DM: Carbohydrates (2007)

Citation:
 
Study Design:
Class:
- Click here for explanation of classification scheme.
Quality Rating:
Research Purpose:

To describe appropriate methods for determining and classifying the glycemic responses to different foods.

Inclusion Criteria:

Inclusion articles for studies not mentioned.

Exclusion Criteria:
None specified.
Description of Study Protocol:

Recruitment:

Article selection methods not specified.

Design

Narrative Review.

Blinding Used (if applicable):

Not applicable.

Intervention:

Not applicable.

Statistical Analysis:

Statistical analysis not completed.

Data Collection Summary:

Timing of Measurements:  not applicable

Dependent Variables:  not applicable

Independent Variables:  not applicable

Control Variables:  not applicable

Description of Actual Data Sample:

Initial N:  not applicable

Attrition (Final N):  not applicable

Age:  not applicable

Ethnicity:  not applicable

Other Relevant Demographics:  not applicable

Anthropometrics:  not applicable

Location:  not applicable

Summary of Results:

Glycemic Index should fulfill four criteria to be of clinical utility:

1. Consistency of values for the same food across space and time.

2. Application in individual subjects

3. Application to mixed meals

4. Demonstration of clinically significant therapeutic improvements by practical dietary changes.

 Glycemic Index Methodology:

1. Portions of test foods and white bread containing 50 g available carbohydrate are fed to normal or diabetic patients in random order on separate occasions after an overnight fast.

2. To reduce variability the standard food (white bread) should be repeated at least 3 times by each subject.

3. Capillary finger-prick blood samples are taken for normal subjects fasting and at 15, 30, 45, 60, 90 and 120 minutes after the start of the test meal, and for diabetic subjects fasting and at 30-minute intervals for 3 hr. The normal dose of insulin or oral hypoglycemic agent, if any, is taken after the fasting blood sample and 5-10 minutes before starting to eat the test meal.

4. The area under the glycemic-response curve for each food is expressed as a percent of the mean response to the standard food taken as a percent of the mean response to the standard food taken by the same subject, and the resulting values are averaged to obtain the GI value for the food.

5. Subjects with outliers (values >2 SD from the mean) should be retested twice. If the values are still outliers, the results should be discarded for that subject.

Food Portion Size

1. Food portion size has a major effect on the GI value because glycemic responses are related to the carbohydrate load.

2. The dose response for an intermediate GI food, bread, and for glucose appears to be nearly linear up to 50 g available carbohydrate, but the dose response flattens between 50 and 100 g.

3. Food-portion sizes based on food tables or food analyses that do not give accurate dietary fiber values may result in portion sizes that contain <50 g available carbohydrate, especially for high fiber foods such as legumes.

Choice of standard food:

1. Originally the GI was based on the response of 50 g glucose equals 100.

2. Because of the sweetness and osmotic effect of glucose, 50 g white bread has been used as the standard.

3. Other foods can be used as the standard but first need to be compared to 50 g glucose or white bread in an amount to provide 50 g glucose.

Number of standard tests:

Each subject should do at least 3 bread tests with the mean result used for GI testing of other foods to reduce the mean, variability, and skewness of the resulting GI distributions.

Length of time of blood sampling:

Measurements of area under glycemic-response curve need only be continued until blood glucose returns to baseline.

If measurements continue beyond the return to baseline, tends to reduce the differences in GI between foods.

Clinical utility of the glycemic index

Consistency of values across space and time

  • the mean CV for the 11 foods tested in different centers is 16%, similar to that seen in type 2 patients doing repeated tests of the same food, and less than that for normal subjects or subjects with type 1 diabetes

Application to mixed meals

  • The GI of mixed meals is expressed as the weighted mean of the GI values of each of the component foods, with the weighting based on the proportion of the total meal carbohydrate provided by each food.
  • the approach can be extended to total diets

Quantitative prediction of relative glycemic responses

  • the percent difference between meal GI closely predicts the percent difference between the mean meal glycemic responses

Qualitative prediction: ranking glycemic responses

  • in individual subjects the GI can be used to predict which of two mixed meals of equivalent macronutrient composition will have the greater glycemic response.

Therapeutic effects of low-GI diets

  • reducing the GI of the diet with no change in macronutrient or dietary fiber content results in modest but significant reductions in blood glucose in normal subjects and those with type 1 and type 2 diabetes, as measured by glycosylated hemoglobin.
  • a modest reduction in GI (~12) in hypertriglyceridic patients reduced triglycerides by 20% and cholesterol by 9%
  • low-GI foods may induce higher satiety

 

 

Author Conclusion:
  • Within limits determined by the expected GI dofference and by the day-to-day variation of glycemic responses, ranking the glycemic potential of different meals is possible.
  • Reducing the fluctuations in blood glucose after meals has only a modest effect on blood glucose.
  • The ability of low-GI diets to induce useful reductions in blood lipids in hypertriglyceridic patients is of therapeutic importance.

 

Funding Source:
University/Hospital: St. Michael's Hospital-University of Toronto
Reviewer Comments:
Narrative review.
Quality Criteria Checklist: Review Articles
Relevance Questions
  1. Will the answer if true, have a direct bearing on the health of patients? Yes
  2. Is the outcome or topic something that patients/clients/population groups would care about? Yes
  3. Is the problem addressed in the review one that is relevant to dietetics practice? Yes
  4. Will the information, if true, require a change in practice? Yes
 
Validity Questions
  1. Was the question for the review clearly focused and appropriate? Yes
  2. Was the search strategy used to locate relevant studies comprehensive? Were the databases searched and the search termsused described? ???
  3. Were explicit methods used to select studies to include in the review? Were inclusion/exclusion criteria specified andappropriate? Wereselectionmethods unbiased? No
  4. Was there an appraisal of the quality and validity of studies included in the review? Were appraisal methodsspecified,appropriate, andreproducible? No
  5. Were specific treatments/interventions/exposures described? Were treatments similar enough to be combined? Yes
  6. Was the outcome of interest clearly indicated? Were other potential harms and benefits considered? Yes
  7. Were processes for data abstraction, synthesis, and analysis described? Were they applied consistently acrossstudies and groups? Was thereappropriate use of qualitative and/or quantitative synthesis? Was variation in findings among studies analyzed? Were heterogeneity issued considered? If data from studies were aggregated for meta-analysis, was the procedure described? No
  8. Are the results clearly presented in narrative and/or quantitative terms? If summary statistics are used, are levels ofsignificance and/or confidence intervals included? Yes
  9. Are conclusions supported by results with biases and limitations taken into consideration? Are limitations ofthe review identified anddiscussed? No
  10. Was bias due to the review's funding or sponsorship unlikely? Yes