You can lose weight either with a low-calorie, low-fat diet, or with a low-carb, high-fat diet. With the Harcombe diet, you can switch these up. Either have a "fat meal", where carbs are restricted as much as possible, or have a "carb meal", where fat is restricted as much as possible, and separate the meals by at least 4 hours to give your body time to digest it. Protein can be eaten with either kind of meal, but the carb meals should have lean protein.
The only foods that are prohibited are those that are high carb and high fat, for example, pizza, doughnuts, french fries, ice cream and some kinds of yogurt. Also, sugar and corn syrup. So you can eat both bread and butter, but not together. Veggies (with the exception of potatoes, corn, beans, rice and wheat which are all carb-heavy) can be eaten at either meal.
Also, forbidden would be any kind of soft drink (including diet), ketchup and hoisan sauce (because of the sugar), and salad dressings. Yogurt would not be allowed unless it is low-fat and thus eaten with carbs, or low-carb and thus eaten with fat.
Interesting theory, worth a try.
Arggh. There is another theory that protein and carbs shouldn't be combined in the same meal and that fat and carbs should be combined, in order to lower the glycemic index. I don't know what to say. Under this theory, there is a category called "starch" which includes bread, bananas, rice, corn and potatoes, which shouldn't be eaten with "protein" (meat, cheese and beans).
This actually makes more sense to me, but it is harder to describe. I could see a study with 4 options - a low-carb, meat-heavy diet; a low-fat, restricted-calorie diet; a Harcombe fat vs carb diet, and a food-combining protein vs starch diet.
These are contradictory. In the Harcombe diet, meat can be eaten with bread, but butter can't be eaten with bread. In the food-combining diet, bread and butter can be eaten together but not bread and meat.
Under the food-combining diet, pizza is forbidden, not because of the carbs and fat, but because of the carbs and protein (meat + cheese). Doughnuts are forbidden, not because of the carbs and fat, but because of the sugar. French fries are not forbidden, but should not be eat with ketchup (sugar) or meat. Ice cream is forbidden because of the sugar. Yogurt is allowed as long as it is low in sugar.
And yet another thought. The most passionate defender of starch, Dr. John McDougall is also in favor of low-fat.
So, bottom-line, if you must have bread, eat it without meat, cheese, or butter. If you must have potatoes, eat them baked, not fried, and without butter, cheese or meat. If you must have rice, eat it veggies, not with meat.
I am sure this isn't the final word. Time for experiments.