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. Not necessarily. It’s common for LC folks to have FBG in that range. Presuming your A1c and post-meal numbers are good (which it sounds like they are), I probably wouldn’t worry about it. As your body becomes accustomed to burning fat for fuel, insulin sensitivity decreases. This can cause a “dawn effect” where FBG is higher than you’d expect it to be.

    Some people find that adding a moderate amount of carbs in the form of safe starch, for example, helps normalize their blood sugar.

    • I am a 51-year-old female, I weigh 108 lbs, and on most days I eat between 70-80 grams of carbs per day, most of which come from white rice. Before finally landing upon this eating routine, I would feel light-headed and disoriented within a couple hours of eating a meal (of meat, whole grains, beans, veggies, fruits, dairy). With my current eating routine of mostly meat and fish and white rice my blood sugar problem (if that’s what it was) seems to be under control. As added benefits, I am able to maintain my weight quite easily and my digestive problems have resolved.

  2. Another great article, Chris. I’m puzzled by my FBS being around 95-99 but frequently my 2 hr post prandial will be below 90. Shouldn’t the FBS come in lower? Is this a morning cortisol stress response?

  3. Chris

    Just read your replies on Whole Health. Here’s something to test on people you know who spike easily. Give them a NSAID for two or three days. If their blood sugars cease to spike then it might be a glucose desensitization issue. The has to do with inhibiting COX-2 and PGE 2. I’m reporting on this on my blog.

  4. Chris

    Thank you for doing this topic. I write a diabetes blog and I try not to club the ADA all the time but those guidelines always seem to pop up. They are a major problem because they give people a since of security that they shouldn’t have.

    I was actually thinking about this just before I read your blog. I’m a type 2 ketosis prone diabetic with a family history of diabetes and because of this I view the ADA guidelines as nearly tragic for people like me. I’ve come to a decision that I’m going to tell people in my family what are the real guidelines for diabetes. Basically, I’m taking the FBS ADA guidelines and using them for everything. Less than 100 is normal. 100 to 120 is the prediabetic range and above 120 is diabetic. I know that sounds pretty drastic for postprandials and the like but I would rather my children and grandchildren have a chance for a healthy future.

    I wrote a long blog on hemoglobin and A1c and put up charts of the people who are most likely to have these sorts of problem, whether they are diabetic or not. Interestingly enough, this seems to match up with peoples that live where Malaria or Malaria like diseases occur.
    http://ketosisprone.blogspot.com/2010/10/a1c-glycation-problems-and-dka.html

  5. Julie: also note that some people actually experience better blood sugar control on a moderate carb. diet than a low-carb diet. I suspect this is related to the phenomenon I described above, where introducing some carbohydrate gets the body accustomed to burning it again, and probably prevents cortisol and/or epinephrine from getting involved.

  6. Julie:

    I didn’t mean to imply that it is never a concern; just that it may not be a concern. It depends a lot on what your post-meal and A1c levels are. For example, if you wake up at 105 mg/dL but drop down in the 80s soon after, and stay between 80-120 for the rest of the day, I may not be concerned (I’d also have to consider your symptoms, micronutrient status and other clinical variables).

    However, if you wake up at 105, never drop below that level, and experience post-meal BS of above 140, then I’d be concerned. Those are two totally different patterns.

    The studies that were done showing FBG >95 causing harm were likely done in people eating the Standard American Diet (for the most part). I suspect had the researchers tested their post-meal blood sugars, they would have been high. So I’m not sure we can extrapolate those results to someone eating a low-carb diet. I believe there are some studies showing that higher FBG with normal post-meal BG doesn’t predict future diabetes, but the opposite is not true, i.e. studies show that you can have a normal FBG, but if you have elevated post-meal BG you’re still at risk.

    This is a complicated topic and several variables are involved. Check out an interesting article and discussion in progress at Stephan Guyenet’s blog. Make sure to read the comments.

    • I stumbled on this thread quite by accident, but this could be a total paradigm shift for me. My blood sugars have been out of whack for years – usually low. But stayed having high FIG readings so I went low carb. But the numbers get worse. I’m going to scour and devour this site. First thing – your 3 day increased carb challenge. Question: menopause seems to have really mad things word. Advice?

  7. Chris: if, as you state, “low-carb diets induce insulin resistance,” why isnt it a cause for concern? I went low carb some time ago, no processed foods or bad oils, lost 20 lbs, do regular strength and resistance training, A1c 5.3, post-prandials <120, usually <100, very low BMI and BFP.

    But now my FBG levels are as high as 125! (Used to be 87) I guess I don't get the logic behind the assurances that a low-carb diet "may not be a cause for concern" when in the same article you quote studies showing diabetes risk in people with FBG levels above 95. Type II diabetes runs in my family, including those who are not overweight. Does someone like me need to be concerned? What are the botanicals and nutrients you refer to that can improve insulin sensitivity?

  8. Could also try high intensity strength training, to make sure you’re really depleting muscle and liver glycogen occasionally. Something like Body By Science.

  9. There are a number of botanicals and nutrients that can help improve insulin sensitivity. You could also try intermittent fasting, provided your cortisol levels are not out of whack.

    • In February 2011, my cortisol level was 17.3. My naturopath at the time told me it was too high.
      My insulin tested at 3.0 and my A1c was 5.8.
      Is there any suggestion on how to lower AM cortisol? He never really gave me any help on that and he is no longer in my employ for a myriad of reasons.

      My new practitioner is a typical MD and he told me I didn’t need a fasting insulin test, a CRP or homocysteine test. He gave me some lame explanation about my other numbers indicating the information. Finding a good doctor is almost impossible these days. Even the naturopaths really don’t know what they are doing.

      My last total cholesterol (Sept. 2011) was 236. HDL 68, LDL 153, Tris 75.

      I do have stubborn belly fat that I have had most of my adult life but I’m losing some of it as I work on my diet.

  10. Any other thoughts on correcting IR? I read all your posts and I watch my carbs, eat a strict gluten free diet, limit vegtable oils (I have recently gone further and embraced an extremly low PUFA diet, so no more bacon etc. for me) and get good sleep. I also take metformin and natural thyroid.

    Despite all of this, my insulin is never below the magic 10. Any ideas on what I can do?

  11. Lynn: I think there’s wiggle room here, and it depends somewhat on the metabolic function of the individual. But I don’t consider 150g to be low-carb. I’d say that’s more in the realm of “moderate”. I’m thinking more like 100g and below.

  12. Great article. You mention that FBG can be elevated on a low carb diet. What do you consider a low carb diet to be in terms of grams? Anything under 150g a day or do you mean a keto diet?

  13. Thanks, Lacie. As for your BF, it could be a reactive hypoglycemia pattern, where the insulin surge after meals is too high, and he goes into a hypoglycemic state (which produces the extreme hunger). Cortisol dysregulation is often involved in this situation.

    • I think I might have this issue also, extreme hunger shortly after meals(carb cravings) and also lately extreme fatigue 1 1/2 – 2 hours after some meals. If it is reactive hypoglycemia, what does one do to fix it?

      • I was diagnosed with reactive hypoglycemia when stroke like symptoms presented themselves after eating quinoa for breakfast. (I had been eating more whole grains than I usually did that week.) The only thing that helped me was to cut out all sugars completely, (fructose, lactose, natural, artificial – if it was sweet, I didn’t eat it or drink it) and not eat any carbs EXCEPT if they were from certain types of vegetables like broccoli (no potatoes, or other overly starchy veggies). And, when I ate a veggie, I had to pair it with a protein. No nuts, except for a banana with peanut butter prior to a 4 mile run since then my body would use the sugars from both over time and I wouldn’t pass out. :0)

        Basically, all I ate was meat and dairy for three months straight (beans act like carbs, so they are also a no-go).

        My body reacted very well to this modified Atkins diet, but I could see someone having a heart attack if they stay on it too long. My sugars have been better for over 9 months since being that super strict for 3 months.

    • Hey Chris I have type 2 and have been on metformin but here lately I have had issues where my blood sugars drop after I eat they latey has not went over 110 and an hour later they are down to 83 I’m not on low carb but I don’t eat a ton of carbs I follow the servng guidelines I have been scared to take my metformin due the low numbers any idea [email protected]

  14. Chris, this is some of your best work. My BF and I are two months into a low carb diet and my blood sugar has stabilized, but he still has extreme hunger between meals and has to eat 4x a day. Can’t wait for your third installment where you show how to check blood sugar between meals–I suspect he has some insulin issues that I don’t have.

  15. Did you read the article? The whole point is that the mainstream targets you learned in your CEU class are not supported by the scientific literature. An A1c of 7% maps to an average blood sugar of 172 mg/dL. Studies clearly show blood sugar that high dramatically increases the risk of cardiovascular disease and diabetic complications. The same is true for a fasting blood sugar above 95 mg/dL and 2-hour post-meal / OGTT readings above 140 mg/dL.

      • Actually Laurie, it depends on which formula you use.

        Chris’ measurement was from the DCCT formula. There are SEVERAL formulas for determing eAG (Estimated Average Glucose) from A1c, including the ADAG formula you mentioned.

        Unfortunately, none are perfect or correct in every instance. I’d rather estimate high than low, personally.

  16. K… Don’t know where or how old this info is. I’m work in pharmacy and just took a CEU on Diabetes that the info is good on for the next two yrs. Normal is 70-130 and a1c of 7%. So… wondering what source is being used for these statistics. You can view mine at wwww.powerpak.com look for the CEU on Diabetes.

    • The AACE (American Association of Clinical Endocrinologists) and the IDF (International Diabetes Federation) are BOTH recommending Lower targets than the ADA (American Diabetes Association) which is where you get your numbers from, Bridwell.

      The IDF mentions the following targets:

      HbA1c 6.5%
      BP 130/80 mmHg
      Total cholesterol 4.5 mmol/L (174 mg/dl)
      LDL-cholesterol 2.5 mmol/L (97 mg/dl)
      HDL-cholesterol 1.0 mmol/L (39 mg/dl)
      Triglycerides 1.5 mmol/L (133 mg/dl)
      Urinary albumin:creatinine 2.5 mg/mmol (22 mg/g) – men
      3.5 mg/mmol (31 mg/g) – women
      Exercise 150 min/week

      Both the IDF and the AACE also recognize the importance of reducing refined carbohydrates and starches to achieve these goals. They don’t come outright and recommend Paleo or LCHF diets, but they do recognize better glucose control in those that adopt those diets than in those that maintain a typical (western) diet.

      BTW, the primary source of funding for the American Diabetes Association is Pharmaceutical and Processed food companies. I don’t have stats for every year, but in 2005 alone Big Pharma and Big Agro/Food companies gave them 23 MILLION DOLLARS. It’s only been increasing since then.

      Regarding post-meal glucose, in the IDF “Guideline for the management of post-meal glucose” (the most-recent version) they clearly state: “The new IDF guideline recommends that people with diabetes try to keep post-meal blood glucose below 7.8 mmol/l (140 mg/dl) during the 2 hours following a meal.” Not just AFTER two hours following, but DURING. In other words, no spikes at 45 minutes, one hour or two hours…

      The ADA still states 10.0 mmol/l (180 mg/dl) is a safe spike, which is ludicrous since every study done shows damage starts at 7.8 mmol/l (140 mg/dl)

      Interestingly, the ADA tries to look good by being a member of the IDF … but they’ve yet to adopt their protocols, as doing so would mean they could no longer accept money from companies that make money of cheap cereals that spike sugar. I mean, Honey-Nut Cheerios – one of the highest glycemic cereals around – covered in high-fructose corn syrup and other sugars had the ADA seal on it…

      • The ADA is sponsored by drug and food companies, they want you to keep using their products so of course they are going to say higher limits are “ok”, take anything the ADA says with a grain of salt, remember its your health, not theirs!

      • This is an interesting post, Glen. Thank you for your diligent research. Can you post the source for the numbers you post? I am above each of those numbers but below the US guidelines in all cases. Would like to better understand the difference. Thanks,

      • I’m not 100% sure about this, but even if over 140 causes nerve damage… The rationale for ADA I think is that at that rate nerve damage will take 100 years to translate into an actual problem for you. Vast majority of people are not going to be in this world 100 years from now, so whats the point of limiting yourself for no reason.

        180 is likely the number they came up with beyond which you are likely to see complications in THIS life.

    • My Doctor says I am Diabetic. My A1C was 5.1 and my average fasting is 105-109
      2 hrs after a meal is 95-112 (lowest to highest)

      I am confused with these numbers and would like to know why she is considering me diabetic.

      • Linda there is no way these numbers are diagnostic for diabetes. The fasting readings imply impaired fasting glycemia, a possible pre-diabetic condition but not necessarily if you control risk factors. The A1C & post meal figures look completely normal.

    • Remember that the drug and insurance companies benefit by classifying as many people as diabetic as possible, particularly people who are, in fact, not diabetic. This will aid in (a) selling drugs and meters/supplies, etc., and (b) increase the amount of premiums coming in.

      • Mike, the ADA has the levels set ridiculously high because they observed one population and at the levels they selected it was guaranteed to have diabetic complications which were irreversible.

        The ADA is effectively a gatekeeper of who will and won’t be diagnosed with diabetes based on their observations in that singular population.

        I would argue if your blood sugar is already running above 100 then your diabetic and should be take steps to correct it. The 200+ level is asinine.

        • I didn’t want to argue against the ADA because I figured everyone would come to the same conclusion. The ADA will kill you. Their diet promotes maintaining a BG above 140 which will damage, over time, every organ, nerve, muscle, eyeball to the point you will be disabled or die. My dad has followed their diet for 15 years and cannot control his BG. I followed a low-carb/quality carb diet for 3 months, and I can maintain mine. Don’t follow the ADA diet, it is suicide!