Once I noticed the first, I saw them everywhere – a curious white disc stuck to the upper arm. People were wearing them at the gym, on the underground, in the office. An internet search told me they were continuous glucose monitors, which give an instant read-out of your blood sugar level.
Around the same time, I noticed media coverage of ZOE, a dietary advice programme based partly on your blood sugar responses to food, whose podcast . And my social media feeds were filled with posts about how to hack your glucose levels. Clearly, glucose is having a moment.
We normally think about blood sugar in relation to diabetes, in which the body can’t regulate glucose levels, with serious health implications. Continuous glucose monitors were invented to make it easier for people with diabetes to find out – and hence control – their blood sugar levels.
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Now, however, some companies offer them to people without diabetes as part of a programme of personalised nutrition advice, seeing them as a window into an individual’s response to diet that can guide the foods they should eat for optimum health. The idea behind monitoring glucose is to reduce the “spikes” that happen after eating, as this will help with weight loss and boost mood, energy levels and sleep, or so the story goes. So, what do we know about the health effects of glucose spikes? And is monitoring them a good idea for everyone or could there be any downsides? I decided I needed to dig into the research and try these monitors for myself.
Glucose certainly plays a vital role in our health. This highly soluble sugar molecule is one of the chief ways that fuel is transported around the body because it is an easy way to store and release energy. When we eat, its levels in the blood naturally rise and so the pancreas makes the hormone insulin, triggering some of the glucose to be locked down in bulky storage molecules, such as glycogen. Several hours after eating, as glucose is used up as fuel, the glycogen is converted back into glucose.
Diabetes occurs when this glucose control goes awry. In type 1 diabetes, the pancreas stops being able to make insulin, so people need to inject themselves with the hormone several times a day to prevent their sugar levels from skyrocketing. In the more common type 2 diabetes, which is often linked with being overweight, the body’s cells become less sensitive to insulin. People with this version are often advised to lose weight and to avoid foods high in sugar or starch, which the body quickly breaks down into sugar.
Continuous glucose monitors
Both types can have health consequences. If someone’s blood sugar falls too low, they may faint or even fall into a coma. But if it is consistently too high, glucose can stick onto larger proteins inside their cells and even their DNA, fouling up myriad metabolic processes. Over time, this can lead to kidney disease, blindness and blood vessel damage that can cause heart attacks and strokes. A pivotal study in 2000 showed that people with type 2 diabetes who keep their glucose levels lower, on average, had .
Continuous glucose monitors arrived in 1999 to help those with type 1 diabetes know how much insulin to inject without doing lots of blood tests a day. Now, a sensor on the arm estimates the glucose level in the fluid between cells, which is very similar to that in the blood, and sends continuous signals to a phone app. “They have been game changers for people with type 1 diabetes,” says , a diabetes specialist at Imperial College London. They can also help people with type 2 diabetes. David Unwin, a doctor based in Southport in the UK, says glucose monitors help some of his patients with diabetes maintain a good diet. “It causes you to think a lot more about what you’re eating,” he says. “You see that those biscuits put up your blood sugar.”
Intrigued to see if there were any interesting patterns in how my blood sugar levels changed throughout the day, I decided to try out two glucose monitors, one provided by , which offers glucose monitoring along with some other tests, including a gut microbiome analysis, and one by a service called from Abbott that focuses on blood glucose. When the first arrived, I was taken aback by the almost 1-centimetre-long needle. But applying it was easy and surprisingly painless, and soon I was brandishing two monitors, one stuck to each arm. I should say that I didn’t try the diet coaching aspect of either programme.
The good news is that for the two weeks I wore the monitors, my glucose values stayed well within the target range of 3.9 to 10 millimoles per litre, usually taken as the ideal lower and upper limits for people with diabetes. And my personal report from ZOE said my glucose control was excellent. But that didn’t mean I could necessarily relax. Within that range, I experienced many ups and downs. Occasionally after eating a big meal, such as a pasta dish or a curry with rice and naan bread, my glucose rose by more than 1.7 mmol/L, which is one of Lingo’s definitions of a spike.
Seeing these spikes was worrying, so it was time to take a closer look at the evidence that they are bad for people without diabetes. The firms behind these programmes typically point to long lists of studies that suggest links between higher blood sugar and poor health. “If you are having multiple spikes in a day, your mean glucose level will be raised. We know that raises your risk of diabetes and heart disease,” says at King’s College London, one of the founders of ZOE. A spokesperson for Lingo pointed me to a review of 38 studies showing that in people without diabetes, those with higher glucose levels were , arguably the most serious risk of poor blood sugar control.
But there are two caveats for anyone taking this kind of research as evidence that we should all be trying to minimise our glucose spikes. The first is that most of these studies merely observe correlations between high glucose and poor health outcomes – and that doesn’t show the former causes the latter. The correlation could be caused by a third factor: for instance, people who have a more sugary diet could be more unhealthy in other ways, and that might be behind their poor health. This is a wide-ranging problem with dietary research.
The second caveat is more subtle. Glucose levels jump up and down many times over the day, so there are several ways they can be quantified. The studies in the review that found a higher risk of heart disease used several common measures, including the “A1C” test – which reflects someone’s average blood sugar over many weeks – and people’s response to a standardised sugary drink, which measures increases in blood sugar that last for at least 2 hours. But much of the negative talk about spikes implies that any rises are harmful, even brief ones, says Misra. “It is totally normal for your blood sugar to go up if you eat carbohydrates, and then to come down again, because the pancreas has made insulin.”
Blood glucose spikes
What’s more, we still don’t know how high a spike is too high. Most of our knowledge about blood glucose comes from studying people with diabetes and it is unknown if the same figures apply to those without the condition, says Misra. “We do not have that data for healthy people because we’ve never had these devices before,” says , a dietitian at the University of Oxford.
When it comes to the other claimed benefits of reducing spikes, research has begun to accrue suggesting that high glucose levels are linked with and . But it is still early days, with most of the studies so far being small ones that observe correlations. The best kind of evidence would be a large, randomised trial showing that people who limit their glucose spikes have better sleep, for instance.
Another central claim is that avoiding glucose spikes and subsequent dips can help with weight loss. One study of 1070 ZOE users found that some people are prone to big dips in glucose 2 to 3 hours after eating. This correlated with them overall that day, says study co-author at King’s College London, who is also chief scientist at ZOE. In fact, using a glucose monitor told Berry that she experienced this effect. “It was a real eye-opener, putting together how I felt with what’s going on inside my body,” she says.
Sceptics say that this is something we don’t need a blood test to measure. “People already have their own bodies to tell them: ‘I’m hungry. I didn’t sleep well,'” says , a diabetes specialist at the University of Wisconsin-Madison. “The reality is most of the information they’re going to get from [a monitor] is probably things that they could have figured out on their own.”
Others counter that monitors help people see what is happening. “These devices can really train you to see what’s going on,” says at Stanford University in California, who is co-founder of a glucose coaching app called . Using them lets people avoid the foods that cause the biggest spikes, he says.
Given we already know that sugary foods cause a rise and then dip in blood glucose, why do we need monitors to tell us individually which foods have the biggest effects? The argument here is that different people have different blood sugar responses to the same foods. One study by ZOE of 1002 people without diabetes, for example, . That was borne out by my own experience, when I had a slightly higher spike compared to that of my partner, who was also wearing a monitor, after eating a roast chicken dinner.
Yet it is unclear whether these divergences are consistent or even meaningful. One study that has yet to be peer-reviewed looked at 30 people without diabetes and found that a person’s glucose rise in response to the same meal on two different days could be as different as their . “It’s probably more variable than people think,” says study co-author Aaron Hengist at the US National Institutes of Health. Even differences between the monitors could have an effect – Lingo’s device generally gave me higher readings than ZOE’s, occasionally by as much as 0.8 mmol/L.
A few studies have started to look specifically at whether glucose monitors or personalised nutrition advice can help with weight loss. One, in 40 people who were overweight and didn’t have diabetes, compared a group who received in-person dietary advice alongside a continuous glucose monitor with a group who received only the advice. After eight weeks, the group with monitors each lost 3.1 kilograms, on average, compared with an average of 2.3 kg in the other group. The team concluded that the monitors .
Data from a much larger randomised trial carried out by ZOE was presented at the 2023 European Nutrition Conference in Belgrade, Serbia, comparing people who followed its full programme, logging their food intake every day and receiving ongoing advice through the app, with people who received a leaflet and online resources containing healthy dietary advice, a video lesson explaining the advice and regular check-ins by email to encourage adherence. The control group also had access to coaches by email, but not to the same level as those on the ZOE programme.
The study found that those who used the ZOE programme improved slightly on several metabolic parameters compared with the control group, and that after four months they had lost 2 kg compared with a 0.3-kg gain in the control group. But this arguably wasn’t really a fair comparison, as trials of different weight-loss programmes inevitably find that people succeed in losing more weight when they get more advice, attention and support, says Guess. Spector says that this inequity is inevitable when comparing any personalised nutrition programme to standard dietary advice. “It’s common practice in trials to compare with the industry standard.”
Some might argue that a 2-kg weight loss after four months isn’t that impressive. Spector says the goal of ZOE isn’t rapid weight loss but to get people eating more healthily long term.
Low-carb diet
To be fair, most diets don’t produce much weight loss in randomised trials, so you could argue that this approach is on par with any other diet. But the focus on blood sugar concerns some dietitians, such as Guess, because to prevent spikes, people might be tempted to avoid foods that are high in sugar and starch – a low-carb diet, in other words.
The question of whether a low-carb diet is healthy has long been debated. says we should base meals around starchy foods like wholemeal bread, pasta, rice and potatoes. People on low-carb diets, however, tend to get more calories from proteins and fats, whether from meat, dairy or plant sources. “I worry that dietary advice that actually does something that we need it to – lowering blood pressure, lowering cholesterol – is being subverted by [concerns over] blood sugar in people who have no reason to be concerned about their blood sugar,” says Guess. “People are being prompted to focus on a non-existent risk factor, potentially to the detriment of actual risk factors.”
“If you focus on any single metric, such as only blood sugar or only blood fat, you are only seeing a very small piece of a huge puzzle,” says Berry. “That’s why, at ZOE, we use multiple inputs to generate our food-based advice.”
Where glucose monitors are less contentious is in potentially helping to identify people with diabetes and pre-diabetes, in which glucose levels are higher than average, but below the threshold for diabetes. Doctors offer cheap and simple tests for these conditions, but many people don’t receive them as they don’t have regular check-ups. It is possible that the craze for glucose monitoring could help spot extra cases by encouraging people to see a doctor.
As for me, I wasn’t sorry to take the monitors off and stop worrying about my spikes. While the evidence about the benefits of limiting them in people without diabetes is still unclear, there is more certainty around A1C tests, because they reflect long-term glucose averages and, fortunately for me, my A1C estimate from ZOE was below the level indicating pre-diabetes. I’ll take that as a reassuring sign – although I suspect I will be checking in on my score in the years ahead – not with a commercial dietary programme though, but with my doctor.
