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.

I feel like we owe it to Americans to be smarter than to just say ‘eat less.’

“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.