It’s the Sugar, Folks

Sugar is indeed toxic. It may not be the only problem with the Standard American Diet, but it’s fast becoming clear that it’s the major one. [.. T]he researchers found that increased sugar in a population’s food supply was linked to higher diabetes rates independent of rates of obesity.

In other words, according to this study, it’s not just obesity that can cause diabetes: sugar can cause it, too, irrespective of obesity. And obesity does not always lead to diabetes.

To clarify..

Sugar is indeed a catalyst for many unwanted "side effects" such as weight gain (with or without diabetes). This observation is the cornerstone of the Montignac diet, M. claims the so-called Calorie Theory is defunct. From his Eat Yourself Slim:

The Origins of the Calorie Theory: In 1930 two American doctors, Newburgh and Johnson, of the  University of Michigan, suggested in one of their papers that ”obesity results from a diet too high in calories, rather than from any metabolic deficiency". Their study on energy balance was based on very limited data and, above all, had been conducted over too short a period to deserve serious scientific acceptance.

This did not prevent their study from being immediately and widely acclaimed as irrefutable scientific truth, and it has been treated as ”gospel” ever since.  A few years later, however, Newburgh and Johnson, concerned at the publicity which had been given to their discovery, somewhat hesitantly published some serious reservations they had concerning their previous findings. These went entirely unnoticed. Their initial theory was already integrated into the syllabus of most Western medical schools, and there it remains to this day [..]

[W]e may wonder how people with big appetites who consume 4,000 to 5,000 calories a day are not fatter than they are (some even remain skinny). If the calorie theory was correct, these hearty eaters would come to weigh 60 to 80 stone within a few years. Furthermore, how can you explain why some people continue to put on weight even when they reduce their daily calorie intake by eating less? Thousands of people go on gaining weight like this while starving themselves to death.

Montignac's explanation of weight gain is entirely different. There are bad carbohydrates (potato, corn) and good ones (leeks, most fruits). Bad carbohydrates will 1) increase glucose 2) which triggers massive amount of insulin 3) whose side effect is to direct the fat, present in the bloodstream at that time, to be stored as fat.

Then, ingesting fat (lipids) on their own will not cause weight gain.

I heard of variations of M.'s approach, Atkins Diet, Paleolithic Diet, etc. None of these variations are entirely on par with Montignac's teaching, it's not about eliminating grains entirely - there are good grains. Montignac does not say "eat proteins all the time" either which a lot of Atkins followers seem to do.