Carbohydrate-rich foods are classified according to glycemic index (GI), a ranking system for carbohydrates based on their effect on blood glucose levels after meals. High-GI carbohydrates are considered "bad" (with the exception of those foodstuffs like carrots that, even though they have high GIs, have a quite low carbohydrate content and should not significantly affect blood sugar levels).

"Bad carbohydrates", such as those in sweets, potatoes, rice, and white bread, may not be taken together with fats, especially during Phase 1 of the Method. According to Montignac's theory, these combinations will lead to the fats in the food being stored as body fat. Some kinds of pasta, such as "al dente" durum wheat spaghetti, some varieties of rice, such as long-grain Basmati, whole grains and foods rich in fiber, have a not so high GI.

Besides, the quality of fat foods depends on the nature of their fatty acids: polyunsaturated omega 3 acids (fish fat) as well as monounsaturated fatty acids (olive oil) are the best choice, while saturated fatty acids (butter, fat meats) are to be limited. Fried foods and cooked butter should be avoided.