The theory is rather simple: people tend to eat the same amount of food every day (3 pounds). What determines whether they gain or lose weight is the number of calories in that three pounds of food. Those who eat high calorie foods gain weight. Those who eat low calorie foods lose weight.
What makes a food high calorie? Forget everything you've ever heard because the truth is surprising. The more water in the food, the lower calorie it will be. That's because there are no calories in water (0 calories / gram). Fiber is the next best thing at 0 or 1 calorie per gram. The more fiber the better. Protein and carbohydrates are in the middle. They have 4 calories per gram. The real problem is fat, which has 9 calories per gram, twice as much as carbohydrates and protein. Fat is the biggest enemy of the dieter because it is so loaded with calories.
Pritkin's method is very simple. Weigh the food you eat and count the number of calories. As long as calories per pound is less than 400 you will lose weight. Calories per pound between 400 and 600 leads to weight maintenance. More than 600 calories per pound causes weight gain. (The only trick is that for liquids count just the calories, not the weight. Which means avoid liquids which have more than a few calories.)
Nothing else matters, according to Pritikin. The glycemic index is worthless, and weight gain has nothing to do with insulin rushes or food intolerances. There is no need to count carbs, fats, or eat small portions. Ultimately weight gain is caused by eating more calories than we burn, and that is determined by whether we eat high-calorie foods or not.
Fruits and vegetables are the stars of a low calorie diet. Their calorie density is very low so they can be eaten in unlimited quantities, along with the delicious complex carb foods like pasta and potatoes which make up the rest of the diet.
The result is a low fat, high carb diet, which as Pritikin documents, has been shown in 25 years of scientific studies to cure diabetes, allow people to lose weight and keep it off, go off blood pressure medicines, and avoid bypass surgery.
In case someone cares about my credentials, I have no financial ties to Pritikin. I have however followed their diets for 21 years with excellent results. I am 70 pounds lighter now, going from an obese 32 BMI to a slim BMI 20 and body fat under 10%. My cholesterol, triglycerides, and glucose levels & tolerance are all superb, and in spite of 21 years on a high-carb diet I show absolutely none of the bad side effects like insulin resistance which the high-fat diet authors claim I should have.
I am in my 40's, at my dream weight, and finished a marathon last year. Anyone who wants to join me can. Being fat is optional. If someone doesn't want to buy Pritikin, Barbara Roll's "Volumetrics" book has the same kind of diet.