Context matters - why there's more to blood sugar control than food

More and more people without diabetes are using continuous glucose monitors (CGM) to understand their blood sugar levels. Many companies tout CGMs as a way to find the “right food for you” based on your blood sugar responses to specific foods. This is being marketed under the title of ‘precision nutrition’, the holy grail of the nutritional research world for over 20 years.

But does the science actually support this approach? Is there more to blood sugar control than food?

When we’re thinking about the effects of food on blood sugar responses we can look at the actual meal composition itself, such as the amount of carbohydrate, fat, protein and energy (calories). But it turns out that other factors matter as much, if not more than what you eat.

There are factors related to the individual, such as their metabolic fitness. An elite Tour de France cyclist will have a different blood sugar response than us mere mortals to the same food. But there’s also so-called meal context - shorter-term factors such as sleep, meal timing, recent physical activity and stress. The flagship “PREDICT” publication for personalised nutrition company Zoe, found that meal context caused as much variation in a person’s glucose response as the actual food itself¹!

In a provocatively titled research article awaiting publication - “Imprecision nutrition?”² - the idea that data from CGMs can inform our diet was challenged. They demonstrated that the same people, eating the same meal can have very different glucose responses from one day to the next.

When people wear a CGM, they might be tempted to avoid foods that cause a large or prolonged spike in glucose levels. They might preferentially eat more foods that don't cause big changes to their blood glucose. The problem with this approach is that, as this study showed, some meals which caused a high spike at one sitting had a much less pronounced effect the second time the meal was eaten. Similarly, meals that on one sitting produced a low glucose spike caused a much higher spike on the second sitting.

A recent study using the results of CGM data to create a personalised diet for people with prediabetes, which minimised the glucose peaks after eating, did not improve glucose control over a 6-month period compared to a generic low-fat diet³.

So exactly how does meal context affect our blood sugar levels?

The big question is how do you interpret your blood glucose data and how can you use it to improve your metabolic health?

Let’s say you eat a meal one day and you get a small rise in your blood glucose levels, maybe to around 6.5mmol/l which doesn’t last more than an hour. That meal goes in the list of things that are fine to eat and you think no more about it. Until you eat the exact same meal a week later and notice your blood glucose has gone up to 9.1mmol/l. What’s happened? What do you do about it and should you stop eating that food? All great questions. Let’s think this through, step-by-step.

Broadly speaking, the more simple sugars and digestible carbohydrates you eat, the larger a blood sugar response you can expect from your food. But what else you eat with those carbohydrates matters. For example, eating fat, fibre and protein with carbohydrate tends to slow absorption of sugars from the gut, reducing post-meal glucose peaks.

The order of what you eat matters too. Fats and protein before carbohydrates tend to reduce glucose peaks, whereas carbohydrates first will typically cause a higher peak. From a glucose control perspective, a salad with olive-oil dressing starter, a protein, carbohydrate and fat-based main course followed by a sugar and fat-rich dessert makes sense.

So pay attention to exactly what you’re eating and the order in which you’re eating it.

The ability of the body to manage glucose responses tends to be higher in the morning than other times of the day. Several studies have shown that when people eat the same meal in the morning and evening, it will take longer for blood sugar to go back to normal and the peak will be higher in the evening.

It’s important to pay attention to the timing of your meals. Eating close to when you’re going to sleep leads to both worse glucose control and worse sleep quality⁴. Ideally you should be leaving at least 3 hours between your last meal and going to sleep.

Multiple studies have confirmed that poor sleep quality and reduced quantity leads to higher glucose peaks after eating ⁵. Just 4 days of sleeping 4.5 hours is enough to cause insulin resistance in young otherwise fit individuals⁶. We also know that poor sleep makes us eat more and choose high-calorie comfort foods.

Cortisol is one of our main “stress” hormones and tends to be increased in periods of sustained high stress. Difficulties at work, challenging relationships and feeling overwhelmed with day-to-day demands are all likely to result in raised cortisol. This raises blood glucose levels by telling the body to create more glucose, which in turn causes raised insulin levels and stops the body breaking down fat⁷.

It’s important to take your stress levels into account when you’re looking at your CGM data. If you’ve eaten something that you thought should be healthy and you’ve noticed a big glucose spike, is it the food or could it be what else is going on in your life?

When you’ve had an unexpected glucose spike, think about what exercise you’ve done around the time of the meal. We went into this in some detail in our recent article on insulin resistance, but exercise makes your muscles more responsive to insulin and better able to dispose of glucose after eating.

The more you exercise the better your blood sugar control will be. Have you had a few days off? Did you have a day of not leaving the house, or even the chair!? We now have good evidence that recent exercise will tend to cause lower and less prolonged glucose peaks after eating⁸.

Remember though that very vigorous exercise can cause a temporary spike in blood glucose due to the breakdown of glycogen into glucose in your liver. This doesn’t mean you shouldn’t exercise but just be wary of basing dietary changes on glucose spikes without taking exercise into account.

CGMs are an incredibly useful tool when used in the right way. But using them as a way to decide whether a food is “good” or “bad” for you, especially on a one-off basis is not hugely helpful. Do you really need CGM data to tell you that cakes, biscuits and Coke are bad for your blood sugar?

There’s also way more to health than just your glucose readings. A bowl of oats and fruit for breakfast will probably lead to a higher glucose spike than a plate of bacon, sausage and egg but do you really think that makes for a healthy daily breakfast? Of course not. Don’t abandon common sense in favour of glucose data. Doing so may prompt you to avoid foods which have other health benefits.

Our advice? If you want to use CGM data to inform your diet, ask yourself the following questions:

• Am I well rested, have I been sleeping well the last few days?
• Am I feeling calm and in control?
• Have I been doing my normal amount of exercise in the last week?
• Am I eating this food at the time I normally would?
• Is this a food that I know has no other health benefits?

If the answer is yes to all of these questions, particularly if you’ve noticed glucose spikes more than once, then maybe it’s a good idea to minimise that food's presence in your life.

The last question in particular, is important. Many people notice that they have a glucose spike after eating foods with health-promoting properties like fruit. This is normal, fruits contain fructose. But they also contain fibre, vitamins and polyphenols all of which have proven health benefits and fruits have been shown to reduce your risk of developing type 2 diabetes⁹. But the form of fruit matters. Fresh fruit is a much better option than dried fruits and fruit juices.

Wearing a CGM can tell you a lot about your metabolic health. It can give you helpful insights into your diet and will likely reinforce what you already know, i.e too many cakes, biscuits and crisps are bad for you. It might provide surprising insights into your dietary patterns that for some people prompts radical change.

But there’s so much more to your blood sugar responses than just what you eat. We would urge you to really think about the rest of your life before cutting out too many types of food. In particular, it’s important to think twice before cutting out foods with very clear health benefits like fruit.

If you do have significant glucose spikes after eating, it’s probably better to treat it as a sign that you need to change your “metabolic engine” rather than fuel that you’re putting in it. In the short-term, you can improve your glucose responses by optimising sleep, stress management and increasing movement. In the longer-term, the way to improve glucose responses is by improving your metabolism, largely through targeted aerobic and strength training.