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


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.

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.

As I mentioned in Part 1 of this article series, there are potential problems with how well these tests are utilized to diagnose diabetes. This is an area that needs further study, but continuous glucose monitoring for the general population may be a better marker.

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, fasting insulin, 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 89 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.

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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:

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.

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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Join the conversation

  1. Organ damage etc caused by spikes by themselves (with HBA1C below 6), or prolonged high levels (HBA1cs of 6.5 and above)?

    I’m being told only the latter, but you seem to be saying *both*

    With my deranged metabolism (slightly more deranged than your ‘average’ diabetic!), the only way I can never got above 140 is to never eat more than 10g of carb in one sitting.

  2. I don’t see why it would be different, since the high blood sugar spikes are what cause the organ damage, complications and elevated CVD risk. Perhaps Micheal Barker will chime in here. He has a blog specifically for T2KPD.

    • Dear Chris
      Thank you for the nice insight. I was told after a routine blood test in November, 2013 that I am a prediabetic (106mg/dl FBG). However, since last 2 years I have been running 5-6Km almost everyday @ 11-11.5Km/hr. I was told to follow up after a month; my mom is a diabetic. However, recently my my BG trends were found to be interesting without major change in diet. Average general trend: Fasting BG: 77-80mg/dl. PP 40 mins after meal: 134-155mg/dl. PP 1 hour after meal: 125-127mg/dl. PP 2 hours after meal: 87-89mg/dl. Is that test on November, 2013 can be disregarded now?

  3. Hi – despite living my life like a T1, I am officially a ‘ketosis prone type 2’. So I have the double joy of MDI/hypos _and_ the T2 baggage of ‘sloth and gluttony’ etc.

    So does your advice on never going above 140 also apply to shooters like me?

  4. Lila,

    All of the research I’ve read, and people whose opinions I respect (like Jenny Ruhl & Dr. Bernstein), suggests that keeping blood sugar below 140 mg/dL is the best way to prevent future diabetic complications. I don’t have much experience with T1, so you might want to contact Jenny and see what she thinks. She’s usually pretty responsive.

  5. Hi, I have read both cover to cover.

    I’m just worried about constantly going over 140. My diabetes team isn’t too concerned about this because my BG is back in range by the next meal, and they say my A1C is fine and control is excellent. But from what I read from Jenny Ruhl and also from yourself (as in this article), BG shouldn’t go over 140 *ever* and I just wonder how concerned I should be as mine often does.

  6. Recently diagnosed with insulin-dependent diabetes, and told to check BG only before meals and bedtime. These numbers were usually nothing to worry about.

    But then I started post-prandial testing and was horrified to find out how worrying those were. I am on insulin but the ‘quick’-acting does absolutely nothing for the first two hours, then starts working about three hours later and is done in five hours. (A pattern verified by two weeks of testing hourly while awake, from which my fingers are just recovering, ouch.)

    My FBG is normally in the 90s and each 10g of carb will raise by BG by about 50. Anything I eat will stick around until my insulin gets going 2-3 hours later, so it only takes a piddling 10g to put me in the >140 danger zone. If I do a 2-hour PP reading, it is *always* high but if at that point I add a small bolus, I’ll hypo about 3 hours later.

    I have tried injecting further and further in advance of meals (up to 2 hours before eating, despite being warned not to do so by my diabetes team) but this hasn’t always worked as it is really hard to estimate how much you’re going to eat 2 hours later. Plus some days, the insulin does do what it says on the tin and leads to dangerous hypos (in the 20s).

    Most days when the bolus is matched to my food, I’m back in normal range 5 hours later. Evidently I would prefer never to go above 140 but since it only takes 10g of carb to do that, that is easier said than done even on a low-carb diet. How much do I need to worry about those hours when my BG is above 140, as it almost always comes back down later?

    My A1C is 5.9 which is a bit higher than what I would like; however my diabetes team doesn’t think it should go any lower.

    • Lila: Have you read Dr. Bernstein’s “Diabetes Solution”? He’s T1 himself and it’s by far the best book I know of for T1s because he goes into great deal on how to use insulin properly with an LC diet. You might also want to check out bloodsugar101.com.

  7. Good point, Michael. It would be so much easier if the U.S. caught up with everyone else in this regard.

  8. Chris

    One thing I just thought about in relation to Khalid. Most of the world uses mmol/l for A1c. A 6.5 would translate into about a 5.5 mg/dl A1c which would be about the right number given his daily readings. He might just need to check the units.

  9. Khalid:

    What are your highest readings at 1-hour after meals? With an A1c of 6.5, and FBG of 110, it would appear you must be having some spikes somewhere throughout the day.

    OTOH, A1c isn’t always accurate for a variety of reasons. You could try getting fructosamine tested to see how it compares.

    Blood sugar regulation is complex, and individual. It’s hard for me to say more without doing a more extensive intake.

  10. Dear Chris it is a great article. Two issues here (49y old)
    1. My blood sugar after 2 hrs from normal meal is 100-120
    However fasting is 110. I have low bg variance around 110 all the day.
    2. My a1c is 6.5 and does not match with my average of 110

  11. Hi Emily,

    With reactive hypoglycemia (RH), it’s typical to see a big spike in blood sugar after meals, followed by a hypo. In your case, I’m not seeing any spikes at all so I’d be more likely to call what you’re dealing with plain old hypoglycemia.

    If you’ve been able to manage it with diet, that’s fantastic. Be aware that hypoglycemia is often associated with low cortisol levels or cortisol dysregulation, so that may be something you want to investigate.

  12. Hi Chris, thank you so much for your informative articles. I would be very interested to hear more of your thoughts on reactive hypoglycemia. I’ve had symptoms for as long as I can remember – eating moderate carb or low carb paleo helps a great deal. Not once in my life have I had a (fasting or otherwise) blood glucose reading over 100, though I haven’t done a careful two and three hour post-prandial test all together. Fasting is typically in the 70s, 1 hr after 50 g dextrose is low 80s, and other than that a few 90s over the years when not fasting. My symptoms of shakiness, sweating, and weakness occur approximately 2&1/2 hour after a high sugar meal (sugary cereal and skim milk, for example, back in the day, or a coke, which I haven’t had in 20 years due to this issue) and resolve with juice consumption or eating a piece of fruit. Once my blood glucose was checked then and it was high 60s, another time low 70s. (A aspartame diet drink while fasting, especially “cherry-flavored” will cause the same symptoms after 2 hours, though I haven’t checked the blood glucose then).

    Anyway, your more detailed thoughts on the whole topic of reactive hypoglycemia would be most welcome. I seem to have found my curative diet in any event.

  13. It would be the inflammation, more than the pain, that could contribute to cortisol dysregulation and blood sugar imbalance. But yes, the end result is the same.

  14. Chris,

    Thank you so much for your response. Could a chronic aggravating pain, such as in a heel spur, possibly be responsible for the inbalance of cortisol and/or glucagon that causes the rise in blood sugar?

    Once again, thank you.

  15. Lynn, I have Hashi’s, IBS, diagnosed with fibro, not yet convinced as I have chronically high inflammatory markers and signs of lupus without the ANA, kidneys at stage 2, elevated liver enzymes etc. I’ve just changed doctors and he is redoing all tests, so I haven’t modified my diet yet as I wanted to have these results without changing anything (to help eliminate factors that may be causing symptoms), then going to reexamine diet.

  16. Hi Simona

    Do you live in Ireland? Snap…..

    It is ILLEGAL for her to withhold your test results. You need to speak to her secretary and demand them. If that fails, send a written letter to the records department, citing the Data Protection Act 1989. If nothing within a month, threaten them with legal action. Worked for me, because they knew that what they were doing was illegal.

    T3 is available in Ireland yep. It’s not illegal.

  17. Oh, my basal temperature was ok when I had a look at it in the morning but I’m freezing all the time especially at night when I go to bed late, however, very often it’s around 14 C in the house. I checked that website too.

  18. Thanks Chris,


    I saw a private endocrinologist in August and I think she asked for these tests, I had them done, but unfortunately I haven’t seen the results although I asked for them. She hasn’t said anything besides the fact that my LDL cholesterol is high. You would think she knows how to interpret them properly. She didn’t ask for a salivary cortisol test, so I’m just guessing there, from my experience. She doesn’t want to prescribe T3 as allegedly it is not licensed in Ireland.

  19. Hi Simona

    Peat is more gung ho anti PUFA’S than Stone is. A max of two eggs a day, olive oil only as a condiment and PUFA free meat such as beef and lamb.

    When one increases carbs, it can expose hidden issues. Have you had a cortisol saliva test, a full thyroid panel (TSH, Free T4, and Free T3, anti-TPO, anti-TgAb and Reverse T3) and sex hormone panel done?

    What are you basal temps? Temps during the day? Have you been to Stop The Thyroid Madness?