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Dietetic Internship

Food for Thought

April Meredick

Glycemic Index: Another Fad Diet?
Many scientists believe that the glycemic index (GI) of foods is an asset in the prevention and treatment of obesity, diabetes mellitus, and heart disease (1).  Others find flaws with the glycemic index due to its irreproducibility of values among foods, its complexity, and its resemblance to other “miracle diets.”

What is the Glycemic Index?
The glycemic index is a scale that measures how quickly foods and beverages raise blood glucose for two to three hours after consumption (2).  With two decades of research available, there are now glycemic indices available for over 300 foods (Table 1.).  Researchers have been testing reproducibility of responses, mixed meals effects, and clinical functionality of GI in the treatment of diabetes, obesity, and coronary heart disease (1).  The glycemic index remains a controversial tool in the United States due to its lack of in-depth research, its complexity, and its true value in treatment.

There are two types of glycemic index currently in use.  The first type of GI uses a value of 100 to 50 grams of pure glucose as its base.  Pure glucose raises blood glucose faster than any other carbohydrate source except maltose.  When one uses this index, white bread has a GI of 70.

The second GI uses a value of 100 to 50 grams of white bread.  In this index, glucose has a GI of 138.  In both types of glycemic index, foods that raise glucose concentration at a rapid rate have a high GI, whereas foods that raise glucose at a slower rate have a lower GI.

Food

Glycemic
Index

Food

Glycemic
Index

Pinto beans42Pineapple66
Oatmeal cookie55Watermelon72
Whole wheat bread72Apple38
Cheerios cereal74Brown rice59
Special K cereal54Sweet corn55
Bran muffin60Parboiled white rice47
Pound cake54Orange juice55
Fruit cocktail55Milk44
Grapes43Brown rice pasta92
Dried Apricot30Spaghetti40
Table sugar65Carrot95
Baked potato93Jellybeans80


Pros in Using the GI

Research shows that foods producing the highest glycemic response are typically starchy or convenient foods in our society such as bread, breakfast cereals, and potatoes (1). The glycemic response of starchy foods is independent of their fiber content. Since the starch is fully gelatinized and is digested very quickly, researchers believe the high glycemic response is responsible for many health problems (1). Diets high in rapidly digested carbohydrates increase blood glucose levels and insulin response; therefore, people with diets high in rapidly digested carbohydrate may be prone to Type 2 diabetes mellitus, obesity, and coronary heart disease (2).



People with diabetes show greater insulin resistance than those without diabetes, and frequent consumption of high-GI foods may exacerbate glycemic and insulin responses. Higher insulin levels throughout the day produce carbohydrate oxidation at the expense of fatty acid oxidation, promoting synthesis of very low density lipoproteins (VLDLs) in the liver and fat storage in adipose tissue (1).  Since triglycerides are most concentrated in VLDLs, excessive production of VLDLs may lead to coronary artery disease. Some research even “blames” hypertension specifically on diets abundant in high GI foods (3).

Low GI diets may have a positive effect on weight control since they may enhance satiety and reduce insulinemia (1). Research shows that isocaloric meals of different glycemic indices produce different responses in blood glucose levels. Consumption of high GI meals was found to have different effects on metabolism, perceived hunger, and subsequent food intake on various subjects. After a high GI meal, there was generally an increase in counterregulatory hormones, producing increased hunger and food intake (4). Due to hormonal elevations after consuming a high GI diet, one may be more prone to overeating and becoming obese than those who consume a lower GI diet.

Cons in Using the GI

Since a food’s effect on blood sugar levels differ with a food’s ripeness, cooking time, amount eaten, GI of other foods in the meal, fiber content, time of day, a person’s blood insulin levels, and recent activity, glycemic index values are not absolute (2). For example, the carbohydrate content of a banana changes as it ripens (Table 2). A green/yellow banana has a lower GI than a yellow/brown banana (5). Instant white rice boiled for one minute has a GI of 65; however, it has a GI of 128 when boiled for 6 minutes (2).

Table 2.  Change in carbohydrate composition of bananas during ripening. (5)

Time (days) and Appearance

0 – green with some yellow

2 – yellow with some green

4 - yellow

6 – yellow with few black spots

8 – yellow with many black spots

Starch82412693
Sugar748638188


Another major problem with GI is that it labels foods as “good” or “bad” (6). However, foods from all the food groups make up a healthy and well balanced diet.  For people with diabetes, it is more important to regulate how much carbohydrate, rather than which type of carbohydrate, is eaten per meal (6).  In counseling patients, we can’t say that there are “good” versus “bad” foods based on the GI.  If we classify some carbohydrate foods as “better” than others, clients may overeat or overuse the “good” choices.  People with diabetes might eat unlimited portions of low GI foods only to wonder why their blood glucose levels have “mysteriously” increased.  Or they may substitute potato chips (low GI) for boiled potatoes (high GI) thinking that they have made an excellent dietary choice (1).

Carbohydrate counting, which entails obtaining the content of carbohydrate from food labels or from having knowledge of average portion sizes containing 15 grams of carbohydrate, can be difficult to learn for people with diabetes.  The addition of low, moderate, and high glycemic index foods would add complexity and another step to meal planning (6).

There are many other potential problems with the glycemic index.  For example, reproducibility of glucose response in the same subject has not been studied extensively for many foods.  There is variability in glucose response that remains unexplained.  Also, subjects should be studied when their glucose concentrations return to basal levels (6).  This has not yet been done.

Subjects’ glucose concentration prior to eating a meal affects their gastric motility and gastric emptying.  Therefore, there is much variance in subjects’ glucose concentrations after the ingestion of equal glucose loads.  In fact, the gastric emptying rate accounts for 36% of variance in glucose concentration (6).

Another commonly recognized flaw of GI is mixed meal response.  Research shows that mixed meals created using the GI did not result in the same post-meal glucose response that is used for the individual food (6).

American Diabetes Association

The American Diabetes Association believes that the glycemic index is a difficult diet concept to follow, and it questions the clinical utility of the glycemic index (7).  The American Diabetes Association recommends that people with diabetes concentrate on the amount of carbohydrate they consume per meal, rather than the type of carbohydrate.  Medical nutrition therapy for people with diabetes is based on the individual, with consideration given to eating habits and lifestyle factors (7).  Treatment goals then determine nutrition recommendations.

Is the glycemic index just another fad?
Many clinicians believe in the research on GI and educate patients with obesity, diabetes, and heart disease on how to utilize this food index.  According to Table 1, however, it appears that clients should avoid carrots and choose jellybeans instead.  As Table 2 demonstrates, there is much variation in food at different stages whether it is related to the ripeness, preparation, or processing.  While the glycemic index may have some merit, interpreting its usefulness and validity would be difficult for most clients and clinicians.

The GI is not a valid tool for determining glucose response of foods.  It is important to remember the total amount of carbohydrate eaten, not the carbohydrate source, is the critical factor affecting blood glucose levels (8).  For example, in diabetes mellitus, it is the total carbohydrate consumed per meal that impacts blood glucose levels.  Similarly, in obesity and coronary artery disease, it is the total amount of food eaten per day that impacts weight.  Excess calories are converted to fat.  Therefore, the more calories people eat, the more weight they will gain regardless of which type of carbohydrate they choose.

References:

  1. Brand-Miller R, Foster-Powell K.  Diets with a low glycemic index: from theory to practice.  Nutr Today 1999; 34(2):64-72.
  2. Englyst K, Englyst H, Hudson G, et al.  Rapidly available glucose in foods: an in vitro measurement that reflects the glycemic response.  Am J Clin Nutr 1999; 69:448-54.
  3. Liu S, Willett W, Stampfer M, et al.  A prospective study of dietary glycemic load, carbohydrate intake, and risk of coronary heart disease in US women.  Am J Clin Nutr 2000; 71:1455-61.
  4. Ludwig D, Majzoub J, Al-Zahrani A, et al.  High glycemic index foods, overeating, and obesity.  Peds 1999; 103(3):656-68.
  5. Englyst H, Cummings J.  Digestion of the carbohydrates of the banana by the human small intestines.  Am J Clin Nutr 1986; 44:42-50.
  6. Franz M.  In defense of the American Diabetes Association’s recommendations on the glycemic index.  Nutr Today 1999; 34(2):78-81.
  7. American Diabetes Association.  Position statement: Nutrition recommendations and principles for people with diabetes mellitus.  http://journal.diabetes.org.
  8. Franz M.  Current therapies for diabetes: lifestyle modifications for diabetes management.  Endo and Metab Clin 1997; 26(3):499-510.