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Glycemic Index A valuable measur

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Glycemic Index A valuable measur Powered By Docstoc
					   Glycemic Index:
Research Meets Reality
                             Outline
       Putting the GI into perspective
    -      History
    -      Definition(s)
    -      Measurement
  Limitations of the GI
  - Processing/preparation
  - Ripeness
  - Origin/variety
  - Inclusion of other macronutrients
  - Variability in the measurement
 The bottom line
                History of the GI

 The GI was initially developed to “standardize” the
  body’s glycemic response after eating a carbohydrate-
  rich food


 Today the GI is being touted as a dietary planning tool
  to promote weight loss, prevent obesity, and reduce the
  risk for chronic diseases despite a lack of scientific
  evidence to support such claims
                    Definitions of GI

 Often used (but inaccurate) definitions:
   - Rate of digestion and absorption of a carbohydrate-rich food
   - Blood glucose response to a carbohydrate rich food
 Accurate definition of GI is much more complicated:
   - The incremental area under the blood glucose response curve
     of a 50g portion of available carbohydrate of a test food
     expressed as a percentage of the response to the same amount of
     the reference food (i.e., white bread or glucose)
                       Measuring GI

 Standardized methodology for measuring GI
   - Subjects:
      - Typical sample size is 10-12 subjects
      - Subjects in a given sample must be matched for glucose tolerance
        (i.e., all subjects must be diabetic or glucose intolerant, or
        “normal”)
   - Protocol:
      - Subjects come into a laboratory after an overnight fast
      - Subjects consume, in random order, 50g of carbohydrate of the
        test food (e.g., red potatoes ) or reference food (e.g.,glucose) and
        come back another day to test the alternate food.
                             Measuring GI
- Protocol (cont.)
   - Blood glucose is measured every 15-30 minutes for at
     least 2 hours and values are plotted on a graph.
                     9                                   red
                     8                                   potatoes
                                                         glucose
    Plasma glucose




                     7
                     6
       (mmol/L)




                     5
                     4
                     3
                     2
                     1
                     0
                         0     30       60       90      120

                                     Minutes
   - Area under the curve (AUC) is calculated and GI is
     determined:
              - GI = [AUC of test food (e.g., red potatoes) ÷ AUC of reference
                food (e.g., glucose] ÷ 100
                       GI Classification

      Category           Glucose Reference         White Bread Reference



 Low                             < 55                         < 60

 Medium                         55-70                         60-85
 High                            > 70                         > 85

Brand-Miller. The New Glucose Revolution. 1996
Note: There was no research conducted to establish the cut-off values for the GI.
                             Outline
       Putting the GI into perspective
    -      History
    -      Definition(s)
    -      Measurement
  Limitations of the GI
  - Processing/preparation
  - Ripeness
  - Origin/variety
  - Inclusion of other macronutrients
  - Variability in the measurement
 The bottom line
                  Limitations of the GI
    Many factors affect the GI of a carbohydrate -rich food,
     limiting its practicality and applicability
    1.   Processing/preparation
    2.   Ripeness
    3.   Variety or origin
    4.   Inclusion of other foods or condiments
    5.   Variability in the measurement
         –   Time of day
         –   Between subject variation
         –   Within subject variation
                 Limitations of the GI
1. Processing/preparation- Alteration of food form
   -    Changing the physical form or particle size by mashing,
        grinding, rolling, even thoroughly chewing can increase the
        GI of a food
   -    Heating and cooling certain carbohydrates can result in the
        formation of resistant starch and produce a lower GI
       -   Example: Heating and cooling potatoes results in a
           significantly lower GI
   -    Cooking method and duration
   -    Chemical modification:
       -   Example: Acidity
                     Limitations of GI
2. Ripeness
   -    Ripening generally decreases GI
       - Example: the GI of a green banana would be higher than
           that for a ripe banana
3. Origin/Variety
   -    Variable GIs of similar foods from different countries
       - Russet Potato (Canada): 56
       - Russet Potato (USA): 76
       - Russet Potato (Australia): 85
   -    Variable GIs for different varieties of similar food
       - Long-grain instant white rice: 75
       - Long-grain wild rice: 54
                    Limitations of the GI
4. Addition of other macronutrients
   - Protein
      - Lowers the GI but increases insulin response
   - Fat
      - Lowers the GI, but increases the calorie content
   - Fiber
      - Impact of fiber on GI is controversial
             - Soluble vs insoluble
             - Experimental vs practical applications
                  brown rice: 55 vs white rice: 64
                  whole wheat bread: 71 vs white bread: 70
                  wheat spaghetti: 37 vs white spaghetti: 42
               Limitations of the GI

4. Addition of other macronutrients (cont.)
   -   Single foods vs mixed meals
       - Usefulness of GI in the context of mixed meals is
         controversial
       - GI of a single food much less important than
         overall GI of a given meal or snack
                  Limitations of the GI

5. Variability in the measurement:
   -   Time of day
       -   Morning vs afternoon
   -   Between similar foods
       - whole milk: 11-40          - ice cream: 36-68
       - glucose: 85-111
   -   Between & within subjects:
                               Standard        Coefficient of
                               Deviation        Variation(%)
       Within-subject:            15.9                   21
       Between subject:           5.8                    8
               Limitations of the GI


 Despite recent claims, research does not support the
  use of GI for weight loss
   - GI does not determine satiety
   - A low GI diet does not promote weight loss
   - GI is not correlated with nutrient density
                     Limitations of the GI

   GI does not determine satiety
    -   Holt et al. A satiety index of common foods. EJCN;1995:675-690.
        - Purpose: To examine the satiety value of different foods.
        - Methods:
            -   Subjects consumed 240 kcal portions of 38 common foods
                grouped into 6 categories:
                 -    Fruits (n=4)
                 -    Bakery products (n=5)
                 -    Snack foods (n=7)
                 -    Protein-rich foods (n=6)
                 -    CHO-rich foods (n=9)
                 -    Breakfast cereals (n=7)
            -   Subjective experience of satiety collected every 15 minutes
                over 120 minutes + ad libitum intake at a subsequent meal.
                       Limitations of the GI


 GI does not determine satiety (Holt et al. 1995)
             Results
                                           Glycemic Index (GI)
 GI was not associated with
  Satiety Index (SI)                   Relative to Satiety Index (SI)
                                        Food          GI        SI
 SI scores most strongly
  related to:
                                    Potatoes         85        323
  * serving size (+)
  * simple sugar content (-)        Lentils          31        133
 Protein, fiber, & water content   Ice cream        62         96
  was positively correlated with
  SI                                Orange           44        202
 Fat content was negatively
  correlated with SI
                           Limitations of the GI
 A low GI diet does not promote weight loss
             Reference       Duration              Diets                Weight Loss Differences
                                             (Isoenergetic)
  Jenkins et al. 1985        4 weeks    Low-GI vs Low Fat diet             Low GI > High GI
  Jenkins et al. 1987a       2 weeks      Low-GI vs High-GI              Not Significant (NS)
  Jenkins et al. 1988        2 weeks      Low GI vs. High-GI               High GI > Low GI

  Jenkins et al. 1987b       4 weeks      Low-GI vs High-GI                Low GI > High GI

  Brand et al. 1991          12 weeks     Low-GI vs High-GI                       NS
  Fontvielle et al. 1992     5 weeks    Low-GI vs High-GI foods                   NS
  Frost et al. 1994          12 weeks   Low-GI vs High-GI foods                   NS
  Frost et al. 1998          3 weeks    Low-GI vs High-GI foods                   NS
  Luscombe et al. 1999       4 weeks    Low-GI vs High-GI foods                   NS
  Jarvi et al. 1999          24 days    Low-GI vs High-GI diets                   NS
  Tshillas et al. 2000        6 mo       Low-GI vs High-GI breakfasts             NS
  Giacco et al. 2000         24 weeks   Low-GI vs High-GI foods                   NS

  Alfenas & Mattes 2004       8 days    Low-GI vs High-GI diets                   NS
                        Limitations of the GI

 A low GI diet does not promote weight loss (cont’d)

         Author           Duration                 Diets                 Weight Loss
                                            (energy restricted)          Differences
Wolever et al. 1992         6 weeks        Low-GI vs High-GI foods             NS

Slabber et al. 1994         12 weeks       Low-GI vs High-GI diets     Low-GI > High-GI

Spieth et al. 2000            4 mo         Low-GI vs Low-fat diets     Low-GI > Low Fat

Agus et al. 2000             6 days        Low-GI vs High-GI diets             NS

Heilbronn et al. 2002       12 weeks       Low-GI vs High-GI diets             NS

Ebbling et al. 2003           6 mo         Low-GI vs. Low-fat diets            NS

Pereira et al. 2004      10% weight loss   Low-GL vs High-GL diets    NS time to achieve wt
                                                                               loss
Frost et al. 2004           12 week        Low-GI vs Low-fat diets             NS
               Limitations of the GI

   GI  Nutrient Density


                       vs


                        vs

                            vs
                            Bottom Line

 GI is NOT practical
   - A complex physiological measure
   - Laboratory assessment does not readily translate to how we
     normally eat
   - GI is not an inherent property of a food
       - Readily altered by variety, origin, cooking, processing, and the
         addition of other nutrients and foods
   - Limitations of GI are numerous
 GI has not been shown to be associated with:
   - Satiety or satiation
   - Weight loss
   - Nutrient density
                      Bottom Line

 If not GI, then what?
       Small Steps to a
        Healthier You




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