The latest - and, some would say, most credible - diet is based on howeasily we digest different foods. Is it real science or just another fad?Lucy Atkins reports.
The glycaemic index of the food we eat is fast becoming one of the hottest topics in nutrition. It is basically a system that nutritionists use to rank foods based on the rate at which they raise your blood- sugar level. Foods with a low glycaemic index, or GI, such as wholegrains or whole lentils, make the blood sugar rise and fall gently. High-GI foods, such as white bread, on the other hand, are easily broken down by your body; they therefore cause your blood-sugar levels to surge, spike and then crash.
Many nutrionists now believe this process may affect our long-term health. And diet gurus are leaping on to this lucrative bandwagon, claiming that the GI of your diet has implications for your waistline, too. So will GI be our next big health concern? Will it replace fat, fibre or protein in our lexicon of dietary dos and don'ts? Or is it just the latest fad in a growing field of "nutribabble", the misapplied pseudoscience used to sell a zillion diet books?
There is certainly some evidence to link the GI of the diet to various health matters. Some research topics are newer and more controversial than others. According to a study in last month's American Journal Of Clinical Nutrition, women who in early pregnancy eat a diet full of high-GI foods, such as white bread, cake and mashed potato, may have a slightly higher risk of having babies with neural-tube defects such as spina bifida.
Researchers found the risk of birth defects doubled in women who ate lots of high-GI foods and quadrupled among women who were obese. The idea is that the sugar surge you get from eating high-GI foods may interfere with important stages of fetal development, such as the formation of bone around the spinal column.
Andrew Russell, chief executive of Britain's Association for Spina Bifida and Hydrocephalus (ASBAH), says this is the first research to find such a link and should therefore be regarded with great caution. "To tell a mother who ate cream cakes early in pregnancy that she is responsible for her child's birth defects is simply not justified at this stage," he warns.
"Neural-tube-defect research is very complicated. The idea that a sugar surge in the maternal blood could cause spina bifida, while not impossible, would need a lot of corroboration from follow-up studies. What we do know for certain is that a shortage of certain vitamins, most notably folic acid, in the early days of pregnancy can cause neural-tube defects. But the link to the GI of your diet, though worthy of research, is by no means proven."
Many scientists agree with one Harvard University medical school nutritionist who calls the GI "the most fascinating and promising area in nutritional research today". But the most vociferous champions of this nascent research field are the authors of weight-loss books. Indeed, a new diet in-crowd, proclaiming the vital importance of a food's GI, is threatening to oust the controversial Atkins diet entirely.
The leader of the GI pack is The South Beach Diet, by a US cardiologist, Dr Arthur Agatston. He advocates an Atkins-style high-protein regime but, unlike Atkins, he allows his followers - who reportedly include Bill and Hillary Clinton and the Sex And The City star Kim Cattrall - to eat "good" carbohydrates but not "bad" ones.
Bad carbs have high GI levels. They make you feel temporarily full when you eat them because they make your blood-sugar levels rise rapidly. But they soon fall again, leaving you lethargic and hungry. This cycle, he argues, may cause us to overeat and to crave sugary foods, which will make us fat.
The New Glucose Revolution, by Jennie Brand-Miller, Johanna Burani, Kaye Foster-Powell and Susanna Holt, and The GI Diet, by Rick Gallop, are two other fast-selling diet books that expound similar ideas. The GI crowd advocate a diet of healthy, low-GI foods, such as lentils, oats, wholegrains and beans, which fill you up for longer and do not leave you with unmanageable carb cravings. So should we all get a GI index and chuck out that pesky Dr Atkins New Diet Revolution once and for all?
Dr Azmina Govindji of the British Dietetic Association says GI is far more complicated than it looks. "Not all low-GI foods are healthy, for a start. Chocolate, for instance, actually has a relatively low GI," he says. "So does ice cream. But you wouldn't lose much weight if they were your dietary staples. Much of a food's GI is to do with how it is processed by the body. This can be incredibly complicated. For instance, your body breaks down wholemeal bread quickly, so it has quite a high GI, but it takes a long time to break down the wholegrains in granary bread, which therefore has a low GI."
There are infinite, seemingly illogical variations. Tinned kidney beans have a higher GI than soaked and boiled dried ones; the GI of pasta (surprisingly low) depends on what shape it is (macaroni is higher than spaghetti). What you eat with a food alters its GI (a fatty pudding can change the GI of the pasta you had for your main course), and how you cook a food also matters (mashed potato has a higher GI when hot than cold).
Still, research has shown that a food's GI may well affect appetite. "Researchers are currently investigating the role of a food's GI in satiety," says Dr Toni Steer of MRC Human Nutrition Research, in Cambridge. "Though there has been no single major study to link the GI of food to weight loss, various small studies have had promising results." For example, researchers at Oxford Brookes University have found that children who eat a high-GI breakfast cereal, such as Sugar Puffs, Rice Krispies or Cornflakes, get hungrier before lunchtime, and are more likely to snack and eat more calories overall, than other children.
Further research has been carried out into the broader health implications of GI eating. "Studies have shown that people who eat more low-GI foods have better HDL ('good' cholesterol) levels in their blood and so are at a lower risk of developing coronary heart disease," says Steer. And according to Govindji, who also wrote the book Healthy Eating For Diabetes with the chef Antony Worrall Thompson: "Some studies suggest that if you have 'syndrome X', a kind of insulin resistance, eating high-GI foods can be unhelpful." If you have this condition, as Worrall Thompson does, your body tissues become less sensitive to insulin. "People with syndrome X are more prone to heart problems and diabetes," says Govindji. "So focusing on more low- to medium-GI foods can help stabilise blood-sugar levels and manage the condition." The GI of what you eat is also important for those who already have diabetes. "In diabetes, blood-sugar and insulin levels need to be very well controlled," says Govindji. "So the GI index of your diet can be helpful as part of an overall diabetes treatment plan."
So it looks as if GI madness is on its way. GI-speak will soon be as common as carb-talk is now. Yogurt pots will be labelled "low GI", snack bars will be GI-indexed and we will all know the rating of gnocchi against noodles. Far-fetched? Let's hope not. After all, GI-friendly muesli is a lot better for you at breakfast than Atkins-style lard.
... - Guardian service
What the experts say: Dr Eva Orsmond, a weight-management and obesity specialist who runs the Slievemore Clinic in south Dublin, believes the glycaemic-index system could work for people who eat healthily but find it hard to lose weight. She stresses, however, that concentrating on low-GI foods should be a lifestyle choice, not a short-term diet.
Although low-GI foods should help people to feel full for longer and prevent snacking, as they increase insulin levels only moderately, an important influence is how we digest food, she says. "Everyone has a different metabolism, and so you've got to look at every one uniquely."
Orsmond also warns that Irish people still eat too much fat and too little fibre - only half of us eat the necessary 30 grams a day. Don't get obsessed by the glycaemic index of foods: we all need calories for energy, and a balanced diet is all-important.
She also points out that food companies that wanted to label their products as scoring low on the glycaemic index would run into problems. Referring to the glycaemic index amounts to making a medicinal claim, which means it would contravene Irish Medicines Board rules.
Sarah Keogh, a consultant dietician at the Albany Clinic in central Dublin, believes the broad principles behind the GI diet could benefit its followers' health. "I would like to see more people eating low-glycaemic-index foods, such as beans and lentils, at least three times a week, but preferably every day," she says.
"They are good for keeping blood-sugar levels lower, they are just generally very healthy and they lower cholesterol. A tablespoon of linseeds should be eaten every day, going well with porridge for breakfast. Linseeds contain omega-6 fatty acids that help lower your cholesterol, and also phyto-oestrogens, which are linked to preventing breast cancer."
Keogh is concerned about the effects of regimes such as the Atkins diet, which involves cutting out carbohydrates in favour of protein. She says a comparison of the Atkins diet with a more balanced diet has shown that after a year weight loss is similar between the two groups - and that, after four years, 95 per cent of people on the Atkins diet have put the weight back on.
-Jennifer Hurley
Low-GI foods
Fish, seafood, fruit, green vegetables, tomatoes, oats, porridge, bran flakes, wholegrain bread, lean meat, chicken.
High-GI foods
Alcohol, white bread, chips, fruit juice, soft drinks, processed breakfast cereal, sugar.