipoic acid.27–31 Make sure to talk with your healthcare provider before introducing any new supplements. 

Wrapping Up

Fasting insulin outside of the normal range can indicate trouble with glucose regulation before other markers detect it. It also allows you to make timely changes to your lifestyle and prevent more serious metabolic conditions. 

At Opt Health, we are big advocates of early detection and prevention. Our physicians utilize the latest, evidence-based biomarkers to assess your metabolic health. If your test results suggest that your body is managing glucose poorly, together we can develop a comprehensive plan to help you get back on track.

References

1. Attia P. Assessing metabolic health: where HbA1c falls short and how it compares to fasting glucose, CGM, and OGTT. Peter Attia. July 10, 2024. Accessed November 11, 2024. https://peterattiamd.com/problems-with-hba1c/

2. Wilcox G. Insulin and insulin resistance. Clin Biochem Rev. 2005;26(2):19-39.

3. Insulin Resistance & Prediabetes - NIDDK. National Institute of Diabetes and Digestive and Kidney Diseases. Accessed November 11, 2024. https://www.niddk.nih.gov/health-information/diabetes/overview/what-is-diabetes/prediabetes-insulin-resistance

4. Eyth E, Basit H, Swift CJ. Glucose Tolerance Test. In: StatPearls. StatPearls Publishing; 2024. Accessed November 12, 2024. http://www.ncbi.nlm.nih.gov/books/NBK532915/

5. DiNicolantonio JJ, Bhutani J, OKeefe JH, Crofts C. Postprandial insulin assay as the earliest biomarker for diagnosing pre-diabetes, type 2 diabetes and increased cardiovascular risk. Open Heart. 2017;4(2):e000656. doi:10.1136/openhrt-2017-000656

6. Brodows RG. Starvation enhances the ability of insulin to inhibit its own secretion. Metabolism. 1985;34(1):53-57. doi:10.1016/0026-0495(85)90060-5

7. Carmina E, Stanczyk FZ, Lobo RA. Chapter 34 - Laboratory Assessment. In: Strauss JF, Barbieri RL, eds. Yen & Jaffe’s Reproductive Endocrinology (Seventh Edition). W.B. Saunders; 2014:822-850.e3. doi:10.1016/B978-1-4557-2758-2.00034-2

8. Insulin: Background, Serum Insulin Measurement, Interpretation. Medscape. October 29, 2024. Accessed November 12, 2024. https://emedicine.medscape.com/article/2089224-overview#a1

9. Atkinson MA, Eisenbarth GS, Michels AW. Type 1 diabetes. The Lancet. 2014;383(9911):69-82. doi:10.1016/S0140-6736(13)60591-7

10. Malecka-Panas E, Gasiorowska A, Kropiwnicka A, Zlobinska A, Drzewoski J. Endocrine pancreatic function in patients after acute pancreatitis. Hepatogastroenterology. 2002;49(48):1707-1712.

11. Effects of hemipancreatectomy on insulin secretion and glucose tolerance in healthy humans - PubMed. Accessed November 11, 2024. https://pubmed.ncbi.nlm.nih.gov/2179721/

12. Castillo AR, Zantut-Wittmann DE, Neto AM, Jales RM, Garmes HM. Panhypopituitarism Without GH Replacement: About Insulin Sensitivity, CRP Levels, and Metabolic Syndrome. Horm Metab Res. 2018;50(9):690-695. doi:10.1055/a-0649-8010

13. Ling JCY, Mohamed MNA, Jalaludin MY, Rampal S, Zaharan NL, Mohamed Z. Determinants of High Fasting Insulin and Insulin Resistance Among Overweight/Obese Adolescents. Sci Rep. 2016;6:36270. doi:10.1038/srep36270

14. Cătoi AF, Pârvu A, Mureşan A, Busetto L. Metabolic Mechanisms in Obesity and Type 2 Diabetes: Insights from Bariatric/Metabolic Surgery. Obesity Facts. 2015;8(6):350. doi:10.1159/000441259

15. Okabayashi T, Shima Y, Sumiyoshi T, et al. Diagnosis and management of insulinoma. World Journal of Gastroenterology : WJG. 2013;19(6):829. doi:10.3748/wjg.v19.i6.829

16. Ouyang D, Dhall D, Yu R. Pathologic pancreatic endocrine cell hyperplasia. World Journal of Gastroenterology : WJG. 2011;17(2):137. doi:10.3748/wjg.v17.i2.137

17. Barbot M, Ceccato F, Scaroni C. Diabetes Mellitus Secondary to Cushing’s Disease. Frontiers in Endocrinology. 2018;9:284. doi:10.3389/fendo.2018.00284

18. Ferraù F, Albani A, Ciresi A, Giordano C, Cannavò S. Diabetes Secondary to Acromegaly: Physiopathology, Clinical Features and Effects of Treatment. Frontiers in Endocrinology. 2018;9:358. doi:10.3389/fendo.2018.00358

19. Geer EB, Islam J, Buettner C. Mechanisms of Glucocorticoid-Induced Insulin Resistance: Focus on Adipose Tissue Function and Lipid Metabolism. Endocrinology and metabolism clinics of North America. 2014;43(1):75. doi:10.1016/j.ecl.2013.10.005

20. Ahrén B, Scherstén B. Effect of sulfonylurea on glucose, insulin and C-peptide responses to a meal stimulus in a patient with type 2 diabetes and liver disease. Acta Med Scand. 1984;215(5):487-491. doi:10.1111/j.0954-6820.1984.tb17683.x

21. Cortés ME, Alfaro AA. The effects of hormonal contraceptives on glycemic regulation. The Linacre Quarterly. 2014;81(3):209. doi:10.1179/2050854914Y.0000000023

22. Sonagra AD, Biradar SM, K D, Ds JM. Normal Pregnancy- A State of Insulin Resistance. Journal of Clinical and Diagnostic Research : JCDR. 2014;8(11):CC01. doi:10.7860/JCDR/2014/10068.5081

23. Impact of High-Carbohydrate Diet on Metabolic Parameters in Patients with Type 2 Diabetes - PMC. Accessed November 12, 2024. https://pmc.ncbi.nlm.nih.gov/articles/PMC5409661/

24. Martyn JAJ, Kaneki M, Yasuhara S. Obesity-induced insulin resistance and hyperglycemia: etiologic factors and molecular mechanisms. Anesthesiology. 2008;109(1):137-148. doi:10.1097/ALN.0b013e3181799d45

25. Fedewa MV, Gist NH, Evans EM, Dishman RK. Exercise and insulin resistance in youth: a meta-analysis. Pediatrics. 2014;133(1):e163-174. doi:10.1542/peds.2013-2718

26. Leproult R, Deliens G, Gilson M, Peigneux P. Beneficial impact of sleep extension on fasting insulin sensitivity in adults with habitual sleep restriction. Sleep. 2015;38(5):707-715. doi:10.5665/sleep.4660

27. Lau FC, Bagchi M, Sen CK, Bagchi D. Nutrigenomic basis of beneficial effects of chromium(III) on obesity and diabetes. Mol Cell Biochem. 2008;317(1-2):1-10. doi:10.1007/s11010-008-9744-2

28. Hlebowicz J, Hlebowicz A, Lindstedt S, et al. Effects of 1 and 3 g cinnamon on gastric emptying, satiety, and postprandial blood glucose, insulin, glucose-dependent insulinotropic polypeptide, glucagon-like peptide 1, and ghrelin concentrations in healthy subjects. Am J Clin Nutr. 2009;89(3):815-821. doi:10.3945/ajcn.2008.26807

29. Liu K, Zhou R, Wang B, et al. Effect of green tea on glucose control and insulin sensitivity: a meta-analysis of 17 randomized controlled trials. Am J Clin Nutr. 2013;98(2):340-348. doi:10.3945/ajcn.112.052746

30. Fung TT, Manson JE, Solomon CG, Liu S, Willett WC, Hu FB. The association between magnesium intake and fasting insulin concentration in healthy middle-aged women. J Am Coll Nutr. 2003;22(6):533-538. doi:10.1080/07315724.2003.10719332

31. Henriksen EJ. Exercise training and the antioxidant alpha-lipoic acid in the treatment of insulin resistance and type 2 diabetes. Free Radic Biol Med. 2006;40(1):3-12. doi:10.1016/j.freeradbiomed.2005.04.002

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