The Glycaemic Index (GI) is a method of classifying the glycaemic (i.e. blood sugar) response to carbohydrate-rich foods. Put another way, it is a measure of how quickly carbohydrate reaches the bloodstream. Foods can be tested and ranked according to a measure of their GI. The GI is calculated by measuring the area under the curve (of glycaemia) versus time, after consuming a food containing 50g of available carbohydrate by comparison with 50g of reference food (either glucose or white bread). Blood glucose testing is carried out every 15-30 minutes for 2-3 hours.

The lower the GI of a food, the flatter the curve of blood glucose response, meaning the slower this is digested and converted to blood sugar.

It does not necessarily follow that all sweet sugary foods will be quickly absorbed and therefore have a high GI. It is much more complex and depends on a number of characteristics of the food consumed, e.g.:

Type of sugar—Glucose (GI = 100) has a higher GI than fructose (GI = 20). Temperate fruits (e.g. apples, pears, oranges) have a higher fructose content and therefore lower GI than tropical fruits (e.g. watermelon, rockmelon, mangoes).

Nature of the starch—The ratio of two types of starches (amylose: amylopectin) can affect the GI of foods. Amylopectin is more easily digested than amylose. Therefore, foods such as legumes, with a high amylose: amylopectin ratio have a lower GI than rice which has a higher amylopectin content.

Processing and particle size—Food particle size is a determinant of GI. This is shown with bread processing. Whole meal bread has a finer texture than rye bread and thus a higher GI.

Cooking methods—Cooking affects the degree of gelatinisation of fruits and thus GI. Stewed fruits have a higher GI than fresh fruits.

Starch-nutrient interactions—Carbohydrate forms molecular complexes with fats and proteins. This happens, for example, in a baked cake where GI will be reduced.

Fat—Large amounts can reduce GI (e.g. in ice cream) and hence high-fat foods may have a desirable GI but still be undesirable in terms of energy content.

Fibre—This depends on type of fibre. Insoluble fibre appears to have no immediate effects. However, soluble fibre (e.g. guar gum) can decrease the rate of absorption of glucose and hence decrease GI. Fibre supplements will not have the same effect because the fibre is not packaged into the food. Resistant starch will also lower the GI of a food.

Taking these properties into account we can see why, in Table 9.3, wholemeal bread has a higher GI than rye bread and a pineapple has a higher GI than an apple. GI is not a simple function of the sugar content of foods and detailed tables showing the GIs of a wide variety of foods are now available.

There are day-to-day variations in GI due to physiological effects, and the GI of component foods can predict the likely effect of a meal. Sydney University studies have shown that low-GI foods may be beneficial for fat loss as they decrease hunger ratings. Due to the slower absorption of low-GI foods, they may result in less rapid fluctuations in blood glucose and appetite changes for some individuals.

Australian researchers are leading the world in GI research. For more information on this evolving topic, please refer to The GI Factor by Sydney University’s Professor Jennie Brand Miller.

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Like a lock that accepts only a certain key, cells of the central nervous system have receptors that accept and respond only to particular endorphins. For example, it appears that one receptor involved in stimulating appetite (called the “kappa” receptor) is designed to work only with the substance known as dynorphin. Dynorphin molecules fit into the kappa receptors. Nerve impulses then travel to the hypothalamus, which interprets and relays them as hunger cues. Appetite is thus stimulated, particularly for sweet-tasting foods.

Scientists soon found that rats given morphine (which increases endorphin activity) and allowed to choose from among the macronutrients tended to increase their fat intake while ignoring the carbohydrates. The investigators then administered drugs known to block the opiate receptors. Doing so, they found, tended to suppress eating. They found the same effect in humans. They then theorized that use of opiate blockers (also known as opiate antagonists) suppresses appetite by producing feelings of fullness or satiety.

But what triggers the release of endorphins in the first place? Further experiments found that food deprivation or stress can play a role. In other words, if you are under stress-pressure at work or school, for example-your body secretes endorphins to control the damage. Some of those endorphins ease any physical pain you might be feeling, while others work to stimulate your appetite, especially for sweet foods. You may respond to these signals from your hypothalamus by gobbling down a slice of pie, for example.

But why should eating sugar help relieve stress? Why doesn’t lettuce (to pick a food at random) have the same effect?

As it turns out, the very act of eating sugar stimulates the opiate-releasing process even further. Not only does sugar make the body release more endorphins, it also enhances the ability of the receptor to bind with the substance-like oiling a lock to make a key work better. Thus, eating sweet foods does indeed relieve stress, producing feelings of relaxation and contentment by enhancing the amount of natural painkillers floating around inside the body.

You might have spotted the flaw in this otherwise tidy little system. Here’s the problem: When a person is under stress, the body releases endorphins that stimulate the appetite for sweet foods. The person then eats a candy bar. The sugar in turn stimulates further secretion of endorphins, triggering greater appetite and leading to more consumption of sweets. Where does the cycle end?

As you might have suspected, it may not end. Theory has it that some people with bulimia might be caught up in the vicious cycle represented by the opiate-receptor feedback loop.

A bulimic will typically skip meals in the belief that doing so will keep her weight under control. But skipping meals only defers appetite; it doesn’t eliminate it. Eventually the urge to eat becomes overpowering, leading to a binge. Food deprivation itself can trigger the release of endorphins, which in turn stimulate appetite. Thus a bulimic who deprives herself of meals causes her body to produce a powerful natural appetite stimulant.

Many a patient reports that her binges occur in times of stress. Research has shown that stress also cranks up the endorphin system. Finally, when a binge leads to consumption of high-sugar items-ice cream, cookies, candy-even more opiates are released, stimulating appetite even further, and the vicious cycle kicks into high gear. For many, the only thing that disrupts this process is a drastic measure: self-induced vomiting.

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When Kay, age 55, recalls how she started losing weight, she laughs, “It was an accident.” Not that she didn’t wantto lose weight. Sure, she was carrying an unwanted 60 pounds on her frame; and of course she was tired of being mistaken for her daughter’s grandmother. But she simply couldn’t find the motivation to exercise.

Then Kay decided to organize her family’s books—all 5,400 of them—by cataloging them on her new computer. “I teach college geography, so I do a lot of reading,” she explains. “Besides, my family just loves books.” Each night, she’d carry several armloads upstairs, type up the data, and tote the books back downstairs. She’d stretch for 15 to 20 minutes to prevent next-day stiffness, then take a hot bath. By the time the project was complete, she had lost 5 pounds!

Kay decided that this was too easy not to continue. “Nothing breeds success like success,” she says. She stripped and repainted her daughter’s room and rearranged the attic. She shoveled snow in the winter and turned her garden into a “gym” in the summer. She took up walking, too, usually getting a 40- to 50-minute workout 5 days a week.

These days, Kay is a svelte 112 pounds, down from her heaviest of 172 pounds. Her family’s book collection is as organized as ever. And her house is so tidy that she’s even prepared for unexpected guests.

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