"Researchers find a western-style diet can impair brain function," reports The Guardian.
An experiment on 110 students at an Australian university found that those asked to eat Belgian waffles and fast food performed worse on learning and memory tests after a week than those who ate their regular diet.
The students eating fast food were also more likely to want to eat sugary breakfast cereal immediately after eating a full breakfast, than they were before they began the week-long experiment.
The researchers say both outcomes may be a result of changes to the hippocampus (a region of the brain associated with memory). They say the hippocampus usually suppresses memories of how good food tastes when we become full, so we do not eat more than we need.
However, the researchers consider that eating a western-style diet may impair the ability of the hippocampus to do this job. That would mean people continue to crave tasty food even when they're not hungry. This might explain why it's difficult for people who often eat fast food to resist the temptation to eat fatty or sugary foods. But it's still only a hypothesis.
The findings from the memory tests are also not too convincing. Those who ate fast food scored 92% on a memory recall test at 1 week compared with 97% in controls. Whether this would make a meaningful difference in everyday life is unclear. There was no longer a difference at one month. The study also used a small sample of people, and the findings may not be the same in another group.
Nevertheless, the findings support the general understanding that a diet high in saturated fat, sugar and salt is not good for health.
Find out more about eating a healthy, balanced diet.
Where did the story come from?
The researchers who carried out the study were from Macquarie University and Griffith University in Australia, Yale University and American University in the US, and the University of Sussex in the UK.
The Guardian sets out a balanced and accurate report of the study. However, Mail Online overstates the certainty of the results, saying that "the hippocampus normally stops us from gorging on more food when we are full by suppressing memories of how tasty it is." However, that's just one hypothesis for how the system works and the study does not prove it's correct.
What kind of research was this?
This was a randomised controlled trial. This is usually the best type of study to measure the effect of a treatment or other intervention, such as diet. However, to give the best results, trials usually need to include a large number of people and follow up results over a reasonable period of time.
The researchers wanted to find out the effect of a western-style diet, which is high in sugar and fat, on people's cognitive abilities and their ability to control appetite. However, they only studied 110 people who were asked to follow a diet for 1 week and were followed up a maximum of 1 month later. So, the results are never going to provide strong evidence.
What did the research involve?
Researchers recruited 110 volunteer students aged 20 to 25 from Macquarie University in Australia. The students were all a healthy weight and ate a healthier-than-average diet. There were slightly more women than men in the study.
People who were vegetarian or vegan, dieting, pregnant or had a range of illnesses or were taking prescription medicines were excluded from the study. Participants were randomly assigned to either a "western-style" diet group or a control group.
Those allocated to the western-style diet group were given a breakfast which included a toasted sandwich and a milkshake, high in fat and sugar, on the 1st day of the study.
Between days 2 to 7 they were asked to eat 2 Belgian waffles (provided by the researchers) on 4 days, and to eat a main meal and a dessert or drink from a fast food chain on the other 2 days. They were given cash to buy the food and asked to provide receipts, and photos of themselves eating the waffles.
Those allocated to the control group were given a breakfast of a toasted sandwich and a low-fat, low-sugar milkshake on the 1st day of the study. For the rest of the study they were asked to follow their normal diet.
On the 1st day, researchers asked all the students to not eat anything before arriving at the centre. The students took tests including:
- the Hopkins verbal learning test (HVLT) of learning and memory (recall) skills
- a "wanting and liking test" in which they were shown a range of snack foods and asked to rate how much they wanted to eat them, and then asked to eat them and say how much they liked them
The wanting and liking test, which involved sugary breakfast cereals and toast with a range of toppings, was taken before and immediately after the students ate breakfast. This was intended as a test of appetite control. People who feel full (for example, immediately after breakfast) may still like a food, but are less likely to want to eat it.
The breakfast and test routine was repeated at the end of the study (day 8). The learning and memory test was repeated after a further 3 weeks (day 29).
Researchers also asked people to record their food intake in diaries, and took measures of blood sugar and cholesterol.
They compared the results of the tests between the groups, before and after eating breakfast, and on day 1 and day 8. They looked to see whether changes in results for appetite were linked to changes in results for learning and memory tests.
What were the basic results?
At the start of the study, the students from the western-style diet and the control group performed about the same on the learning and memory and the liking and wanting test. Unsurprisingly, students in both groups were less likely to want to eat immediately after eating breakfast, compared to before breakfast.
Learning and memory test results (based on delayed retention of words):
- 93.1% on day 1, 92% on day 8 and 92.4% on day 29 for those on the western-style diet
- 93.8% on day 1, 97% on day 8 and 94.2% on day 29 for those in the control group
The researchers said the difference between groups on day 8 represented a "small to intermediate" effect size. However, there was no longer any significant difference between groups at follow-up on day 29.
The students on the western-style diet also showed changes in their wanting and liking test. While their pre-breakfast results were similar to the start of the study, after breakfast there was less of a gap between how much they liked the food and how much they wanted to eat it. Breakfast did not seem to have affected their desire to eat food as much as it did for the control group.
When they looked at the results of the wanting and liking tests, researchers found that the changes from start to end of the study mirrored the changes in the learning and memory results. In other words, learning and memory decline in performance mapped against lessening of appetite control.
How did the researchers interpret the results?
The researchers say their results "demonstrate that a western-style diet can rapidly impair appetitive control in humans – an effect that could promote overeating in consumers of a western-style diet. The study also suggests a functional role for the hippocampus in appetitive control."
The headlines suggesting that eating a western-style diet for 1 week could damage the brain are overstating the findings of this study. The researchers did not look at the brain, but at tests of learning and memory, and any effects appeared to have reversed by the end of the study.
Looking at these learning and memory tests, the study found those following the western-style diet scored a few percentage points lower at 1 week. However, the results show both groups varied over time, at baseline, 1 week and follow-up. It's possible that the difference between the groups at day 8 could have come about by chance, although the researchers say it was a significant difference. If it was a true difference, there were still many unknowns.
We do not know:
- if the participants would have noticed a difference themselves from this small score difference – for example, in their everyday lives
- if they would have continued to deteriorate in score, had they continued to follow the diet
- if a difference may have been seen had they used other cognitive tests, rather than just the HVLT test
- whether the same results would be seen in other population samples, rather than just this small sample of students in Australia
The changes to the wanting and liking test do not directly prove that students who had higher "wanting" scores after eating breakfast would have aten more. The test only looks at how much people are motivated to eat, not whether they do so. The researchers say the wanting and liking test has not been tried to see whether it translates directly into eating more.
Setting these concerns aside, the study does reinforce the point that a healthier diet is known to have many benefits for health and wellbeing, and this may include brain function. The western-style diet the researchers talk about includes a lot of highly processed food, with high levels of salt, sugar and saturated fat. Eating too much of these types of foods, and too often, is likely to increase your chances of becoming overweight and getting conditions such as diabetes and heart disease.
Some experts also argue that western-style diets could become addictive, saying the more a person eats of these types of foods, the more they want to eat them.
A healthier diet includes:
- at least 5 servings of fruit and vegetables each day,
- pulses and wholegrains and food with plenty of fibre
- less sugar, salt and saturated fat
Find out more about healthy eating.
Analysis by Bazian
Edited by NHS Website
Links to the headlines
The Guardian, 19 February 2020
Mail Online, 19 February 2020
Links to the science
Royal Society Open Science. Published online 19 February 2020