When "Normal" Blood Sugar Isn't Normal (Part 2) | Kresser Institute

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When “Normal” Blood Sugar Isn’t Normal (Part 2)

on October 31, 2016

by Chris Kresser

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. 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 diabetes and cardiovascular disease. 

 Marker  Normal  Pre-diabetes  Diabetes
 Fasting blood glucose (mg/dL)  <99  100-125  >126
 OGGT / post-meal (mg/dL after 2 hours)  <140  140-199  >200
 Hemoglobin A1c (%)  <6  6-6.4  >6.4

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.

This study showed that even after a high-carb meal, normal people’s blood sugar rises to about 125 mg/dL for a brief period, with the peak blood sugar being measured at 45 minutes after eating, and then drops back under 100 mg/dL by the two hour mark.

Another continuous glucose monitoring study confirmed these results. 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. One in two “pre-diabetics” get retinopathy, a 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:

 Marker  Normal  Pre-diabetes  Diabetes
 Fasting blood glucose (mg/dL)  <99  100-125  >126
 OGGT / post-meal (mg/dL after 2 hours)  <140  140-199  >200
 Hemoglobin A1c (%)  <6  6-6.4  >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.

 Marker  Ideal
 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.


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  1. Thanks! As a lowish carb, low fat high raw eater, I suspected this was the case when my FBG was showing up 88 to 95 yet my post meal BG never went higher than that either! I couldn’t figure out how to reduce my levels any more, since even my low glycemic fruit intake is only once a day, and my other foods are VEGS. Besides exercise , yuk, lol.

  2. Hi Chris, I have a question regarding popcorn lol, so I will first let you know some information about me, I am 34 years old 177lbs 6’0″ I was overweight in my 20’s and when I turned 31 I peaked to 300lbs! but not for a long time was I this heavy, maybe 4 months, I was however still in the 270’s for about 5 years after college, I started putting weight on after high school from 217, to finishing college around 230lbs, but I am also very muscular, I decided to gain control of my health after some blood work I had when I was 31, and I discovered that I had high tryglicerides 388, my fasting blood sugar was 126, and A1C 5.5 my blood pressure was pre hypertensive, oddly my LDL was not super high around 120’s over the past 3 years I have lost 123lbs! doing intermittent fasting, and following a Mediterranean style diet. I have consistant A1C’s 5.1, fasting sugars are usually in the low 70’s to low 80’s I balance everything pretty well, but here is my question , I often test myself after meals just to see how my body is reacting, and to be honest even when I was 300lbs my two hour would never be above 140, usually around the 120’s after eating pasta and chocolate cake! , however the one food, as of today, my wife and I went out for our anniversary back in october, and split a hot fudge sunday, it was really big, and then I ate half of a really large M and M cookie, after one hour my BS was 100, two hours 92, then in the morning it was 77 fasting, I will tell you that this is routein anytime I eat processed sugar or carbs, but here is my question, for some reason if I eat popcorn during the day I am fine, but if I eat it before bed, and only popcorn will do this, my bs will go up to say 120’s , and two or three hours later its still in the 120s! , and the next day it might read over a hundred, like I get pre-diabetic reading 105 110 at 8 hours, by 12 hours its back below 100. now I typically do not eat a ton of carbs, so I would consider myself to be on a low carb diet. I have read that sometimes people will have this reaction to a low carb diet if they have carbs on occasion, popcorn would be a treat once a month maybe. its the only food that does this. I can eat Ice cream, peanut butter cups , and indulge in a lot , and handle the sugar fine. what is the deal with popcorn? this is air popped with butter or olive oil, this only seems to occur if I eat it late at night, if I eat it in the evening its not an issue. should I totally avoid popcorn? or am I just overreacting about this. I really appreciate taking the time to read this. just FYI, my other blood work is superb, cholesterol is 145, LDL 79 Tryglicerides 44, HDL 70. I mostly eat high fat low carb, moderate fruit and meat.


    • That is weird. Popcorn doesn’t cause me any major problems- slight increase in BG, as would be expected, but nothing out of the ordinary. It’s so weird how some foods affect some people more than others. After I found out I was pre-diabetic, I started writing down everything I ate and testing after 1 hour and 2 hours every meal for a few weeks and then asked to see a diabetes educator about how I could continue to improve my diet. I told her I was worried about spending the upcoming Christmas with my husband’s family because they are Norwegian and there is always copious amounts of lefse and other potato based foods, and potatoes are supposed to be one of the worst foods for diabetics. She asked if I had noticed a strong reaction to potatoes, and I said no, that potatoes, in fact, didn’t seem to affect me much at all. She raised her eyebrows and asked if I was also from a Scandinavian background. I said, yes, that my family was from Sweden. She said she was not surprised, and that she had noticed that her American clients from northern European ancestry could often eat potatoes while her American clients from African or Asian heritage usually had great difficulty eating potatoes. Meanwhile, other grains like rice or quinoa seem to affect those clients less than they did her clients from northern European backgrounds. I know for certain that both rice and quinoa affect me much more easily than potatoes do. So, maybe there are certain people who just react strongly rice and others who react strongly to potatoes, and maybe another group who reacts strongly to popcorn, as well?

  3. Am 68 y/o woman from large extended family and my 73 sister is only relative w/diabetes (type2). She has poor eating habits and is inactive. Now my 11 y/o dog just developed diabetes so I have to give her insulin tw ice a day and it is scary. I’m afraid I will do it wrong. Why can’t she take Metformin instead? Anywho, my blood sugars have always been 70,72 but last bloodwork (fasting) was 105. My doc did not seem concerned but now as I am reafing about doggie and people numbers I started worrying about the 105.I felt from reading your articles you are reliable so….

  4. For the sake of diagnosis, though, don’t you have to do the OGTT? I’m pre-diabetic and almost never have a 2 hour post-prandial over 140. I don’t eat an extremely low carb diet, but I am trying to eat healthier, and I do limit my carbs to around 60g per meal, which is still quite a bit. If I have a snack, I try to keep carbs under 30 grams. This is probably where my diet has changed the most – getting rid of the junky snacking. I haven’t found anything I can’t eat in moderation, though. If I ate an entire bag of candy with no fat or protein, or, if I took the OGTT again, I would undoubtedly fail it. But, I don’t do this, so my post-prandials never show evidence of pre-diabetes. My fasting bg was in the 90s when I was diagnosed, and my A1C was 5.5. The ONLY way I would have been diagnosed is with OGTT.

  5. i am turning 64. my parents were not diabetics. my most recent A1C was 5.5 but the fasting glucose has always been about 100. this time it was 102. I’m very busy during the day and usually eat my main meal in the evening like at about 7 or 8 plus other snacking. Someone told me that if I spread out the eating during the day better, my fasting blood glucose level would be better. I stopped eating at about 8:00 pm the night before the blood work but I still got a 102. I’m going to eliminate some stuff from my diet for the next test but I still think it’s the pattern of eating over a 24 hour period. any thoughts on this??

  6. Doc, while a fasting glucose can be impacted by a very low carb diet, does the a1c also get skewed higher on a low carb diet?

  7. I’m not overweight at all. Fit. 55 year old female. Started low carb (prior 12 years vegan, no dairy, no processed foods) after I did a glucose tolerance test and found my fasting was 97, 1 hr 149 and 2 hour 85. So, two months later on a low carb plant based diet, nothing has changed. In fact my two hour at times is worse. I exercise every day, walk 5 miles a day, swim 3 days a week, do arm weights and HIIT workouts 3 days a week. What On earth is going on?

  8. Daniela and Robin, I too wish I could do something for A1c 5.7 and elevating fasting blood sugars 95-100. Normal weight and lots of exercise. I eat VLC and have recently gone to 20 gm TOTAL carb per day. I have flexible doctors who have trialed metformin and victoza which I can’t tolerate due to side effects. I tried insulin low doses and started to gain weight around my middle so know that extra insulin was contributing to metabolic resistance. Recent labs show fasting insulin 12 so HOMA calculation shows severe insulin resistance.

    I have been experimenting with probiotics, acacia fiber, red wine, vinegar and LOTS of olive oil (1 cup a day) and all seem to help. Now I wonder if lowering animal products would help. Would love feedback.

    • OMG you’re killing your cells with fat. They are blocking your glucose from entering. THAT’S the problem! STOP THE OILS and fats. There is no such thing as a healthy oil.

      See: Dr Barnard.

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