Often, when we try to shed excess weight, our bodies decline to cooperate.

Deep-rooted biological mechanisms partially explain why we often regain lost pounds.

People who lose significant weight tend to regain one-third to one-half of the lost pounds within a year of their initial success — typically, much faster than that.

Meanwhile, a debate has raged about which diet is best for losing weight and keeping it off: low-carb or low-fat.

Now, a clinical trial's startling, encouraging findings may have rendered that debate irrelevant and obsolete.

The trial's results suggest that the nature of the foods we eat matters more to weight control, versus calorie-counting or a diet’s proportions of fat or carbs.

Stanford study sought the key to healthy weight control
A scientific team from the Stanford University School of Medicine recruited 609 people between the ages of 18 and 50 — about half men and half women — for a 12-month clinical study (Gardner CD et al. 2018).

The participants were randomly assigned to one of two diet groups:

  • Low-fat — cut back on cooking oils, fatty meats, whole-fat dairy foods, and nuts.
  • Low-carbohydrate — cut back on cereals, grains, breads, pastries, starchy vegetables (e.g., potatoes), and beans.

Both groups were told to adjust their diets gradually, until they reached the lowest level of carb or fat intake they felt comfortable with and could maintain indefinitely.

Critically, all the participants were instructed to follow these guidelines:

  • Avoid processed, nutrient-poor fare.
  • Minimize added sugars, refined flours, and trans fats.
  • Focus on fresh, whole, minimally processed, nutrient-dense foods — prepared at home whenever possible — and maximize vegetable intake.

As lead author Christopher Gardner, Ph.D., said, “We made sure to tell everybody 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.”

Over the 12-month period, researchers measured the participants' weight, body composition, insulin levels, and blood lipid (fat and cholesterol) profiles, and periodically surveyed the volunteers to monitor the amounts of fat or carbohydrates consumed daily.

The participants were encouraged to follow official U.S. guidelines for physical activity, while dietitians held 22 classes in which they taught the volunteers about food quality and gave them behavior modification techniques to help them stay on track.

Compliance with the assigned diets was assessed by periodic diet surveys and by measuring the participants' blood lipids, respiratory exchange ratio, and other indicators of diet composition — and the results indicated a remarkably high degree of adherence.

Whole foods produced weight loss, regardless of a diet's carb-fat profile
After 12 months, all the participants enjoyed the same average weight loss — 13 pounds — regardless of whether they’d followed a low-fat or low-carb diet.

Likewise, average calorie intakes among the participants in both groups ended up being closely comparable by the end of the study.

Despite the identical average weight loss achieved in both groups, there was substantial variability: some participants dropped more than 60 pounds while others gained as many as 20 pounds.

In other words, the strategy of favoring vegetables and whole foods — without counting calories or paring back portion sizes — resulted in similar average weight loss, regardless of whether people were assigned to cut back on carbs or fat.

Dr. Gardner believes the biggest takeaway is a simple strategy for losing weight: eat less sugar and refined flour, eat lots of vegetables, and favor whole foods in general, whether that’s a wheatberry salad or grass-fed beef.

And, as he said, “On both sides, we heard from the people who'd 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.”

Trial looked for links between genes, insulin, and diet success
Past research has suggested that diets based on each person’s individual genetics, insulin responses, and gut microbiome makeup might improve success at weight loss and control — an idea lacking hard proof, but heavily promoted by companies that sell testing programs.

So, the participants had part of their genome sequenced, so the scientists could look for genetic patterns previously linked to changes in carbohydrate or fat metabolism.

In addition, the participants underwent a test of their insulin response to sugar.

The purpose of the genetic and insulin-response tests was to see if those factors would predict whether a low-fat or low-carb diet would produce more weight loss in people with specific characteristics.

But in the end, neither diet group's success at weight loss could be tied to their genetics or their insulin-response to sugar.

This outcome clearly undermines claims that — to achieve optimal weight loss and control — people should eat diets that "match" their gene profile and/or response to dietary sugars.

As the study authors wrote, “... neither of the two hypothesized predisposing factors [genetic profiles or insulin-response profiles] was helpful in identifying which diet was better for whom.”

Both diets improved key health markers, but the low-carb group enjoyed an edge
By the end of the 12-month study, both diets had improved all the participants’ lipid profiles and lowered their blood pressure, insulin, and blood sugar levels.

However, LDL cholesterol levels rose — an undesirable outcome — among participants in the low-fat group.

In contrast, people in the low-carb group enjoyed greater (desirable) rises in HDL cholesterol levels and bigger (also desirable) drops in blood triglyceride levels.

Both groups enjoyed a drop in the prevalence of metabolic syndrome, with no significant difference between the low-fat and low-carb diets.

Stanford team will keep mining the data
As Dr. Gardner said, “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.”

Moving forward, he and his team will continue to analyze the data collected during the study, and they hope to partner with other Stanford scientists to uncover keys to individual weight loss.

“I'm hoping that we can come up with signatures of sorts,” Gardner 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.”

Interestingly, the senior authors of the study included Stanford professor John Ioannidis, MD, DSc, who's famed for his searing critiques of the quality of biomedical research. His reputation for scientific rigor lends this study extra weight.


  • Gardner CD, Trepanowski JF, Del Gobbo LC, Hauser ME, Rigdon J, Ioannidis JPA, Desai M, King AC. Effect of Low-Fat vs Low-Carbohydrate Diet on 12-Month Weight Loss in Overweight Adults and the Association With Genotype Pattern or Insulin Secretion: The DIETFITS Randomized Clinical Trial. JAMA. 2018 Feb 20;319(7):667-679. doi: 10.1001/jama.2018.0245.
  • Gardner CD, Offringa LC, Hartle JC, Kapphahn K, Cherin R. Weight loss on low-fat vs. low-carbohydrate diets by insulin resistance status among overweight adults and adults with obesity: A randomized pilot trial. Obesity (Silver Spring). 2016 Jan;24(1):79-86. doi: 10.1002/oby.21331. Epub 2015 Dec 6.
  • Stanford Medicine. Low-fat or low-carb? It's a draw, Stanford study finds. February 20, 2018. Accessed at https://www.eurekalert.org/pub_releases/2018-02/sm-lol021518.php/