If you enjoy a low-carbohydrate or ketogenic diet, you should be aware of the phenomenon I call temporary carbohydrate intolerance. You already know that carbohydrates—especially sugars and simple starches—will reliably increase the blood sugar of even the healthiest person. What you might not realize is that when you follow a keto diet, those same carbs are likely to spike your blood sugar even more.
Temporary carbohydrate intolerance is a predictable result of the healthy metabolic changes that accompany the ketogenic diet. Unfortunately, the phenomenon is only rarely discussed, both in the halls of medicine and in the online community of keto dieters. Even keto-friendly doctors and nutritionists may be unaware that it occurs. The reason this is a problem is that when temporary carbohydrate intolerance is noticed, many—doctors, patients and dieters alike—confuse it with chronic insulin resistance. This can sound distressing—isn’t a keto diet supposed to improve insulin sensitivity? This confusion may result in false diagnoses of Type 2 diabetes, gestational diabetes, and pre-diabetes.
The Name: Temporary Carbohydrate Intolerance
I propose we call this phenomenon “temporary carbohydrate intolerance” because I think it is the most accurate possible description of exactly what occurs: a temporary and quickly reversible state which worsens the body’s ability to efficiently metabolize carbohydrates and which does not in any other way indicate insulin resistance.
Why does the phenomenon need a new name? Well, experts haven’t even really agreed upon a name to begin with, and the terms they do use are generally unhelpful: adaptive insulin resistance, impaired glucose tolerance, physiological insulin resistance, peripheral insulin resistance, and perhaps others. Each of these is misleading and/or incomplete.
I don’t think we should use the words “insulin resistance” at all. Our condition has almost nothing at all to do with the insulin resistance that defines Type 2 diabetes, a chronic and debilitating condition that is also highly related to obesity, metabolic syndrome, compromised immunity and cardiovascular illness. I know from the experience of my own patients that they very often experience temporary carbohydrate intolerance even while they are otherwise enjoying the best insulin sensitivity of their adult lives.
My Story
I have specialized in metabolic disorders for my entire career, but even I was only dimly aware of temporary carbohydrate intolerance until about five years ago. That was when I experienced it myself.
When I was pregnant with my first child, I was diagnosed with gestational diabetes. The experience of frequently testing my own blood sugar, and seeing how carbohydrates led directly to blood sugar highs, was what inspired me to adopt a ketogenic diet after giving birth. It worked well, and I felt healthy, but I was shocked to see that cheat meals—even just a croissant or a cookie—would spike my blood sugar higher than I had ever seen it before. My body had obviously forgotten in some way how to tolerate moderate or large doses of carbohydrates.
A few years later I had an unrelated bout of illness, and because I had lost so much weight I decided to add carbs back into my diet. For the first few days my post-prandial glucose levels were close to 200 md/dL. But soon after that, my highest post-prandial glucose levels (after eating 4 chocolate chip cookies!) were only as high as 120 mg/dL. It took a few days, but my body evidently reengineered itself quickly to deal with carbs again.
If you test your blood sugar often, you may have already noticed that the same thing has happened to you. After several days or weeks on a low-carb diet, it seems that small amounts of carbohydrate begin to have an outsized effect, spiking blood sugar to a surprising height. If you have insulin-treated diabetes, you might learn that the carbohydrate-to-insulin dosing ratio you once relied upon has become inaccurate. If you knew that a banana or a cookie required a certain amount insulin beforehand, now you need more insulin for the same exact food. And if you eat at least a moderate amount carbs more than a couple days in a row, you might again see your carbohydrate-to-insulin dosing ratio shift back towards where it used to be.
The Science
Doctors have known about temporary carbohydrate intolerance for over 60 years. Perhaps our best evidence establishing it as a known phenomenon is a study that was published way back in 1960. In “The Effect of Prior Carbohydrate Intake on the Oral Glucose Tolerance Test,” Dr. Hugh Wilkerson and two co-authors proved that healthy adults on a reduced carbohydrate diet experienced a “loss of carbohydrate tolerance” that was so dramatic as to occasionally result in a false diagnosis of diabetes.
At the start of the study, eighteen young men and women took an Oral Glucose Tolerance Test (OGTT) to prove that they were metabolically healthy. Two hours after ingesting a sugary drink, the participants’ blood glucose levels had all returned to a normal and healthy level, as expected.
They were then instructed to adhere to a very low-carbohydrate diet for five days—the phrase “ketogenic diet” had not yet been coined—and repeat the OGTT. This time, two hours after enjoying the same sugary drink, the first sugar they’d eaten in days, nearly all of the participants exhibited wild glucose spikes. The men had peak blood glucose levels about 80% higher than they had in the previous tests, and the women were 100% higher. According to the standards of the day, two of the participants would have been diagnosed—falsely—with full-fledged Type 2 diabetes, and seven met the criteria of “possible diabetes.”
How do we know that the dieters didn’t actually have chronic insulin resistance? Because they exhibited a perfectly normal carbohydrate tolerance both before and after the experiment with the very low-carb diet. The last step of the study asked the participants to eat a moderately-carby diet, 150g per day for a few days. A third and final OGTT showed that their metabolism had returned back to normal.
By the way, the authors of the 1960 study used the phrase “impaired glucose tolerance” to describe our phenomenon. While that might have been a good name, we can’t use it anymore because it is now almost universally used to mean prediabetes, which indicates a state of chronic insulin resistance that is measurable and worrying, but not yet at diabetic levels.
Why it Happens
Why does it happen? Temporary glucose intolerance is likely a natural consequence of the body’s transition to fat adaptation. In making this shift, the body makes innumerable metabolic changes to prefer fat to carbohydrates. Then, when you eat a large dose of concentrated carbs, the body is simply unprepared. It hasn’t turned on the carb-burning apparatus in some time. It’s caught unaware.
For the precise mechanisms of this change, we have to rely to some extent on animal studies. A 2010 study found that rats on a ketogenic diet indeed displayed dramatic and temporary glucose intolerance. This work confirms exactly what I see in my own patients that adopt the keto diet. While temporary carbohydrate intolerance may suggest a limited measure of peripheral insulin resistance, they are otherwise more insulin sensitive than they’ve been in years. They see improvements in A1c and post prandial glucose, and they take less insulin and other glucose-lowering medications.
Patients with Type 1 diabetes—who must inject all or most of the insulin their body requires—may be the most reliable indicator of what’s going on. The typical patient with T1D on a keto diet may see their carbohydrate-to-insulin dosing ratio change for the worse, but at the same time they will need significantly less basal insulin, and fast-acting insulin ‘corrections’ will be more powerful than ever. These are strong indication that they are in fact increasingly sensitive to insulin.
Not every researcher shares my interpretation. The authors of this 2013 study from the American Journal of Physiology, although noting the keto diet causes a beneficial “reduction of fasting insulin, C-peptide, and glucose levels,” contend that glucose intolerance and peripheral insulin resistance are “potentially harmful metabolic consequences.” The same journal quickly published a rebuttal arguing that glucose intolerance and peripheral insulin resistance are healthy responses to ketosis and not necessarily equivalent to the glucose intolerance and insulin resistance seen in metabolic disorders like diabetes. In this telling, the body’s reduced need for glucose during a ketogenic diet (or fasting) is “especially useful,” a natural adaptation to a different metabolic state.
Temporary Carbohydrate Intolerance and the OGTT
Meanwhile, back in the medical mainstream, the nearest that most doctors come to understanding temporary carbohydrate intolerance is a vague awareness that diet might influence an Oral Glucose Tolerance Test.
The understanding that a low-carb diet could throw off an OGTT is old enough to have originally inspired the 1960 study discussed above. In the years since, researchers have continued to study whether a special diet is called for to reduce the incidence of false diabetes diagnoses. Unfortunately, some of this work has made the guidelines less useful for keto dieters, not more, because few or none of these more recent experiments have been performed with participants on a true keto diet.
Consider, for example, a 2004 study which rather absurdly tested a “low-carbohydrate diet” that only had 20% fewer carbs than the “high-carbohydrate diet.” Unsurprisingly, the authors of this study that concluded that “the carbohydrate percentage of the preparatory diet did not influence the results of an OGTT,” and that therefore there was likely no need for a special preparatory diet. This affirms official guidelines that now advise patients to eat “normally” before they take the test, guidelines which simply will not apply to anyone who normally enjoys a low-carbohydrate diet.
Any Oral Glucose Tolerance Test taken on a low-carbohydrate diet will be invalid.
If you are on a keto diet and you need to take an OGTT, as is routinely done to check for gestational diabetes, you could receive a false diagnosis of diabetes or pre-diabetes.
What to Do
So, if you think you have temporary glucose intolerance, should you do anything about it?
Many keto dieters will decide that the occasional blood sugar spike, in the context of an otherwise insulin- and glucose-lowering diet, isn’t worth worrying much about. Others may find inspiration to adhere even more rigidly to a low-carb approach, avoiding or entirely eliminating those glucose-spiking cheat meals. A third group may find the idea of temporary glucose intolerance uncomfortable, and seek to add more healthy carbohydrates to prevent temporary carbohydrate intolerance.
We simply don’t have enough data to estimate how damaging the infrequent blood sugar spikes from the occasional splurge meal on keto might be.
To learn more about metabolic health and low carb diets, sign up now for the upcoming Metabolix lecture series, which begins February 4, 2021.
I thought patients were told to eat 150 g of carbs for 3 days before an OGTT.