Glucose monitors could feed you misinformation

Continuous glucose monitors are becoming all the rage: endorsed by celebrities, these devices resemble a coin-sized patch that sticks on the skin and beams a read-out of predicted blood sugar levels to an app on a mobile phone. Many people have signed up to programmes that claim they’ll help improve your health by guiding your nutritional intake. And although these devices can be very helpful for people with diabetes, where they can be reasonably accurately calibrated to the user’s blood sugar, the evidence is, according to a new study, when the average person straps one on, they may be being fed misinformation about the impact of their diet, potentially leading them to make worse food choices.  Javier Gonzalez is at the University of Bath…

Javier - We were interested in the accuracy of these continuous glucose monitors, sometimes called CGMs, and these CGMs are devices that attempt to estimate our blood sugar level at any one time, and how they do this is they don't measure the sugar in the blood, they measure the sugar in something called interstitial fluid, and that's the fluid that bathes our cells and it's slightly separate to the bloodstream, and it then uses algorithms to try to estimate what the true level is in the bloodstream, and because of this difference we don't expect exactly the same value from the CGM to the blood sugar level, and because some influencers on social media and TV shows and also in research are tending to use these CGMs to try to understand the blood sugar responses to foods, so for example does a banana increase your blood sugar level more than another food, then in this scenario there are potentially changes in factors that influence the accuracy of CGM, and we wanted to better understand that.

Chris - I must admit I work in a hospital, I've had colleagues roll up their sleeves and show one of these patches, they look like a sort of sticking plaster stuck on that they've worn for a couple of weeks, and then they whip out their mobile phone and show you a trace saying - “Well, that was my breakfast and look what it did to my glucose level.” So, this is not just the odd social media influencer, this appears to be quite penetrating through society in terms of the uptake of these devices.

Javier - Yeah definitely, they were originally developed for people living with diabetes and can be a great tool for people with diabetes to manage their blood sugar level, but as you say they're increasingly used by the general population.

Chris - What did you do then to try to probe this? How accurate these things are compared with, and I presume that pricking your finger like a diabetic would, that's the gold standard because that's really blood?

Javier - That's right, yeah, so what we did is we used the finger prick test, the finger stick test, as our gold standard measure, and at the same time people wore a continuous glucose monitor, and what we did is we studied people across a variety of different foods that should lead to different levels of blood sugar, so we gave people pure glucose and then we gave people different foods and different fruit smoothies, sometimes it was whole fruit, sometimes it was in the liquid form, to try and understand whether the accuracy of the CGM differed across these variety of different foods and beverages.

Chris - How many people did you look at? 

Javier - This was a study of generally healthy people, it was a sample size of 15, which for some types of studies can be relatively small, but what we did was a crossover design, so people act as their own control, and for those types of studies you can get much more statistical power for a very small sample size, so it was in some senses proof of concept, it's not necessarily aiming to extrapolate across a whole load of people, but it does demonstrate the principle in a tightly controlled laboratory setting.

Chris - And were they any good? Did the devices turn out to be an accurate reflection of what was really going on, or are we being misled?

Javier - The general finding was that on average they tend to overestimate the true blood sugar level, but what complicates this is that that difference was varying between people and between the different foods and beverages that were ingested, and that's trickier because if it was just a clear systematic bias then we could easily just subtract that and correct for it across everyone and across all the meals and it would be rectified, but because it varies between people and the different foods and beverages, then it's not easy to apply such a simple correction factor to improve the CGM accuracy.

Chris - Now, if these things were condemning bad for you stuff, you wouldn't mind if it told people that it was spiking their glucose higher, because it might deter them from resorting to that extra can of fizzy drink or that bar of chocolate, but was it condemning things that actually we quite like people to be eating more of?

Javier - Well it was certainly misclassifying certain things that we would describe as low glycemic index. So, the glycemic index is a way to classify foods by the degree to which they raise your blood sugar level, and it was classifying some foods as medium to high glycemic index when they're actually classified as low glycemic index based on the gold standard measure, and I guess one other thing that might be causing a bit of unnecessary concern is that because they overestimate the blood sugar level systematically, then it also means that when we're aiming to keep our blood sugar level within a certain range, the CGM overestimates the time we spend outside of this healthy range. So, you could have someone who has perfectly healthy blood sugar levels, but they're unnecessarily worried because their CGM is telling them that they have a higher blood sugar level than they actually have.

Chris - I was just going to say, is this going to feed into people's health anxieties, and is it actually going to make some people potentially eat less well because they'll be thinking that some things are worse for them than they really are?

Javier - Potentially, and I think that's something we should be aware of anyway, in the sense that we should be not focusing on one single metric. Our health is dependent on a variety of different things, and glucose is just one of those factors, so even if this was a perfect measurement, there are other things that contribute, and we might make dietary decisions that improve our blood sugar level at a detriment to other aspects of our health.