Whether you are just starting a New Year's diet or struggling to maintain a healthy weight, a provocative new study offers some timely guidance. It isn't so much what you eat, the study suggests, but how much you eat that counts when it comes to accumulating body fat.

The findings are the latest in a string of studies to challenge claims that the secret to healthy weight loss lies in adjusting the amount of nutritional components of a diet—protein, fat and carbohydrates.

The...