As a physician working at the intersection of preventive and longevity medicine, one of the most common—but often overlooked—questions I get is: “How do I know if I’m insulin resistant?”
Many people assume that a fasting glucose or even a hemoglobin A1c is enough to answer this. While these tests are useful, they often miss the subtle, early stages of metabolic dysfunction. This is where the oral glucose tolerance test (OGTT) becomes indispensable.
What Is Insulin Resistance, and Why Does It Matter?
Insulin resistance is a state where your body’s cells don’t respond effectively to insulin, the hormone that moves glucose from your bloodstream into your tissues. Over time, this leads to higher circulating insulin levels, fluctuations in blood sugar, and ultimately an increased risk of type 2 diabetes, cardiovascular disease, dementia, and accelerated aging.
The tragedy is that insulin resistance develops silently—often for years before it shows up in the routine labs your doctor is likely checking.
Data from NHANES shows insulin resistance prevalence as high as 45% in individuals 18-441. And this is likely an underestimation. Almost half of young people in our country are insulin resistant! The numbers for older age groups are most assuredly higher.
Why Fasting Glucose and A1c Aren’t Enough
Fasting glucose can remain normal until very late in the disease process. The pancreas simply produces more insulin to keep fasting numbers in range. I’ll show an example of this below.
Hemoglobin A1c reflects an average of blood sugars over three months, but it can also stay deceptively “normal” even as insulin resistance worsens - especially if someone does a good job managing their diet. Again - you’ll see an example of this below.
Both of these can miss the subtle metabolic dysfunction that starts a decade before type 2 diabetes is diagnosed. I’ll show you an example of this below from a real patient in my practice.
The Value of the Glucose Tolerance Test
The OGTT involves drinking a 75-gram glucose solution after an overnight fast, followed by a series of blood draws (usually at fasting, 1 hour, and 2 hours).
Here’s why it’s powerful:
It shows how your body handles a glucose challenge, rather than just a resting state.
Measuring both glucose and insulin at each time point provides even deeper insight. For example, a patient may have normal glucose values at 1 and 2 hours, but at the cost of abnormally high insulin levels. That’s a red flag for early insulin resistance.
It helps stratify risk early, when lifestyle and targeted interventions can make the biggest difference.
To put this in perspective I’ll show a recent example from our practice of a woman who came to see us and was otherwise very healthy but we diagnosed her with VERY significant insulin resistance (technically met criteria for type 2 diabetes!). She was shocked needless to say.
First we see her insulin levels at fasting, 1 hr and 2 hr. Fasting and 1 hr are ok but we begin to see an issue with the 2 hr. We know that at 2 hours the insulin should be back down close to the baseline. Here her insulin is still rising at the 2 hour mark - a sign that something is not right with her insulin glucose regulation. Seeing the glucose numbers below will put it all together.
Her fasting glucose is near perfect at 86. But at 1 hr and 2 hr marks we see how bad things get. On a OGTT a 2 hour glucose >200 is diagnostic of type 2 diabetes. She has such severe insulin resistance she is technically type a 2 diabetic. Now she has done a remarkable job controlling this as her A1c (3 month blood sugar average) was only 5.8 (normal considered <5.7). But if she were to go out and eat a slice of cake or snack on a bag of chips or eat a bowl of rice she could easily spike her sugars above 200 again.
Most doctors would have only checked her fasting glucose (for her was 86) and told her she was perfect. Some docs may have also done an A1c and may have told her she was barely in the pre diabetic range (with the caveat that is is normal for her age). But each of these massively misses the mark and misdiagnoses her. This is the utility of the glucose tolerance test.
Some might say that you could easily diagnose this by wearing a continuous glucose monitor (CGM). You would be partially correct. Yes you could potentially diagnose this from a CGM by seeing a large spike in glucose after a big carb or sugar load. But the much more subtle forms of insulin resistance won’t show a big spike in glucose. You might keep your glucose <130 but your insulin goes up 10x at 1 hr and is still elevated at 2 hr indicating there is resistance. Something you can only see with the OGTT.
Why This Matters for Longevity
If our goal is not only to prevent disease but to extend healthspan, then early detection of metabolic dysfunction is non-negotiable. A well-timed OGTT can identify issues long before diabetes or vascular damage occur. It allows us to:
Personalize nutrition and exercise prescriptions.
Use supplements or medications (like metformin or GLP-1s, if indicated) more strategically.
Track progress in reversing insulin resistance over time.
My Take as a Physician
We have all our clients undergo an OGTT with insulin measurement. It’s a relatively simple, inexpensive test that can reveal a great deal about metabolic health.
We cannot afford to wait until diabetes is “official” before acting. Prevention and reversal begin by identifying the earliest cracks in the foundation—and the OGTT is one of our sharpest diagnostic tools for doing just that.
In my next post we’ll take a look at the best ways to reverse insulin resistance.
👉 If you’ve never had a glucose tolerance test, or if you’ve been told your fasting glucose and A1c are “fine” but still have concerns, it may be worth a deeper look. Your future self will thank you.
References
Parcha V, Heindl B, Kalra R, Li P, Gower B, Arora G, Arora P. Insulin Resistance and Cardiometabolic Risk Profile Among Nondiabetic American Young Adults: Insights From NHANES. J Clin Endocrinol Metab. 2022 Jan 1;107(1):e25-e37. doi: 10.1210/clinem/dgab645. PMID: 34473288; PMCID: PMC8684535.
