Understanding the Glycemic Index
July 24, 2010
The Glycemic Index (GI) is one of the most helpful and straight-forward tools available when it comes to choosing which foods to consume and which foods to avoid. Utilized primarily by people to help treat their Type 2 diabetes, the Glycemic Index lists many common foods and their impact on blood sugar levels. The Glycemic Index rates foods on a scale of 1 to 100 (sometimes the scale can go up to ~140). A high score indicates that eating that food will cause a large increase in your blood sugar, while a low score indicates that a food will have a minimal effect. Generally speaking, foods that are high in sugar, simple carbohydrates and are highly processed have high GI scores and foods that are low in carbohydrates and are relatively unprocessed have low GI scores. To the left is an example Glycemic Index chart showing the GI values of some common foods.
So why is blood sugar important in the first place and what role does it play in your weight loss efforts? The answer to this question, like so many others, can be better understood through the lens of human history. For the vast majority of human history the problem has been too little available food, rather than too much. Furthermore, the widespread availability of refined sugar is a relatively recent phenomena, beginning in the mid 1800's. For thousands of years of human development, there was no scoop of sugar to sweeten that hot cup of java (for most cultures, there was no coffee, either). There were no sweetened cereals, no high-fructose corn syrup, no cookies, no candy and no soda.
In short, there was very little sugar in the human
diet, with the exception of the fructose that naturally occurs in fruit and the lactose that naturally occurs in milk. This increase in sugar consumption is illustrated by the chart at right. Instead, the human diet was composed of whole grains, meats, fish, eggs, vegetables, fruits, milk, tubers and other basic foods. Against this dietary backdrop, thousands of years of natural selection optimized the human body to utilize this diverse array of basic foods. However, in our recent history, advancements in farming, shipping, food processing and lifestyle have led to dramatic changes in the average diet, changes that are inconsistent with our historical diet. Many of the metabolic functions that humans have evolved to survive cycles of abundance and famine wreak havoc on our bodies when confronted by our modern diet. While occasional peaks in blood sugar levels are completely normal and well tolerated by the body, consistently elevated blood sugar and frequent spiking of blood sugar levels has a wide range of deleterious effects. The most well known effects of excessive blood sugar levels are weight gain and insulin insensitivity, also known as Type 2 Diabetes.
So how can utilizing the Glycemic Index help you control your weight? Because using the glycemic index can help you moderate your blood sugar levels, and high blood sugar levels are directly responsible for weight gain. High blood sugar creates the metabolic conditions which stimulate your body to store energy as fat and glycogen. Specifically, high levels of glucose in your blood stimulate your pancreas to release the natural
hormone, insulin. Insulin causes fat cells (adipocytes) to absorb the glucose in the blood and convert it into fat for storage. At the same time, high levels of blood sugar and insulin repress another natural hormone, glucagon. Glucagon stimulates your body to produce glucose by burning stored energy, such as fat. So basically, when you have high sugar levels in your blood, your body is focusing on storing energy instead of burning it. This makes it extremely difficult to lose weight. The solution is to use the Glycemic Index to help you make better choices about what to eat and what to avoid. Avoiding high GI foods will help make your other efforts, like exercising or watching your total caloric intake, that much more effective. It will also help you avoid getting Type 2 diabetes, which can have a devastating impact on your quality of life.
Like anything else, the Glycemic Index is not perfect and there are a couple of important caveats to be aware of. First, the glycemic index is based on blood glucose levels, so it can underestimate the negative impact of some foods, particularly those with high levels of similar sugars, like fructose. Take for example your average non-diet soda. It is basically carbonated sugar water, and you would logically assume that non-diet soda would have a GI value near 100. However, soda is sweetened with high fructose corn syrup, which must be converted into glucose by the enzymes in your body. As a result soda has a GI value near 65 because it doesn't raise your blood sugar as high as if it were sweetened by glucose. But it's a hollow victory, because while the peak may not be as high, it will elevate your blood sugar for a longer period of time because it takes time to convert all of the fructose into glucose. In general, if you are trying to regulate your blood sugar and lose weight, you should do your best to limit sugar intake across the board. The other caveat is that certain very high fat foods are low in sugars and simple carbohydrates and appear to have very attractive GI values. Eating vast amounts of bacon, for example, is going to have negative consequences, regardless of what the Atkins Diet plan says. The best approach is to use the Glycemic Index with a healthy dose of common sense to help you make intelligent dietary decisions.
In summary, the Glycemic Index can help you avoid foods that can unhealthily raise your blood sugar. Low GI value foods are most often those foods which are less processed and contain lower levels of sugars and refined carbohydrates, like flour. Additionally, foods with low GI values are more likely to be more nutritious, more filling and slower digesting, helping you better regulate your hunger levels. All of these factors can help you make progress towards gaining control over your weight.
References, Background Reading and Additional Information
Books:
Merl Raffetto
Jennie Miller, Kaye Foster-Powell and David Mendosa
Dana Carpender
Online Resources:
Scientific Research Papers:
Dietary Determinants of Changes in Waist Circumference Adjusted for Body Mass Index – a Proxy Measure of Visceral Adiposity. Romaguera, et al. PLoS One. 2010
Association of glycemic index and glycemic load with risk of incident coronary heart disease among Whites and African Americans with and without type 2 diabetes: the Atherosclerosis Risk in Communities study. Hardy, et al. Annals of Epidemiology. 2010
