Posted December 14, 2022
Many people have gotten very interested in how their diet affects their blood sugar levels. And as a result, continuous glucose monitors, or CGMs, have gotten very popular not just among people with diabetes, but also among people who are interested in optimizing their health.
In general, I think this is a good development, as I am all in favor of people taking an active role in maintaining – or improving – their health. But: many researchers and healthcare professionals, myself included, feel that how some people use their CGM is unlikely to actually benefit them.
In this blog post, I will discuss four points that are key to making a CGM a tool for improved health.
Brief Introduction to Continuous Glucose Monitors
CGMs are small devices that attach to the skin with a very thin filament inserted INTO the skin. This filament measures the concentration of glucose in the interstitial fluid, which is the fluid around the body’s cells. Glucose levels in the interstitial fluid provide a reasonably good approximation of blood sugar levels. These CGMs typically measure glucose levels once every minute, thereby providing an almost continuous measurement of the ups and downs of glucose levels throughout the day. So this is a very convenient and useful alternative to measuring blood glucose the traditional way, which involves a finger prick with a little needle to obtain a drop of blood, and then using a glucose meter to get a 1-time snapshot of your blood sugar concentration.
Blood Sugar Response: Only One of Many Health Effects of a Food
The first point I suggest we need to keep in mind when we are using a CGM is that our blood sugar response is only one of many health effects of a food.
As a long-time clinical diabetes researcher, I am the first to tell you that our blood sugar level is an important measurement to keep an eye on, but it’s just one of many, and we should not use it by itself to judge whether a food is ‘healthy’.
So when someone, be it on social media or in a scientific paper, argues that we should use our blood sugar response to foods to ‘personalize’ what we eat, I suggest we are at least a little bit critical. The specific suggestion I have heard most often is that we should use a CGM to figure out which foods give us personally a blood sugar spike, and then stay away from these foods.
That sounds very reasonable, but I confess that I haven’t jumped on that bandwagon. Why not?
Just imagine a world that is maybe a little bit more technologically advanced than ours currently is, and in that world we also have, let’s say, continuous fat monitors that tell us minute-by-minute the concentrations and composition of all the different fats in our blood. Or a continuous vitamin monitor that tells us the concentrations of all of the vitamins in our blood, and tissues. Or a continuous gut microbiota monitor that provides information on the composition and function of our gut bacteria. So every minute, we can see how happy those little guys in our gut are.
You can probably imagine that the foods we eat would register very differently on the ‘healthy’ vs. ‘unhealthy’ scale when it comes to these measurements, right? We don’t have these technologies available, but just because we can measure our sugar levels so easily now, and we can’t measure all this other stuff as well, should that mean that we base all of our food choices on blood sugar responses alone? Of course not, so please don’t fall into this trap.
At the very least, we could look at those other measures that ARE already available to us. The fasting lipid profile, blood pressure, biomarkers of inflammation, our micronutrient status, and many more, all of these are established risk factors for one disease or another. If you use a CGM to figure out which way of eating keeps your sugar levels stable, that’s great, but don’t forget to also consider how that diet affects all of the well-established biomarkers of disease risk.
There is another reason to not just use your blood sugar responses to a food as the only criterion to decide what you should and should not eat: your blood sugar response to foods is not set in stone. As we will discuss in detail in the future, you can affect your blood sugar response to foods by, for example, eating different combinations of foods, in different order, or by going on a walk after a meal. Or, and that brings us to the second point I’d like to make, you could try to improve your glucose tolerance so that your body is better able to handle whatever carbs you do eat.
A CGM Does Not Measure Effects on Glucose Tolerance
The second point I’d like to bring up is one that is almost never discussed: a CGM measures the acute effects of a food or meal on blood sugar levels, but it doesn’t measure the long-term effects of a food or meal on glucose tolerance.
Let me explain.
What you measure with a CGM is the acute short-term blood sugar response to a specific food or meal. However, as explained in our most recent blog post, our bodies actively regulate our blood sugar levels, and the ability to keep blood sugar in the normal range is called glucose tolerance. Now, we’ll get to this very point in a lot more detail in the future, but the fact is that a food may be very good for your short-term blood sugar levels, but it may cause you to become glucose intolerant from eating it repeatedly.
The case I am making is that eating in a way that optimizes glucose tolerance will also, in the long term, reduce your blood sugar levels throughout the day. But the effect of a food or meal on glucose tolerance doesn’t immediately show up in your CGM data. It’ll take some time.
Now, to illustrate this point more, let me show you some data from my own CGM. I regularly eat quite a bit of carbs simply because I eat a lot of minimally processed plant foods. My blood sugar is usually in the 85-125 mg/dL range, and almost never exceeds 140 mg/dL.
Let me show you an example: the screenshot below shows my blood sugar response to a dinner of a large serving of veggies, including starchy veggies such as carrots, a few potatoes, and a little bit of chicken. My blood sugar increased from around 100 to about 110 mg/dL.
Or the second screenshot, which shows my glucose response to a breakfast of four slices of bread with cheese and two boiled eggs. If you believe the low-carb gurus online, my blood sugar level should have gone through the roof after eating four slices of bread, but it barely increased and topped out at about 120 mg/dL.
So even on my pretty high-carb diet, my HbA1c is around 4.8%, suggesting that my average blood sugar level throughout the day and night is somewhere around 90/95 mg/dL.
So the fact that I can eat a large amount of carbs in one meal and not experience a major glucose spike is because my way of eating has made me very glucose tolerant. This is the result of several factors, and we’ll discuss these over the coming weeks and months on this website.
The main point I’d like to make here is that using the CGM to identify and then cut out foods that give you blood sugar spikes addresses only part of the problem: in my opinion, if you care about your blood sugar levels and your metabolic health, you may also want to improve your glucose tolerance, because glucose intolerance really is the core problem underlying excessive blood sugar responses to meals.
So please be clear that the CGM will not acutely help you understand what the foods you eat do to your glucose tolerance.
Keep it Real!
With that, let’s get to the third point, and that is that I strongly suggest you keep your experiments with your CGM real.
One temptation I see is that people, once they get their CGM, eat large quantities of isolated foods to figure out their glucose response to them.
As an example, I was watching this fitness influencer on YouTube the other day. She ate 3 or 4 bell peppers, raw, all by themselves. That gave her a blood sugar spike, and then she concluded she should not eat bell peppers.
Well, I eat a lot of vegetables, but I don’t think I have ever eaten more than one or maybe one and a half bell peppers in a single meal. And, more importantly, I almost always eat them in a mixed dish like a salad or stir fry, with fat and lots of other stuff. The glucose response to these mixed meals would be very different from eating large quantities of bell peppers by themselves, and that is a much more relevant measurement than creating an artificial experimental eating situation.
So this YouTuber who now avoids bell peppers, if she ate a more reasonable amount of bell peppers with some fat and protein in a normal mixed dish, she would almost certainly not experience a similarly major blood sugar response. So, in my opinion, she’ll miss out on a delicious and nutritious food for – likely – no good reason.
So my suggestion is to use the CGM to test the blood sugar response to the meals you actually eat, not some huge quantities of isolated foods that you otherwise have never eaten in your life.
Do Not Obsess Over Every Little Increase in Your Blood Sugar Levels
With that, I’d like to close with the fourth point I’d like to make, and that is – to me – the most important one of these four: please do not obsess over every little increase in your blood sugar levels!
I see a lot of people on social media who share their CGM data totally freak out whenever their blood sugar level jumps from 100 to 120 mg/dL. They immediately start talking about ‘blood sugar spike’ and needing to avoid these foods in the future.
If you find yourself in a similar state of mind, staring at every little increase in your blood sugar data with horror, I have a suggestion: please, chill a little bit!
Well, how can I say that? Isn’t it clear, you may ask, that elevated blood sugar levels can cause health problems in the long term? Yes, that is true indeed, but I do think that it is very important to be clear about what we mean when we say high or elevated blood sugar.
Now, as we discussed in the last blog post, we have clear clinical guidance about what normal FASTING glucose levels should be (less than 100 mg/dL), what normal glucose levels should be 2 hours after drinking a 75-g glucose beverage as part of an oral glucose tolerance test (less than 140 mg/dL), and what our average blood glucose should be, as measured by the glycated hemoglobin or HbA1c test (less than 5.7% in the HbA1c test, which is equivalent to an average blood glucose level of 117 mg/dL).
At the same time, we have little clear guidance on what kind of blood sugar spikes are OK, and what a normal range of blood sugar values throughout the day is. That is partly because the American Diabetes Association criteria shown in the table above are based on very standardized tests, specifically an oral glucose tolerance test that includes drinking a beverage containing 75g of pure glucose. Blood sugar responses to normal meals are quite different.
Also, the criteria don’t tell us anything about what a normal level would be 30 min or 1 hour after a meal. For example, we may wonder whether a spike up to 180 mg/dL or even 200 mg/dL is OK and still consistent with normal glucose tolerance, as long as our level comes down below 140 mg/dL within two hours of the meal.
So allow me to share my framework of what I consider elevated vs. normal blood sugar levels.
So, first let’s take a look at people who have diabetes. Usually, their fasting blood sugar is 126 mg/dL or higher, and they typically spend a significant part of the day in the range above 180 mg/dL. In this group, it’s not rare for patients to have blood sugar levels of 250 or 300 mg/dL after a meal. For people with blood sugar in that range, we have a lot of conclusive and strong evidence that lowering their blood sugar levels will benefit their long-term health.
Next, we have people who have pre-diabetes. They often may have fasting glucose above 100 mg/dL, but not necessarily so, and they are likely to have sugar levels well above 140 mg/dL at least some of the time, sometimes maybe even exceeding 180 mg/dL. Again, we have good scientific evidence that those individuals would benefit from lowering their blood sugar levels.
Then we have a third group, less well-defined clinically, that does not yet meet the criteria for pre-diabetes, but that does suffer from regular spikes in blood sugar levels to 180 mg/dL or even higher. This is based on scientific literature showing that some supposedly healthy people unknowingly, meaning if they don’t wear a CGM, have repeated blood sugar spikes after a meal to 180 mg/dL or even higher.
There is a lot less scientific evidence available to know whether this group also suffers long-term health consequences from these excessive blood sugar spikes, but I would judge it as likely that this carries some health risks. So, yes, in my estimation, this group would also benefit from lowering blood glucose levels and specifically working towards avoiding these blood sugar spikes. I think here is where a CGM is a particularly powerful tool to detect these spiking patterns, and enable you to do something about it.
And then we have a fourth group with normal glucose tolerance and blood sugar in the 70-140 mg/dL range. That range, 70-140 mg/dL is based on looking at CGM data from healthy people with normal glucose tolerance. Based on several studies, such healthy people have blood sugar levels in that 70-140 mg/dL range at least 95% of the time, so I think it’s reasonable to consider this as normal.
Now, the big question that I am sure many of you are wondering about: if you fall into this category, you have normal glucose tolerance and your blood sugar level is in the 70-140 mg/dL range at least 95% of the time, do you still benefit from lowering your blood sugar levels and specifically minimizing blood sugar increases after a meal? Well, at this point, we just don’t have a lot of strong data that have looked into that question.
For now, let me say that my best guess is that, all other things being equal, it may be better to have smaller blood sugar bumps and to maybe have sugar levels fluctuate in a tighter range, such as 70-120 mg/dL. However, that is somewhat uncertain, and I’d argue that the benefit of ‘optimizing’ your blood sugar levels in this group is likely pretty small compared to reducing your blood sugar levels in these three other groups.
So this is why I suggest that if you use a CGM and you find that your sugar levels fall into this normal range, chill and develop a bit of a relaxed relationship with your CGM. You are already in a great place, and there just isn’t that much more that you could optimize at this point. And – in my opinion – the little bit of health benefits that MAY result from further lowering your already close-to-optimal glucose levels are almost certainly not worth stressing out about.
One issue is that I think there is a very real risk here that our relationship with food becomes an unhappy one if we obsess too much over every little change in an individual health marker like blood glucose.
I don’t want to be misunderstood, so let me reiterate that there is no doubt that long-term health benefits greatly from normalizing blood sugar levels if we have diabetes, pre-diabetes, or experience major blood sugar spikes. There may also be benefits to keeping your blood sugar fairly level if you have normal glucose tolerance, with glucose usually ranging between 70 and 140 mg/dL. Still, I want to emphasize that agonizing over every little up and down in your sugar levels is likely not healthy. I want to encourage you to consider your whole body, your entire health, including your mental health, and not reduce your health to a single number based on a single health metric. Keep this in perspective, look at the big picture, and don’t stress out over your CGM data. Then I think that wearing a continuous glucose monitor can be a great, empowering tool for better health.
As always, remember that the content of this blog post is not medical advice, and I suggest discussing your CGM data or any changes in your diet with a qualified professional such as a registered dietitian, nutritionist, or physician.
I am currently using the Veri app and an Abbott FreeStyle Libre sensor to help me share how some of the science we are discussing can be used to improve blood sugar levels. However, I am not affiliated with Veri, Abbott, or any of the other CGM manufacturers, and I have not received any payment from any such company for this post, the associated video, or any other service.
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