Glycemic Index and Glycemic Load

Most carbohydrates we eat, from starch to table sugar, share a basic biological property: they can be digested or converted into glucose in the body. Glucose, in turn, enters the blood stream (where it is commonly called blood sugar) and from there reaches all body tissues and provides an efficient energy source.

Not all carbohydrates are the same

The digestion and absorption rate of consumed carbohydrates is a major factor that determines the blood glucose (blood sugar) response. The faster the digestion rate, the higher the blood glucose spike. It used to be thought that the chain length of the carbohydrate molecule determined the digestion rate, and blood sugar response of consumed carbohydrates. The saying was, the longer the chain length, and the more complex the carbohydrate molecule, the slower the digestion rate. This gave rise to the classification of carbohydrates as complex (which are long chains of glucose units joined together, also called starch or polysaccarides) and simple (single glucose or sugar units, also called monosaccarides). This view has been reflected in nutritional recommendations that advocate increased consumption of starchy foods and decreased consumption of sugar (1).

However, throughout the past 30 years, the relevance of the chain length in determining carbohydrate digestion rate has been questioned. Today it is well known that many starch rich foods raise blood sugar (and insulin, which is secreted by the pancreas in response to increases in blood sugar levels) as much, or even more, than comparable amounts of table sugar (2-4). Also, common starchy foods often result in different blood glucose responses (5). Thus, this structural classification of carbohydrates into simple and complex, which was popular in the 1980s and 1990s, does not reflect the physiological impact of the carbohydrates (that is, how our body reacts to them). For this reason, the glycemic index (GI) was developed for classifying carbohydrate-containing foods according to their ability to raise blood sugar levels after consumption, also called the glycemic response (6, 7).

The glycemic index

The glycemic index is defined as “the incremental area under the blood glucose response curve after a standard amount of carbohydrate from a test food relative to that of a control food (either white bread or glucose) is consumed” (8, 9). A simpler description of the glycemic index is that it classifies carbohydrates based on their effect on blood sugar levels.

The recommendation is to choose carbohydrate containing food with a low GI-value (that is, “slow” carbohydrates) (8, 10-13). Slow carbohydrates (foods with low GI-values) are beneficial because they result in a slow, sustained increase in blood sugar levels and keep it more stable, than “fast” carbohydrates (foods with high GI-values), which cause a fast, transient spike in blood sugar levels, and also increased insulin demands.

When comparing tables with GI values, make sure to first check whether the reference food is glucose or white bread, because tables that use glucose as a reference against which the foods are compared (that is, 50 g glucose = 100) give GI values that are about 30% lower than tables that use white bread as a reference (that is, 50 g white bread = 100). To convert between the two, just multiply the white bread based GI values by 0.7, or divide the glucose based GI values by 0.7.

It is important to remember that the glycemic index value for a given food is based on the standard amount 50 g carbohydrates. This means, that in order to obtain the glycemic effect that is indicated in a glycemic index value, one needs to eat an amount of that food which contains 50 g carbohydrates. Thus, the glycemic impact of any given food is dependant also on the amount of carbohydrates that it contains (14-17). Therefore, the glycemic load was introduced in order to allow comparisons of the glycemic effects of realistic portions of different foods (17-22).

The glycemic load

The glycemic load is calculated by multiplying the GI value of a food with the number of grams of carbohydrates (not including fiber) in a serving of that food, and dividing the total by 100 (14, 17-19, 22).

The table below shows the classification boundaries of low, medium and high glycemic index and glycemic load values (19, 22).

  Glycemic Index Glycemic Load
Low 55 and below 10 and below
Medium 56 - 59 11- 19
High 70 and higher 20 and higher

The importance of the glycemic load can be exemplified with carrots. Although the glycemic index for carrots is reported to be as high as 131 (7), the glycemic load for one serving of carrots is small because the amount of carbohydrate in one serving of carrots is minimal (about 7 g carbohydrate). Indeed, on would have to eat 1.5 lb (0.7 kg) of carrots (which provides 50 g carbohydrate) to produce a glucose response of 131!

Because the glycemic load of a food is determined by multiplying its glycemic index by its available carbohydrate content (in grams) per serving, the primary question becomes which of these factors is the stronger determinant of the food’s glycemic load. Is the amount of carbohydrate in a food or its glycemic index the most important part of its glycemic load?

To answer this question, scientist performed a statistical analysis of the relations between GI, GL, and carbohydrate content. By doing so they were able to show that the carbohydrate content alone explained 68% of the variance in glycemic load values, whereas the glycemic index value alone explained 49% of the variance in GL values. But although the carbohydrate content of a food has a great impact on the glycemic load than does its glycemic index, one has to remember that the glycemic index is a better general marker for a foods nutrient quality, since whole, unprocessed foods have a lower glycemic index value and lower nutrient content, than do their processed counterparts (23-26). Hence, although carbohydrate is indeed the greater determinant of GL, it makes no sense to ignore a factor that accounts for almost 50% of the variation in glycemic load (22).

Knowing that the glycemic load reflects the overall glycemic impact and insulin demand of eaten foods, it is time to turn our attention its implications for disease risk and health promotion.

Why should I care about the glycemic index and glycemic load?

Regular consumption of high-glycemic index (or high-glycemic load) meals, compared to low glycemic index (or low-glycemic load) meals containing the same amount of calories and nutrients, results in higher 24-hour average blood sugar and insulin levels (27, 28). And the other way around, substituting foods with low a glycemic index (low glycemic load) for those with a high index (high glycemic load) lowers blood sugar and insulin responses (28-30), and reduces insulin production/secretion and glycated hemoglobin concentrations, in both diabetic and non-diabetic subjects (15, 16, 30).

White this has obvious implications for management of diabetes (in fact, the glycemic index was first developed to aid in dietary management of diabetics7) (14, 31-33), the glycemic impact and diet induced insulin demand is also of importance to healthy non-diabetic individuals. The reason for this that eating foods with a low glycemic index (and low glycemic load) results in more even blood glucose levels, less insulin demand and improved blood lipid profiles. This in turn offers several health benefits that reduce the risk for developing many of the major chronic diseases, notably diabetes, heart disease, obesity and cancer (10, 13, 34-36).

Prevention of diabetes

Calorie for calorie, high glycemic index (and high glycemic load) meals result in higher blood sugar levels and stimulate more insulin secretion than low glycemic index (low glycemic load) meals (9, 11, 27, 28). High blood sugar levels and excessive insulin secretion in turn both contribute to the loss of the insulin-secreting function of the pancreas, which leads to insulin resistance and can cause irreversible diabetes (37). Support for the importance of the glycemic index and glycemic load for blood sugar and insulin control comes from studies which have shown that a high dietary glycemic load (i.e., the product of the glycemic index of a specific food and its carbohydrate content) has been associated with an increased risk of type 2 diabetes, while a lower dietary glycemic load is associated with a reduced risk of type 2 diabetes (20, 21, 33).

Prevention of heart disease

High intakes of carbohydrate rich foods with a high glycemic index and high glycemic load have been associated with insulin resistance (28, 29,38) a lower concentration of HDL (the “good” blood cholesterol) (39-44), a higher concentration of LDL (the “bad” blood cholesterol) (43), and elevated blood triglycerides (also called hypertriglyceridemia) (28, 39, 41, 43). It is well known that high blood sugar levels (hyper-glycemia), high insulin levels (hyper-insulinemia), insulin resistance and associated disorders of lipid metabolism are important risk factors for coronary heart disease (45-52), even in previously healthy individuals (49, 53, 54). It has also been shown that chronically high insulin levels increases cardiovascular morbidity and mortality (55), and that consumption of high glycemic index and high glycemic load foods from refined carbohydrates increases the risk of coronary heart disease, independent of known coronary disease risk factors (56). People who consumed diets with the highest glycemic loads had a risk of developing coronary heart disease over the next 10 years that was almost twice as high as those who consumed the lowest glycemic loads (56). Other studies support the protective effect of a low dietary glycemic load on the risk of coronary heart disease (57).

Prevention/Treatment of Obesity

Obesity can be described as the "New World Syndrome", and its prevalence is on continuous rise in all age groups (58-66). Thus, effective strategies to combat this trend are urgently needed. An increased physical activity and improved dietary habits are cornerstones in our battle against expanding waistlines (67, 68). When it comes to diet, more and more studies show the importance of the glycemic index and glycemic load for body fat loss and weight control.

Low-GI foods may benefit weight control in at least two ways (69):

1) by promoting satiety and,

2) by stimulating liberation of stored body fat and promoting fat burning (fat oxidation).

These beneficial effects of low-GI foods stem from the slower rates at which they are digested and absorbed, and the ensuing effects on blood sugar levels and insulin responses (12, 69). In the first two hours after a meal, blood glucose and insulin levels rise higher after a high-glycemic load meal than they do after a low-glycemic load meal containing equal calories (9, 14-16, 30). However, in response to the excess insulin secretion, blood glucose levels drop lower (often to below fasting levels) over the next few hours after a high-glycemic load meal than they do after a low-glycemic load meal (12, 69-71). The high insulin levels after consumption of high glycemic index and high glycemic load meals also drastically reduce the levels in the blood of another energy fuel, free fatty acids (12, 69, 70). When blood glucose levels drop fast and too low, together with lowered free fatty acids, the body interprets this as a "low fuel status" (12, 69, 70), and takes action to restore the readily available energy supply in the blood stream. One major way of doing this is to stimulate appetite, hunger and food intake. Support for this come from many studies, which have all shown that decreases in blood sugar (blood glucose) levels, even if they are modest and transient, are associated with hunger and spontaneous meal requests (72-77). This is one explanation for how low-glycemic index foods delay the return of hunger, decrease subsequent food intake, and increased satiety when compared to high-glycemic index foods. Additional mechanisms may also account for the differences in the satiating effects of high- and low-GI foods. Because low glycemic index (and low glycemic load) foods are characterized by slower rates of digestion and absorption in the small intestine, nutrient receptors in the gastrointestinal tract are stimulated for a longer period of time, resulting in prolonged feedback (through signals such as cholecystokinin and glucagon-like peptide-1) to the satiety center in the brain (72, 78-80). In line with those mechanisms, the results of several studies indicate that compared to high glycemic load meals, low-glycemic load meals are significantly more satiating (12, 70, 71, 81-83).

Differences in glycemic index and glycemic load also dictate differences in fuel partitioning in the body. When glucose and insulin concentrations spike in the blood after food ingestion, the ability of the body to liberate stored fat and to burn the fat for energy, is severely inhibited 84-86. This is an important effect since reduced rates of fat burning (fat oxidation) has been linked with greater body fat gain (87, 88).

Thus, studies on both appetite regulation and energy metabolism suggest that low glycemic-load food habits may be useful in promoting weight (fat) loss and weight control, and decreasing the prevalence of obesity. Further support for this comes from a study which showed that consumption of a low-GI diet for 5 wk, compared with a high-GI diet of equal caloric content, decreased body fat mass by 1.5-2.2 lb (0.7-1.0 kg) despite no difference in exercise or other lifestyle factors (89). It is interesting that the fat reduction was mostly from around the waistline (89).

You might wonder how much you need to reduce your glycemic load in order to achieve a beneficial effect on fat loss. While there is no one-size-fits-all, a good starting point is to reduce your dietary glycemic load with at least 17 gram glucose equivalents per day (90).

Washboard waffles as part of a low glycemic index and low glycemic load diet

Because of the importance of the glycemic effect and insulin for health promotion and disease prevention, the washboard waffle and pancake mix was formulated with unrefined whole grains that have a low glycemic index. Together with a reduced carbohydrate content, the washboard waffle also has a low glycemic load. This makes it an ideal food for both prevention and management of the above mentioned diseases, and for health promotion!

Numbers within parentheses refer to references