Low-carb Lab Testing – Part 2 – Fasting Insulin Test


This is the second installment in a series of articles exploring pertinent lab tests for people following low-carb diets, and how a slightly different perspective is needed when interpreting the results compared to results from people following high-carb diets.

In the previous post in this series, we looked at three measurements related to blood glucose: fasting glucose, hemoglobin A1c, and fructosamine. We left off saying that while these are important to monitor regularly, they offer a limited view of a much larger metabolic control system. Blood glucose, hemoglobin A1c (HbA1c), and fructosamine indicate only what’s happening with blood glucose. They reveal nothing about insulin, which we will explore in this post.

Knowing your numbers is an important step for anyone who wants to transform their health. Heads Up Health was designed to empower you to manage all of your health data, including your lab test results, in one secure location. You can learn more on our homepage or by clicking below to create your account and start building your own centralized health portfolio.


The Fasting Insulin Test

We said it last time, and it’s worth repeating:

A fasting insulin test is the most important test your doctor probably isn’t ordering. 

The reason it’s so important to track insulin is that in many cases, fasting glucose and A1c remain normal due to chronically elevated insulin—that is, sky-high insulin is keeping the glucose “in check.” Fasting glucose and HbA1c are often the last things to rise, and they become elevated only after one of two things has happened:

  1. The pancreas can no longer pump out the inordinate amounts of insulin required to keep blood glucose within a safe range (sometimes called “beta cell burnout”). This is relatively rare, except in type-1 diabetes, which is an autoimmune condition and not driven by a poor diet.
  2. The pancreas still secretes large amounts of insulin but some of the body’s cells no longer respond to it properly, resulting in high blood glucose. (These cells become resistant to the presence of insulin.) This is far more common.

This explains why many people are surprised by a diagnosis of type 2 diabetes or pre-diabetes. They—and their doctors—had been lulled into a false sense of security by glucose measurements that fell within normal ranges for years, because no one was measuring insulin.

Medical professionals who are aware of the wide-ranging effects of chronically elevated insulin would agree that a fasting insulin test should be included as a standard part of routine bloodwork. But until that happens, if you’re concerned with getting and remaining metabolically healthy, you will need to specifically request it from your doctor or order it on your own from a direct-to-consumer lab testing service.

Fasting insulin test

Insulin helps control blood glucose.

Here’s how to use a fasting insulin test as a gauge for metabolic health: 

  • Optimal range: 1- 9 μU/mL
  • Intermediate risk range: 10 -11 μU/mL
  • High risk range: ≥ 12 μU/mL

If your fasting insulin is in the double digits, it’s a sure sign something is awry. However, just as we explained regarding fasting glucose, if your fasting insulin falls within the optimal range, it doesn’t automatically mean everything’s fine. In some people, the fasting level is normal, but the level after meals rises very high and takes an extended length of time to come back to baseline—if it even does come down fully before the next meal. So it’s possible to have a fasting insulin level in the optimal range but have high insulin throughout most of the rest of the day.

Chronically elevated insulin ( called “hyperinsulinemia”) should be suspected when fasting glucose, HbA1c, and possibly fasting insulin are normal, but you experience unexplained or “idiopathic” health issues, such as:

  • Stubborn fat loss
  • Hypertension (high blood pressure)
  • Headaches; migraines
  • Gout
  • Skin tags
  • Vertigo
  • Tinnitus
  • Infertility (in women and men)
  • Erectile dysfunction
  • Benign prostatic hyperplasia
  • Polycystic ovarian syndrome (PCOS)
  • Gynecomastia (enlargement of breast tissue in males)

Unfortunately, owing to the complexity of the chemical assay used to measure insulin, there’s currently no way to measure insulin at home, the way you can do with glucose and HbA1c.

Why Track Insulin?

Medical professionals—particularly physicians, nutritionists, and researchers who work with individuals with obesity, type-2 diabetes, and metabolic syndrome—increasingly recognize that it is elevated insulin, rather than blood glucose, that’s responsible for many of the chronic illnesses that plague millions of people, robbing them of quality and quantity of life. Chronically elevated blood glucose (“hyperglycemia”) is dangerous and, over time, results in damage to the eyes, kidneys, liver, blood vessels, and extremities. Much of the organ and tissue damage that occurs in type-2 diabetics with poor blood sugar control results from chronic hyperglycemia.

But, many non-diabetics will experience physical deterioration in the absence of high blood glucose. In these individuals, it’s the insulin that’s the problem. These folks are essentially diabetic, but because their blood glucose is normal, they won’t be officially diagnosed. This is what Dr. Joseph Kraft called “diabetes in-situ,” or “occult diabetes”—occult, meaning hidden. The high blood sugar is hidden or masked by the pathologically high insulin.

A large and still growing body of scientific research indicates that chronic hyperinsulinemia is the unifying factor behind some of the most common chronic illnesses of our time. Michael Eades, MD, Mary Dan Eades, MD, and Loren Cordain, PhD, some of the earliest proponents of low-carb and Paleo diets, explained over a decade ago that hyperinsulinemia may be the driving force behind acne, skin tags, PCOS, myopia, and male pattern baldness (it’s not all genetic!). Chronically elevated insulin is also a risk factor for Alzheimer’s disease, infertility and sexual dysfunction, inner-ear and balance disorders (e.g., vertigo, tinnitus, Ménière’s disease), some forms of cancer (coupled with worse prognosis in those undergoing treatment), and cardiovascular disease. In fact, Dr. Kraft wrote, “Those with cardiovascular disease not identified with diabetes are simply undiagnosed.”

Test fasting insulin to remain healthy.

Want to stay healthy? Measure insulin!

Something to keep in mind is that hyperinsulinemia occurs in people of all shapes and sizes. Obesity is more often an effect, rather than a cause, of disturbed insulin and glucose signaling in the body. So individuals who are lean and appear healthy on the outside are not immune to the adverse effects of high insulin. These individuals have personal body fat set points that prevent them from becoming overweight or obese, but they’re not spared the other undesirable outcomes from derailed metabolism. (Researchers call this “normal weight obesity,” but more casually it’s referred to as TOFI – thin outside, fat inside. Even though these folks remain at a “normal” weight, their biomarkers indicate metabolic syndrome or insulin resistance.)

Tracking insulin is the canary in the coal mine – the “check engine light.” It’s an early warning sign that your diet and lifestyle need adjustments. You don’t have to wait until your blood glucose is high enough to prompt a type-2 or pre-diabetes diagnosis. Elevated fasting insulin might be one of the first indicators that something’s amiss, and you can take action to correct it.

Track your progress

The Heads Up Health app was designed specifically for individuals to take control of their health data so it can be used for better decision making. For our users within the United States, you can electronically link your medical facility to your Heads Up account and instantly import your lab test results. If we can’t connect to your medical facility or you live outside the US, you can easily enter your results manually:

Track your fasting insulin results with Heads Up Health

Track your fasting insulin results with Heads Up Health

With subsequent tests, you can also trend your results over time to see how your low-carb lifestyle is impacting important markers like fasting insulin:

Trending your fasting insulin results over time

Trending your fasting insulin results over time

Heads Up can also integrate the data you are collecting at home – everything from weight to blood sugar and steps per day – so you can compare how your healthy lifestyle choices are impacting your lab test results.


Coming up next…

In the next post, we’ll introduce you to the HOMA-IR test that will help you connect your fasting insulin and glucose levels for a deeper analysis of your level of metabolic health.

About the Author

Amy Berger, MS, CNS, NTP, is a USAF veteran, Certified Nutrition Specialist and Nutritional Therapy Practitioner who specializes in using low-carbohydrate nutrition to help people reclaim their vitality through eating delicious foods, and showing them that getting and staying well doesn't require starvation, deprivation, or living at the gym. Her motto is, “Real people need real food!” She blogs at www.tuitnutrition.com, where she writes about a wide range of health and nutrition-related topics, such as insulin, metabolism, weight loss, thyroid function, and more.
22 Responses to "Low-carb Lab Testing – Part 2 – Fasting Insulin Test"
  1. Miche says:

    Thanks, Amy. My latest fasing insulin test was 3.7 uIU/mL. Does fasting insulin fluctuate based on what you’ve eaten? My previous results were 5.4 and even though I had fasted the requisite 12 hours, I had dinner in a restaurant that included a glass of wine and higher protein (for me) the night before.

    • Amy Berger says:

      Hi Miche, the fasting insulin level shouldn’t be affected much by what you eat the night before, especially if you were fasted for 12 hours before the blood draw. Both of your results – 3.7 and 5.4 are great, so I wouldn’t be concerned if I were you. (But I am not a doctor, so you may want to consult with your MD.)

      • Miche says:

        Thanks for clarification, Amy. I was just curious, because I’ve had 3 fasting insulin tests, first in Nov. was 3.7, in Feb 5.4 and then 3.7 last week. I am not worried but was just curious; since I’ve now had two at 3.7, I’ll assume that the 5.4 was the anomaly 🙂

        Thanks for all your work! I love to read all of your writing!

  2. Eunice Dockrell says:

    Forgive my ignorance, is fasting glucose, HbA1c and fasting insulin two different blood tests that needs to be done to determine the insulin level? I live a low carb lifestyle and generally do intermittent fasting (16:8) most days. I have blood tests done every year and have a blood test monitor at home (where I just prick my finger) and my last reading was 5.3 mmol/L

    • Amy Berger says:

      Hi Eunice,
      Thanks for reading. Fasting glucose, HbA1c and fasting insulin are *three* different tests. Fasting glucose is usually included as a standard part of routine bloodwork in the U.S., but you have to specifically ask for the A1c and fasting insulin, as they are not usually included unless you ask for them.

  3. tosjac says:

    “This is relatively rare, except in type-1 diabetes, which is an autoimmune condition and not driven by a poor diet.
    The pancreas still secretes large amounts of insulin but some of the body’s cells no longer respond to it properly, resulting in high blood glucose”
    I don’t agree. Read

  4. Julie says:

    Hi Amy, I have been on low carb diet and daily intermittent fasting (eating only 2x a day) for approx 2 & 1/2 months. Had blood work last month with following results: fasting insulin: 1.9; fasting glucose 88; A1C of 5.9. Should I be concerned with such a low fasting insulin and high A1C that I have LADA/diabetes 1? I have been checking my sugar levels at home and fasting glucose usually in mid to high 90s. My primary doctor made no note of these levels when reviewing my labs so any guidance would be greatly appreciated. Thanks.

    • Amy Berger says:

      Hi Julie,
      It would be difficult for me to comment without knowing more about your situation. Do you have an endocrinologist? Do you test your BG at other times during the day besides fasting? What was your A1c prior to starting low carb? There’s a chance it’s actually come down quite a bit; perhaps it was even higher before you adopted this way of eating. It’s possible your fasting glucose is normal, but it’s going higher after meals and/or remaining elevated throughout some of the rest of the day, which would account for the higher A1c despite a low fasting level. It’s hard for me to say more. Lab values really have to evaluated as a mosaic — a whole made up of many smaller individual parts. Having a fuller picture would give a better indication as to what might be going on besides just seeing these 3 levels related to blood glucose and insulin. If you have any lab values from *before* going low carb, I would compare to those, because you might actually already be doing better than you were.

      • Julie says:

        Thank you so much for your response. I forgot to mention perhaps an important factor in that my A1C in April before starting in June with low carb diet and intermittent fasting was 5.6. Looking back through recent years my A1C levels have consistently been at 5.6 or 5.7 since the year 2014 with one reading of 5.9 in 2015. I July i have taken BS readings at 1 hour and 2 hours after eating and usually n the 105 to 110 range with couple readings at approx 125 range.
        My lipid panel are in normal to good range. Normal weight and am 61 years old. This was the first time I had fasting insulin test ever, at my direct request to my doctor, after realizing it is necessary to get the big picture. I can’t believe it is not routinely done. I don’t know if 1.9 insulin level is anything to be alarmed about. I have not been to an endro doctor yet. Thanks for any additional guidance.

  5. Ian says:

    Unfortunately different insulin assays give different results, so it is not possible to use the cut-offs in this article reliably. If you send a sample to different laboratories, the values that come back will be quite different. Each laboratory will have its own reference interval, and this needs to be taken into account when interpreting results.

  6. Chrissy says:

    I was wondering how accurate insulin levels are when your NOT fasted. I had my blood drawn about an hour after I had eaten and the insulin number was rather high so I’m thinking I need to have it redone when im fasted to be more accurate.

  7. Amy Berger says:

    Hi Chrissy,
    Thanks for reading. Yes, have the test redone when you are fasted. It’s very important that it be fasted because insulin will rise significantly after a meal even when you eat low-carb. Not as much as if you were eating a high carb meal, but it will still be significantly higher than it would have been if you were fasted.

  8. Samantha says:

    Hi Amy,
    Thanks for the article. I was recently diagnosed with PCOS/metob syndrome, after having 5 kids…my periods stopped. My dad was type 1 diabetic. The Dr gave me metformin, which I am hesitant to take. I requested an insulin test which he did, and the results were :
    GLUCOSE: 89 (70-99)
    INSULIN: 13 uIU/mL (4-13)
    Much of what I’ve read is saying despite the lab range, anything double digit is dangerous. I’ve also read metformin shouldn’t be used so often. What do you think?
    Also, I just got a bowflex trainer which is high intensity. I’ve read high intensity increases testosterone production. Should I not do high intensity since i suspect high insulin has caused my PCOS, which means my male hormones are already too high?

    • Amy Berger says:

      Hey Samantha,
      I’m not a physician, so all I can do is give you my thoughts. This is not medical advice.
      Fasting insulin of 13 is higher than most of us would like to see, but I’ve seen much, much higher. Are you doing a low carb diet? That would be the place to start…more effective than exercise, and more effective than metformin, although those other things can help. Starting with some variation of a low carb diet would be the first and most important step, in my opinion. Metformin actually seems to be one of the drugs that actually helps people and is not riddled with a ton of dangerous and undesirable side-effects. It has *some* (no drug doesn’t), but on balance, metformin does seem pretty helpful. I don’t think it’s a problem to take it, but hopefully the MD didn’t prescribe some kind of super high dose.

      For the most part, exercise — even high intensity exercise — is beneficial for insulin issues. I don’t think you’ll be messing with your testosterone from intense exercise alone. The #1 thing contributing to the high testosterone is the high insulin, so the way to address the T is with that combination of low carb diet, metformin, and exercise. Also make sure you get enough quality and quantity of sleep. Probably a tall order with 5 kids, but you really do need to rest and repair, and your body does that best during sleep. If this is difficult for you, or you are constantly overwhelmed and stressed out, see if you can work something out with your spouse/SO, friends, or family members to give you a night off once a week. (At least!)

  9. Anna says:

    Thank you for these wonderful posts! I have struggled with weight especially over the last couple of years. Had success on ketogenic diet, but gained most back when I went back to moderate carb, lower calorie diet. I just recently started Intermittent Fasting and keto again, about 2 weeks before blood panels. My #s look like this: Fasting blood glucose = 101 (low pre-diabetes). Insulin = 11 (show “normal” on MD range and “intermediate risk” based on your post). HA1C = 5.1 (normal). And if I calculated correctly, HOMA-IR of 2.74 (high range of “early insulin resistance”). My doctor was not concerned with me developing pre-diabetes based on my HA1C and fasting insulin, but my gut tells me that I am insulin resistant based on my history of gaining weight with even moderately low carbohydrate diets and losing only on ketogenic diet. Would you consider these numbers in the pre-diabetic range, and do you believe intermittent fasting combined with a low carb or ketogenic diet would be beneficial?

    • Amy Berger says:

      Hi Anna, I am not a physician, so I absolutely cannot “diagnose” anything. In my opinion, a fasting glucose of 101 and a fasting insulin of 11 are not terrible, but they’re obviously not ideal. They’re both a little higher than I’d be happy with, and also higher than with what most of the low carb and keto-oriented MDs I know would like to see as well. But I wouldn’t worry at all if I were you. If I’m understanding your comment correctly, you were only back to a low carb way of eating for 2 weeks before having this blood test. Give things a bit more time to take effect. I would imagine if you re-test in 60 or 90 days, both measurements will have come down a little.

      In the meantime, I think you’ve already proven to yourself by your own experiment here (although you didn’t intend it as one) that a lower-carb way of eating seems to be best for you. You might not have to be strictly ketogenic all the time, but *some* variation of a reduced carb diet is probably the way to go.

      • Amy Berger says:

        Also, you don’t “have to” wait 60 or 90 days to re-test. You might even see a difference in 30-45 days, but I just wouldn’t drive yourself crazy over it. Stay the course, keep carbs low, and you should be fine. And the glucose of 101 and insulin of 11 really aren’t that bad. Yes, you’d like them to come down a little (and I’m sure they will), but you’re starting back to low carb from a very good place. Honestly, some people out there are in really dangerous territory…fasting glucose in the 200s, insulin in the 20s or even 30s…so you’re doing yourself a world of good by getting back on the horse *now,* before things got further out of hand. 🙂

        • Anna says:

          Thank you so much for sharing your thoughts and experience with me. I feel much better about my numbers now and am committed to making changes to bring those numbers down into the “good” range. I am so glad I found your blog and excellent information!

  10. Alex says:

    Great article! Unfortunately I live in Sweden and it doesn’t look like I can order a fasting insulin test from anywhere, even privately. Can you think of other proxies I could potentially use to assess if I’m doing OK or not?

    • Amy Berger says:

      Hey Alex, look at your triglycerides and HDL. You want trigs low and HDL high. That ratio is usually a good sign of where insulin is. Different experts cite a different ratio as being the cutoff point for being at lower risk for cardiovascular problems and being metabolically healthy, so it’s hard to give an exact ratio, but certainly no more than 3.5 (trigs no higher than 3.5 times the HDL). Many people who do a low carb or ketogenic diet have a ratio <1.0 -- HDL is actually higher than trigs. But I would say under 2.0 is probably okay for most people.

  11. Anonymous says:

    Hi. What’s the highest you’ve seen insulin go on a test? I’m trying to find other cases of super high insulin levels. I have reactive hyperinsulinemia caused by an insulinoma. I’m strictly reactive. I have no problem fasting. During a 5 ht ogtt my insulin spiked to 522. Sugar went to 143 before dropping to the 70’s. Then we did a mixed meal test. Sugar peaked at 167 ( it as a super high carb meal. More than I would eat in a week) and my insulin went to 780. This is in universal units.

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