The mainstays of most of the diet regimens of the last 30 years have been the GI (glycaemic index) rating score as well as its cousin the glycaemic load. Famous best-selling diet books such as the G-Plan Diet, the South Beach diet all used the index in some way and changed the way we thought about carbohydrates. Now a detailed new study published in Cell pays this score – and how we use it – some closer scrutiny.

The GI theory goes that there are many different types of carbs and they can be graded into how rapidly the body converts them into glucose. The faster the burn rate, the higher the index and the more rapid the rise in blood sugar. This surge in blood sugar also triggers a rise in insulin and the combination of these events if sustained over time is believed to lead to unhealthy metabolic changes leading eventually to obesity and diabetes.

This nutritional dogma has been the backbone of the advice to avoid eating high GI foods such as pasta, rice and potatoes and replace them with low GI alternatives such as beans or lentils. In many cases people give up carbs completely or avoid refined carbs and instead go for approaches like the Atkins Diet. This cutting down on “bad” carbs also sounds like common sense and most doctors, health professionals and members of the public, including myself when I started writing my bookThe Diet Myth, assume that proper scientific and clinical trials have been carried out that back up these ideas. But what if none of it were true and we had been misled?

When I started researching four years ago I could find plenty of laboratory animal studies showing the benefits of a low GI diet. But while studies of diabetic subjects generally confirmed some advantage of a low GI diet, in non-diseased people it was a different story. There was a distinct lack of any decent human trial that proved convincingly that a GI-based diet was any better than equivalent diets based on cutting calories.

While studies did show that low GI foods and diets could alter blood sugar profiles no study has shown that when the amount of carbs and calories are kept constant the lower GI score for the food didn’t influence body weight. So humans seem to be responding differently to laboratory mice, and although many people do lose weight in the short-term on these diets, it may be because they are paying more attention to what they eat rather than any validity of the food scoring process.

It’s those microbes again

What the new study – from an Israeli team led by Eran Segal – found was that by closely studying 800 volunteers who kept careful food diaries and had wearable devices that continually monitored their blood glucose levels, an amazing pattern of diversity emerged.