Low-fat or low-carb? It’s a draw, study finds
Stanford researchers have found that, contrary to
previous studies, insulin levels and a specific genotype pattern don’t predict
weight-loss success.
New evidence from a study at the Stanford University
School of Medicine might dismay those who have chosen sides in the low-fat
versus low-carb diet debate.
Neither option is superior: Cutting either carbs or
fats shaves off excess weight in about the same proportion, according to the
study. What’s more, the study inquired whether insulin levels or a specific
genotype pattern could predict an individual’s success on either diet. The
answer, in both cases, was no.
“We’ve all heard stories of a friend who went on one
diet — it worked great — and then another friend tried the same diet, and it
didn’t work at all,” said Christopher Gardner, PhD, professor of medicine and
the lead author of the study. “It’s because we’re all very different, and we’re
just starting to understand the reasons for this diversity. Maybe we shouldn’t
be asking what’s the best diet, but what’s the best diet for whom?”
Past research has shown that a range of factors,
including genetics, insulin levels (which helps regulate glucose in the body)
and the microbiome, might tip the scales when it comes to weight loss. The new
study, published Feb. 20 in JAMA, homed in on genetics and insulin, seeking to
discover if these nuances of biology would encourage an individual’s body to
favor a low-carbohydrate diet or a low-fat diet. The senior authors of the
study are Gardner; Abby King, PhD, professor of health research and policy and
of medicine; Manisha Desai, PhD, professor of medicine and of biomedical data
science; and John Ioannidis, MD, DSc, professor of medicine.
A TALE OF TWO DIETS
In his quest to find out if individual biological
factors dictate weight loss, Gardner recruited 609 participants between the
ages of 18 and 50. About half were men and half were women. All were randomized
into one of two dietary groups: low-carbohydrate or low-fat. Each group was
instructed to maintain their diet for one year. (By the end of that year, about
20 percent of participants had dropped out of the study, due to outside
circumstances, Gardner noted.)
Individuals participated in two pre-study activities,
the results of which were later tested as predictors of weight loss.
Participants got part of their genome sequenced, allowing scientists to look
for specific gene patterns associated with producing proteins that modify
carbohydrate or fat metabolism. Then, participants took a baseline insulin
test, in which they drank a shot of glucose (think corn syrup) on an empty
stomach, and researchers measured their bodies’ insulin outputs.
In the initial eight weeks of the study, participants
were told to limit their daily carbohydrate or fat intake to just 20 grams,
which is about what can be found in a 1½ slices of whole wheat bread or in a
generous handful of nuts, respectively. After the second month, Gardner’s team
instructed the groups to make incremental small adjustments as needed, adding
back 5-15 grams of fat or carbs gradually, aiming to reach a balance they
believed they could maintain for the rest of their lives. At the end of the 12
months, those on a low-fat diet reported a daily average fat intake of 57
grams; those on low-carb ingested about 132 grams of carbohydrates per day.
Those statistics pleased Gardner, given that average fat consumption for the
participants before the study started was around 87 grams a day, and average
carbohydrate intake was about 247 grams.
What’s key, Gardner said, was emphasizing that these
were healthy low-fat and low-carb diets: A soda might be low-fat, but it’s
certainly not healthy. Lard may be low-carb, but an avocado would be healthier.
“We made sure to tell everybody, regardless of which diet they were on, to go
to the farmer’s market, and don’t buy processed convenience food crap. Also, we
advised them to diet in a way that didn’t make them feel hungry or deprived —
otherwise it’s hard to maintain the diet in the long run,” said Gardner, who
holds the Rehnborg Farquhar Professorship. “We wanted them to choose a low-fat
or low-carb diet plan that they could potentially follow forever, rather than a
diet that they’d drop when the study ended.”
CONTINUING TO MINE THE DATA
Over the 12-month period, researchers tracked the
progress of participants, logging information about weight, body composition,
baseline insulin levels and how many grams of fat or carbohydrate they consumed
daily. By the end of the study, individuals in the two groups had lost, on
average, 13 pounds. There was still, however, immense weight loss variability
among them; some dropped upward of 60 pounds, while others gained close to 15
or 20. But, contrary to the study hypotheses, Gardner found no associations
between the genotype pattern or baseline insulin levels and a propensity to
succeed on either diet.
“This study closes the door on some questions — but it
opens the door to others. We have gobs of data that we can use in secondary,
exploratory studies,” he said. Gardner and his team are continuing to delve
into their databanks, now asking if the microbiome, epigenetics or a different
gene expression pattern can clue them in to why there’s such drastic
variability between dieting individuals.
Perhaps the biggest takeaway from this study, Gardner
said, is that the fundamental strategy for losing weight with either a low-fat
or a low-carb approach is similar. Eat less sugar, less refined flour and as
many vegetables as possible. Go for whole foods, whether that is a wheatberry
salad or grass-fed beef. “On both sides, we heard from people who had lost the
most weight that we had helped them change their relationship to food, and that
now they were more thoughtful about how they ate,” said Gardner.
Moving forward, he and his team will continue to
analyze the reams of data collected during the yearlong study, and they hope to
partner with scientists across Stanford to uncover keys to individual weight
loss.
“I’m hoping that we can come up with signatures of
sorts,” he said. “I feel like we owe it to Americans to be smarter than to just
say ‘eat less.’ I still think there is an opportunity to discover some
personalization to it — now we just need to work on tying the pieces together.”
The study’s other Stanford co-authors are postdoctoral
scholars John Trepanowski, PhD, and Michelle Hauser MD; research fellow Liana
Del Gobbo; and senior biostatistician, Joseph Rigdon, PhD.
Gardner, Desai and Ioannidis are members of the
Stanford Cancer Institute. Gardner and Ioannidis are members of the Stanford
Cardiovascular Institute. Gardner and Desai are members of the Stanford Child
Health Research Institute. Gardner is a member of Stanford ChEM-H. Ioannidis is
a member of Stanford Bio-X.
The study was funded by the National Institutes of
Health (grants 1R01DK091831, T32HL007034 and 1K12GM088033), the Nutrition
Science Initiative and Stanford’s Clinical and Translational Science Award
(grant UL1TR001085).
Stanford’s departments of Medicine and of Health
Research and Policy also supported the work.
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