The Role of Glycemic Index in Fat Loss and Disease Prevention

What is glycemic index ?

Glycemic index or GI is a system of grading for carbohydrates based on the response of blood glucose to them. The blood sugar response of a standard amount (usually 50 grams) of a given food item is compared with that of the same amount of glucose. The concept of glycemic index was invented at the University of Toronto by Dr. David J. Jenkins and his co-workers in 1981.

Carbohydrates that rapidly convert to glucose in the digestive system causing a rapid rise in blood sugar have a higher glycemic index. On the other hand carbohydrates that break down into glucose relatively slowly causing gradual rise in blood sugar have lower glycemic index.

The official definition of glycemic index of a food is the area under the two hour blood glucose response curve (AUC) following the ingestion of a standard amount of carbohydrate (usually 50 g). The AUC of the test food is divided by the AUC of the standard (usually glucose, whose glycemic index is fixed at 100, but earlier it was white bread) and multiplied by 100.[1]

Why is glycemic index important for my health?

For a healthy person high glycemic index food means higher demand of insulin in a shorter time which requires the pancreas to overwork resulting potentially in insulin resistance and probably other disease conditions of the Metabolic Syndrome including type 2 diabetes. This process of disease development is long term an may take several years. Whereas for a diabetic, the cost of high glycemic index food is immediate hyperglycemia (high blood sugar). Because the primary concern of a diabetic is to keep blood sugar in control, he or she should avoid high glycemic index food items.

Glycemic index of foods

Factors affecting the glycemic index of a food

Type of starch

Starch comes in many different configurations (amylose vs. amylopectin). Some are easier to break down into simple sugars than others. The starch in potatoes, for example, is broken down and absorbed into the blood rapidly

Fiber content

The sugars in fiber have bonding structure in ways that the body is unable to break. The more fiber a food has, the lower is its glycemic index.

Ripeness

Ripe fruits and vegetables have more simple sugar than unripe ones, and therefore tend to have a higher glycemic index.

Fat content and acid content

The more fat a food or meal contains, the slower its carbohydrates are converted to simple sugar and absorbed into the bloodstream and thus has lower glycemic index. In case of acid or vinegar the reverse is true.

Physical form

Finely ground grain is more rapidly digested, and so has a higher glycemic index, than more coarsely ground grain.

Entrapment

Trapping of the starch molecules within the food lowers the glycemic index.

Brown versus white bread

Brown breads are generally made from whole wheat which has higher amount of fiber than white breads. Therefore brown breads are expected to have lower glycemic index. However if enzymes are used in the preparation of brown bread, in order to soften the crust, the glycemic index of such brown breads is very high.

Disease prevention and glycemic index

There is ample scientific evidence to support the benefits of low glycemic index foods over the high glycemic index ones. Populations eating low glycemic foods for several years were shown to have lower incidences of type-2 diabetes and heart disease as compared to populations eating high glycemic index foods. This can be easily explained as follows: High glycemic index foods will cause many spikes of high blood sugar which will demand higher and frequent insulin levels leading to insulin resistance and metabolic syndrome including diabetes. Direct effect of the blood sugar spikes is the oxidative damage to blood vessels.[2]

The concept of glycemic index is supported by leading international health organizations including the American Diabetes Association.[3] 

Fat loss and glycemic index

Recent research on animals has provided strong evidence of association between high glycemic index carbohydrate and increased risk of obesity. In human trials, it is difficult to separate the effects from glycemic index and other factors such as fiber content, palatability, and compliance. In the study by Pawlak et al, 2004, male rats were separated into high and low glycemic index groups over 18 weeks while mean bodyweight was maintained. Rats fed the high glycemic index diet were 71% fatter and had 8% less lean body mass than the low glycemic index group. Postmeal glycemia and insulin levels were significantly higher and plasma triglycerides were threefold greater in the high glycemic index fed rats. Furthermore, pancreatic islet cells suffered "severely disorganised architecture and extensive fibrosis". The evidence in this study showed that continued consumption of high glycemic index carbohydrates would likely have led to the development of severe metabolic abnormalities.

Glycemic Load

Glycemic load categorization adapted from Foster-Powell K, Holt SH, Brand-Miller JC. International table of glycemic index and glycemic load values: 2002. Am J Clin Nutr 2002; 76:5-56. 

Points against Glycemic Index

The glycemic index has been criticized for the following reasons:

1. The glycemic index does not take into consideration other factors besides glycemic response, such as insulin response.
2. The glycemic index of a food varies depending on the kind of food, its ripeness, the length of time it was stored, how it was cooked, and its variety [1]).
3. The glycemic index of a food varies from person to person and even in a single individual from day to day, depending on blood glucose levels, insulin resistance, and other factors.
4. The glycemic index of a mixed meal is very difficult to predict. For example, fats and proteins can make a meal sit in the stomach longer, which reduces a food's glycemic index.
5. The glycemic index value is based on a portion that contains 50 grams of carbohydrate only. This criticism can be addressed by taking the Glycemic load into account.

References:

1. Brouns et al (2005). "Glycaemic index methodology." Nutrition Research Reviews 18; 145-171

2. Balkau et al (1998) "High blood glucose concentration is a risk factor for mortality in middle-aged nondiabetic men. 20-year follow-up in the Whitehall Study, the Paris Prospective Study, and the Helsinki Policemen Study." Diabetes Care 1998 Mar;21(3):360-7

3.  Sheard et al (2004). "Dietary carbohydrate (amount and type) in the prevention and management of diabetes: a statement by the american diabetes association." Diabetes Care;27(9):2266-71

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