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Unraveling Blood Sugar Part I: The Truth about ‘Normal’ Levels & Understanding Your Test Results

Updated: Jun 9, 2020

New patients will often tell me their blood sugar is 'normal'.

When I ask what this means to them – in terms of actual numbers – the variability among responses speaks volumes to the general public's lack of clarity when it comes to defining target ranges for physiologic parameters such as blood sugar.

World Health Organisation Definition

The WHO defines diabetes as a fasting blood glucose ≥7mmol/L. But have you ever questioned where that number comes from? Why not 6.8, or 7.25? Does any result below 7 mean my blood sugar is a non-issue?

Turns out ideal levels may be a lot lower than the guidelines suggest.

We'll look to answer these questions and more in this series as we deep dive into the finer details of blood sugar and it's control.

Nature is not Digital

It's easy to forget nature doesn't function in a digital manner. There's no internal sensor which triggers an alarm when blood sugar tiptoes above 7mmol/L, nor does a result of 6.9 – 0.1mmol/L within the recommended range – mean we're out of hot water.

The so-called “Normal”

Our body functions as a continuum and it's naive to think we should only treat patients who are categorically diabetic. Although guidelines are established to inform best practice, it's important to keep in mind their man-made and somewhat arbitrary nature.

My own survey of GPs found the majority considered a fasting blood sugar of 5.5 or less to be normal, a belief most of my new patients seem to share. Pathology reports usually come with a reference range or guideline listed in brackets next to each result and many patients will take these as gospel, not concerning themselves with any results not flagged as outside the indicated parameters. Let's delve a little deeper into patients who fall within this category (blood glucose <5.5mmol/L) and take a closer look at whether they indeed have naught to worry about.

A group of 46,578 Californians aged 40+ with a fasting blood sugar of 5.5mmol/L or less (ie. within “normal” range) were divided by researchers into 2 groups, 4.8mmol/L or higher (80% of the cohort) and a smaller group with fasting sugar <4.8 (18.7%).

n=46,578. 1977-2007. Mean f/u 7 years. 40% male, age: 57.5years.

In the 6-7 year follow up period, results of this mammoth study showed that even in a cohort of adults classified as having 'normal' blood sugar, those with levels of 5.3-5.5 are 2.3 fold more likely to become diabetic than those <4.8. For every 0.1mmol/L rise in blood sugar above 4.8, the corresponding increase in risk of becoming a diabetic is a whopping 12% (1).

Being ‘within range’ might not be the all clear we assume it to be.

Are Asians more at Risk?

Among the Asian population it’s been shown that the same findings hold true.

Of 11,369 Japanese non-diabetic citizens aged 40-79 whom were studied (all with fasting blood sugar <6), 85% had readings ≥4.8. The researchers followed participants over a period of 7 years and found that individuals with blood sugars of 5.5-5.7 were 3.8x – and those at 5.8-6, were 8x – more likely to become diabetic when compared to those with a blood sugar under 4.8.

It's widely accepted that Asians are at higher risk of becoming diabetic than their Caucasian peers, and hence at the same blood sugar levels tend to experience more potent adverse effects as a result – with those from Southern Asia identified as the worst sufferers.

In a group of 10,000 Chinese adults aged 35-59 not known to be diabetic, researchers found that over a 20-year period of follow up, fasting blood sugar was correlated to death rate.

Individuals with a fasting glucose of 7 or higher carry a 2-fold higher risk, whilst those between 5-6.9 a 20-40% higher risk of death compared to those with readings of 5 mmol/L or less (3).

What Levels are Safe? Should 4.7 to 4.8mmol/L be our new benchmark?

It’s clear that the “normal” or safe range for blood sugar is nowhere near as high as 5.5mmol/L. Blood sugar levels of higher than 4.7 to 4.8 are a potential early signal of your body’s impaired ability to handle ingested sugar and can be associated with a higher likelihood of progressing with time, to fulfill WHO definition of diabetes.

If your fasting blood sugar results are greater than 4.7 to 4.8, further assessment of your glycaemic status may be warranted to rule out “prediabetic” status. The “prediabetic” label is an umbrella term used to describe an array of medical terms including fasting hyperglycemia, insulin resistance, impaired glucose tolerance etc. (all of which are also notably by and large arbitrarily defined).

My Fasting Blood Glucose Levels are OK What next?

One of the dangers of complacency is what happens when we stop after just looking at fasting glucose.

Our body is a complex, nondigital system constantly operating underneath the surface to maintain optimal function and keep itself adequately regulated. A lot of the time, keeping things moving can come at a hidden cost.

A case study may help illustrate my point.

Mr KA is a 65-year-old Chinese Australian with no noted history of issues with sugar handling. He has suffered high blood pressure for several years (well controlled with medications) and led a mostly sedentary life as a successful businessman. His parents were both diagnosed with diabetes in their 70s and as a result, Mr KA has always been conscientious in monitoring blood glucose.

Fasting blood sugar:

When we looked at his fasting glucose, that is, his blood sugar following 12+ hours void of caloric intake, it was indeed normal at 4.8 mmol/L.

Fasting insulin:

Then we looked at his fasting insulin, it was 9.1 mIU/L.

Insulin is released into blood circulation by the pancreas after eating and regulates blood sugar by facilitating glucose uptake into – and providing energy for – each and every one of the 37.2 trillion or so cells in our body (more on this process in part II). After over 12 hours of fasting and inactivity, we would expect insulin in blood circulation to be low.

With baseline levels anticipated to be 1-2 mIU/L, what was all this insulin doing in his blood? Insulin shouldn’t have any work to do under these circumstances.

In Mr KA's case, his elevated fasting blood insulin levels were indicative of his body's need for insulin, even without the added stimulus of a glucose load. Essentially, his blood sugar wasn't normal at all - just kept misleadingly so by colossal amounts of insulin being churned out by an overworked pancreas. In effect, the compensation is a lot like Auspost at Christmas time. To the consumer, the delivery service is functioning as expected, with packages getting delivered on time. In reality, this higher output is only able to be maintained by a corresponding influx of extra workers in distribution centres pulling longer shifts.

Now imagine its Christmas at Auspost in your body all year roundand this is the level your organs are required to perform at continually, it's np wonder burn out of the pancreas eventuates.

GTT (Glucose Tolerance Test):

Sometimes the full picture isn't yet clear and a Glucose Tolerance Test may be of use for further investigation of blood sugar handling.

We gave Mr KA a 75g dose of glucose to ingest and rechecked his blood sugar and insulin levels 1- and 2-hours post. The subsequent rise and fall of his blood sugar levels are as expected don't immediately alert us that anything is dramatically amiss.

Insulin however, stayed in circulation even 2 hours after the sugar drink as opposed to curving down to baseline levels. Mr KA's blood glucose was kept regular at the expense of an exorbitant amount of insulin production, in addition to the elevated baseline levels. Let's visualise this happening every day, each time Mr KA eats a meal or drinks an orange juice, year in year out. Whilst it's great the pancreas has the capacity to kick into this higher gear, there's only so long it can stay there before total burn out. Eventually, the cells in our body will become more and more resistant to insulin, desensitised, in the same way antibiotics lose their effectiveness against targeted bacteria with frequent exposure. Ultimately, this eventually leads to a dependency on injectable artificial insulin for survival.

Take Home Message:

Balancing and understanding your sugar and insulin levels is a lot more complex than it appears. Although the safe range for fasting blood sugar is often quoted as 5.5, we would all do well to refer to this as a minimum reference point, rather than the gold standard. Though most extreme sugar-related illnesses can be avoided under this threshold, it's best to see levels just below this as still in the 'orange zone' rather than the green.

Rather than aiming for a figure to get below and taking your foot off the pedal upon reaching it, focus on constant maintenance, if not improvement of your results from one test to the next.

In Part II of this series we will dive into how to improve the efficiency of your cells’ uptake of sugar and some interesting medical methods to prevent your pancreas from working overtime.


  1. Nichols GA et al. Normal Fasting Plasma Glucose and Risk of Type 2 Diabetes Diagnosis. Am J Med. 2008 Jun;121(6):519-24.

  2. Kato M et al. Fasting Plasma Glucose and Incidence of Diabetes --- Implication for the Threshold for Impaired Fasting Glucose: Results From the Population-Based Omiya MA Cohort Study. J Atheroscler Thromb. 2009;16(6):857-61.

  3. Zhou L et al. Fasting glucose and its association with 20-year all-cause and cause-specific mortality in Chinese general population. Chronic Diseases and Translational Medicine 2019; 5(2):89-96.

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