Everything you need to know about continuous glucose monitors (CGMs)
Table of Contents:
What is a CGM?
How do I apply a CGM?
What should I do my first two weeks wearing a CGM?
Why continuous monitoring is better than “point-in-time”.
Who should wear a CGM?
How frequently should I scan my CGM?
What is my ideal blood glucose range?
How much of a glucose spike is “too big”?
After your first week, try for a no-hitter day!
Treat yourself like one of Sherlock Holmes’ cases, where you’re the detective!
Definitions of A1c; GMI; Glycemic/Glucose Variability; Time in Range; % Time CGM is effective (Data Sufficiency)
#1 What is a CGM?
A CGM is an incredible technological advancement that is revolutionizing blood glucose monitoring. Since the 1960s if you wanted to test your blood glucose level the way you did it was by pricking your finger and placing a drop of blood on a paper strip. Thanks to the proliferation of smartphones, sensors have gotten cheap enough to make modern miracles like CGMs possible. Rather than rely on incomplete “point-in-time” data, CGMs monitor your blood glucose levels 24/7. Here’s a snapshot of data from when Laura ate a bagel as an experiment to test the impact on glucose:
Here’s data from a low-carb meal:
#2 How do I apply my CGM?
#3 What should I do my first two weeks wearing a CGM?
The primary thing we want to accomplish in the first two weeks is establishing your baseline. We want to understand what “normal” means for your unique body. This means taking note of:
Ranges during the day and at night
How large glucose spikes are in response to different types of meals
What your glucose registers when you wake up in the morning
How your body responds to physical activity like taking a walk or doing exercise
We don’t need to worry about optimizing things or minimizing your glucose right out the gate, there will be time for that. What’s important is being able to track improvement and/or changes from the baseline that is established over the first two weeks.
#4 Why continuous monitoring is better than “point-in-time”.
Using “point-in-time” data to figure out what’s going on with your blood glucose is like trying to figure out what a movie is about by scanning through pictures of individual scenes. You might pick up on the gist of things but you aren’t learning the full story. When it comes to blood glucose, it is important to know:
How far it’s moving from trough to peak to trough
How long it takes to recover back to baseline after a spike
How long it stays in the target range
What it’s doing over night
What’s more, because it’s easy to get immediate feedback, you can very easily start to connect blood glucose levels to specific foods, activities (e.g. taking a walk after a meal), and even stress levels.
Here’s another analogy we think is useful. You wouldn’t drive a car if it only started accelerating minutes or hours after you pressed the gas pedal. Likewise, using a weight scale to measure health is really showing you how your body reacted to lifestyle changes you made weeks or even months ago.
#5 Who should wear a CGM?
A very common misconception is that CGMs are only for people living with diabetes. Nothing could be further from the truth. For example, understanding your glucose patterns is valuable to athletes (link to article from Nutrisense with images below) who are trying to optimize their macronutrients (e.g. carbs, fats, proteins) to improve performance.
Glucose patterns can affect energy levels and sleep quality, which is leading people from all backgrounds to wear them if they’re interested in making improvements there. CGMs are also an invaluable resource in deciphering the true impact of different foods on blood glucose levels. For example, there was a type of “healthy”, whole-grain oat cookie that Laura used to eat all the time. Then after wearing a CGM she realized that the cookies, despite what ingredients they claimed to be made of, were spiking her glucose to 180. You can’t truly understand what most foods are going to do to your blood sugar without wearing a CGM. What’s even crazier, not all foods have the same impact on all people - so even if we could trust what was on the packages it still isn’t enough.
Stress is another really interesting reason to wear a CGM. When we’re stressed out our bodies enter “fight or flight” mode, spiking our cortisol and epinephrine. In response, our body floods us with glucose (for energy) so that we can be ready to “run from a lion”. Obviously, there aren’t any lions in the modern world so what ends up happening is the glucose our body creates in reaction to stress just stays in our system, which contributes to weight gain. A CGM will tell you if your body is responding to stress in this way.
There are already tons of reasons to wear CGMs and people are figuring out new reasons on a seemingly daily basis. Anecdotally, Ben mentioned that when he’s in the men’s locker room at the gym he’s now seeing them all the time on a wide variety of people, including people who have the physiques of elite athletes.
Lastly, we’re aware of a company that just announced they’re going to start subsidizing the cost of CGMs for all of their employees, not because all of their employees are living with obesity or diabetes, but because they realize how useful CGMs are for maintaining health and learning about our bodies.
Ben, Matt and Laura now wear CGMs regularly just as part of their normal healthcare routine.
As to the answer of: Who should wear a CGM? We think the answer is most people. Soon the better question to ask might be: Who shouldn’t wear a CGM?
#6 How frequently should I scan my CGM?
Your CGM should be scanned at least once every 8 hours. Some devices can only hold 8 hours of data so you need to transmit the data from the device itself to your smartphone for long term storage and analysis. However, we’d also suggest scanning your glucose:
After meals
Whenever you feel “off” or “super great”
After walking or exercise
In the morning
Whenever you eat a new food or drink
This will help you to familiarize yourself with the impact these activities have.
#7 What is my ideal blood glucose range?
In the beginning your ideal range is dependent on your starting point and should be determined in conjunction with your doctor. If you are living diabetes what will likely happen is you will use the first two weeks to establish a baseline. Then, over time, you will target a lower range.
For reference, a healthy blood glucose ecosystem is considered to be one where glucose levels stay between [70 and 120] 90% of the time.
Exercise is an extenuating factor, and can sometimes spike your glucose outside of the range even though its healthy.
#8 How much of a glucose spike is “too big”?
The science tells us that when our body's glucose levels spike beyond 70 points damage is being done. So, at the least we’d like to try and keep spikes below 70 points.
However, it is important to remember that when we exercise our body produces glucose to give us energy. This extra production of glucose sometimes causes our blood sugar to rise even more than it does after a carb heavy meal. That’s okay! Glucose spikes from exercise should not be worried about or avoided.
#9 After your first two weeks, try for a no-hitter day!
A no-hitter day is when you stay within your target range for a full 24 hours. You don’t need to manage no-hitter days all the time, but see if you can manage one after you’ve been wearing a CGM for a couple weeks and have gotten a good idea about the impact of your lifestyle and diet. If you go on a walk or do other exercise that causes your glucose to spike outside of the range don’t worry - that doesn’t count!
#10 Treat yourself like one of Sherlock Holmes’ cases, where you’re the detective!
The list of things that can impact your blood sugar levels is long:
Foods
Stress
Sleep quality
Exercise
Medications
What’s more, one spike is not like another. Sometimes a glucose spike (like from taking a strenuous walk) will result in lower glucose levels for the rest of the day. Also, exercise induced spikes usually recover to baseline more quickly than spikes from a big meal.
When it comes to monitoring your glucose levels, it’s important not just to notice when it spikes, but also how high it spikes and how long it takes to recover. It’s also important to monitor your fasting glucose levels and how they are changing over time.
At the end of the day you are Sherlock Holmes and the set of clues you have to work with are your daily activities and the data outputted by your CGM. Most CGM apps have a note-taking function, use it! Especially for the first few weeks, make a point of taking notes:
When you wake up
When you eat a meal
When you eat a new food
When you feel lousy and tired
When you feel amazing and energetic
When you take a walk or do other forms of exercise
Over time patterns will emerge, it’s up to you to identify them!
Don’t forget you have a doctor. Treat your doctor like Watson, always there to help you solve the case if answers aren’t forthcoming.
#11 Definitions:
A1c. Your A1c is a measure of your average blood sugar levels over the past 3 months. This chart from the Mayo clinic shows the ranges for “No diabetes” and “Pre-diabetes”:
GMI stands for Glucose Management Index. This is what your CGM data would predict to be your A1c score. Note that differences between GMI and A1c are likely, and are usually due to not wearing the CGM consistently enough. For example, if you only wear your CGM 50% of the time, then A1c will be measuring your actual average blood sugar, whereas the GMI prediction will be based on only having half of the data.
Glycemic/Glucose Variability. This is a measure of how far glucose readings vary from the mean (average) or median (midpoint) glucose readings. It’s looking both at the degree of up and down fluctuations (amplitude) and the frequency of variation.
Time in Range is the percent of time spent inside your target range - which will be represented on your CGM app by the green shaded area. Higher is better.
% Time CGM is effective (Data Sufficiency). Ideally, a CGM should be worn and scanned at least 70% of the time. This makes sure that the data the CGM captures is accurate and holistic enough to make health decisions. This means, for example, that over a 2 week period you would want to have data for at least 10 days.
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