High blood glucose is the most common metabolic disturbance seen in many obese patients and it can cause severe health consequences. Fasting blood glucose is a critical clinical test used to screen for diabetes and assist with diagnosis. Blood glucose is monitored after an overnight fast and it normally falls after the fast is completed. Pre-diabetes is a condition where blood sugar levels may still be above normal, but a person has impaired insulin sensitivity and a high risk of developing diabetes.

Blood glucose level is the reason for high fasting blood glucose on low carb or keto diet. If your fasting blood glucose is high it means you have very less carbs and less fat. If this is true then why fasting blood glucose levels are high on a low carb or keto diet?

Last spring, after 18 months of successfully following the keto diet, I checked my blood sugar with my home glucose meter for the first time in many months. The result shocked me.

I bought this device, which also tests for ketones, when I was diagnosed with pre-diabetes in the fall of 2015. When I started following the low-carb keto diet, I had my blood tested regularly. Soon my blood sugar was back in the normal range. Day after day, I was in optimal ketosis. On top of that, I lost 5 kg and felt great – full of energy, no hunger or cravings. Soon I could predict the results of measurements based on what I ate or did. I set the counter aside and continued with my happy, healthy keto life.

When my doctor had lab tests done in the spring, I pulled out the device again. Although I have no health problems, my blood pressure is excellent and my weight stable, she wanted to know about my cholesterol, lipids, HbA1c and fasting blood sugar on the keto diet – and I was curious too.

To compare the accuracy of my meter with the lab’s results, I got in my car in front of the clinic at 7:30 am on the morning of the test and pricked my finger. I expected an excellent fasting blood glucose (FBG) of 4.7 or 4.8 mmol/L (85 mg/dL).

It was 5.8! (103 mg/dl). What?

It was 5.8! (103 mg/dl). What? I refused the tests and went home – I didn’t want my doctor to warn me again that I had pre-diabetes when I had no explanation for the high result.

The next morning, I took the test again: 5.9 ! (104). А ????

I tested every morning for the next two weeks. No matter what I did, my blood sugar was 5.7-6.0 (102-106 mg/dL), which is in the pre-diabetic range.

One morning, after a restless sleep, it was as high as 6.2 mmol/l (113 mg/dl). But my ketones still showed 1.5-2.5 mmol/l. I was still burning fat, not glucose. When I checked my blood after dinner, there was hardly any movement in my blood sugar. The graphs I entered into SugarStats were almost a straight line, with the highest FBG during the day, 12-14 hours after each meal.

What happened?

Many people ask this: Why is my blood sugar higher on a low-carb diet? There are dozens of ketogenic and paleo forums discussing this topic. Some of this information raises concern because unsubstantiated claims are made that a low-carb diet can trigger, rather than reverse, diabetes. Some forum users even say they gave up the keto diet because they were so intimidated by the high FBG numbers.

But don’t be afraid. We have reviewed the medical literature and consulted leading experts. Here’s what you need to know:

1. Relax, it’s normal – call it adaptive glucose maintenance

We find that in people who follow a low-carb diet for an extended period of time, blood sugar levels are highest during the day, says Dr. Sarah Hallberg. You don’t really have a blood sugar problem. You’re doing very well. But if you look at the 24-hour blood glucose log, you will see that the glucose level is high in the morning and then drops steadily throughout the day, with no [blood glucose] spikes, even after meals.

The scientific name is physiological insulin resistance, and that’s a good thing…..

The scientific name is physiological insulin resistance, and that’s a good thing – as opposed to pathological insulin resistance.

As regular visitors to Dr. Jason Fung and Dr. Ted Nyman’s website know, a pathological form of insulin resistance is caused by ever-increasing insulin levels – hyperinsulinemia – that attempt to force glucose into overcrowded cells. Insulin resistance is a hallmark of type 2 diabetes, polycystic ovarian syndrome (PCOS) and other chronic diseases.

So instead of physiological insulin resistance, we call it adaptive glycemic sparing – a name that many have suggested to avoid confusion. Dr. Ted Nyman describes this as the muscles being in glucose denial mode.

Before you switched to a ketogenic diet, your muscles were the main place where blood glucose was absorbed and used for energy. However, on a long-term keto diet, they prefer fat as a fuel. So the muscles resist the action of insulin, which delivers sugar to the cells for energy, essentially saying this: We don’t want or need your sugar anymore, so move on. Hence the slightly elevated but generally stable level of circulating glucose in the blood.

Where does the glucose come from if you don’t eat sugar and only carbohydrates from leafy vegetables are on the menu? Your liver, via gluconeogenesis – the formation of glucose from non-carbohydrate sources such as lactate, glycerol and glucogenic amino acids into proteins. It is a natural protective process by which Homo sapiens have survived hundreds of thousands of years of feasting and starving.

According to Dr. Jeff Wolek, there is no great need for carbohydrates in the diet because people can adapt well to a low-carb diet. In the livers of people adapted to the keto diet, he notes: Ketone production increases dramatically to replace glucose as the main energy source for the brain, while fatty acids provide most of the energy for skeletal muscle. The production of glucose from non-carbohydrate sources via gluconeogenesis supplies carbohydrates to the few cells that rely on glycolysis [use of sugar as an energy source].

Why is blood sugar higher in the morning? This is the phenomenon of dawn, when cortisol, growth hormone, adrenaline and the enzyme glucagon give the liver a boost to get through the day and stimulate gluconeogenesis for cells that need glucose.

It’s your body forcing you to eat breakfast – one of the posts on this common phenomenon – except that your muscles, when adjusted to fat, don’t want to.

The HbA1c test determines the average blood glucose level over the past three months by counting the number of glucose molecules bound to red blood cells. With a low-carb diet, the HbA1c is almost always below the FBG, indicating that good glycemic control or type 2 diabetes is not an issue for that person.

2. Insulin low – significant difference

As Dr. Nyman wrote recently, a fasting blood glucose measurement alone without a fasting insulin measurement doesn’t tell us much. Two people can have exactly the same fasting blood sugar and totally different circulating insulin levels.

It’s about the relationship between glucose and insulin and how they work together. This is called the homeostatic model of insulin resistance or HOMA-IR. The name is polysemous, but it simply means that the body is always trying to keep its basic systems in balance, which is called homeostasis. Insulin works against glucose and tries to keep blood sugar levels stable, i.e. in homeostasis.

In the case of one person – e.g. B. in pre-diabetes, type 2 diabetes or polycystic ovarian syndrome – insulin may be administered in increasing amounts over a longer period of time to keep blood sugar levels relatively stable. Although blood sugar levels can remain within normal limits, increasing amounts of insulin are required to maintain them. When insulin resistance develops, that is, when insulin becomes less and less effective at lowering blood sugar, blood sugar levels eventually become too high.

In the second person – the one who has been on a keto diet for several months and is now burning fat for energy – the pancreas releases only a small amount of insulin to keep blood sugar stable. This means that you are no longer insulin resistant, but very sensitive to insulin and only need a small amount of insulin to control your blood sugar.

Dr. Nyman’s chart shows that if you know your fasting blood glucose and insulin levels, the HOMA-IR equation can determine whether you are insulin sensitive or insulin resistant. If your fasting blood glucose is 5.7 (103) and your insulin level is also high (more than 12 µU/ml), you are insulin resistant and on the way to type 2 diabetes. If your blood glucose is 5.7 but your fasting insulin level is less than 9 µU/ml, you are insulin sensitive and probably in glucose refusal mode due to a low-carb diet.

Most doctors still do not test fast-acting insulin with a fasting blood glucose test. In general, you should ask.

3. Understanding cortisol and its effect on blood sugar

We all live with stress, but some of us have a harder time reducing or alleviating it. This can manifest as restlessness, poor sleep, insomnia, feelings of anxiety or constant tension. Cortisol is a stress hormone involved in the physiological fight-or-flight response. And this directly affects our body’s blood sugar levels.

Sustained cortisol stimulation raises blood sugar levels, notes Dr. Jason Fung, who devoted an entire chapter to the stress hormone in his 2016 bestseller, The Code for Obesity. He describes how cortisol released from the adrenal glands signals the body to release glucose in preparation for a perceived threat by stimulating gluconeogenesis in the liver. In prehistoric times, this burst of energy from carbohydrates was usually used in battle or on the run from a threat. But these days, that stress is often not released, the energy is not used, which can lead to persistent blood sugar spikes (and high insulin levels to try to bring them down).

American nurse Kelly Pound, a low-carb diabetes counselor who blogs at LowcarbRN, wore a continuous glucometer that checks blood sugar every 5 minutes to see how different foods and stimuli affect blood sugar throughout the day. She found that even on a low-carb diet, stress had the biggest impact on her performance, much more than any low-carb diet. My blood sugar is on average 20 to 30 points higher in the morning if I haven’t slept for at least seven hours, says Pound, who decided to focus on stress reduction as an important part of diabetes management. Working so hard to control my blood sugar with a diet, only to have it spike when I’m stressed, is detrimental to my goals.

I too have noticed the same effect of stress, especially on sleep. Since this spring, I’ve been doing yoga early in the evening, focusing on other stress-relieving activities (painting and playing guitar at night instead of surfing Facebook or the news), and wearing earplugs and a headband to sleep better. This brings my FBG back into the normal range the next morning.

A glass or two of wine in the evening can also lower the level because the liver breaks down the alcohol first and then switches to gluconeogenesis, but I don’t want to drink every day.

4. Blood glucose meters may give false readings

While there are physiological reasons for elevated blood sugar, there may also be technical reasons. The U.S. Federal Drug Administration admits that the results of home blood glucose meters can vary by up to 15%. This means that a value of 100 mg/dL can actually be as low as 85 mg/dL and as high as 115 mg/dL – a huge variation!

Dave Feldman, the software engineer who cracked the cholesterol code, wrote on his blog about the variations in glucometers and made diagrams and photos of his two glucometers measuring the same drop of blood. Although he expected higher blood sugar levels as a result of his long-term low-carb keto diet, he was surprised to find that some results were quite different. He notes that every time he gets an unexpected result, he takes three measurements, all within a few minutes of each other, and then averages the results.

Other researchers point out that pricking a finger to draw blood with hands on which tiny traces of sugar or food are present (e.g. just touching a piece of fruit) can lead to erroneous measurements. Dehydration also increases results. You should always wash your hands before taking the test, but some soaps contain additives like honey or perfume that can stick to your fingers and confuse the reading. In addition, tires that are too hot or too cold and other environmental factors, such as high altitude, result in higher or lower readings. Even different batches of test strips can give different results on the same meter.

Now, like Feldman, I take another reading when I get an abnormally high reading. It is often a few points lower.

5. A little about LADA – latent autoimmune disease in adults

If all the information in this article hasn’t helped you understand how blood sugar rises on a low-carb diet, and if your blood sugar rises not only in the morning but also at other times of the day, you may have a problem with latent autoimmune diabetes in adults (LADA), sometimes called type 1 diabetes.5

Both Hallberg and Fung say they regularly see LADA patients who have been incorrectly diagnosed with type 2 diabetes.

LADA is similar to type 1 diabetes in that antibodies attack the insulin-producing cells of the pancreas, but the disease occurs in adulthood rather than in childhood or adolescence. Diabetes researchers note that LADA also shows signs of type 2 diabetes, but patients tend to lose weight and need insulin faster because their insulin-producing beta cells have been attacked by the antibodies. The highest rates of LADA are found in Northern Europe, where up to 14% of people with type 2 diabetes actually have the disease.

I test serum C-peptide and insulin in everyone who follows my crash diet, says Dr. Fung. I examine them over time, and it often takes 6 months to a year to make a definitive diagnosis. I suspect that about 5-10% of my type 2 patients are actually LADA patients. Dr. Nyman has a similar experience and routine.

Fortunately, as with type 1 or type 2 diabetes, the low-carbohydrate keto diet is an effective way for people with LADA to keep their blood glucose levels as low as possible. When they eventually need to inject insulin, a low-carb diet allows them to use the minimal amount of insulin needed to keep blood sugar levels high. It may not slow the progression of beta cell destruction, but low-carb is a highly effective treatment strategy for reducing drug dependence, notes Dr. Fung.


After a sober blood sugar test on a low-carb keto diet, I no longer worry about my morning jitters. Now that I am focusing more on stress reduction and better sleep, they are still in the normal range and I am still in optimal ketosis and loving my keto diet and life.

And by the way, when I finally took the test, I got great results. My lipid and cholesterol levels were excellent and my HbA1c was 5.3%. Keep doing what you’re doing, my doctor said. You’re a model of good health.

Ann Mullens

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Frequently Asked Questions

Why is my fasting blood sugar higher on keto?

The ketogenic diet is a high-fat, low-carbohydrate diet. When you eat a lot of fat, your body produces ketones in the liver to use for energy. Ketones are produced when the body uses fat as its primary fuel source instead of glucose. The higher levels of ketones in your blood can cause your fasting blood sugar to be higher than normal on a ketogenic diet.

Why is my blood sugar so high when I’m not eating any carbs?

Your blood sugar is high because you are not eating any carbs.

Can keto cause high glucose?

No, keto does not cause high glucose. Can keto cause high cholesterol? No, keto does not cause high cholesterol. Can keto cause high triglycerides? No, keto does not cause high triglycerides. Can keto cause high blood pressure? No, keto does not cause high blood pressure. Can keto cause high blood sugar? No, keto does not cause high blood sugar.

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