Why Your “Normal” Blood Sugar Isn’t Normal (Part 2) | Chris Kresser
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Why Your “Normal” Blood Sugar Isn’t Normal (Part 2)

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In the last article I explained the three primary markers we use to track blood sugar: fasting blood glucose (FBG), oral glucose tolerance test (OGTT) and hemoglobin A1c (A1c). We also looked at what the medical establishment considers as “normal” for these markers. The table below summarizes those values.

MarkerNormalPre-diabetesDiabetes
Fasting blood glucose (mg/dL)<99100-125>126
OGGT / post-meal (mg/dL after 2 hours)<140140-199>200
Hemoglobin A1c (%)<66-6.4>6.4

In this article, we’re going to look at just how “normal” those normal levels are – according to the scientific literature. We’ll also consider which of these three markers is most important in preventing diabesity, diabetes, and cardiovascular disease. But before we do that, I’d like to make an important point: context is everything.

In my work with patients, I never use any single marker alone to determine whether someone has a blood sugar issue. I run a full blood panel that includes fasting glucose, A1c, fructosamine, uric acid and triglycerides (along with other lipids), and I also have them do post-meal testing at home over a period of 3 days with a range of foods.

If they have a few post-meal spikes and all other markers or normal, I’m not concerned. If their fasting BG, A1c and fructosamine are all elevated, and they’re having spikes, then I’m concerned and I will investigate further.

On a similar note, I’ve written that A1c is not a reliable marker for individuals because of context: there are many non-blood sugar-related conditions that can make A1c appear high or low. So if someone is normal on all of the other blood sugar markers, but has high A1c, I’m usually not concerned.

With all of that said, let’s take a look at some of the research.

Fasting Blood Sugar

According to continuous glucose monitoring studies of healthy people, a normal fasting blood sugar is 83 mg/dL or less. Many normal people have fasting blood sugar in the mid-to-high 70s.

While most doctors will tell you that anything under 100 mg/dL is normal, it may not be. In this study, people with FBG levels above 95 had more than 3x the risk of developing future diabetes than people with FBG levels below 90. This study showed progressively increasing risk of heart disease in men with FBG levels above 85 mg/dL, as compared to those with FBG levels of 81 mg/dL or lower.

What’s even more important to understand about FBG is that it’s the least sensitive marker for predicting future diabetes and heart disease. Several studies show that a “normal” FBG level in the mid-90s predicts diabetes diagnosed a decade later.

Far more important than a single fasting blood glucose reading is the number of hours a day our blood sugar spends elevated over the level known to cause complications, which is roughly 140 mg/dl (7.7 mmol/L). I’ll discuss this in more detail in the OGGT section.

One caveat here is that very low-carb diets will produce elevated fasting blood glucose levels. Why? Because low-carb diets induce insulin resistance. Restricting carbohydrates produces a natural drop in insulin levels, which in turn activates hormone sensitive lipase. Fat tissue is then broken down, and non-esterified fatty acids (a.k.a. “free fatty acids” or NEFA) are released into the bloodstream. These NEFA are taken up by the muscles, which use them as fuel. And since the muscle’s needs for fuel has been met, it decreases sensitivity to insulin. You can read more about this at Hyperlipid.

So, if you eat a low-carb diet and have borderline high FBG (i.e. 90-105), it may not be cause for concern. Your post-meal blood sugars and A1c levels are more important.

Hemoglobin A1c

In spite of what the American Diabetes Association (ADA) tells us, a truly normal A1c is between 4.6% and 5.3%.

But while A1c is a good way to measure blood sugar in large population studies, it’s not as accurate for individuals. An A1c of 5.1% maps to an average blood sugar of about 100 mg/dL. But some people’s A1c results are always a little higher than their FBG and OGTT numbers would predict, and other people’s are always a little lower.

This is probably due to the fact that several factors can influence red blood cells.

Remember, A1c is a measure of how much hemoglobin in red blood cells is bonded (glycated) to glucose. Anything that affects red blood cells and hemoglobin – such as anemia, dehydration and genetic disorders – will skew A1c results.

A number of studies show that A1c levels below the diabetic range are associated with cardiovascular disease. This study showed that A1c levels lower than 5% had the lowest rates of cardiovascular disease (CVD) and that a 1% increase (to 6%) significantly increased CVD risk. Another study showed an even tighter correlation between A1c and CVD, indicating a linear increase in CVD as A1c rose above 4.6% – a level that corresponds to a fasting blood glucose of just 86 mg/dL. Finally, this study showed that the risk of heart disease in people without diabetes doubles for every percentage point increase above 4.6%.

Studies also consistently show that A1c levels considered “normal” by the ADA fail to predict future diabetes. This study found that using the ADA criteria of an A1c of 6% as normal missed 70% of individuals with diabetes, 71-84% with dysglycemia, and 82-94% with pre-diabetes. How’s that for accuracy?

What we’ve learned so far, then, is that the fasting blood glucose and A1c levels recommended by the ADA are not reliable cut-offs for predicting or preventing future diabetes and heart disease. This is problematic, to say the least, because the A1c and FBG are the only glucose tests the vast majority of people get from their doctors.

OGTT / Post-Meal Blood Sugars

If you recall, the oral glucose tolerance test (OGTT) measures how our blood sugar responds to drinking a challenge solution of 75 grams of glucose. I don’t recommend this test, because A) it’s not realistic (no one ever drinks 75 grams of pure glucose), and B) it can produce horrible side effects for people with poor glucose control.

However, there’s another more realistic and convenient way to achieve a similar measurement, and that is simply using a glucometer to test your blood sugar one and two hours after you eat a meal. This is called post-prandial (post-meal) blood sugar testing. As we go through this section, the numbers I use apply to both OGTT and post-meal testing.

As the table at the beginning of this article indicates, the ADA considers OGTT of between 140 – 199 two hours after the challenge to be pre-diabetic, and levels above 200 to be diabetic.

But once again, continuous glucose monitoring studies suggest that the ADA levels are far too high. Most people’s blood sugar drops below 120 mg/dL two hours after a meal, and many healthy people drop below 100 mg/dL or return to baseline.

A continuous glucose monitoring study showed that sensor glucose concentrations were between 71 – 120 mg/dL for 91% of the day. Sensor values were less than or equal to 60 or 140 mg/dL for only 0.2% and 0.4% of the day, respectively.

On the other hand, some studies suggest that even healthy people with no known blood sugar problems can experience post-meal spikes above 140 mg/dL at one hour. As I said in the beginning of the article, context is everything and all of the markers for blood sugar must be interpreted together.

If post-meal blood sugars do rise above 140 mg/dL and stay there for a significant period of time, the consequences are severe. Prolonged exposure to blood sugars above 140 mg/dL causes irreversible beta cell loss (the beta cells produce insulin) and nerve damage. Diabetic retinopathy is an extremely common (and serious) diabetic complication. Cancer rates increase as post-meal blood sugars rise above 160 mg/dL. This study showed stroke risk increased by 25% for every 18 mg/dL rise in post-meal blood sugars. Finally, 1-hour OGTT readings above 155 mg/dL correlate strongly with increased CVD risk.

What does it all mean?

Let’s take a look again at what the ADA thinks is “normal” blood sugar:

MarkerNormalPre-diabetesDiabetes
Fasting blood glucose (mg/dL)<99100-125>126
OGGT / post-meal (mg/dL after 2 hours)<140140-199>200
Hemoglobin A1c (%)<66-6.4>6.4

But as we’ve seen in this article, these levels depend highly on context and whether all markers are elevated, or just a few of them.

If you’re interested in health and longevity – instead of just slowing the onset of serious disease by a few years – you might consider shooting for these targets. But remember to interpret the numbers together, and also remember that blood sugar is highly variable. If you wake up one morning and have a fasting blood sugar of 95, but your A1c and post-meal numbers are still normal, that’s usually no cause for concern. Likewise, if you see a one-hour post-meal spike of 145 mg/dL, but all of your other numbers are normal, that is also usually no cause for concern.

MarkerIdeal
Fasting blood glucose (mg/dL)<86*
OGGT / post-meal (mg/dL after 2 hours)<120
Hemoglobin A1c (%)<5.3

*If you’re following a low-carb diet, fasting blood sugars in the 90s and even low 100s may not be a problem, provided your A1c and post-meal blood sugars are within the normal range.

Another key takeaway from this article is that fasting blood glucose and A1 are not often reliable for predicting diabetes or CVD risk. Post-meal blood sugars are a more accurate marker for this purpose.

And the good news is that this can be done cheaply, safely and conveniently at home, without a doctor’s order and without subjecting yourself to the brutality of an OGTT.

I’ll describe exactly how to do this in the next article.

867 Comments

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  1. Why would you test for blood sugar when diabetes is an insulin problem?
    Why not test insulin levels?

  2. I’m a 62 year old woman who’s always been healthy and active. 5’7″ 150#. I have always eaten a lot of sugar and had no obvious adverse effects though i know that’s a bad idea. i could also go nearly all day without eating, even exercise vigorously, and feel fine.

    however in the past year, and especially in the past few months I’ve had episodes of what i call “the wobblies.” legs and arms feel like they’re trembling and/or vibrating, and feel weak. sometimes i feel clammy and sweaty and overheated, but not always. that’s pretty much it for symptoms.

    my doctor suspects hypoglycemia and gave me a glucometer.

    since i began testing at home, my blood glucose has never been below 81.

    one time when i ate did sugar jellies it spiked to 155 But was down to 112 less than an hour later. it’s usually 120 (ish) after a meal and is between 80 to 110 throughout the day.

    i seem to have the wobblies almost all the time, though sometimes they’re gone, or very mild. they ate usually worst in the morning.

    i don’t know that they’re correlated with blood sugar at all! i feel them more when I’m standing or exerting myself. and yes, two times when my blood glucose was 81 and 82, the episodes were acute.

    is it possible that i am someone whose “normal” is higher than usual?

    my doctor said she’d send me to a neurologist if I’m not hypoglycemic.

    my recent blood work is completely normal including thyroid. however past bloodwork had shown me to be slightly anemic.

    I’m wondering if it’s neurological, hormonal, psychosomatic, or tested to my blood sugar, even with what appear to be normal readings.

    • Simple reliance on a fasting blood sugar or even an A1C test may not be the most reliable diagnostic tool considering your symptoms. A1C tests are skewed when a person is even slightly anemic. Fasting blood sugar tests didn’t work to discover my hypoglycemia; but a 4 or 6 hour glucose tolerance test did because it actually charted or graphed a measured glucose solution by the hour and how my body specifically responded to the insulin produced. Mild hypoglycemics most always fail the test. Severe hypoglycemics usually “crash” before the test is completed.
      I’m no doctor, but this actually happened to me.
      Good luck!

    • What’s this business about going without eating all day? Your symptoms could have NOTHING to do with glucose. For example, sodium/potassium ratio.

      What are you eating? Super low carb? That’s the first symptom you’ll see on KETO dieting.

      Download Cronometer and enter and track every single thing you injest. You’ll see what’s going on nutritionally. Your doctor should be taking this more seriously.

        • Here’s the link, I use it on my desktop. My autocorrect misspelled the word. It’s CRONometer not CHronometer LOL.

          https://cronometer.com/

          There’s a free version, and a (inexpensive) Gold version which gives additional functions.

          It allows you to enter your biometrics all day then generate a report. IE BG reading. Waist size etc.

          It also allows you to make a “note” any time any place. So you can note the times you took your BG. Or one big note with all your comments.

          Should be testing and tracking:

          Morning fasting glucose level
          Pre meals
          MINIMALLY Post meals 1 hour
          Post meals 2 hours

          You should actually start tracking every 15 minutes with BG because people peak different times. Until you see a pattern emerge. Obviously this means no snacking or your BG will be all over the place. Which is natural. But not helpful for potential or existing diabetics.

          If you happen to wake up in the middle of the night, track it also. For the Dawn Phenomenon.

          My BG can be 75 at 3 am and when it releases glucose for the day, it starts rising in the 80s by 5-6 AM peaking at 7-8 am 93 then back down in the low 80s by 9-10 am. Which is a BUMMER and I’m working on it by fasting longer not eating after dinner at 5-6.

          People with peaking higher morning BG over low 80s have a challenge when going for labwork at 8 am.

          It means my insulin resistance has not healed. Yet. Even if all my other criteria is good. I never EVER spike over around 100-105 BUT I eat a very healthy diet. Dr Fuhrman.

          Also do not drink liquids with meals or directly after. Abstain from liquids 15 minutes before.

          Some people have spiking BG when eating weird like the OP “going all day” because the body is fighting that!

          Lastly, if you have a fatty liver, you need to fix weight and be a thin person so your liver can continue to get rid of all that unhealthy stuff.

          Your waist should at LEAST be less than half your height.

          • “Your waist should at LEAST be less than half your height.” — no matter how much (or rather, how little) i weigh, my waistline does not go below 36″. i’m 66″ tall, 125 lb, waistline currently 37.5″. any thoughts on why or what i should do about this?

            i mentioned this recently to my doctor, who was shocked, but still didn’t really follow up on it, and i’m so frustrated with him. i’m concerned that i have high cortisol. no matter what i do, my waistline does not go down. and i have skinny limbs, which no matter what i do remain quite skinny.

            up until very recently i was eating keto, doing intermittent fasting, and got my weight down to 122 lb, but i was having *extreme* dawn phenomenon. waking up at 4am with heart racing at 100 bpm, dry mouth, need to pee. (and normal blood glucose — in the 80s — i would actually measure it just to be sure, at 4a when this would happen.) it was awful — nothing i did helped me get back to sleep.

            so i’ve started adding some carbs back into my diet in the evening. but i can’t go too high because i’m prediabetic. besides which, i don’t think it really helps that much anyway, because my body doesn’t process the carbs well, even slow carbs. (and really, i think i tend to swing between hyper- and hypoglycemic — years ago when i had the hyperglycemic episode that led to my pre-d dx, i swung into hypoglycemia too.)

            i’m not sure WHAT to do, and i’m incredibly frustrated with my doctor. (who wants to send me to a THERAPIST for my sleep problems. talk about treating me like a hysterical female patient.)

            anyway — any thoughts on what i can do?

    • Did your doctor explain WHY he/she suspects hypoglycemia? What you call “the wobblies” (legs and arms feel like they’re trembling and/or vibrating, and feel weak) sounds like classic symptoms of the adrenaline rush that accompanies a hypoglycemic episode. Not eating and/or vigorous exercise could cause or exacerbate a hypoglycemic episode. You say that your BG is never below 81, but is it often in the low 80s or only when you feel “wobbly?” What are your highest BG values when you feel wobbly?

      Could you be one of those people who’s normal is high? Yes. Could your wobbly feeling be unrelated to BG, or be related something else in your blood chemistry? Yes. Could it be neurological, hormonal or psychosomatic? Yes.

      But if what you’re feeling really are hypoglycemic episodes then consuming glucose should cause the wobbly feeling to go away, as well as being reflected in your BG values. I speak of episodes because if you are often hypoglycemic then you would likely lose your “feeling” for hypoglycemia as you become increasingly acclimated, and/or have less and less adrenaline rush.

      In short if your “wobblies” ARE due to BG, it should be easy to determine by manipulating/observing your glucose levels. Using a CGM might tell you a lot more than you can learn from a glucometer. If you really think it IS hormonal (because BG is controlled by hormones e.g. insulin) you may prefer to follow up with an endocrinologist before going another direction with your doctor.

  3. I had H1ac of 6.3. I heard about how statins raise blood sugar, so I quit and 3 months later, it was 5.2. Since my cholesterol was higher, doc wanted me to start statins again. 3 months later. my H1ac was 6.5. What’s a body to do?

    • overall cholesterol numbers are useless, just as many people 200 chol…….Statins are poison. the way to reduce Lp(a) [the part of LDL that kills you] is to do the Linus Pauling protocol……….3grams of lysine and 3 grams of VitC 2-3x day.

  4. While all of this discussion is trying to drive patients to get ever lower blood sugar results, the fact is that studies of type 2 diabetics have repeatedly demonstrated that striving for strict control actually kills people compared to taking a more relaxed approach. In fact, the scientific proof that stricter blood sugar control was killing people was so dramatic that the study had to be stopped on ethical grounds, which is something that only happens very rarely in scientific studies. There is a lot about diabetes and blood sugar control that conventional diabetes theory simply does not yet understand, so before ruining my life striving for strict control, I would take a close look at the Accord study and the many, many others that have confirmed it.