The Importance of the Glycemic Index on the 5:2 Diet

Understanding the blood sugar's effect on the body and the glycemic index of various foods will go a long way in ensuring the successful completion of a fast. The glycemic index (GI) is an indicator of how quickly your body's blood sugar will rise after eating a particular food.

Because carbohydrates affect blood sugar to a much greater effect than proteins or fats, the GI deals mostly with foods that are higher in carbs but don't have a sizable protein or fat content (of course, there are exceptions to this rule).

Blood sugar levels matter because the higher your blood sugar is, the more insulin your body produces. And when your insulin is turned on, your body simply can't — and won't — burn fat. So instead, it stores it, increasing your risk for weight gain, diabetes, metabolic syndrome, and even cancer.

The GI attributes a number that is affected not only by what types of carbohydrates you eat, but also by how much you eat. This is called the glycemic load (GL). You can derive the GL with this equation:

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In the GI, each food gets a score out of 100. Stick to food choices that have a score of less than 50 or a GL score less than 20 because foods with a low GI/GL don't cause such a rapid surge in blood sugar. They're slower to digest and absorb in the body, thus helping to avoid a flood of insulin entering the bloodstream.

Furthermore, when talking about GI/GL, not all carbohydrates are created equal. It probably doesn't come as a surprise to anyone that a pear has a considerably lower GI than, say, pancakes slathered in maple syrup. Some foods have GI and GL scores that you might not expect.

For example, take dark chocolate. Fifty grams, or 1.8 ounces, of the bittersweet stuff has one of the lowest GI scores — a mere 23. On the other hand, one ounce of breakfast rice cereal has a GI score of 89.

The GI is important when you're fasting on the 5:2 Diet because the fact that higher GI/GL–scoring foods will encourage your body to release insulin and store fat; the inevitable blood sugar crash that comes after eating a high glycemic food will only make fasting that much more difficult.

You'll wind up with that uncomfortable empty feeling in your stomach and want to eat again, rather than enjoying longer levels of feeling full and satisfied if you choose foods that are lower on the GI/GL scale.

Fasting paired well with the Paleo Diet can help improve your results. You can't go wrong trying to stick to Paleo-friendly foods while on both your fast and nonfast days.

Only foods that have sizable carbohydrate levels are measured with the GI. That means that foods such as most nuts, eggs, and other protein- and fat-rich foods don't have a GI or GL score, which also means that they won't wreak havoc on your blood sugar levels.

Following are some Paleo-friendly foods with low GI/GL scores that are good for your two fasting days each week on the 5:2 Diet. On your nonfasting days, you can still reach for these foods as a snack to keep your GI/GL levels low.

Fasting Day Food Choices with Low GI/GL Scores
Food GI GL Portion Size (grams)
Apple 34 5 120
Banana 47 14 120
Carrots 35 2 80
Cashews 25 5 50
Grapefruit 25 3 120
Orange 31 3 120
Prunes 29 10 60

When planning out your fasting day foods, be sure to check serving sizes. Because you're only allowed 500 calories (for women) or 600 calories (for men) while following this diet, it can be all too easy to go over your allotted calories, if you don't keep serving sizes in check. You can also search online for food calculators that can help you figure sizes of different foods.

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