If you have been following the news in the world of nutrition, you are no stranger to the all too familiar pendulum swing of the use of the glycemic index or GI for meal planning. This article will:
- Define the GI and GL (glycemic load),
- Explore the controversy
- Outline the current research
- State the American Diabetes Association position statement and
- Summarize the findings
What is the GI? : The GI is a means of ranking carbohydrate containing foods based upon how they affect blood sugars after consumption. To rank a food, 50 grams of digestible carbohydrate from that food is compared with a reference food such as glucose or bread, and then assigned a value1.
Low GI foods: In general, foods with a GI value lower than 55 are considered “low GI”, and include not surprisingly, most all fruits and vegetables, legumes and lentils, oat bran, all bran and other whole grains such as bulgur and barley. (For an extended list of GI values, please visit this site: http://ajcn.nutrition.org/content/76/1/5.long#sec-5 and click on Table 1).
High GI foods: Not surprisingly, white bread or a bagel, and corn flakes (all refined, low fiber foods) have a high GI (>70), and instant oatmeal, unlike it’s superior, steel cut cousin has a higher GI value. However, there are a few surprises as noted further down in this article.
Controversy surrounding the GI: From the time of it’s inception in 1981, the GI has sparked controversy among healthcare professionals. For starters, the variability in the glucose response within and among individuals1 to apparently similar foods has raised eyebrows among a few.
Why the variability?
Different ingredients and different methods: Two different commercial brands of the same type of food may have been processed differently, using slightly different ingredients in slightly different proportions, so that the rate at which each food is digested and absorbed in the GI tract varies and in turn this can potentially impact the GI of that food.
Different GI testing methods : A difference in the methods used to test the GI of test foods in different parts of the world may also cause a significant variation in the GI of apparently similar foods.
Other factors impacting GI:
- Fat and fiber: Fat and fiber in general, slow down digestion; hence high fiber foods for the most part have a lower GI, as do high fat foods such as ice-cream.
- Degree of processing and cooking: Pasta cooked al dente has a lower GI than pasta cooked till it’s all mushy. Refined grains that are pulverized to fine white flour have a higher GI because of the greater surface area for digestive enzymes to act on, whereas coarse grains that have an intact outer bran and germ layer have a lower GI as it takes your digestive enzymes longer to break down these foods. This creates a case for eating grains in their whole form, rather than processed whole grain bread.
- Ripeness: The riper the fruit or vegetable, the higher the GI.
- Acid content: Foods containing acid have a lower GI. For e.g., acetic acid, the biologically active component of vinegar, inhibits the activity of enzymes that digest carbs, thus fewer carbs are converted to sugar, leading to a lower GI.
GI surprises: Also, the GI may not always be an indicator of healthy food choices. For e.g. Coke has a moderate GI of 581, despite the astronomical amount of sugar, premium ice-cream (high fat) has a very low GI of 371 and a Snickers bar a moderate GI of 561. As noted above, the higher fat content in icecream contributes to it’s lower GI value by slowing down digestion. Ironically, instant oatmeal, albeit not stellar, actually has a higher GI than these foods. Go figure!
Clearly, the GI system is not perfect and if your meal choices are based solely upon the GI, you could see how you could potentially end up with food choices that are less than desirable!
Quality and quantity: While the GI of a food may have some value in assessing it’s impact on your blood sugar, it does not tell the whole story. The amount of the carbohydrate that is actually consumed ultimately determines the rise in sugar and the subsequent insulin response.
Glycemic load: A more accurate representation of a food’s impact on blood sugar can be obtained by estimating the glycemic load of a food, obtained by multiplying the glycemic index by the amount of carbohydrate in grams provided by a food, and dividing the total by 100. GL = (GI x the amount of carbohydrate)/100.
Putting the GL into practice: For e.g. although the GI of carrots is high at 921, a 1/2 cup of carrots has a scant 5 grams of carbs, thus driving its GL into the ground at 5! As a rule of thumb, glycemic loads below 10 are considered low, and glycemic loads above 20 are considered high. The dietary glycemic load is the sum of the glycemic loads for all foods consumed in the diet. This also implies that by combining foods with low and high GI, you may be able to balance food choices in a meal.
That said, published GI values for carrots can vary from 92, (one of the oldest study) to 32, once again underscoring how GI tables may display inconsistent values.
Sifting through the evidence: Given all the controversy, this warrants a quick review of the research so far:
Cooper Center Longitudinal Study: Among other studies, a large epidemiological study of 10,912 men and women, demonstrated that a higher glycemic load diet may be linked with high triglycerides (form of fat in blood), low HDL (healthy cholesterol), a larger waistline and elevated blood sugar. These associations were free of confounding variables. Although a cross-sectional study cannot prove a causal relation, the outcomes from this study suggest that consuming a low glycemic index and glycemic load diet is associated with an improved metabolic risk profile.
Pendulum swings back: The Omnicarb Randomized Clinical trial : Of course, more recently, a 5-week controlled feeding study in 163 overweight subjects demonstrated that low GI diets compared with high Gi diets did not improve insulin sensitivity, lipid levels, or systolic blood pressure. The study concluded that in the context of an overall DASH-type diet, (a healthy diet high in fruits, vegetables, low in saturated fat with low fat dairy products), using the glycemic index to select specific foods may not improve heart disease risk factors or insulin resistance. (body’s inability to utilize available insulin, increasing the risk of diabetes down the road)
Systematic review of literature: The evidence from the Clinical trial above gets a nod from this 2010 systematic review of the literature, which showed that in general, there is little difference in glucose levels and heart disease risk factors between low-GI and high-GI or other diets. A slight improvement in glucose may be seen with a lower GI diet, but it is unclear if fiber may have skewed the results of these studies. The review also emphasized the need for a standardized definition of low GI.
ADA statement: In a position statement published in Jan 2014, The ADA recognizes that the literature on GI is complex.
For individuals with DM: Both the type and amount of carbs in a meal affect blood glucose levels. However the total carbs may be the primary driver of the glucose response, hence for individuals with diabetes, monitoring carb intake by carb counting or some form of carbohydrate estimation as a primary strategy to achieve optimal glucose control, is recommended. There is some limited evidence that substituting low glycemic load foods with high glycemic load foods might modestly improve glucose control.2 The GI may be used as an additional tool to “fine tune” blood sugars further.
Putting the GI in perspective:
If you have diabetes: The broader body of nutrition research shows that a modest 5-10% wt loss, portion control, and emphasizing a diet rich in whole grains, legumes, fruits, vegetables and decreased saturated fat would be key, and this is easier for consumers to follow than looking up complex tables and calculating glycemic loads. Consumers may have a hard time grasping the fact that a low GI may not equate to “healthy” as seen with the few examples above.
However, with portion control and higher fiber foods, you would by default, positively impact glucose levels as well as cholesterol and weight. Use the GI/GL as a “back up” to these primary goals noted above. An individual consultation with a Registered Dietitian is advised to create a meal plan tailored to your specific medical needs
If you do not have diabetes, the same principles noted above would apply. Choosing low GI foods does not offer any additional benefit if you are already following a healthy eating pattern high in fruits, veggies and whole grains and low in saturated and trans fats. And what’s more, you do not need an app or calculator to do that!
- The Art and Science of Diabetes Self Management Education, Desk Reference, Second Edition.
Disclaimer:This blog is strictly for informational purposes only, and should not be construed as medical advice. Please consult your doctor or dietitian for recommendations tailored to your unique needs.