The GI Diet is one of several diets which focus on eating low Glycemic Index foods. If you think you can go it alone with just this book by Rick Gallop, feel free to try, as it does all the work for you, designing customized menus and shopping lists for only $1.50 per week. As stated in the Sun, "For people who never want to diet again... the pounds will drop...the only diet you'll ever need."
The GI diet has its basis in the 1981 discovery by nutrition professor Dr. David Jenkins that various carbohydrates affect blood sugar levels in various ways, developing the Glycemic Index. All GI diets have one thing in common, the focus on eating low Glycemic Index foods, that is, those which take longer for the body to break down to glucose. Low GI foods include mainly whole grains and unprocessed foods. Anything with a GI of 0-55 is considered low (and marked with a green light in Rick Gallop's GI Diet reviewed here), those between 56-69 are medium and rate a yellow light, 70 to 100 is high GI, or a red light. For instance, comparing cereals, muesli is a 40, shredded wheat is a 67, and rice krispies is an 82. WIth the traffic light system, you don't need to know the numbers, just which of the three categories they are in.
You want to avoid eating high Glycemic Index foods which give you a quick glucose high or sugar rush, followed by a slump. Eating low GI foods reduces hunger cravings and makes you feel full for longer. So you'll be avoiding things like Instant White Rice (87), Scones (92), Watermelon (80), and White Bread (71). Some measures will surprise you-- branflakes weight in at a hefty 74, while sweet cherries are only 22, so it does take some learning to get up to speed with the system. This is an excellent system, but it is not perfect--one drawback nutritionists point out is the tables refer only to one food eaten on their own, but combined and cooked or prepared in various ways, the values can vary.
Keep in mind that GI only involves carbs, so you won't see indexes listed for high fat/protein foods like cheese, eggs, meat, etc., but you will need to eat these in moderation as they can be calorie dense foods. It's important not to overdo it on high calorie foods like nuts and such just because they have a low GI index. If the GI diet is used sensibly, studies suggest it may have a role in preventing Type 2 diabetes or maturity-onset diabetes.
Performance is mixed; this is not a diet to lose lots of weight quick on (1-2 pounds per week would be typical), but is more a slow and steady method which changes the way you eat, for the better, over the long-term. If you have the patience, then it's a great choice, though if you have lots to lose, you may want to start with something more dramatic for a few weeks/months, then graduate to the GI diet to complete the loss and maintain. It is as much a lifestyle as a diet plan, and it is certainly refreshing to see a wildly popular diet with a real scientific grounding and real health benefits.
Menu: The menu includes many whole grain and unprocessed foods.
Prices: If you're confident enough to just buy the Rick Gallop book (~$22), then this can be a reasonably priced diet, though you will pay a bit more for whole grain and fresh ingredients.