Understanding insulin's role in metabolic health is vital for overall well-being and maintaining a healthy lifestyle. In current research and clinical practice, there is no consensus (yet) on what optimal insulin levels are, and testing is often inaccessible. We can, however, use glucose levels as a proxy for insulin sensitivity.
Insulin: the hormone that regulates glucose
Insulin is a hormone produced by the pancreas that plays a critical role in regulating blood glucose levels by facilitating its absorption. Insulin acts as a key that unlocks the cells, allowing glucose from the bloodstream to enter and provide energy for cells . Maintaining your blood glucose levels within a normal range is crucial for metabolism and overall health, so it makes sense that having stable insulin levels is also important for metabolic health.
When the relationship between insulin and glucose becomes dysregulated, metabolic issues can arise . Insulin resistance occurs when cells in your body become less responsive to the action of insulin. This lack of responsiveness makes it difficult for cells to absorb glucose, causing an increase in blood glucose levels. Insulin resistance is often associated with conditions such as Type 2 diabetes and metabolic syndrome .
Contrary to what has been previously thought, hyperinsulinemia (high insulin levels) can precede insulin resistance and is starting to be more heavily researched as an initial indicator of metabolic dysregulation. If insulin levels are constantly too high, then something may be awry with your metabolism [4, 5].
How we test for insulin levels
This test measures the amount of insulin in a person's blood after an overnight fast (around 8-12 hours without eating). Similar to the process in an A1C test, your healthcare professional will collect a small sample of blood, then send it to a lab for analysis. Results of the fasting insulin test indicate the concentration of insulin in the blood during the fasting state and are reported in micro International Units per milliliter (uIU/mL) .
By looking at fasting insulin levels alongside fasting plasma glucose levels, clinicians can better understand an individual’s level of insulin sensitivity.
Common ways to show results from these tests include:
- Glucose/insulin ratio .
- HOMA-IR, or homeostasis model of insulin resistance . This is an equation that assesses an individual’s insulin resistance status and estimates beta cell function (the cells that produce insulin).
- QUICKI, or quantitative insulin sensitivity check index . This is a variation of the HOMA-IR equation that is used to determine insulin sensitivity.
As we’ll discuss, higher levels of insulin can indicate insulin resistance or hyperinsulinemia. Conversely, low levels might point toward Type 1 diabetes or hyperglycemia [6, 7].
Why the fasting insulin test isn’t done more often
If we can test for insulin sensitivity, why isn’t this done at every annual physical? The reason is that we don’t have good benchmarks for what is “normal” when it comes to insulin levels.
Fasting insulin tests are not done as frequently as tests for glucose levels because the reference ranges for insulin levels are not as well established. There is also a lack of standardization in methods for testing and analyzing fasting insulin (which could definitely add to the issue of establishing reference ranges), and potential issues with health insurance coverage for the lab test itself (lab tests can be expensive).
On the other hand, glucose testing has been extensively researched and validated over decades of research, with clear thresholds and guidelines for diagnosing diabetes and assessing glycemic control. Research on testing glucose in started the mid-1800s, with urine glucose testing becoming available in 1908 and blood glucose testing first developed in 1965 . With over 50 years of patient use, blood glucose testing is a standard practice in healthcare settings.
While fasting insulin levels can provide valuable insights into insulin resistance and early-stage disease, the interpretation of these levels is not standardized. There is less consensus among medical professionals on diagnosing insulin resistance based on fasting insulin levels.
One major reason is the variability in testing and analysis of fasting insulin tests, as found by the American Diabetes Association . As confirmed by a separate study, testing of identical blood samples resulted in varied reported insulin level values. This continued lack of consensus in testing and standardized guidelines makes it difficult for clinicians to use insulin levels as a measure for diagnosis of disease and prescribing appropriate interventions .
Additionally, major health organizations, such as the American Diabetes Association (ADA) and the United States Preventive Services Task Force (USPSTF), do not currently include insulin level measurements or direct recommendations for measuring fasting insulin levels in their guidelines [15, 16]. Much of the focus in medicine is on detecting late-stage insulin resistance, which is seen with hyperglycemia (high blood sugar levels).
This is largely a byproduct of the way our healthcare system is structured, where clinical care emphasizes putting out fires (only offering treatment once a patient has been diagnosed with Type 2 diabetes or pre-diabetes) rather than preventing them. The problem is that in its early stages, insulin resistance can manifest as normal blood sugar levels but high fasting insulin levels (called hyperinsulinemic normoglycemia) .
Since the focus is often on high blood sugar (hyperglycemia), testing for fasting insulin levels may not be prioritized — meaning that you won’t even have a chance to identify insulin resistance early on and take steps to reverse it before it disrupts your metabolism in bigger ways.
Who should get a fasting insulin test
A fasting insulin test is particularly valuable for individuals who are at risk for developing insulin resistance, such as those who are overweight or obese but do not yet have prediabetes or diabetes.
The test can help identify if your body is producing higher levels of insulin to compensate for a reduced response to its action — a key marker of developing insulin resistance and other related metabolic issues .
In other words, understanding insulin levels can be useful in both early and late-stage disease. In early-stage insulin resistance, where blood sugar levels are still within the normal range, elevated fasting insulin levels can indicate underlying issues. Knowing early could prompt intervention to prevent the progression of metabolic dysregulation.
Despite this, fasting insulin testing may not be immediately recommended by your healthcare provider unless there is a specific clinical indication or suspicion of metabolic dysfunction.
Glucose tests, rather than fasting insulin tests, are generally the first-line approach because of their accessibility, reliability, and convenience. As we’ll discuss, blood glucose tests can be used as a proxy for determining insulin levels, and can actually be more powerful for understanding your individual metabolism.
What are normal or optimal fasting insulin levels?
When it comes to insulin levels, there are no universally established normal values. The concept of "optimal" fasting insulin levels is not as straightforward as it may seem–which adds further to the lack of use and reliability.
Insulin levels, like glucose levels, are context-dependent and can vary depending on various factors such as the time of measurement, individual characteristics, or fed state (fasting vs. after eating) . Also, like glucose, insulin levels fluctuate throughout the day to accommodate for stimuli like stress or physical activity. Even the type of test used can influence the measured insulin levels [13, 14].
Research in has shown promise for establishing optimal ranges. One study in healthy Iranian people found fasting insulin levels to be 2-12 uIU/mL (with a range of 1.6-11.4 uIU/mL in men and 2.3-12uIU/mL in women) . Research in Chinese men found that fasting insulin levels fell in the range of 1.57-16.32 uIU/mL, with a median level of 5.79 uIU/mL .
Another study, looking at over 4,000 non-diabetic people with low blood glucose levels, showed women aged 30-49 years old had a median fasting insulin level of 5.1 uIU/mL, and men aged 30-64 and women aged 50-64 had a median fasting insulin level of 5.6uIU/mL . In older adults — 75-year-old non-diabetics — the interval for fasting insulin levels was found to be 1.74-18.27 uIU/mL, with a slightly lower range of 1.66-15.05 uIU/mL when only considering individuals with a BMI under 30 .
That said, there is some consensus in the medical community around what might be considered ideal, good, fair, and probable insulin resistance in terms of fasting insulin levels:
- Less than 10 uIU/mL is often seen as optimal or ideal.
- Less than 20 uIU/mL is considered good.
- Around 25-35 uIU/mL is typically regarded as fair.
- Above 40 uIU/mL could indicate early insulin resistance.
However, it's important to remember that these ranges are not definitive, and results should always be interpreted in the context of the individual's overall health, symptoms, risk factors, and other test results.
Using glucose levels as a marker of insulin resistance and metabolic health
While getting an insulin test may be challenging due to factors like out-of-pocket cost, accessibility, or reliability in results, glucose monitoring can be a practical alternative to understanding your insulin sensitivity.
Blood glucose levels are a key indicator of how your body processes food for energy and how effectively insulin is functioning in your body. Sustained high glucose levels often indicate that your body is struggling to effectively utilize insulin, which can suggest insulin resistance .
Continuous glucose monitors (CGMs) make it easy to track glucose levels throughout the day. Unlike a fasting insulin test, which provides a single snapshot in time, CGMs provide real-time information about glucose levels. By constantly monitoring your glucose, you can gain insights into how different factors such as meals, physical activity, stress, and sleep affect your glucose levels. This gives a comprehensive picture of your body's metabolic response and can help pinpoint areas for improvement.
CGMs allow you to monitor several critical markers of glucose regulation: average glucose levels, glucose variability, and fasting glucose levels. A higher average glucose level or drastic variability can be indicative of metabolic issues.
By correlating these glucose metrics with the Four Pillars of metabolic health — nutrition, exercise, stress management, and sleep—you can build healthier habits and make targeted lifestyle modifications. For example, you might notice certain foods cause a spike in glucose levels and opt to limit them in your diet, or you might find that light physical activity, like walking, helps in maintaining more stable glucose levels.
Insulin's critical role in glucose regulation, the implications of insulin resistance, and the relevance of fasting insulin tests encompass an essential part of metabolic health.
- Fasting insulin tests are valuable tools for detecting early insulin resistance and monitoring your body’s insulin regulation. It is particularly valuable for individuals who are at risk for insulin resistance but do not have prediabetes or diabetes.
- Insulin tests are not conducted as frequently as glucose tests due to various factors including less established reference ranges and the focus of current guidelines on later-stage insulin resistance.
- Currently, there are no clinical guidelines for “normal” values of fasting insulin levels. It is typically considered that insulin levels less than 10 uIU/mL are optimal, while levels above 40 uIU/mL could indicate insulin resistance.
- When a fasting insulin test isn't feasible, tracking glucose levels can provide valuable insights into insulin sensitivity, resistance, and overall metabolic health. Glucose monitoring can serve as a valuable proxy for understanding insulin sensitivity.
- Continuous glucose monitors (CGMs) allow real-time tracking of glucose levels, enabling the correlation of glucose metrics with lifestyle factors and helping to prevent insulin resistance.