Mediterranean-style diet matched WeightWatchers for weight loss in trial

What happened

A randomized trial published in JAMA Network Open compared two free, 2-year weight loss programs in adults with obesity. One was a Mediterranean-style diet adapted to Southern food traditions. The other was a calorie-focused commercial program. Weight loss came out the same.

The headline question, as first reported by Healthline News (Healthline News), is whether diet quality matters more than counting calories. The trial does not answer that in the affirmative. Even the Healthline article quotes an outside expert, Stanford's Christopher Gardner, saying calories still matter for weight loss and that the study wasn't designed to test diet quality.

So the supportable story is narrower: two different approaches landed in the same place on the scale.

What the study actually found

The DELISH trial, led by researchers at UNC-Chapel Hill, enrolled 360 adults with a BMI of 30 or higher from one academic and six community primary care practices in central North Carolina. Of those, 182 were assigned to the Mediterranean-style program (called Med-South) and 178 to the control. Mean age was 54.7 years, and 55.6% of participants were female (JAMA Network Open).

Med-South ran in three phases: diet quality in months 1–4, weight loss in months 5–12, and maintenance in months 13–24. The control group received the workshop and digital components of WeightWatchers, which the paper describes as a commercially available and effective weight loss program. Both groups got advice on moderate physical activity.

The between-group difference in weight change was −0.11 percentage points (95% CI −1.79 to 1.57; P=.90). The authors concluded there was no material difference.

Diet quality is where Med-South pulled ahead. Among participants who returned for follow-up, the Alternative Healthy Eating Index-2010 score was 3.62 points higher in the Med-South group (95% CI 1.11–6.12; P=.005). A UNC press release on EurekAlert adds that the Mediterranean-style group reported more mono- and polyunsaturated fat and less carbohydrate, and that blood pressure reductions did not differ between groups.

What it does not show

The diet-quality result was a secondary outcome, and it covers only people who attended the 24-month visit. That is not a randomized, intention-to-treat comparison. About 35% of the Med-South group and 27.5% of controls were lost by 24 months, and the dropout rates differed. People who stay in a study may eat differently from those who leave.

• It does not show diet quality drives weight loss. Weight loss was the same in both arms, so the trial offers no evidence that the better-scoring diet worked better for weight.
• It does not show calories don't count. The control program was calorie-focused and performed as well, and the trial wasn't built to isolate calories from food quality.
• It may not generalize. Primary care clinicians reviewed eligible patient lists and referred people they considered appropriate, which could filter who ended up in the trial.
• It was open-label, meaning participants and staff knew who was in which program (PMC version). Weight is an objective measure, but self-reported diet intake is more vulnerable to that kind of awareness.
• Diet quality was measured through intake questionnaires plus a skin-carotenoid device, per the press release. Skin-carotenoid scores improved in both groups.

What it means for you

If you've been told you must choose between counting calories and eating better, this trial suggests you don't have to pick a winner for the scale. Two quite different programs produced roughly 3.4% weight loss at 2 years. For someone weighing 103 kg (the study's median), that is a few kilograms.

The trial's lead author, Dr. Keyserling, says similar weight loss can be achieved with calorie-restricted diets, and that a healthful dietary pattern may have broader health effects, which this trial was not designed to test. That second part is a claim about long-term health, not weight loss, and this trial wasn't designed to prove it.

• Choose a structure you can keep up. Both programs were free, included group or digital support, and ran for two years.
• If you like tracking, tracking is fine. If you don't, a pattern built around foods you enjoy is a reasonable alternative.
• Talk to your clinician before making big changes, especially if you manage a condition such as diabetes or high blood pressure.

Context: what else Healthline cited

The Healthline piece also points to a 2024 analysis of DIETFITS, a trial of 609 adults comparing low-carbohydrate and low-fat diets. That paper grouped participants by 12-month change in diet quality and by how closely they followed their assigned macronutrient targets (PMC).

We couldn't confirm its specific findings, including the claim that high adherence plus high diet quality produced the greatest loss. It is also a secondary analysis, which is observational, and DIETFITS did not pit diet quality against calorie counting. Treat it as a hint, not a verdict.

The honest summary: the headline question is interesting, but this trial's answer was a tie.