Why tracking blood glucose with a CGM isn’t a good idea

Wearing a continuous glucose monitor (CGM) when you’re not diabetic has become more popular.

Instagram influencers warn us about the danger of blood glucose spikes to your health. Personalised nutrition programmes tell you they can predict your response to a meal, and tell you what to eat for better wellbeing. But is it all it’s cracked up to be? 

In this blog post I’ll explain why I don’t recommend using a CGM if you don’t have diabetes.

Let’s start looking at what we’re actually talking about with monitoring blood glucose.

What is considered a problem range for your blood sugar?

In order to check for diabetes or confirm a diagnosis a doctor will look at your blood tests.

They check if you have high blood sugar when you haven’t eaten (fasted), or what your markers of blood glucose are like over a 3 month period (Hba1c).

The NICE guidelines for diabetes diagnosis highlights the following levels of concern:

• HbA1c of 48 mmol/mol (6.5%) or more.
• Fasting plasma glucose level of 7.0 mmol/L or more.
• Random plasma glucose of 11.1 mmol/L or more in the presence of symptoms or signs of diabetes. 

These are measures that show you have prolonged exposure to high levels of glucose in the blood, and could indicate diabetes. 

What’s wrong with high blood sugar?

Having too much glucose in your blood for long periods of time can put you at risk of several serious health issues. In people who have diabetes the pancreas doesn’t produce enough insulin so glucose can’t get into cells, and stays in your blood.

What is diabetes again?

• Type 1 diabetes is an autoimmune condition where the immune cells attack the pancreas and your body cannot make insulin
• Type 2 diabetes is where your body doesn’t produce enough insulin. Or insulin resistance is where your cells don’t respond to the insulin to allow glucose in, so the glucose stays higher in the blood.

If your blood glucose level is consistently & repeatedly high for long periods (more than 2 hours) you will be at higher risk from stroke, heart disease, nerve damage, kidney problems and retina damage. In uncontrolled diabetes where blood sugar is high for long periods then the glucose in your blood damages tissues, nerves and cells. 

Is it bad to have high blood sugars? 

There is a difference between high blood sugar levels in diabetes and the ‘sugar spikes’ described by the ‘Glucose Goddess’ or personal nutrition programmes like Zoe.  

Your blood sugar shouldn’t be high for more than two hours after a meal.  However, it’s normal for you to get an increase in blood sugar within the first 20-90 minutes after eating. 

What about glucose spikes?

Some advice from the online influencers like the ‘Glucose Goddess’ suggests people should be trying to keep their blood sugar level low at all times. That any ‘glucose spike’ is damaging for your health, and we should all adapt our diets to avoid them. 

By the way – There isn’t a clear medical definition of what is meant by glucose spike. 

If you’re using a CGM to track blood sugars you might think any excursion into the higher level of glucose in the blood must be avoided completely. 

Is lower blood glucose better?

In reality, as long as your body returns the blood glucose to a normal level within two hours, you’re ok. 

Diabetes can be diagnosed when you have blood sugar level over 7.8mmol/L after two hours of a high glucose meal.

But that doesn’t mean that you need to avoid your blood glucose ever getting to that level. A rise and fall is normal as we eat, and then our body packs the glucose away into cells. 

What is a “bad” blood sugar spike?

If we’re thinking about how to control blood sugars then these are the kind of metrics that you might take note of:

1. How big is the spike?

A post-meal (postprandial) blood sugar level consistently rising above 140–180 mg/dL (7.8–10 mmol/L) may indicate your body isn’t managing blood glucose well.  

For non-diabetics we don’t have consistent data about what is good or bad. Most research points to post-prandial hyperglycaemia beginning when your blood glucose rises above the level of 140 mg/dL (7.8 mmol/L) 1 to 2 h after eating. 

Spikes over 200 mg/dL (11.1 mmol/L) are typically considered excessive. But we don’t have long term data from non-diabetics on whether this is actually a problem.

E.g. how many of these are considered bad or ok.  What does it mean if you have one a week, one a day? 

2. How long does it last?

Persistently high blood sugar after meals (for more than 2 hours) indicates poor glucose metabolism. This might stress the pancreas and lead to long-term complications with other body systems.

You want your blood sugars to fall within two hours after eating. 

Is lower blood glucose always better, even within the normal range?

There is no evidence that a normal rise in blood glucose is bad for you. 

No-one is suggesting you should have extended glucose levels for a long time, but as long as you’re under getting a normal response to meals you’re ok. 

Why CGMs aren’t helpful if you don’t have diabetes

If you’re looking for to use a CGM as a non-diabetic here’s what you need to know:

Your blood sugar responses are meant to change throughout the day

Daily (diurnal) variation in blood glucose response to food is normal. People eating the same meal at 8am, 8pm and midnight had different responses of their blood glucose in one study, as an example.

This shows that meal timing has an effect on glucose metabolism, even if you’re eating the same food at each meal.

The other meals you’ve eaten in a day also affect blood glucose response to meals later on. Most people get ‘worse’ blood sugar profile to meals as the day progresses. 

This means we can’t predict what your blood sugar response to foods might be in the future. So you can’t make decisions on what to eat based on these kind of responses. 

Blood glucose levels change depending on what else you eat 

We rarely eat a certain food on its own. Meals of mixed foods (including protein, fats, fibres) are more likely to be the thing that causes your blood glucose to change, rather than one food on its own.

This is why making predictions about what you can and can’t eat based on CGM data doesn’t make sense. 

Exercise changes your blood sugar response

How active you are will affect your response to food. Exercise will improve your insulin sensitivity. 

But actually glucose levels can increase during high intensity exercise in the short term. You might see your blood glucose going up during exercise and feel disheartened.

After exercise your blood glucose response may be lower for 1-2 days (in both diabetics and non-diabetics).

If you consistently exercise you may improve your insulin sensitivity long term. So one day your response to a certain food might be better because you went for a run last night, but next time you eat it your response is different. 

What are the drawbacks to using CGMs in non-diabetics?

While CGMs are valuable tools for people with diabetes to monitor and manage their blood glucose levels. using them without a medical need can have potential drawbacks and risks:

Inaccurate readings in non-diabetics

Firstly, CGMs are calibrated to measure glucose levels within a certain range. If you don’t have diabetes, the readings may not be accurate. Especially at the low end leading to unnecessary concern. Or, conversely, a false sense of security.

This study from 2022 showed a 13% difference in the average readings from a CGM to a blood draw. The researchers used different meal combinations, and tested the blood glucose with both CGM and took blood from healthy adults. The CGM underestimated the time spent in a normal range of blood glucose.  

Secondly, sometimes different CGMs will show different results in the same person, at the same time. This study took 16 healthy people and fitted them with two different CGMs to track their measurements over 4 weeks in an in-patient study.

So the two CGMs showed different measurements. In addition, there were also differences between in the same person eating the same meal from one week to the next. So this means it would be hard to make any judgements about how your body will respond to food.  

Interestingly they also found the difference in glucose readings between the two CGMs increased as the body fat level of the participant increased.

The researchers concluded:

“Misclassifying meals or foods as “bad” or “good” in terms of their CGM responses may affect overall eating behaviors with potentially deleterious consequences.” 

Another study from 2015 found in healthy 65 year olds the CGM data gave slightly higher readings during the day, and slightly lower readings at night.

Unnecessary stress

Continuous monitoring of blood glucose can feel stressful, especially if the readings are misinterpreted.

This can lead to unnecessary anxiety and worry about what you’re eating, or what’s going on inside. After all you can’t feel your blood sugars going up and down most of the time, so it can be scary imagining it spiralling out of control. 

I’ve seen the reports from personalised nutrition programmes like Zoe, explaining the client has ‘Poor blood sugar control’ in response to eating their standardised muffin. When I looked at the data they had a perfectly normal blood glucose response to eating! 

But this client then had a lot of worry about whether they were causing inflammation and lasting damage with their eating pattern, which was very healthy.  

Money could be spent on other things

CGMs are expensive, and if you don’t need it, what else could you spend your money on? 

Too much data can be a bad thing

Constant monitoring of blood glucose levels can lead to information overload. If you don’t have diabetes, this continuous stream of data may not be useful and may contribute to unnecessary anxiety about foods being healthy or bad for you. 

You might be missing other issues with your health

Glucose levels are one measurement. But what about your blood pressure? Or triglycerides? Or cholesterol levels?

These are important markers for health as well and fixating on one of them may lead to issues with another.

For example, noticing when you eat a high fat meal your blood sugar spike is lower, perhaps you choose higher fat meals from now on. Over time this could have an effect on your weight, or blood lipid levels, or nutrient intake.  

Outsourcing your body senses to a device

When you’re looking at your blood sugar rather than thinking about how you’re feeling it doesn’t help you tune in to your own hunger levels, or sense of wellbeing. 

What about if you thought how you feel in general, rather than after each specific food.

Creates fear of foods

We can easily jump to the wrong conclusions about foods by looking at this one data point. If you think “when I had sweet potatoes my blood sugars went up” you might start to avoid them. But if the ‘blood sugar spike’ isn’t even something you need to worry about then you’re cutting them out of your diet unnecessarily. 

Importantly, as I mentioned above CGMs are not good at reproducing the same results. Studies have shown that the same person eating the same meal one week to the next will have different blood sugar profiles. 

This means that programmes promising personalised nutrition like Zoe can’t be accurate.

They make predictions about your future blood glucose response based on a defined period when you wear a CGM.  If the CGM devices can’t reproduce the same response, in the same person, then should we take their recommendations and avoid foods like oats, fruit or coffee? 

Next steps

If you’re looking for more information about using a CGM I really recommend checking out the work of Dr Guess, who writes a brilliant blog with easy to understand information about blood glucose, insulin resistance and CGMs. I thank her for help with understanding this topic.

If you want help figuring out your IBS triggers, start your Gut Reset with me over 3 months to find a diet that works for you. Just set up a free call to discuss what you need some help with. Email info@goodnessme-nutrition.com