So does a low-carbohydrate diet help people burn more calories? Or is the composition of the diet irrelevant if the calories are the same? Does it matter if the question is how to lose weight or how to keep it off? There was no consensus at the end of the session. But here are a few certainties about dieting amid the sea of unknowns.

What we know

People vary — a lot — in how they respond to dieting.

Some people thrive on low-fat diets, others do best on low-carb diets. Still others succeed with gluten-free diets or Paleo diets or periodic fasts or ketogenic diets or other options on the seemingly endless menu of weight-loss plans.

Most studies comparing diets have produced results like Dr. Gardner’s: no difference in weight loss between study groups as long as the calorie intake was kept equal. But within each group, there always have been a few individuals who lost a lot of weight, a few who did not lose any weight, and a few who actually gained.

Dr. George Bray, an obesity researcher who is emeritus professor at the Pennington Biomedical Research Center in Baton Rouge, La., sums it up this way: “Eat the diet you like and stay with it.”

There is nothing new in the diet universe.

Many of the diets people swear by today have been around in various incarnations for decades. More than a century ago, a best-selling book, “How to Live,” told Americans that the only way to lose weight was to count calories.