Research & Education

Higher Fasting Glucose on Ketogenic Diets: Reason to Worry?

The vast majority of people who adopt a low carb or ketogenic diet experience stunning improvements in blood glucose control and insulin levels.  Carbohydrate restriction is so effective for type 2 diabetes and metabolic syndrome, in fact, that researchers have said it should be “the default treatment” for these issues, and those who follow it are typically able to reduce or eliminate many medications, including insulin.  However, individual variability being what it is, a small percentage of patients see a rise in fasting blood glucose after some length of time on a very low carb diet.  Considering that elevated fasting glucose is part of the diagnostic criteria for both type 2 diabetes (T2D) and metabolic syndrome, is this seemingly paradoxical rise a reason for concern?

As is true for so much of functional medicine, it’s all about context.  A fasting blood glucose (FBG) that’s higher than one would typically expect in someone adhering long-term to a very low carb diet doesn’t automatically indicate anything nefarious.  There are many reasons why FBG might be elevated, and many ways to assess metabolic health and glucoregulation beyond just this one measurement.

“Dawn Phenomenon”

The dawn phenomenon will be well known to any physician who treats patients with diabetes or insulin resistance (IR).  Blood glucose naturally rises in response to surges of cortisol and other energy-mobilizing hormones in the early hours of the morning.  This happens in everyone, not just those with diabetes or IR, but for healthy people with good insulin sensitivity, by the time they wake up, glucose has come back to a normal level.  In those with diabetes or IR, on the other hand, the glucose remains elevated for a bit longer, so when they test it first thing in the morning, it’s on the high side.  This can happen even when these individuals follow a low carb diet.  Glucoregulation as a whole, however, will typically be vastly improved even if the first morning glucose reading is still higher than normal.  

“Adaptive Glucose Sparing” 

Another reason for higher than expected fasting glucose is something called physiological insulin resistance.  This is a phenomenon that occurs in people who’ve followed a very low carb or ketogenic diet for a significant length of time, and it’s somewhat related to the dawn phenomenon.  In people who are keto-adapted or fat-adapted—that is, they’ve adhered to a very low carbohydrate intake for a long time—most of the body’s cells run happily on fatty acids and ketones, with a much lower requirement for glucose than in people on a higher carb diet.  Since these cells are fueled effectively and efficiently by fats and ketones, they need only minimal glucose. 

The majority of glucose is spared for tissues with an absolute requirement for it, such as the brain.  With muscle tissue “refusing” the glucose in order to keep it available for the brain, the blood glucose rises, especially first thing in the morning.  This is sometimes called “physiological insulin resistance” in order to differentiate it from pathological insulin resistance, but a better name for it is adaptive glucose sparing—an adaptation some people’s bodies make as a healthy, normal, and to-be-expected response to a very low carbohydrate intake.  

Fasting Glucose – Limited and Misleading!

One measurement in isolation should rarely be used to diagnose something or to prescribe medication—especially when it’s something as variable as fasting glucose.  A better tool to gauge glucoregulation is hemoglobin A1c.  For most people, as long as A1c is still normal, a slightly elevated FBG isn’t cause for alarm.  While the fasting level may be on the high side, the A1c is a better indicator of glucose levels throughout the rest of the day, over the course of weeks and months.  If a patient is particularly worried about a slightly high FBG, they can use a home glucometer to test multiple times throughout the day for a few days or weeks in order to get a more accurate picture of their glucose control.  People who experience dawn phenomenon or physiological insulin resistance often find that their glucose levels throughout the day are well within the normal/low-normal range they expect on a ketogenic diet, and this can put unfounded fears to rest.

However, A1c is fraught with liabilities and is not always a reliable indicator of blood glucose levels.  There may be an even better way to assess metabolic health and insulin sensitivity.  The real bang for the buck with regard to insulin sensitivity and healthy glucose control is HOMA-IR.


The name says it all: homeostatic model assessment of insulin resistance.  Where HOMA-IR shines over fasting glucose and A1c is that it takes insulin into account—that is, how hard the body needs to work in order to maintain homeostasis: how much insulin is required to keep blood glucose in a normal range?

Fasting glucose and A1c are both measurements solely of blood glucose.  However, in a staggering number of people, FBG and A1c are normal because dangerously high insulin levels are keeping them in check.  (The researcher who uncovered the astounding scope of this was Joseph Kraft, MD, who chronicled these findings in his book, Diabetes Epidemic & You.)  

By testing only FBG and A1c, potentially millions of people with impaired insulin sensitivity are lulled into a false sense of security with regard to their metabolic health.  HOMA-IR is what really tells the tale, and it can help identify patients at risk for myriad conditions stemming from chronically elevated insulin, even when blood glucose is normal.  There are numerous such conditions, but the short list includes PCOS, hypertension, gout, obesity, BPH, erectile dysfunction, and Alzheimer’s disease.  

Here’s how HOMA-IR is calculated:

Glucose in mass units (mg/dL)

 HOMA - IR =

(Glucose x Insulin) / 405

Glucose in molar units (mmol/L)

HOMA - IR  =

 (Glucose x Insulin) / 22.5


  • Let’s see HOMA-IR in action:
    Excellent insulin sensitivity:  ≤ 1
  • Average insulin sensitivity:  1.75
  • Insulin resistant:  ≥ 2.75

Patient A

      Fasting glucose: 92 mg/dL

      Fasting insulin: 4 μIU/mL

 HOMA-IR: (92 x 4) / 405 = 0.90

Patient B

     Fasting glucose: 82 mg/dL

     Fasting insulin: 14 μIU/mL

 HOMA-IR: (82 x 14) / 405 = 2.83


Patient A’s fasting glucose is higher than Patient B’s, but Patient A’s insulin is much lower.  By taking both glucose and insulin into account, the HOMA-IR scores shows that even with a lower fasting glucose, Patient B is at greater risk for metabolic complications down the road.  Their body has to work harder and they require much more insulin in order to maintain a healthy glucose level.  

Bottom Line

A slightly elevated fasting glucose in long-time low carbers or keto dieters is typically not cause for concern.  As always, the full picture should be considered so that each measurement—be it glucose, A1c, insulin, triglycerides, LDL, or something else—can be interpreted as part of a dynamic system and within the proper context.

By Amy Berger, MS, CNS