Blood Sugar 140: Where did the 140 mg/dL threshold come from?

This post (that is going to be a series to keep post lengths manageable) has been brewing for quite a while, but I was reminded of it because Jenny Ruhl came out with a diet book recently, and appeared recently as an "expert" on Jimmy Moore's Ask the Low Carb Experts podcast.  After listening to her previous podcast with Jimmy, I had some mixed reviews.  Jenny is certainly articulate, well-read (though I disagree, often strongly, with many of her interpretations) on the topic, and quite a bit more moderate/measured than most in her views on controlling diabetes with low carb.   But she also seems to view all diabetes through her own MODY eyes.  MODY (Mature Onset Diabetes of the Young) is a rare genetic form (there are actually several rare genetic forms classified as MODY) that is lumped quite often under the category of Type 1.5.  One of these days I need to address some other things on Jenny's website, as the site (and her book) is one of the commonly linked to internet resources for diabetes info in LC circles.    Her book, Blood Sugar 101, more specifically this Amazon review , as well as this page on her website, is where I took the title of this post from.  On the website she writes:

The studies you will read below, some of which are not cited in the AACE guidelines, make a cogent case that post-meal blood sugars of 140 mg/dl (7.8 mmol/L) and higher and fasting blood sugars over 100 mg/dl (5.6 mmol/L) cause permanent organ damage and cause diabetes to progress.

It is the first reference on this page that seems to be the original primary research source for the 140 mg/dL threshold ... a threshold that seems to have become somewhat ingrained as fact.

Increased Prevalence of Impaired Glucose Tolerance in Patients With Painful Sensory Neuropathy.   This study is cited under the bolded header and description that follows:

Nerve Damage Occurs when Blood Sugars Rise Over 140 mg/dl (7.8 mmol/L) After Meals

The University of Utah neurologists found that patients who were not known to be diabetic, but who registered 140/mg or higher on the 2-hour sample taken during a glucose tolerance test were much more likely to have a diabetic form of neuropathy than those who had lower blood sugars. Even more telling, the researchers found that the length of time a patient had experienced this nerve pain correlated with how high their blood sugar had risen over 140 mg/dl on the 2-hour glucose tolerance test reading.

It is important to note that this study also showed that only the glucose tolerance test results corresponded to the incidence of neuropathy in these patients, not their fasting blood sugar levelsmor their results on the HbA1c test.  This is significant because most American doctors do not offer their patients glucose tolerance tests, only the fasting glucose and HbA1c tests that fail to diagnose these obviously damaging post-meal blood sugars.

She is not alone in over-reaching with the results of this study.  In the Kindle version of Perfect Health Diet, the Jaminets write:

Nerve Damage Occurs When Blood Sugars Rise Over 140 mg/dl 

In a study of patients with peripheral neuropathy of unknown origin, neurologists found that many people who don’t have diabetes nevertheless have “diabetic neuropathy” – nerve damage from excess glucose. Moreover, when given a glucose tolerance test, how high blood sugars rose over 140 mg/dl was correlated with severity of the neuropathy. 30

Jaminet, Paul; Jaminet, Shou-Ching (2011-12-07). Perfect Health Diet: Four Steps to Renewed Health, Youthful Vitality, and Long Life (Kindle Locations 712-716). YinYang Press. Kindle Edition. 

There is a hat tip to Ruhl in the footnotes here. This is something Paul has repeated on the website and evoked during the "safe starches" debate::

In Safe Starches Symposium: Dr Ron Rosedale

In my main reply, I had written:  What is a dangerous level of blood glucose?

In diabetics, there seems to be no detectable health risk from glucose levels up to 140 mg/dl, but higher levels have risks. Neurons seem to be the most sensitive cells to high glucose levels, and the severity of neuropathy in diabetes is correlated with how high blood glucose rises above 140 mg/dl in response to a glucose tolerance test. [1] In people not diagnosed with diabetes, there is also some evidence for risks above 140 mg/dl. [2]

{emphasis mine}, and

In How to Minimize Hyperglycemic Toxicity:

What’s interesting to me here is that what we really care about is not the glycemic index, but the peak blood glucose level attained after a meal. It is blood glucose levels above 140 mg/dl only that are harmful, and the harm is proportional to how high blood glucose levels rise above 140 mg/dl. So it’s the spikes we want to avoid.

As mentioned previously, during my over three years now in LC internet circles, I've heard this 140 number innumerable times.  I've seen people worrying over doing irreparable damage to their cells and organs whenever  their blood glucose spikes over 140.  This is a case of a combination of what I would call scientific telephone and circular referencing.  Here we have one rather small study that has been blown way beyond all proportion as to its implications.

• By "scientific telephone", I'm referring to the phenomenon that occurs when we play that telephone game as kids.  Remember that?  One person makes up a phrase then whispers it to the next person in a circle.  And around and around it goes until the last person whispers it back to the originator.  I don't recall a single instance from my childhood when the phrase made it around intact, and often it was horribly mangled.  
• By circular referencing I am referring to the phenomenon that can occur sometimes where one source makes a claim and others repeat it enough that the claim becomes attributed to them, and the claim can even reach "I heard it somewhere" status because so many refer to it, formally or informally.  It's like those numerous famous quotes that nobody really knows who said it first.

I think many coming from the paleo side of things in this community associate that 140 mg/dL claim with Paul, if anyone at all ... while low carbers more with Jenny.  Mostly I just hear that number thrown around w/o attribution as if it is accepted common knowledge that blood glucose merely "spiking" over 140 is a bad, bad thing..

Indeed even the rather daunting conclusions of the study fall far short of implicating blood glucose spikes:

Our results suggest that IGT may cause or contribute to small-fiber neuropathy, which is similar in phenotype to the painful sensory neuropathy commonly encountered in diabetes. Two-hour OGTT is more sensitive than other measures of glucose handling in screening these patients.

So let's look at the study, shall we?  

The total study group numbered 107 subjects with "idiopathic neuropathy" -- idiopathic meaning of unknown origin.

The median age for the [...] patients was 64 ± 10.8 years (range 46–92), and 59 (55%) of these patients were women. Most patients (88) had exclusively sensory symptoms and signs; 76 reported neuropathic pain, and 12 reported no significant pain. The remaining 19 patients had symptoms and signs of distal weakness in addition to sensory symptoms.

Of the group only 72 subjects were evaluated with an OGTT (67% = 2/3rds of the study group, likely not relevant except that the effective study size is smaller).  As it turned out of the 72:   13 (18%) had Diabetes  ,  36 (50%) had Impaired GT  ,  23 (32%) had Normal GT

There were a number of other findings I'll address in follow-up posts.  One thing I'll say about the study itself at this time is that a study like this just begs for a control group of similar age.  To me this is a huge flaw in the study design.  This is an older group, and insulin resistance and IGT is prevalent in such a group to begin with.  For example, in this study (I found on a very quick search) of 65-74 year olds, 34-38% of the subjects had abnormal glucose tolerance.    Not a perfect comparison, but enough to get the idea.  Pick a random 65ish year old person off the street, roughly 1/3rd have IGT or worse.  Clearly amongst the painful neuropathy subjects, IGT & diabetes were more prevalent (roughly 2X vs. that study), but in the context of the prevalence of IGT in the aging in general, they are somewhat less "impressive".

In interpreting this study and its implications, I plan to address (in whatever order ends up working best):

• The importance of context in extrapolating findings for measures like blood glucose levels.
• What this study says (or does not say) about glucotoxicity (the actual blood glucose levels per se) vs. the potential underlying causes (e.g. insulin resistance) of the symptom (hyperglycemia)
• What this study says (or does not say) about elevated glucose and the role of glycation in diabetes complications.
• The utility of analyzing an "end point" study group for causative agents.  

In the end, I hope to present my case that this study does not provide much support, if any, for the contention that postprandial or transient glucose spikes over 140 mg/dL are damaging to neurons or other organs.

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Follow-up post:  Blood Sugar 140: Context is Everything I - Diabetic vs. Non