THE GI diet is not only healthier but does help you lose weight, a rigorous animal study suggests.
Most diets emphasise reducing fat intake. Others, most famously the Atkins diet, advocate cutting carbs. The GI diet is instead based on eating carbohydrates that have a low glycaemic index, a measure of how much blood sugar levels rise after eating different foods.
Durum-wheat pasta, basmati rice, fruit and most vegetables have low GIs, while potatoes, white bread and refined breakfast cereals have higher GIs. 鈥淏lood sugar can rise as quickly after a bagel as after a bowl of sugar,鈥� says David Ludwig of the Children鈥檚 Hospital in Boston, whose team carried out the latest study.
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For people with diabetes, the benefits of a low-GI diet are clear. But its effect on obesity is still debated. While small human studies suggest low-GI diets do lead to weight loss, critics say it is not clear that the GI value of foods really is the crucial factor. It might be that low-GI foods are less tempting, or contain more bulky fibre, meaning people eat less. Similarly, recent research suggests the Atkins diet works simply because people eat less.
To try to resolve the issue, Ludwig鈥檚 team fed two groups of rats on diets that were identical apart from their starch content. At first the rats were allowed to eat as much as they liked. But after eight weeks the high-GI group began to gain weight, so to rule out the possibility that they were eating more because the high-GI food is more palatable, their portions were reduced.
At the end of 17 weeks, both groups had similar body weights. But there was a striking difference: the high-GI group had 71 per cent more body fat than the low-GI group, and 8 per cent less muscle (The Lancet, vol 364, p 778). The high-GI group also had higher blood levels of substances linked to heart disease, such as triglycerides.
The team鈥檚 theory is that a low-GI diet works by lowering insulin levels, encouraging muscles to take up and burn food. High-GI diets, by contrast, raise insulin levels and promote food uptake by fat cells. Ludwig points out that there is independent evidence that insulin has such effects.
Not all nutritionists are convinced. 鈥淚t is very difficult to translate to human studies from rat studies,鈥� says Anne Flint of the Royal Veterinary and Agricultural University in Frederiksberg, Denmark.
Only meticulous studies in humans will finally settle the issue of whether eating low-GI food alone, rather than eating less, really does reduce obesity. But while there are worries about the long-term health effects of diets such as the Atkins, experts do agree that a low-GI diet will do you no harm even if you don鈥檛 lose any weight. 鈥淎 low-GI diet is the more moderate answer to a succession of popular diets,鈥� Ludwig says. Compared with low-fat or low-carb diets, he says, it is the perfect compromise.