There are a lot of fad diets out there, high carb, low carb, high protein etc. List as long as my arm.
Inuits eat a diet mostly of meat and fat, with a few berries and seaweed thrown in. They are genetically adapted to do so.
IMO the Thai diet is healthy, a small amount of protein, unsaturated oils, vegetables and rice. As long as I can stop them throwing in MSG and sugar.
The ultimate zero-calorie diet is celery and bamboo. Not fond of celery, but I do enjoy bamboo and chili.
During the U-boat blockade of Britain in World War II, a British dietitian was tasked with finding a diet on which Britons could survive without any long term health effects, and was simple. She concluded wholemeal bread, lightly cooked cabbage and potato were the only foods required to supply enough nutrients, meat was unnecessary. Somewhat boring, though.