Personalised nutrition: A deep dive into continuous glucose monitors

Dr Linia Patel takes a look at the latest gadget in personalised health, the continuous glucose monitor, and asks if it’s worth the hype.

It began in the 1980s with a heart rate monitor. For the first time, an individual could observe changes in heart rate as they happened whenever or wherever they chose. Four decades later we have watches, rings, bands and apps that track, measure and quantify almost every aspect of our fitness, nutrition and health.  The latest gadget in this trend of personalised health is the continuous glucose monitor (CGM). By attaching the CGM device to your upper arm, you can see how your blood sugar reacts to meals and particular foods. It gives real-time feedback, helping you identify the foods that cause the largest spikes and crashes in your blood glucose – therefore enabling you to make better food choices. In the world of personalised nutrition, gone are the days of the one-size-fits all narrative. But the question is, if the technology that claims to be the answer to personalised nutrition lives up to expectations, what is real and what is fantasy? What is evidence based and what is merely a prediction or theory?

What is continuous glucose monitoring?^1,2,3

Measuring blood sugar responses is one of the key pillars of personalised nutrition. One way to measure blood sugar levels is to use a continuous glucose monitor. CGMs were developed for people with type 1 and type 2 diabetes to allow them to monitor their blood sugar levels to give the appropriate insulin dose without having to always prick their finger. It could be a lifesaver for people with diabetes who experience low blood sugar levels at night, which comes with the risk of not waking up the next morning.

The devices typically attach to the upper arm and include a transmitter and a small disposable sensor that is worn under the skin. The CGM tracks blood sugars every few minutes throughout the day/night and shares electronic readings in real time. They are designed to be used for short periods of time (usually two weeks). If you want to go longer, you must replace the device with a new one.

In the past few years, there has been growing interest in monitoring blood sugar levels in individuals without diabetes. Venture capitalists have cottoned on to that and have been investing millions in companies that provide CGM advice and support for health-conscious fitness enthusiasts (all of us!).

Blood sugar control – the physiology^4,5

When we eat a carbohydrate-containing meal, it’s broken down into its simplest form: glucose. Foods that cause a rapid spike in blood glucose levels are said to have a high glycaemic index. Examples of refined carbohydrates are white bread, cakes, cookies, pastries, white sugar and fizzy drinks.

If you tend to eat a lot of sugary foods and refined carbohydrates across the day, it’s likely that you experience the extremes of both high and low blood glucose (otherwise known as blood sugar ‘spikes and crashes’). The rise and fall in blood sugar is normal but, when it occurs too frequently, it can impact on various aspects of health like energy, sleep, appetite, metabolic health and exercise performance. In the long term, this can increase your risk of type 2 diabetes, heart disease and weight gain.

Reasons you might want to use a CGM^1,6,7

Blood glucose is an indicator of metabolic health. Maintaining stable blood sugar levels has big implications for optimal mental and physical functioning, and good health. In the short term, blood sugar peaks and troughs can cause mood swings, energy dips, fatigue, irritability, headaches, cravings for refined (high GI) carbohydrates or coffee or the feeling that you need a nap after eating. Long term, poor blood glucose control can lead to obesity, a loss in muscle mass, diabetes and even increase your risk of neurological disorders like dementia. The key, therefore, is regulating your blood sugars well.

Using a CGM allows you to have real-time blood glucose data. Conventional ways to measure blood glucose control measure fasting glucose or a marker called HbA1c. Fasting glucose gives you an indicator of your blood glucose control at one point in time. HbA1c indicates your average blood sugar levels over the previous three months. Neither of these really tell you what’s going on meal to meal like a CGM.

For some people, tracking health parameters increases motivation and how well they stick to their goals. CGMs, like other health and fitness tracking devices, can be appealing and useful to some people in some circumstances as they offer objective information.

However, there is a flip side to all this.

Reasons why CGM might not be a good idea^3,7,8,9

First and foremost, the accuracy of CGMs has been questioned. CGMs don’t measure actual blood sugar levels and they take measurements from the interstitial fluid (ISF). This is thought to have limitations as body fluid shifts, meaning that ISF and actual blood sugar levels may not always match. Diabetics are always told to use CGM as a supplement to – and not a replacement for – fingerprint testing. There can be lags in glucose values between interstitial results (CGM) and finger pricks.

In addition to this, some research shows that how your blood sugar reacts depends in part on how you expect it to react. In 2020, diabetic participants in the study who had type 2 diabetes were given a drink that was either low sugar or high sugar (30g). Findings of that study showed that those who thought they got the high sugar drink had a much larger glucose response than the ones who thought their drink had no sugar at all. The study concluded that subjective perceptions of sugar intake, even when incorrect, produce measurable biochemical changes. The stress response is probably worse for your health than the carbohydrate itself!

Many nutrition experts also warn that CGM feeds the anti-carbohydrate narrative. Carbohydrates are not inherently unhealthy. Some are healthier than others of course. We need to eat less of the ultra-processed carbohydrates and more of the less-refined whole carbohydrates. As the CGM is looking just at glucose, any carbohydrates will produce a larger increase in blood sugar than a food high in fat, which for some individuals creates the illusion that carbs are ‘bad’ and fats are ‘good’. Taken to the extreme, some people may think that a piece of bacon is better than a piece of fruit. Without the right education, gadgets like CGM can fuel disordered eating.

Eating a dessert occasionally and having a blood sugar spike is not going to ruin your metabolic health. Your body knows how to deal with a surge of glucose. If you are habitually having desserts, looking at how specific foods affect your blood sugar doesn’t help you understand why you are eating those foods in the first place, which is another important aspect to consider.

What about Zoe?^{6,10,11,12,13}

An individualised approach to nutrition is important as we are all different. How our blood sugar changes after a meal varies from person to person. A classic study that is often quoted is from a research team that was using identical twins. The twins were given the same type of food and yet they had completely different responses to the exact same food.

A company called Zoe runs the largest scientific study of nutrition and personalised responses to food, with over 20,000 participants so far. By combining blood sugar analysis (using a CGM) with blood fat measurements, as well as technology that looks at the bacteria in your gut, Zoe claims to provide personalised advice to thousands.

Zoe has based much of its existence on a series of research studies called Personalised Responses to Dietary Composition Trial (PREDICT) studies, which are ongoing. These studies have measured the individual glucose, triglyceride and insulin responses of over 1,000 participants in response to identical meals. Findings from the first PREDICT trial have then been used to establish risk of inflammatory diseases for participants and best food recommendations for participants, which can be accessed on the Zoe app. While some of the recommendations that Zoe gives may be helpful (i.e., eating more whole foods and plants), the other nutrition advice regarding which foods are best for you are based on artificial intelligence, and while this emerging science is very interesting, many nutritional experts still believe the research is limited as it follows the lower carb, higher protein narrative for the majority. When you dig through the PREDICT study data, it seems the correlations they have found between how foods impact on people on a population level were not super strong, which calls into question how accurate and meaningful personalised nutrition monitors like Zoe are currently. Dietitians and nutritionists are already giving advice about the importance of eating more whole plant foods – even without Zoe.

Studies, however, do show that fitness trackers like Zoe motivate inactive people to make changes. However, for some, the novelty effect quickly wears off. In a long-term study looking at Fitbits, men and women who used their devices for over a year saw two main themes emerge. The first group dropped usage after three months and the second remained steady with use for at least six months.

One size doesn’t fit all but are CGMs the solution?^{12, 13, 14}

Whether a CGM or any technology works for you will depend on your goals, mindset and personality. With my clinical practice, I use these three questions to help me navigate the use of a wearable device for my clients:

1. What do you hope to learn from a wearable device?
2. How will you use the data?
3. Is there another way to get equally useful information?

 Top tips for managing blood glucose

• Don’t eat your carbohydrates naked. Dress them. Meals without protein, fat or fibre are rapidly broken down and absorbed into the blood stream. This produces a sharp rise in glucose, followed by a crash.
• Opt for whole foods or high fibre carbohydrates. Fibres slow how quickly the stomach empties, which has a knock-on effect on the rate of absorption of glucose in the blood stream.
• Start your day strong. A balanced breakfast (that contains high fibre carbohydrate and some protein) means you set your blood sugar levels on a steady path for the rest of the day.
• Remember, movement influences your blood glucose levels. Different people respond differently to exercise; however, in general, movement will lower your blood sugar levels. Fuel appropriately for exercise to prevent dips after. After a big meal, use a brisk walk to help rebalance blood sugar regulation.
• Alcohol affects your blood glucose. Alcohol interferes with the liver’s ability to release glucose into the blood stream when levels drop too low. This means you are likely to experience prolonged low blood sugar levels after a few drinks, which makes you crave ‘junk’ food.

Take-home message

Nutrition should be personalised to the individual. Measuring blood sugar responses is one of the key pillars of personalised nutrition. However, the reality is that this technology is still in its infancy when not used for managing diabetes. When it comes to the research, while some results may be interesting, we still have a long way to go before the claims that personalised nutrition companies are currently making are substantiated. If you do choose to use a CGM, keep in mind like most things that it is all about context. Many things affect blood glucose responses, including what and how much you eat.

Read more about evidence-based nutrition in Dr Linia Patel’s post on the FitPro blog: Sports Nutrition Myths Busted 

References

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