Large studies published in the NEJM and other prestigious journals have examined this question extensively. Low glycemic, Mediterranean style eating patterns seem to be best for establishing and maintaining healthy weight. In addition, avoidance of trans fats and excessive saturated fats may be healthier. Eating a lot of fresh or frozen colorful vegetables, lean sources of protein, along with healthy fats like Omega 3 fatty acids are all good ways to stay healthier. Excessive protein over time may have a damaging effect on the kidneys so if any radical eating pattern is to be entertained, consulting with your physician is advised.

In the past, certain organizations have advocated low fat diets which have conclusively been proven to not be the healthiest option because fats are then replaced with carbohydrates, often high glycemic carbohydrates. These high glycemic carbohydrates lead to excessive sugar, excessive insulin, fat deposition and central obesity along with difficulty with weight loss.

It is NOT simply calories in, calories out. I am pleased your are inquiring and learning.

I hope this simplistic answer is of assistance. There will certainly be more questions and more studies on this issue.