Many people ask: “Why is my fasting blood glucose higher on low carb?” Dozens of ketogenic and paleo forums have discussions on the issue. Some of the information flames fears, using unsubstantiated claims, that a low-carb diet could trigger diabetes rather than reverse it. Some posters in forums even say they’ve quit the keto diet because their higher FBGs scared them so much.
But have no fear. We have researched the medical literature and consulted with leading experts. Here’s what you need to know:
Relax, it is normal — call it “adaptive glucose sparing”
“We definitely see that in people who are doing low carb long term, the majority will find that their fasting blood glucose becomes their highest value of the day,” says Dr. Sarah Hallberg. “They are not actually having issues with blood sugar. They are doing really well. But if you are looking at a log of 24 hours of blood glucose you will see a high first thing in the morning and then a steady decline throughout the day, with no big excursions [in glucose levels] even after meals.”
As regular visitors will know from Dr. Jason Fung and Dr. Ted Naiman, the “pathologic” kind of insulin resistance is caused by higher and higher levels of insulin — hyperinsulinemia — trying to force glucose into over-stuffed cells. That insulin resistance is a prominent feature of type 2 diabetes, polycystic ovarian syndrome (PCOS) and other chronic conditions.
So let’s call physiologic insulin resistance instead “adaptive glucose sparing,” a name that has been proposed by many to reduce the confusion. Dr. Ted Naiman describes it as muscles that are in “glucose refusal mode.”
Prior to converting to the ketogenic diet, your muscles were the major sites to soak up and use glucose in the blood for energy. On the long-term keto diet, however, they now prefer fat as fuel. So, the muscles are resisting the action of insulin to bring sugar into cells for energy, saying, in essence: “We don’t want or need your sugar anymore, so move it along.” Hence, the slightly elevated, but generally stable, glucose circulating in the blood.
Where is that glucose coming from when you consume no sugar and only leafy veggie carbs in your diet? Your liver, through gluconeogenesis — the creation of glucose from non-carbohydrate sources such as lactate, glycerol, and glucogenic amino acids from proteins. It is a natural protective process that got homo sapiens through hundreds of thousands of years of feasts and famines.
“There is no essential requirement for dietary carbohydrate because humans possess a robust capacity to adapt to low-carbohydrate availability,” says Dr. Jeff Volek. In the liver of a keto-adapted person, he notes: “ketone production increases dramatically to displace glucose as the brain’s primary energy source, while fatty acids supply the majority of energy for skeletal muscle. Glucose production from non-carbohydrate sources via gluconeogenesis supplies carbons for the few cells dependent on glycolysis [using sugar for energy.]”
Why are blood sugars highest in the morning? It’s the dawn phenomenon, when cortisol, growth hormones, adrenaline and the enzyme glucagon pulse to the liver to get you up and moving for the day — spurring gluconeogenesis for the cells that need glucose.
“It’s your body making you breakfast,” notes one post that discusses the common phenomenon — except that when your muscles are fat-adapted, they don’t want it.
The HbA1c test estimates the average glucose level in the blood over the previous three months by counting the number of glucose molecules stuck on red blood cells. On a low-carb diet the HbA1c will almost always be lower than the FBG suggests, showing that good glucose control, or type 2 diabetes, is not an issue for that individual.
Insulin is low — an important difference
As Dr. Naiman recently posted, only testing fasting blood glucose without testing fasting insulin tells you very little. That’s because two people could have exactly that same fasting blood glucose levels and have very different circulating insulin levels.
It is all about the relationship between glucose and insulin and how they work in concert. This is called the homeostatic model assessment of insulin resistance or HOMA-IR. The name is a mouthful, but it simply means that the body is always trying to keep its essential systems in balance or in an equilibrium — called homeostasis. Insulin is working against glucose in an attempt to keep blood sugar levels stable, a.k.a in homeostasis.
In one individual — for example, someone with pre-diabetes, type 2 diabetes, or polycystic ovarian syndrome — insulin may over time be pulsed out in higher and higher amounts just to keep the blood sugar relatively stable. While fasting blood glucose could still be in the normal range, it is taking increasing amounts of insulin to keep it there. As insulin resistance develops, and insulin becomes increasingly ineffective to bring blood sugars down, blood sugars will eventually rise too high.
In a second individual — someone who has been on the keto diet for a number of months and is now burning fat for energy — only tiny amounts of insulin are being pulsed by the pancreas to keep the glucose stable. You are now, therefore, no longer insulin resistant, but very insulin sensitive and only need small amounts of insulin to keep glucose in check.
Dr. Naiman’s graph shows, if you know your fasting blood glucose and your fasting insulin, the HOMA-IR equation can tell you how insulin sensitive or insulin resistant you are. If your fasting blood sugar is 5.7 (103) and your insulin is high, too, over 12 μU/mL, you are insulin resistant and, on your way, to type 2 diabetes. If your blood sugar is 5.7 but your fasting insulin is under 9 μU/mL, you are insulin sensitive and likely in glucose refusal mode from a low-carb diet.
Most doctors do not yet check for fasting insulin with a fasting blood glucose test. You usually have to ask for it.
Understand cortisol and its impact on blood glucose
We all live stressful lives, but some of us have a harder time reducing or releasing our stress. That fact may play out in restlessness, poor sleep, insomnia, or feelings of anxiety or constant tension. Cortisol is the stress hormone that mediates the “fight or flight” physiologic response. And it has a direct bearing on our glucose levels.
“Prolonged cortisol stimulation will raise blood glucose levels,” notes Dr. Jason Fung, who wrote an entire chapter on the stress hormone in his 2016 bestseller The Obesity Code. In it he describes how cortisol, released from the adrenal glands, sends a message to the body to release glucose to prepare for a perceived threat, spurring gluconeogenesis in the liver. In prehistoric times, that surge of glucose energy was usually used up fighting or fleeing from the threat. But in current days, often that stress goes unreleased, the energy not used, which can lead to prolonged higher glucose levels (and higher insulin levels to try to bring it down.)
US nurse Kelley Pound, a low-carb diabetes educator who blogs at the site LowcarbRN, wore a continuous glucose monitor, which tests blood sugars every 5 minutes, to see how various foods and stimuli impacted her blood glucose throughout the day. She discovered that even while eating low carb, it was stress that had the biggest impact on her readings, far more than any low-carb food. “My blood glucose will average 20-30 points higher on mornings where I have not gotten at least 7 hours of sleep,” said Pound, who decided to focus on stress reduction as a key part of diabetes management. “Working so hard to keep blood glucose controlled with diet, only to have it go haywire with stress, is defeating my goals.”
I, too, discovered the same stress impact, especially with sleep. Since this spring, I am doing yoga in the early evening, focusing on other stress-reduction activities (painting and playing my guitar at night rather than scanning Facebook or watching the news) and wearing earplugs and an eyeshade to help ensure a better night sleep. Doing so drops my FBG back into the normal range the next morning.
Drinking a glass or two of wine at night can lower readings, too, because the liver metabolizes the alcohol first before turning to gluconeogenesis, but I don’t want to drink daily.
After doing all this research on fasting glucose levels in low-carb keto eating, I am no longer worried about my morning highs. Now with a greater focus on stress reduction and better sleep, they are routinely in the normal range anyway, while I continue to be in optimal ketosis and love my keto diet and keto life.
And by the way, when I finally went for my lab test, I got great results. My lipids and cholesterol were excellent and my HbA1c was a healthy 5.3%. “Keep doing what you are doing,” my doctor said. “You are the picture of robust health.”