- ACRONYMS AND DEFINITIONS
- ADA - American Diabetes Association
- CVD - Cardiovascular disease
- GI - Glycemic Index
- GL - Glycemic Load
- RCT - Randomized controlled trial
- SBP - Systolic blood pressure
- USDA - United States Department of Agriculture
- DEFINITION OF GLYCEMIC INDEX (GI) AND GLYCEMIC LOAD (GL)
- Glycemic index (GI)
- The GI is a measure of how much blood sugar levels tend to rise after the ingestion of various carbohydrate-containing foods
- Carbohydrates have the most direct effect on blood sugar levels when compared to fats and proteins
- Carbohydrates come in different forms and their absorption in the intestine is affected by a number of factors
- The GI allows a comparison of different foods based on how they tend to affect blood sugar levels
- Glycemic load (GL)
- The amount of carbohydrate consumed in a meal will also affect the rise in blood sugar levels
- GL combines the GI and the amount of carbohydrates consumed to give a measure of total carbohydrate load
- By accounting for the amount of carbohydrates consumed, the GL can give a more accurate estimate of a food's effect on blood sugar levels than GI alone
- Carbohydrates - review of carbohydrates
- CALCULATING THE GLYCEMIC INDEX AND GLYCEMIC LOAD
- Glycemic index
- The glycemic index is calculated with the following steps
- 1. Ten healthy people are fed an amount of test food so that they receive 50 grams of carbohydrate from the food
- 2. Blood sugars are then measured on the participants at different intervals over the next 2 hours
- 3. The area under the curve for blood sugar response is then calculated for each person
- 4. On a separate occasion, the same people are given a reference food (typically white bread or 50 grams of glucose) and the same calculations are performed
- 5. The area under the curve for the test food is divided by the area under the curve for the reference food
- 6. This ratio is then multiplied by 100 to get the Glycemic Index
- 7. The GI values for each food from each participant are then averaged to give a final GI for that food [1,2]
- Glycemic load (GL)
- GL is equal to the GI of a food multiplied by its carbohydrate content in grams divided by 100
- GL takes into account the amount of the food that is eaten [1,2]
- Example:
- Raw apple has a GI = 52
- One large raw apple has approximately 31 grams of carbs
- If a person eats 2 raw apples their GL = 52 X (31 X 2) = 3224 /100 = 32.24
- GLYCEMIC INDEX CATEGORIES
- Overview
- There is no universally accepted range for what constitutes a low, medium, and high GI food
- The table below shows values that are generally used
GI category | GI value |
---|---|
Low GI | ≤ 55 |
Medium GI | 56 - 69 |
High GI | ≥ 70 |
- DIABETES
- Overview
- Because the GI measures the effect of foods on blood sugars, the people most likely to benefit from its use are diabetics
- A number of studies of varying design have evaluated the effect of the glycemic index on blood sugar control in diabetics. Most studies are very small and of suboptimal design.
- Two meta-analyses found that a low GI diet (average GI around 65) lowered A1C values by an average of 0.3 - 0.5% when compared to a high GI diet (average GI around 83) [3,4]
- ADA recommendations for the GI
- DIABETES PREVENTION
- A number of cohort studies have evaluated the relationship of dietary GI and GL with risk of diabetes
- Some studies have found a positive relationship between higher GI/GL diets and diabetes incidence while others have not
- There are no randomized, controlled trials that have evaluated the association between dietary GI values and diabetes risk
- In general, there is some observational evidence that diets higher in GI and GL may increase the risk of developing diabetes
- The evidence is not conclusive [11,12,13,14,15,16,17,18,19,20,21]
- WEIGHT LOSS
- CVD RISK FACTORS
- Overview
- The OmniCarb trial detailed below evaluated the effects of low and high GI diets on cardiovascular risk factors
- The OmniCarb trial enrolled 163 overweight adults
Main inclusion criteria
- ≥ 30 years old
- SBP 120 - 159 mmHg
- DBP 70 - 99 mmHg
- BMI ≥ 25
Main exclusion criteria
- Cardiovascular disease
- Diabetes
- Chronic kidney disease
- Taking medication that lowers blood pressure or lipids
- Fasting blood glucose level ≥ 125 mg/dL
Baseline characteristics
- Average age 53 years
- Average BMI - 32
- Average triglycerides - 105 mg/dl
- Average LDL - 153 mg/dl
- Average HDL - 58 mg/dl
- Average SBP - 132 mmHg
- Average insulin sensitivity - 7.3
Randomized treatment groups
- Patients crossed over between 4 different diets. Each diet was consumed for 5 weeks with a 2-week washout between diets.
- 1. High-carbohydrate, high-glycemic index
- 2. High-carbohydrate, low-glycemic index
- 3. Low-carbohydrate, high-glycemic index
- 4. Low-carbohydrate, low-glycemic index
- Low carbohydrate = 40% of calories from carbs. High carbohydrate = 58% of calories from carbs
- Low glycemic index = average GI ≤ 45; High glycemic index = average GI ≥ 65
- All of the meals were prepared for the patients and they were consumed under staff supervision
- Calorie intake was adjusted to maintain initial body weight
Primary outcome: Coprimary outcomes were insulin sensitivity (measured by Matsuda index), systolic blood pressure,
low-density lipoprotein (LDL) cholesterol, high density lipoprotein (HDL) cholesterol, and triglyceride levels. Outcomes were measured at the end of each 5-week diet period.
Results
Duration: 5 weeks on each diet | |||||
Outcome | Diet 1 | Diet 2 | Diet 3 | Diet 4 | Comparisons |
---|---|---|---|---|---|
Insulin sensitivity (lower numbers indicate less sensitivity) | 8.9 | 7 | 7.9 | 8.1 | 1 vs 2 p≤0.01 | All other p>0.05 |
LDL (mg/dl) | 139 | 146 | 138 | 138 | 1,3, or 4 vs 2 p≤0.01 | All other p>0.05 |
HDL (mg/dl) | 57 | 57 | 59 | 58 | 1 vs 3 p≤0.01 | all other p>0.05 |
Triglycerides (mg/dl) | 111 | 107 | 90 | 86 | 3 or 4 vs 1 or 2 p≤0.01 | 4 vs 3 p=0.02 | All other p>0.05 |
SBP (mmHg) | 124 | 124 | 123 | 123 | p>0.05 for all comparisons |
Findings: In this 5-week controlled feeding study, diets with low glycemic index of dietary carbohydrate, compared with high glycemic index of dietary carbohydrate, did not result in improvements in insulin sensitivity, lipid levels, or systolic blood pressure. In the context of an overall DASH-type diet, using glycemic index to select specific foods may not improve cardiovascular risk factors or insulin resistance.
- Summary
- This OmniCarb study surprisingly found that at high carbohydrate intake, a low GI diet decreased insulin sensitivity when compared to a high GI diet. The low GI diet also increased LDL cholesterol levels. These findings run contrary to the entire premise of a low GI diet.
- The two diets with lower carbohydrate content were associated with lower triglyceride levels
- A number of other studies have looked at the effects of a low GI diet on CVD and CVD risk factors. Results from these studies have been mixed. [32,33,34,35,36,37,38]
- In conclusion, there is no evidence a low GI diet reduces CVD or improves CVD risk factors
- CANCER RISK
- Insulin and insulin growth factors are thought to play a role in cancer development
- Higher GI foods tend to stimulate higher amounts of insulin secretion
- Because of this relationship, a number of observational studies have looked at the association of GI with the risk of various cancers
- Results from studies have been mixed
- In general, there is no conclusive evidence that higher GI diets increase the risk of any type of cancer [39,40,41,42,43,44,45,46,47,48,49,50]
- GLYCEMIC INDEX DATABASES
- The University of Sydney
- The University of Sydney provides a free online, searchable database of GI values for various foods
- The American Journal of Clinical Nutrition
- The American Journal of Clinical Nutrition issued a table in 2002 that lists the GI for over 750 foods
- The table is available as a free downloadable pdf file
- The table lists two GI values for foods because some labs use a 50 gram glucose load as the reference food and other labs use white bread as a reference food [52]
- CRITICISMS OF THE GLYCEMIC INDEX
- The glycemic index has been criticized for the following:
- Variability in the GI value for individual foods can be high
- The GI of a food is affected by other foods that are consumed with that food
- Following a strict, low GI meal plan is difficult and impractical
- Other dietary interventions (ex. carbohydrate counting, decrease calorie intake) have had better results in trials
- Beneficial effects of a low GI diet are mostly unproven in clinical trials [6,54]
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