Personalized diets are promising, but the science isn’t all there yet

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Microbiologist Lora Hooper wishes she
had a good answer when her mother asks, “What should I eat?”

Hooper could rely on a familiar refrain.
Eat plenty of fruits, vegetables and whole grains, and limit meat and fat
intake. Try to eat foods low on the glycemic index, a measure of how high a
particular food is likely to send a person’s blood sugar after eating it.

Nutrition recommendations have focused
on properties of food, debating whether focusing on calorie counts,
carbohydrates, fats or proteins might be more important. But more studies are
showing that people’s bodies can react very differently to the same foods, and
standardized nutrition advice doesn’t fit everybody. Even identical twins can
have varying responses to identical foods, new research finds, suggesting that the
variety can’t be explained by genes alone.

With genetics being put on the back burner,
researchers are searching for other explanations for why a diet one person
swears by may cause another to gain weight. One big player may be the friendly
bacteria and other microbes in people’s guts.

“Your microbiota really determines how
many calories you take up from your food,” says Hooper, of the University of
Texas Southwestern Medical Center in Dallas. Without a better understanding of
how gut microbes will react, she says, “I don’t think I can read the number of
calories in my food off a box.”

So instead of focusing on the food,
people like Hooper’s mother may have to look within to their own gut microbes or
other personal qualities to find the diet that works best for them, an approach
known as personalized nutrition. But tailoring food regimens to individuals isn’t
likely be a piece of cake, either.

Finding the perfect diet is more
important than ever as epidemics of obesity and type 2 diabetes sweep the
globe. Worldwide, more than 1.9 billion adults and 380 million children and
adolescents — about a quarter of the world’s population — were overweight in
2016, with 650 million of those people obese, according to the World Health
Organization. An estimated 422 million people had diabetes in 2014, and rates
of the disease have continued to grow.  

diet guidelines example
Dietary guidelines may not be tailored enough to individual metabolism, some scientists say.USDA’s Center for Nutrition Policy and Promotion

Follow the guidelines?

Strive as they might to follow healthy
diets, people may be thwarted by what they don’t know about their own response
to food. Geneticist Tim Spector thought the small sandwich and glass of orange
juice he typically bought in the hospital cafeteria was a healthy option. Then he
discovered that both bread and orange juice send his blood sugar levels soaring
into the diabetic range.

And “I get much worse sugar response to
a banana than to an equivalent size of an apple,” says Spector, of King’s
College London. He now chooses apples and pears over bananas. “It’s no hardship,”
he says, “because I like both.”

Standard advice to eat fruits and
vegetables, however, would never clue Spector in to which choice is truly
healthier for him. He is not alone. Previous studies have determined that some
people’s blood sugar may shoot up more after eating a banana than a
cookie
, while another person may have the opposite
reaction
(SN: 11/19/15).

Spector and others have confirmed the
varied landscape of blood sugar peaks and valleys that people achieve after
eating standardized foods full of carbohydrates. In the short term, high blood
sugar spikes might produce headaches and fatigue. Over the long haul, high
blood sugar — the hallmark of diabetes — can lead to damage of blood vessels,
nerves and organs.

It’s not just carbohydrates that produce
a variety of responses. People respond differently
to dietary fats
, too, Spector and colleagues discovered in a new study tracking foods’ effects
on more than 1,000 people
, including hundreds of identical twins. Spector
presented the preliminary, unpublished results June 11 in Baltimore at the American
Society of Nutrition’s annual meeting.

Researchers in the PREDICT I study gave
volunteers standardized foods and tracked participants’ blood levels of the sugar
glucose, insulin and dietary fats called triglycerides. How quickly people
cleared the fats from their blood after a meal couldn’t be predicted by knowing
what happened to their blood sugar or insulin levels after a meal, the team
found.

But perhaps the biggest surprise from
the study was that even identical twins process food differently.

Follow the genes?

Identical twins Julie Hodgson and Diane Portlock look like
each other. Except Portlock, a firefighter in Worcester, England, weighs about
50 pounds more than her twin. The 46-year-old sisters always put their weight
difference down to lifestyle. After all, Hodgson, of Ash Vale, England, is a
competitive rower in the British Army who took a gold medal in the United
Kingdom Invictus Games trials in July. She exercises more than her sister does
and tends to go for salads, while Portlock admits “I like cheese. I like bread.
I like a glass of wine.”

But now the sisters have discovered that their bodies handle
food differently, too. In an extension of Spector’s study, Hodgson’s blood
sugar after a meal would usually rise gradually then taper off “in a nice
little rounded arch,” her sister says. But Portlock’s glucose levels spiked and
quickly fell and then rose again in what she calls “a double dip.”

“My insulin response is really quite
efficient,” Hodgson says. Her body handles carbohydrates well, and eating more
carbs fills her up and gives her more energy to train. But not all carbs work
for her. Spaghetti Bolognese is Hodgson’s worst food for triggering a glucose
spike, even though pasta is considered a low glycemic index food. Now, she
avoids pasta and eats other carbs, including mashed potatoes, her twin’s
nemesis for causing blood sugar spikes. 

Julie Hodgson and Diane Portlock
Julie Hodgson (left) and Diane Portlock (right) are identical twins who recently discovered that they have different reactions to certain foods from each other.Julie Hodgson, Diane Portlock

“Although our DNA and our genetics are the same, the study
has proven that, in fact, we’re very individual as well,” Portlock says.

Genes explain less than half of an individual’s varying
responses to food, Spector and colleagues calculate. That’s bad news for people
who shell out money for consumer
DNA testing companies to tell them what to eat
(SN: 5/22/18), Spector says. “Genes are going to be limited when it
comes to predicting food responses.”

What’s more, the macronutrient content of the food — the
amount of carbs, fat and protein — accounted for only 16 to 32 percent of
varying responses, the researchers found. The rest is still a mystery, and
could relate to a litany of causes. Medications, the amount and quality of
sleep people got, how much they exercised, when people ate and what order they
ate foods in, their overall health and biological rhythms as well as the
microbes living in the volunteers’ guts might all influence their reactions,
Spector says. His team is recruiting
volunteers
for an even larger study aimed at better understanding the
factors that influence metabolism.

Follow the microbes?

Gut microbes are probably the most
important factor, says immunologist and microbiome researcher Eran Elinav of
the Weizmann Institute of Science in Rehovot, Israel. Past research has shown
that microbes determine how much fiber and other complex carbohydrates get
digested. Bacteria in the small intestines may influence how
much fat
people absorb from their food, a study in mice suggests (SN: 7/25/19).

Elinav, Weizmann colleague Eran Segal and other
collaborators have found that gut microbes play a big role in controlling how
much a person’s blood sugar will spike after eating
white or sourdough bread
(SN: 6/6/17)
or a wide variety of other foods.

The researchers have cataloged hundreds
of gut microbiomes — the collection of genes in bacteria, archaea, fungi and
other microbes that live in the intestines — responding to thousands of meals. By
deeply analyzing, or sequencing, the genes of microbes in a person’s stool
samples, and comparing the results with those from people previously studied,
Elinav says he can tell what will happen to someone’s blood sugar after eating
a specific food. Elinav and Segal are scientific consultants to a company
called DayTwo, in Adanim, Israel, that’s based on the researchers’ work.

In February, scientists at DayTwo and
the Mayo Clinic reported in JAMA Network
Open
that they had more
accurately predicted volunteers’ glycemic responses
to eating a bagel and
cream cheese when the team based the prediction on each volunteer’s microbiome
and previous response to the meal than when predicting blood sugars based on
counting carbohydrates.

But here’s where things get sticky,
because what people eat also affects the microbes in the gut in very personal
ways.

“What you eat does determine which
microorganisms are in your gut. There’s no question about that,” Hooper says. But
exactly what about that food influences microbiome changes is a mystery.

It at least seems that microbes
don’t count carbs
, nutrition scientist Abigail Johnson of the University of
Minnesota in Minneapolis and colleagues reported in the June 12 Cell Host & Microbe. Johnson’s colleagues
— 34 students enrolled in a citizen science class at the university — were also
the study’s research subjects. The students kept detailed food records and
collected fecal samples every day for two and half weeks. The team cataloged
the abundance of types of bacteria in each person’s stool samples and tried to
determine which properties of food determined how the microbe mixes would
change.   

“We couldn’t look at nutrients and find
relationships with nutrients and the microbiome,” Johnson says. But looking at
foods eaten over the previous days predicted how the microbiome would change. For
instance, knowing that a food was spaghetti with tomato sauce and meat told the
researchers more about how the microbiome would change than knowing the
carbohydrate, fat and protein content of the food did. Microbes may be more
concerned with trace nutrients or chemical components of food that aren’t
included on labels, Johnson says.

She thinks that the microbiome, at least
the microbes in feces, probably aren’t the major driver of how a person
responds to food. “It’s a player certainly,” but not the whole story, Johnson
says.

To personalize or not to personalize

Other researchers caution against being too quick to discount the properties of the foods themselves. “The glycemic index is still an important worm in this can of worms,” says Jennie Brand-Miller, a nutrition scientist at the University of Sydney.

That’s in part because people’s variability to different
foods is already baked into the glycemic index, Brand-Miller says. She likens
the glycemic index to the tides. Although tides may vary from bay to bay, and
season to season, “On any given day high tide will be higher than low tide.”
Similarly, she says, “on any given day we have a 99 percent chance that high GI
food will give a higher response than a low GI food.”

For most people, she argues, following standard nutritional
advice will help them get and stay healthy.

But, Elinav says, nutritionists tend to
place the blame for the failure of nutritional guidelines to stem the tide of
obesity and diabetes on people not following the guidelines. “Rather than
blaming the public for not following these recommendations, which often change,
maybe the guidelines themselves are not sufficient or not evidence-based
enough,” he says.

Personalizing nutrition may do better
than the one-size-fits-all guidelines for controlling blood sugar, Elinav says.
He and colleagues are pitting the conventional approach and their microbiome
approach against each other in a head-to-head trial. The results may be known
later this year.

Still, there’s probably no harm in
eating within the standard guidelines. “I don’t think we’re going to get 20
years down this line of personalized nutrition research and find out fruits and
vegetables are bad,” Johnson says. But in the future, people may be able to
optimize those recommendations for themselves by adding or avoiding certain
foods.

Until then, when Hooper’s mother asks what to eat, she can only reply, “Mom, I don’t know.” 

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