What the Two-Year Southern U.S. Trial Found

A two-year randomized trial published in JAMA Network Open compared two approaches to weight loss in 360 adults recruited from North Carolina primary care practices. Participants had a body mass index of 30 or higher, an average age of 55, and most were already managing chronic conditions such as diabetes or hypertension. One group followed Med-South, a Mediterranean-style eating pattern adapted for the Southern United States and led by registered dietitians. The other received access to the workshop and digital components of the WeightWatchers program, which centered on calorie reduction while also encouraging more fruits and vegetables.

By the 24-month mark, the two groups lost nearly identical amounts of weight: 3.4% in the Mediterranean-style group and 3.5% in the calorie-focused group. The study also found no significant difference between the groups on outcomes such as blood pressure. The main measurable difference was diet quality. People in the Med-South group increased their consumption of nuts, vegetable oils, whole grains, non-starchy vegetables and fruit significantly more than the control group.

The adherence picture was also different. Ninety-five percent of people following the Mediterranean-style program said they would recommend it, compared with 68% of the control group. That gap is notable because sustained adherence is often the weakest link in weight loss, and the trial design itself spent its first four months on diet quality before turning to weight loss and maintenance.

Independent experts cautioned that the study was not primarily designed to isolate diet quality. Christopher Gardner, professor of medicine at Stanford Prevention Research Center, pointed out that diet quality was a secondary outcome, not the primary one. Both experts quoted in the coverage agreed that calorie balance remains important for weight loss, but that higher-quality foods may improve long-term health and make the process easier to sustain.

Reading the Evidence on Quality, Calories and Adherence

Why the Trial’s Primary Endpoint Limits Diet-Quality Claims

The study was set up to compare two weight-loss programs, not to test diet quality as the main driver of weight loss. Christopher Gardner, a Stanford professor not involved in the research, noted that examining quality as a secondary outcome is interesting, but a proper answer would require a trial designed around that question. This means the similar 3.4% and 3.5% weight reductions can be reported with confidence, while the diet-quality advantages should be read as an observed difference rather than proof that higher quality alone produces more weight loss.

The Med-South Program’s Adherence Advantage

The more striking result may be behavioral. Nearly all participants in the Mediterranean-style group said they would recommend their program, compared with just over two-thirds in the calorie-focused group. The study’s three-phase structure — diet quality first, then weight loss, then maintenance — may have made the eating pattern easier to maintain. The DIETFITS trial referenced in the coverage supports this direction: the greatest weight loss occurred when high adherence and high diet quality went together, not from either factor alone.

Why Experts Still Warn Against Ignoring Calories

Despite the quality benefits, the researchers and outside experts did not conclude that calorie balance is obsolete. Gardner observed that a person could lose weight eating only low-nutrient food if total intake stayed below energy needs. Kristin Kirkpatrick, a registered dietitian at Cleveland Clinic, argued that lower-calorie diets may not be sustainable and can reduce nutrient intake over time, potentially affecting long-term chronic disease risk. The practical reading is that calories set the conditions for weight loss, while food quality shapes what that weight loss does for overall health.

Diet Changes the Study Actually Supports

For people weighing these approaches, the study points to several practical changes rather than a single right diet.

  • Start with the food groups that improved in the Med-South group: add nuts, vegetable oils, whole grains, non-starchy vegetables and fruit while limiting processed foods and refined carbohydrates.
  • Do not abandon calorie awareness if weight loss is the goal. Both groups in the trial lost around 3.4% to 3.5% over two years, so higher-quality eating was not shown to replace the need for an energy deficit.
  • Choose a pattern you can sustain. The 95% versus 68% recommendation gap, combined with DIETFITS findings, suggests adherence and food quality together matter more than a program’s label.
  • If you count calories, upgrade the nutrient density of those calories. Kirkpatrick’s warning about nutrient loss over time supports making vegetables, fruit and whole grains the core of a reduced-calorie plan.