For a long time potatoes have been America's favorite vegetable, so why are diet books, popular magazine and self-proclaimed nutritionists now telling us to stop eating them? We have been persuaded into believing that this controversial glycemic index is the magic bullet to managing our weight!

The glycemic index was originally developed as a tool to help diabetics manage their diet. However, it has and is being promoted still as a dietary tool to help with weight loss and disease prevention. It is a lack of understanding of what the GI measures as well as its limitations that have led Americans to mistakenly believe that all starchy vegetables, such as potatoes, should be severely limited or eliminated completely from the diet.


Defining Glycemic Index

The fact is, regardless of the form in which a carbohydrate is consumed (e.g., starch, lactose, sucrose, fructose), almost all of them are then broken down into glucose and absorbed into the blood stream causing a temporary rise in blood glucose levels.

Let's look more closely at what the glycemic index really tells us. The glycemic index tells us the rate and extent to which a carbohydrate-rich food affects blood glucose levels relative to a reference food. Therefore, a food that causes a fast elevation in blood glucose is referred to as being a "high" GI food.  On the other hand, a food that causes a slow and more gradual elevation in blood glucose is referred to as being a "low" GI food. Foods that fall in the middle, causing a moderate elevation in blood glucose, are referred to as being "moderate" GI foods.


The Carbohydrate Type

Advocates of diets based on the glycemic index try to sell their diet plans by making this concept sound much simpler than it really is and suggesting that most starchy foods and certain fruits and vegetables have a high GI, so they should be avoided.

The main reason that the usefulness of the glycemic index as a tool for diet planning is questionable is that there are several factors that have shown to affect the GI value of a food.  Let's go over these factors.


RIPENESS: As a fruit ripens the GI value tends to decrease. For example, the GI of a green banana is higher than that of a yellow one.


PROCESSING: Grinding, pressing, mashing, rolling and even chewing a food real well can increase the GI value of a food.


PREPARATION METHOD: Believe it or not, the GI value of a starchy food can be significantly decreased by cooking and cooling it. This is why potatoes that are cooled after being cooked have a much lower GI than potatoes eaten immediately after cooking.


VARIETY OR ORIGIN: It is impractical to give a blanket GI value to potatoes because it can vary a lot depending on the variety and where they were grown.


INCLUSION OF OTHER FOODS OR CODIMENTS: Adding protein, fat or increasing the acidity of a carbohydrate food can lower the GI. For example, if you add sour cream, cheese, butter or salsa to a potato the GI will decrease, just like it would also by eating the potato with a piece of meat, fish or chicken. Therefore, since most foods are not eaten alone, the GI value is not very useful.


Another key reason why the GI is not very practical as a diet planning tool is that it does not correspond to nutrient density. For example, the GI of a potato is higher than that of ice cream, yet most of us would agree that a potato is more nutritious than ice cream.

The bottom line is that when it comes to any value for a food and its composition that is obtained in a lab, it is a value that is precise only when given in that lab where it was weighed and measured. Once these foods enter our world where they go through processing, storing, packaging, shipping, displaying, etc. these values will change to some extent.

One thing experts definitely agree with is that greater attention needs to be given to foods that are lower in calories and higher in nutrient density.

Renee Brunetti, LD/N, is a Certified Wellness Coach/Dietitian/Personal Trainer and is American College of Sports Medicine certified. She has 20 years experience in the fitness field as an instructor and personal trainer. For more information, email