What are the benefits of a low GI diet?

A low GI diet is a simple and effective way to:

 Lose body fat and manage weight in the long term
 Reduce the risk of:
 developing type 2 diabetes
 diabetes complications
 heart disease
 Improve physical and mental performance
 Reduce the risk of breast cancer
 Improve pregnancy outcomes

Making low GI choices is not a dramatic change. It’s simply means swapping at least one high GI food for a low GI food at every meal.

Choosing better carbohydrates not only crowds out empty-calorie, glucose-raising foods but also helps battle many health threats. Here’s what eating more high-fibre, low GI foods will do for you:
Lower your risk of diabetes
The so-called diabetes epidemic has made headlines, and for good reason. The number of cases of type 2 diabetes (the most common type) is exploding, and many more people are in line for developing the disease. At the core is a condition called insulin resistance. Most people don’t know they have it — or that the “wrong” kinds of carbs contribute to it.

Insulin resistance occurs when the body stops responding to this critical hormone. Think of insulin as a kind of doorman. Normally, it triggers a reaction that allows glucose to pass through cell walls, where it is burned to produce energy or stored for later use. In some people, however, cells start to ignore insulin, which means the pancreas must manufacture more of it. Insulin resistance is “silent” because often the pancreas can keep up with demand by working overtime. Eventually, however, it may fail to keep up, resulting in type 2 diabetes.

Your genes, excess body fat and lack of exercise cause insulin resistance, but eating the wrong kinds of carbohydrates seems to worsen the problem. In one study, British researchers asked a group of non diabetic women to eat a high-GI diet for three weeks, while a second group ate low-GI foods. An analysis of their fat cells detected more insulin resistance in the women who ate the high-GI diet. It’s hardly surprising, then, that several large population studies have shown that people who eat diets filled with low-fibre, high-GI carbohydrates appear to double their risk of type 2 diabetes.

The good news: You can reverse the threat of diabetes by up to 42 percent simply by trading in your white bread, white rice and sugary breakfast cereal for hearty dark loaves, brown rice and oatmeal, according to a Harvard study of nearly 43,000 men.
Lower your risk of heart disease
Eating more whole grains could also lower your risk of heart attacks—by up to 29 percent, according to James W. Anderson, M.D., the University of Kentucky nutritionist whose research helped make oat bran a nutritional superstar in the 1980s. Dr. Anderson chalked up the benefits to oats’ soluble fibre, which lowers cholesterol. But there’s more going on than that. (In fact, one analysis published in the American Journal of Clinical Nutrition estimated that you’d have to eat three bowls of oatmeal a day to lower total cholesterol by only about two percent.)

Eating lower-GI carbs like oatmeal and whole-wheat pasta also keeps blood sugar levels in check, and there’s little question that persistently high blood sugar increases the threat of heart attacks, perhaps by raising levels of destructive compounds called free radicals. These compounds “oxidize” cholesterol particles, making them more dangerous to your arteries. One study of more than 3,300 subjects found that people with elevated blood sugar levels were nearly three times more likely to develop heart disease.
Glucose begins to build up in the blood as insulin resistance worsens. No one is sure why, but people who have insulin resistance also tend to have high blood pressure, elevated triglycerides (a type of blood fat linked to cardiovascular disease), and low levels of heart-friendly HDL cholesterol—a recipe for disaster. This cluster of problems, along with obesity, comes together in insulin resistance syndrome, also known as metabolic syndrome.

Again, the right diet—and the right carbs—can help. One recent study at the University of Maryland showed that every serving of whole grains that you add to your diet further decreases your odds for developing metabolic syndrome. Another study, by researchers at several hospitals in Boston, found that overweight people who adopted a low-GI diet had lower insulin levels, triglycerides and blood pressure than other dieters given a low-fat meal plan. They had less inflammation, too, which is another benefit of high-quality carbs.
Curb inflammation
When cholesterol assaults artery walls, the immune system responds the way it does to any injury—with inflammation. Choosing carbs rich in soluble fibre may help put the chill on that process by clearing out some of that cholesterol.

You can cool inflammation by filling up your menu with both types of fibre—soluble and insoluble—according to a recent study at the University of Massachusetts Medical School. The researchers followed a group of more than 200 people for a little over a year, measuring their blood periodically for levels of CRP (C-reactive protein, a marker of inflammation). They also asked the subjects to describe their diets. People who ate the most fibre—more than 20 grams per day, which is the minimum amount experts recommend—had 63 percent less inflammation than the others who ate low-fibre diets.
Help you lose weight
Can eating bran cereal and brown rice lead to slimmer hips and thighs? Population studies have shown that people who get most of their carbs from the low end of the GI tend to weigh less than other who gravitate toward sugary or starchy foods. One reason appears to be that meals that include low-GI foods keep you full lover (probably slowing digestion), which helps curb appetite later in the day.

In a recent study, researchers at the University of Sydney in Australia showed that people who ate low-GI diets were twice as likely to lose five percent of their body weight—and keep it off—as people who ate the conventional high-carb, low-fat diet that doctors have been recommending for years. If a five percent weight loss doesn’t sound like much, consider this: If you’re overweight and have even a hint of insulin resistance, that modest amount of slimming can reduce your risk of diabetes by 58 percent.

For some reason, low-GI diets seem to be particularly effective in women. Compared with female subjects who ate a typical high-carb, low-fat diet, women who filled up on low-GI foods lost 80 percent more body fat. They retained more muscle, too. The high-carb, low-GI plan had a bonus: Not only did dieters who adopted this menu keep the weight off, but their LD cholesterol dropped, too.
 

Expanding a bit on one of Arlene's comments:

"In some people, however, cells start to ignore insulin, which means the pancreas must manufacture more of it."

Insulin is made by beta cells in the pancreas and is released with a small protein (a peptide) called amylin or IAPP. Excess production of insulin also means excess production of IAPP.

IAPP readily takes up a “wrong” shape (called amyloid) which can destroy the pancreatic beta cells. The effect of this is that Type 2 diabetes can (unless it is managed well) become insulin-dependent diabetes (no beta cells, no insulin production).