Processed food leads people
to eat more and put on weight, study finds
By health reporter Olivia Willis
It seems like research
that should surprise no-one: when people eat lots of highly-processed food,
they're more likely to gain weight.
Key points
·
Scientists compare calorie consumption and weight
gain in ultra-processed vs unprocessed diet
·
Even when matched for calories, people eat more
and gain weight when they're on an ultra-processed diet
·
Researchers say more research is needed to
understand mechanisms behind food intake
And yet nutrition scientists, who have long
suspected such foods are behind the ballooning obesity epidemic, were recently
surprised to make such a finding.
Why? Well, it turns out the usual suspects —
sugar, salt and fat — aren't solely to blame.
In a small study published in the scientific
journal Cell Metabolism, 20
people spent two weeks eating either a highly-processed or unprocessed diet,
before they swapped to spend two weeks eating the opposite diet.
Despite the two groups' meals and servings being
carefully matched, calorie for calorie, participants consumed more food and
gained weight while on the ultra-processed diet, said lead author Kevin Hall.
"I thought that if we matched the two diets
for components like sugars, fat, carbohydrates, protein and sodium, there
wouldn't be anything magical about the ultra-processed food that would cause
people to eat more," said Dr Hall, senior investigator at the US National
Institute of Diabetes, Digestive and Kidney Diseases.
"We found
that, in fact, people ate many more calories on the ultra-processed diet, and
this caused them to gain weight and body fat."
Studying dietary habits is complicated and often
limited by self-reporting, so it's been difficult for researchers to establish
a direct connection between highly-processed foods and obesity.
Although the study was relatively small, as well
as short, Dr Hall said it was the first to establish a clear cause-and-effect
relationship between processed foods, increased calorie consumption and weight
gain.
"Even more importantly, that causal
relationship didn't necessarily have to do with the nutrients that we always
suspected might be driving that relationship: salt, sugar and fat," he
said.
"It suggests there is something that we
still don't understand about ultra-processed food … that is driving a very
large effect on why people tend to overeat [it]."
The researchers enrolled 20 healthy volunteers
who were given three meals a day and had access to either ultra-processed or
unprocessed snacks, as well as water.
Every meal was carefully matched to a counterpart
meal (in the opposite diet) to ensure carbohydrate, fat, protein, sugar, fibre,
and sodium levels were equal, and the total calorie count was the same in both
meals.
A typical breakfast in the ultra-processed diet
consisted of Honey Nut Cheerios, whole milk with added fibre, a packaged
blueberry muffin, and margarine.
In the unprocessed diet, breakfast included a
parfait made with plain Greek yogurt, strawberries, bananas, walnuts, salt, and
olive oil, and apple slices with freshly-squeezed lemon.
The participants were told they could eat as much
(or as little) as they wanted, and the researchers measured how much they
consumed.
During two weeks on the ultra-processed diet,
participants ate an average of 508 more calories per day (that's about a
quarter of recommend daily consumption). They also gained, on average, almost
one kilogram.
In contrast, they lost the same amount of weight,
on average, during their two weeks on the unprocessed diet.
Importantly, participants reported that both
diets tasted good — so they weren't just eating less on the unprocessed diet
because they didn't like the food.
Tracy Burrows, an associate professor of
nutrition and dietetics at the University of Newcastle, said although the study
was "very novel", the weight differences observed were minimal.
"There's so many influences on somebody's
weight status, that 0.9kg is not very much," said Dr Burrows, who was not
involved in the research.
"Whilst it's reported as significant … the
actual amount is insignificant."
But Dr Hall disagreed, and said the duration of
the study was the reason more considerable weight change wasn't observed.
"The point is, when we look at the calorie
differences between what people are eating, those are persistently different
over the course of the month-long experiment, and they're substantial," he
said.
"You would expect if we were to run this
study out for three or four months, these weight differences would have
continued to accumulate."
Similarly, because the participants were healthy
and the testing period lasted only a month, the researchers didn't observe any
significant differences in other measures of health
Speed eating leads to over eating
As for why the study participants consumed more
food and ultimately gained weight while on the ultra-processed diet, Dr Hall
and his colleagues had several ideas.
They found that when people were on the
ultra-processed diet, they tended to eat faster — potentially not allowing
enough time for their body to signal to their brain that they were full.
"There may be
something about the textural or sensory properties of the food that made them
eat more quickly," Dr Hall said.
This, in turn, he said, could easily lead to
overeating.
Another factor that appeared to play a role in
higher calorie consumption was the higher calorie density of processed food.
"The solid foods were 85 per cent more
calorie dense in the ultra-processed diet compared to the unprocessed
diet," Dr Hall said.
"Given that they're smaller portions, if you
eat anywhere near the same number of grams of those foods, you will receive
naturally more calories."
Less clear, Dr Hall said, was the biological
changes they observed that might also contribute to changes in calorie
consumption.
"When people consumed the unprocessed diet,
for reasons that we don't fully understand, the levels of an appetite
suppressing hormone that's produced by the gut went up," he said.
"Similarly, a hormone that increases hunger
… went down during the unprocessed diet."
Dr Burrows said more research was needed to
support the findings of the trial and make them generalisable to the wider
population.
"It's a really exciting study that provides
preliminary investigation … but because it's a small study in an inpatient
setting, it has its limitations," she said.
Since the food was prepared for the participants,
it didn't take into account convenience and cost — two significant factors when
it comes to peoples' diets.
"We know there are a lot of factors that
contribute to why someone might choose an ultra-processed meal over and
unprocessed on," Dr Hall said.
"For people in lower socio-economic brackets
especially, we need to be mindful of the skills, equipment, knowledge, and
expense needed to create unprocessed meals."
Even while preparing the food for the study, the
researchers found the weekly cost of the ingredients for the unprocessed diet
was nearly 50 per cent higher: $US151 vs $US
Dr Burrows agreed, and said it was important not
to dismiss processed food altogether.
"This study is focused on ultra-processed
foods … but processed food can also mean raw foods that have just been
processed to make them more edible and consumable," she said, offering
Weet-Bix as an example of a processed food that could still be a healthy
choice.
"You can't eat wheat straight from the
field. They've just processed it so it's in a more edible format."
Dr Hall added that the study's findings raised
interesting questions about the role of ultra-processed foods in other diets.
"People seemed to have vehement arguments
about what diet is best for weight loss, whether it be low carb, keto, low fat,
or vegan."
Despite the differences between these diets, he
said, they all have one recommendation in common: to decrease consumption of
ultra-processed food.
"When you have a success story of somebody
on a low carb or low-fat diet, were they successful because they cut the carbs
or the fat, or was it because they reduced the ultra-processed food?"
