Hungry all of the time? It could be in your blood

Researchers working on the largest in-depth nutrition study in the world have found that some of us experience big dips in blood sugar levels after eating, and it makes us hungrier, sooner. We spoke to nutritional scientist Dr Sarah Berry, who worked on the PREDICT programme, to find out more.

Was there a particular reason that breakfast was the meal you chose to focus on for this study?

For the PREDICT study we had two components. One was a very tightly controlled clinic day where people were attending our clinic in the UK at St Thomas’s Hospital, or the clinic in the USA at Massachusetts General Hospital in Boston. And we gave them what we call a ‘metabolic challenge meal’ – a standardised, high-fat, high-carb meal for breakfast, in the form of a muffin and a milkshake. Then we gave them a standardised lunch to really stress their systems so we could start to differentiate people’s responses.

Then they entered a two-week at-home phase where they were fitted with various wearable technology, such as a continuous glucose monitor. During the at-home phase we provided them with standardised breakfasts with different macronutrient compositions for the whole two weeks. They differed each day in terms of the fat, protein, carbohydrate and fibre.

By giving it to them as a breakfast, it meant that everyone came to them standardised, as they’d all fasted overnight. We asked them to avoid exercise because we know exercise can modulate your glycemic response. Then, after three hours they were allowed to eat freely, whichever meals they wanted, and we call this ad libitum. This way we could also look in a very real-life context, as well as in this tightly controlled context.

What did you find?

The key finding from our overall PREDICT programme was that there’s huge variability in how people respond to food. We know that humans are very complicated in how we metabolise food, and we know that food is very complicated, in terms of the thousands of chemicals in it and the complexity of their structure.

And so, because of that, what we saw was that there was a nearly 10-fold difference in how I might respond versus how you might respond to exactly the same food in a tightly controlled setting.

What we also saw was that it wasn’t all about what we ate that determines how we respond to foods, and it also wasn’t all in our genes. About 70 per cent of our participants were identical twins so we could tease apart the genetic contribution to this variability in responses versus the non-genetic.

And we found that genetics actually played a small role and that wasn’t what was really important. Yes, what we ate was obviously important, but equally important was how we ate it – the time of day, when we took our exercise, how much sleep we’d had the night before, and the preceding meal as well.

Because we used continuous glucose monitoring from a sensor that the participants wore on their arm, we were able to move beyond looking at typical features of the glucose response after a meal. What we found was that there were some people that in the two to four hours after consuming a meal had quite a dip in their glucose below their normal baseline level and some people who didn’t.

Read more about diets:

• Dr Michael Mosley: What to eat to beat stress and improve your mood
• A plant-based diet may lower severity of COVID-19 infection by 73 per cent

We found that if we separated people according to whether they were ‘big dippers’ or ‘little dippers’, it impacted how hungry they were. People that have bigger dips were more hungry. On average they consumed their next meal about 30 minutes before people that were little dippers. They also consumed more calories at their next meal.

What I find particularly interesting as a nutritionist is that people who had a bigger dip three to four hours after eating had consumed about 300 more calories over a 24-hour period compared to little dippers.

I think this is really important because what we know from many years of research is that, yes, you might consume less at a subsequent meal if you’ve have a particularly satiating meal, but typically over a 24-hour period it kind of balances itself out.

And we also found that it impacted their alertness. So you had this unfavourable impact that they were consuming more calories if they were dippers, but they also reported less alertness as well.

Were there any trends in the big dippers?

No. We did find that males had slightly larger dips than females. But we found that there was huge variability between individuals just in line with all of our other PREDICT findings. We didn’t see that there was a particular trend in terms of age or BMI, etc.

What can I do if I’m a big dipper?

There are two factors at play here: one, that some people are more prone to dipping than others, which is down to their unique metabolism and biology. And two, that there’ll be some foods that might cause you to have a dip in your glucose, but not cause me to have a dip in mine. And the only way to ascertain that at the moment is to wear a continuous glucose monitor, and to play around and try different foods and see what causes big peaks and dips in your glucose response.

There’s also Zoe, the commercial product from the tech company that has been funding this research, which is a personalised nutrition programme. It enables you to monitor your blood glucose, microbiome and other variables that we know are really important in determining how you respond to food.

How does this research fit into the idea of personalised nutrition?

We’re really starting to understand that we, as individuals, are very complicated, that we have thousands of biological pathways that interact to determine how we respond to food. Likewise, a food has thousands of chemicals, so each and every one of us responds differently.

We’re starting to understand that a ‘one size fits all’ approach to guiding people on what to eat doesn’t work. We know that dietary-related illnesses are a huge problem worldwide, despite detailed guidance on what we should be eating. But is this because the guidance isn’t suitable at that personalised level, or is it that we’re not following it?

In healthy-eating guidelines, there’s huge scope to personalise what we eat according to what’s best for our unique biology. This is really where the field of personalised nutrition is exploding. We’re starting to understand this complexity and, more importantly, we’re starting to be able to measure it.

Until 5 or 10 years ago, we couldn’t. But now with the development of these novel, wearable technologies and the possibility of doing remote home-testing, we’re in a position where we can start to monitor, at an individual level, how we respond to food.

Measuring just your genetics, just your microbiome or just your glucose response has its limitations. And it’s important that we look at the many interrelated factors that determine how we respond to food.

This is exactly what the Zoe personal nutrition product is doing. And I think this is what readers really need to be mindful of, if they’re interested in finding out about their personal responses to food. They need to be looking at the many different factors that impact how they respond to food. They need to be very cautious when undertaking, for example, a genetic-only test because from our research we know that only plays a small role.

• This article first appeared in issue 363 of BBC Science Focus Magazine–find out how to subscribe here