I went on the Zone diet for a health issue (not weight-related). I lost 15 pounds in the first two months, another 25 in the next five. Not quite 80 lbs in 4 months but I do worry that losing that much that quickly wouldn't be healthy. I was actually worried at my initial weight loss but it slowed down.
The Zone is kinda complicated and mathy, and I don't follow it super closely. The basic idea is: eat a lot of vegetables (not starchy ones like potatoes), eat a portion of meat the size and thickness of your palm, use fats (oil, nuts, cheese) sparingly. Avoid bread and rice as much as you can stand. Some vegetables are better than others - aim for low glycemic index vegetables. Google for a list of glycemic index values to get an idea of what these are. The Zone books have references too.
Doing this diet, you have to have the mentality that sugar is bad, and carbs are often bad (unless low glycemic index). The reason is that both of those things spike your blood sugar, and then cause your blood sugar to dip. That equals hunger. Most Americans are on a continuous blood sugar rollercoaster because so much processed food is carb-y.
You can substitute egg whites or tofu for meat. A lot of typical vegetarian fare has a good deal of carbs though. I love beans, but they are more carbs than you'd think. Nuts and cheese have some protein, but a good bit of fat. I was working on having a couple meatless days per week before I started this diet, but found it really difficult to get the high protein level the Zone demands (I have yet to figure out how to cook tofu that I actually want to eat).
I keep track of calories, and have for a long time, and I use my calorie-counting app to get a rough idea of whether something is a source of carbs, fat, protein, if it's not immediately obvious.