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Glucose & Keto: What You Need to Know & Monitor

Madison Holt, MS, RDN, LD

Published in Glucose

11 min read

June 3, 2021
March 8, 2023
a person wearing CGM and looking at their phone
a person wearing CGM and looking at their phone

Whether you are new to the ketogenic world or an avid low carb dieter, you have likely heard the term "keto" before. Ketogenic or "keto" diets have actually been around for years. Recently, the keto diet has become very popular thanks to an ever growing amount of weight loss success stories from keto enthusiasts.

When glucose availability drops so low that the body doesn’t have enough to use for energy, it enters a state called ketogenesis. In this state, ketone bodies are created from fatty acids and replace glucose as the body’s primary source of energy.

Ketone levels are measured via one ketone body in the bloodstream, beta hydroxybutyrate (BHB).

  • BHB levels within a 0.5 mmol/L - 1.0 mmol/L range is considered a “light nutritional ketosis."
  • BHB levels within 1.0 mmol/L- 3.0 mmol/L is considered a more "optimal" ketosis. 

When following a keto diet, most people obsess over their ketone levels. While it is easy to focus on just measuring ketones, doing so would be a disadvantage. The truth is that ketones and glucose go hand in hand -- and monitoring your glucose through a continuous glucose monitor may be more beneficial than just monitoring your ketones alone.

So, let's discuss how a keto diet may impact your blood glucose levels.

How a Ketogenic Diet Impacts Blood Glucose

a couple of people making a salad in the kitchen

A well crafted ketogenic diet can be a great tool for enhancing metabolic flexibility, reducing insulin levels, managing glucose levels, preventing neurological diseases, and treating migraines. Research has found that a well crafted keto diet may even be more effective than low fat diets for treatment of obesity and diabetes.

Before we jump into keto diets, let's take it back to the basics.­ Our bodies break carbohydrates down into glucose to use as fuel, makint the main source of energy for our bodies. There are a couple reasons for this. 

One, our bodies are very efficient at using glucose for energy. Two, we have some organs that require glucose for energy. Keto diets limit carbohydrate intake, which deprives the body of exogenous glucose, meaning glucose sources from outside the body. This forces it to make certain adaptations. 

First, limiting the body’s glucose sources causes the body to switch from primarily exogenous glucose for energy to glycogen—or glucose storages—causing specific metabolic changes.

Because certain organs require glucose for energy, our bodies can produce their own glucose, specifically from the liver. This process is known as gluconeogenesis, and it ensures our organs get the correct amount of glucose to function properly. 

When glucose availability drops so low that the body doesn’t have enough to use for energy, it enters a state called ketogenesis. In this state, Ketone bodies are created from fatty acids and replace glucose as the body’s primary source of energy.

As you can see, the relationship between ketones and glucose is inverse. You can't really get a full picture of what is happening in your body unless you are monitoring your glucose as well. 

Here are a few common glucose trends that we have seen with a keto diet.


Lower Glucose Trends

There is no doubt that carbohydrates have the most influence on glucose levels. So, it makes sense that if you are following a zero to very low carbohydrate diet, your glucose levels will remain more steady. 

Individual glucose baselines will differ from person to person depending on health history, length of time following a keto diet, and activity level. However, average glucose levels for keto dieters usually hover anywhere between 70-­90 mg/dL.

Another metric that we use to evaluate glucose trends is glycemic variability, also known as glucose swings. While a certain degree of variation is normal (and good), we want to prevent large glucose swings. A common trend that we have observed amongst keto diets is low glycemic variability or very stable glucose levels.

Elevated Fasting Glucose Levels

Elevated fasting glucose levels are a very common trend that we see with individuals who have been following a keto diet for a prolonged amount of time. This trend, also known as "glucose sparing," appears to be a natural adaptation. 

Elevated fasting glucose levels usually start to occur with those who have been following a strict keto diet for over a year. This is the body's way of assuring that glucose is present for organs that require it. According to current research, this isn’t necessarily a bad thing. However, we recommend monitoring other health metrics such as glycemic variability, fasting insulin, HDL, triglycerides, and liver enzymes to ensure that everything else remains in a good range.

High Glucose Spikes to Small Amounts of Carbs

Keto diets help to lower insulin levels. But, another common trend that we see is slightly higher glucose responses to a small amount of carbs. Individuals who follow a very low carb diet may have lower insulin sensitivity compared to someone who consumes more carbs. Keto diets can lead to a temporary physiological insulin resistance, in which the body overreacts when carbs are introduced.

Physiological insulin resistance is different from pathological insulin resistance. Pathological insulin resistance is a disease state, whereas physiological insulin resistance is a temporary adaptation.

Similar to the elevated fasting glucose levels with a keto diet, this does not appear to be a bad thing. What is happening is that the muscles, which typically prefer glucose, start to prefer fatty acids for fuel instead. Muscles are a huge sinkhole for glucose. So, when the muscles start to prefer fatty acids over glucose, consuming carbs tend to cause larger glucose elevations.

One way to combat this is to understand your carb threshold and what type of carbs you tolerate best. Understanding your carb threshold will help maintain metabolic flexibility. This will allow your body to quickly shift from using ketones and glucose, without ever being dependent on just glucose.


What CGM (Continuous Glucose Monitoring) Can Do For Keto

a person wearing CGM and looking at their glucose chart

There are many factors that can impact ketosis. The four major components include diet, exercise, sleep and stress. It is no coincidence that the four main factors that impact ketone also impact glucose.

Ketones and glucose are interconnected. Checking your glucose or ketone readings with a meter provides a snapshot of what is happening in the body at that exact moment. Unfortunately, there is no device that measures ketones continuously at the moment.

But, we do have the CGM, which monitors glucose continuously and will help cut through the noise and variables. The CGM gives real time feedback with your diet and daily habits. This provides precision and clarity into what is impacting your glucose levels, which as we know, may potentially be impacting your ketones. For example, for some, getting into ketosis and staying there can be difficult! This is where CGM data can be so useful.

Identify Your Trigger Foods Using a CGM

Everyone has a unique response to carbs. If we were to perform a test with two different people eating a banana, we could see drastically different glucose responses. This is because nutrition is very individualistic.

Identifying how you tolerate specific carbs and identifying the foods that drastically increase your glucose will help optimize your keto diet. With a CGM, you don't have to guess if a certain food is kicking you out of ketosis. The CGM will provide you with the data you need to optimize your diet and give you some peace of mind. You may find that you tolerate more carbs than you think!

Test Out "Keto­-Friendly" Foods

Don't believe those clever marketing labels! Not every "keto snack" is keto-friendly. And if it is, not everyone responds well to them!

Many keto-friendly foods are made with artificial sweeteners and highly processed starches that often raise glucose levels. If you have a go­ to "keto" food, definitely test it out! A CGM will help provide that insight to see how your body responds.

Foods that cause elevated prolonged spikes (think higher than 140 and longer than 3 hrs), are foods we should reduce. For example, one Nutrisense member found that Costco rotisserie chicken was causing huge glucose spikes. This seems odd, since chicken is a go-to food in the keto diet.

After looking into this further, we found that the rotisserie chicken had a lot of additional ingredients that were injected during the cooking process. These additional ingredients caused their glucose levels to spike.

Without the CGM, we would have never been able to figure out that this particular food was causing their glucose to spike and kicking them out of ketosis.

Experiment With Portion Sizes

Getting into ketosis is one thing, but staying in ketosis is an entirely different challenge. The amount of carbs you can eat while staying in ketosis is unique from person to person. Individuals who are physically active and practicing intermittent fasting may be able to consume more carbs.

This is why understanding your personal carb threshold is so important for diet sustainability and metabolic flexibility. The CGM can provide so many insights on your body's carb tolerance. Using a CGM will help you understand how much carbs your body tolerates in a day, at each meal, and when is the best time to eat those carbs.

Test Your Protein Threshold

Too much protein, too little protein --­ this is often a big debate in the keto community. The amount of protein that one person can tolerate is highly personalized. To find out what works best for your body, it is best to test both ketones and glucose.

You likely will have to continue adjusting your macros until you find what works best for you. Typically, the threshold for "excess" protein is very high. Any excess that is not utilized for repair and maintenance of tissue is converted into glucose for energy. Continuously monitoring your glucose will help catch those subtle glucose rises that often occur when too much protein is eaten.

Our Top 3 Tips for Monitoring Glucose on a Keto Diet

fish, meat, eggs, cheese, butter, a bottle of oil, avocado,  cauliflowers, asparagus, cabbage

Many popular keto websites will tell you that eating >30 grams of carbs a day will kick you out of ketosis. For many users at Nutrisense who are following a keto diet, we found this to simply be untrue.

1) Test Your Carb Threshold

One of the first experiments you can do using a CGM on a keto diet is testing your personal carb threshold. Once you are in a steady state of ketosis, you can start testing the 30 gram carb boundaries a bit.

To do this, you would gradually increase your carbohydrate intake while monitoring your ketones and glucose values through your CGM at the same time each day. Next, choose one meal per day that you would include more carbs at and time this meal to be after a workout or some type of movement.

Always pair your carbs with protein and fat for best glucose control. You could start by having 30 grams of carbs per day, then increasing your carbs by 10 g. You can keep bumping up your carbs until your morning ketones drop below 0.5. Once it dips below 0.5, you have found your carb threshold.

 Once you find your threshold, you can consume that many carbs every day or just do it a few days a week. You may be surprised with how many carbs you can consume a day while still maintaining ketosis!

As a pro tip, typically the more fasting and exercise you do will give you more flexibility for carbohydrates. Ketones and glucose should have an inverse relationship. This means that if glucose is high, ketones are likely low.

2) Test Your Trigger Foods

To test your trigger foods you need to first record your pre-prandial (before eating) ketone and glucose value. The best time to test this would be in a fasted state or at least three hours after a meal. Next, eat your experimental food choice.

Two hours after your meal, record your postprandial (after eating) ketone and glucose values. This will allow you to see how quickly your values recovered from your experimental food.

Once you complete the experiment check your results. We want a glucose delta of <30. For ketones, we do not want ketones to drop >1.0 mmol/L and we want it to return back to baseline within 2 hrs. If glucose and ketones are out of the ideal ranges, try retesting with a smaller portion.

3) Test Out Your Protein Threshold

The threshold for protein is typically high, though anything in excess will lead towards energy storage. To find out if you are consuming too much protein at one meal, test your ketones and glucose in a fasted state and make note of it.

Eat a meal and make note of how much protein is in that meal. If the protein amount is too high, we typically see a very gradual rise in glucose data through the CGM. This rise can occur anywhere from 2­4 hours after a meal (depending on the other components of the meal).

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Kara Collier, RDN, LDN, CNSC

Reviewed by: Kara Collier, RDN, LDN, CNSC

Kara Collier is the co-founder and VP of Health at Nutrisense, one of America’s fastest-growing wellness-tech startups, where she leads the health team. She is a Forbes 30 under 30 recipient, frequent podcast guest & conference speaker.

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